a thesis on microquasars

8/14/2019 a thesis on microquasars

1/174

Hard X-ray and Gamma RayProperties of Cosmic Sources

A Thesis submitted to the University of Mumbai for the

Ph. D. (Science) Degreein Physics

Submitted byManojendu Choudhury

Under the Guidance of Prof. A. R. Rao

Tata Institute of Fundamental ResearchMumbai 400 005

June 2004


2/174

ii


3/174

To My Parents


4/174

ii


5/174

Acknowledgements

First and foremost, I wholeheartedly acknowledge the institutional structure, geograph-ical location, general ambiance and the colonnade of TIFR for providing the ideal sitewhere, perhaps, the most eventful years of my life, in both professional and personalfronts, were unfolded. The academic environment of TIFR provided me a very thoroughintroduction to the rigours of the research life, providing a gamut of diverse experiences,transforming me from a green rookie to a level from where (hopefully) I can take off to the next step to establish myself.

I will always remain in gratitude to my thesis supervisor Prof. A. R. Rao, for pickingme up from a situation of dire nothingness, setting me on a path, providing me a goal tostrive for, and for letting me believe in myself when most others had given up.

I am indebted to Prof. P. C. Agrawal, Prof. R. K. Manchanda, Prof. K. P. Singh,Prof. J. S. Yadav, Prof. T. P. Singh and Prof. M. N. Wahia for providing me variousacademic opportunities, discussions, and constant support during the various stages of my doctoral tenure. I would like to mention Dr. B. Paul separately for all the above, plusmany a friendly chatting sessions. I thank the scientic staff of the department for a verycongenial atmosphere in the department, and I should mention Magnes and Shobha fortheir general cheerful disposition and very helpful nature.

The greatest treasures that I will carry with me from my stay here are the friendshipsthat I forged over the years. Santosh, Gulab and Sachi provided a unique environmentof camaraderie and togetherness in the department. Discussions and conversations with

them consisted of a complete package of friendship, understanding and professionalacumen, covering all aspects of science, life and everything else that may follow! I re-ally pride ourselves at creating an atmosphere devoid of petty individualism and jealouscompetition, so common among contemporaries. I can defnitely not fail to mention ourimmediate juniors in the ofce, Vikram and Sarita, who cheerfully sustained the veryfriendly air in the ofce rooms and the tea-tables. In the department, I acknowledgethe companionship of Poonam, Sambaran, Harsha, Surajit, Uddipan, and last but def-initely not the least, Rituparno. In the institute, Pratik-da always provided a sense of togetherness unique to him (even now he is waiting for me to nish my typing, get the

i


6/174

ii Acknowledgements

printout, so that he may guide me to the book-binders). Funda-da (Pranab Sen) left aunique impression in me, Surjeet was always a constant and consistent friend, Yogesh,Yeshpal, Ashok (MP), Arvinder, Amitava, Krishnan, Rahul Jain, Neel made my lateryears in the institute very enjoyable, while Dr. Paul (Dilip), Shubham, Tirtha, Roop, Ra- jesh, Anwesh-da, Bhaswati-di, Arun, Rudrajyoti Palit, Bahniman-da, Soumen-da madethe early years unforgettable. I hope Shankar, Anand, Dipanker, Holla, Eknath, Suman,

Manna, Shamik and others of the footballing fraternity continue with the tradition of kicking the ball regularly. I will always remember Girish Nathan, Kiran and Anjum fortheir company in our rst year (graduate school), and I will never forget Biswajits an-tics, in and out of the cricket eld. I may have, inadvertently forgotten, to name manypeople who made my stay in the hostel a memorable experience. It will be a sacrilege if I fail to mention my Guruji, Shri Namdeo Panchal ji, who introduced Hindustani clas-sical music to me in the last two and a half years, and in the process rendered me a mostcreative and fullling avocation.

I sincerely appreciate the friendship of Dr. C. H. Ishwar-Chandra and the contin-ued academic collaboration with him. I am indebted to Dr. Ashok K. Jain for his sup-port, encouragements, academic discussions and research collaborations. I look forward

to the continued association along with the current ensuing collaboration with Vivek Agrawal and V. Girish. I cherish the friendship and support of Dr. Sergio Mendoza,and I hope to continue my association with him and his beloved country, Mexico, whereI was treated with great warmth and affection. I also acknowledge the hospitality of Dr. Divakara Mayya in Instituto Nacional de Astrosica, Optica y Electronica (INAOE),Puebla, Mexico.

I gladly acknowledge the Kanwal Rekhi Scholarship of the TIFR Endowment Fund,which provided partial support to this thesis. I acknowledge the Department of Astron-omy and Astrophysics, TIFR, and the NATO Advanced Study Institute for providing menancial and other logistical support enabling me to attend the

Summer School atLes Houches, France on, Accretion discs, jets and high energy phenomena in astro-

physics. I thank the IAU for providing partial support for my participation in the IAUColloquium 194: Compact binaries in the Galaxy and beyond, at La Paz, Mexico, aswell as providing complete support for the IAU

Asia Pacic Regional Meeting atTokyo, Japan. I also acknowledge the Kyoto University (Dept. of Physics Yukawa Insti-tute), ISAS and University of Tokyo (Dept. of Physics) for providing the complete sup-port for my participation in the international conference on Stellar-mass, Intermediate-mass and Supermassive Black Holes at Kyoto, Japan. I acknowledge Dr. G. C. De-wangan (Gulab) and Prof. R. Grifths for inviting me at the Carnegie Mellon University,Pittsburgh for a short visit. I should mention that Dr. S. V. Vadawale (Santosh) played


7/174

iii

host to me in Boston during my participation in the international meeting on X-ray Tim-ing: RXTE and beyond, organized by the Harvard University. I also acknowledge theInstituto de Astronomia, Universidad Nacional Autonoma de Mexico (UNAM), as wellas INAOE, Puebla, Mexico, for inviting me to short visits at the respective institutes. Ialso acknowledge the National Centre for Radio Astronomy, for providing support formy participation at the Summer school on radio interferometry and aperture synthesis,

which introduced the rudiments of radio astronomical data analysis, with emphasis onGMRT, which I hope will be of enormous benet to me in near future.No words can do justice to the support of my family, especially my wife, Rajul, who

has been the bed-rock of my support base through all times, good and bad; without herunderstanding and endearing inspiration I wouldnt have reached this day, when I cansee the completion of my thesis. Lastly, I would like to express all my love and wishesto the little bundle of joy, our eleven days old son.


8/174

Contents

Acknowledgements i

Synopsis vii

1 Introduction 11.1 High energy physical processes and phenomena . . . . . . . . . . . . . 1

1.1.1 Accretion disc as a source of high energy emission . . . . . . . 4

1.2 Black hole sources: stellar mass and supermassive . . . . . . . . . . . . 51.3 Microquasars: general properties and behaviour . . . . . . . . . . . . . 8

1.3.1 Transient X-ray blackhole binaries . . . . . . . . . . . . . . . . 91.3.2 Persistent sources: canonical states of X-ray emission . . . . . 10

1.4 Accretion in X-ray binaries . . . . . . . . . . . . . . . . . . . . . . . . 121.4.1 Unication of hydrodynamic solutions of accretion ow . . . . 171.4.2 Hard X-ray emission models . . . . . . . . . . . . . . . . . . . 191.4.3 Geometrical structure of the accretion system . . . . . . . . . . 20

1.5 Outows from microquasars . . . . . . . . . . . . . . . . . . . . . . . 211.6 Aim of this thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

2 X-ray detectors and techniques of instrumentation; Radio astronomy 262.1 X-ray detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272.2 Modern X-ray observatories . . . . . . . . . . . . . . . . . . . . . . . 31

2.2.1 The Rossi X-ray Timing Explorer (RXTE) . . . . . . . . . . . . 312.2.2 Compton Gamma Ray Observatory (CGRO) . . . . . . . . . . . 342.2.3 Other notable X-ray missions . . . . . . . . . . . . . . . . . . 37

2.3 X-ray astronomical data analyses and techniques . . . . . . . . . . . . 382.3.1 RXTE data analysis . . . . . . . . . . . . . . . . . . . . . . . . 402.3.2 Timing analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 41

iv


9/174

CONTENTS v

2.3.3 Spectral analysis . . . . . . . . . . . . . . . . . . . . . . . . . 422.4 Radio astronomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

2.4.1 Green Bank Interferometer ( GBI ) observatory . . . . . . . . . . 462.4.2 Giant Metrewave Radio Telescope ( GMRT ) . . . . . . . . . . . 47

3 Cygnus X-3: spectral studies 49

3.1 Why Cygnus X-3? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493.2 General properties of Cygnus X-3 . . . . . . . . . . . . . . . . . . . . 503.3 General spectral features of Cygnus X-3 . . . . . . . . . . . . . . . . . 53

3.3.1 A historical perspective . . . . . . . . . . . . . . . . . . . . . . 533.3.2 X-ray wide band spectra from RXTE . . . . . . . . . . . . . . . 54

3.4 Correlation of radio & X-ray emission in Cygnus X-3: Spearmans Par-tial Rank Correlation test . . . . . . . . . . . . . . . . . . . . . . . . . 593.4.1 Spearmans Partial Rank Correlation test . . . . . . . . . . . . 60

3.5 X-ray spectral pivoting in the low (hard) state . . . . . . . . . . . . . . 633.6 X-ray spectral evolution driving the radio ares: high (soft) state . . . . 673.7 Complete X-ray spectral evolution . . . . . . . . . . . . . . . . . . . . 71

4 Cygnus X-3: temporal studies 734.1 Binary modulation and correction with a given ephemeris . . . . . . . . 734.2 Radio X-ray correlation of Cygnus X-3 . . . . . . . . . . . . . . . . . 794.3 Power Density Spectrum (PDS) . . . . . . . . . . . . . . . . . . . . . 81

4.3.1 Power Density Spectrum (PDS) of Cygnus X-3 . . . . . . . . . 844.4 Time lag between soft and hard X-rays . . . . . . . . . . . . . . . . . . 91

5 Disc-jet connection in microquasars: low (hard) states 945.1 Radio:X-ray correlation of the persistent sources: hard states . . . . . . 97

5.1.1 Cygnus X-3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 985.1.2 GRS 1915+105 . . . . . . . . . . . . . . . . . . . . . . . . . . 995.1.3 Cygnus X-1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

5.2 Uniform behaviour of X-ray spectral shape with radio emission . . . . . 1065.2.1 The X-ray soft state and suppressed radio emission . . . . . . . 109

5.3 Universal correlation and its origin . . . . . . . . . . . . . . . . . . . . 1115.4 X-ray spectral shape as the driver of the radio emission . . . . . . . . 1145.5 Summary: the generalized picture of the accretion - ejection mechanism

in the low - hard state of Galactic microquasars . . . . . . . . . . . . 115


10/174

vi CONTENTS

6 Two Component Accretion Flow model 1176.1 Two Component Accretion Flow (TCAF) . . . . . . . . . . . . . . . . 1186.2 Outow of mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1206.3 The magnetized TCAF model . . . . . . . . . . . . . . . . . . . . . . 1246.4 Phenomenological picture of accretion and ejection connection . . . . . 125

6.4.1 Cygnus X-3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

6.4.2 Hybrid Comptonization . . . . . . . . . . . . . . . . . . . . . 129

7 Summary and conclusions 1317.1 Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . 131

7.1.1 Cygnus X-3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1317.1.2 Generalized picture of disc-jet connection in Galactic micro-

quasars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1357.2 Future directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138


11/174

Synopsis

An X-ray is a quantum of electromagnetic radiation with an energy, to an order of magni-tude approximation, some 1000 times greater than that of optical photons. Traditionally,the soft X-ray band is dened as the energy range 0.5 12 keV (corresponding to wave-length of 25 1

), the hard X-ray extends to 50 keV and the energy range beyondit till a few MeV is regarded as soft gamma rays, although this classication is not verystringent. High energy astronomy pertains to the observation of the sky in this regimeof the electromagnetic spectrum. The study of cosmic sources at these high energies of X-rays and gamma-rays began only in the early 1960s, after the serendipitous discov-

ery of the low mass X-ray binary (LMXRB) Sco X-1, which houses a neutron star (thecompact object) and a low mass optical companion in the main sequence.

Simple extrapolation from the optical regime suggests that, assuming the physicalprocesses giving rise to these X-ray, gamma ray emissions are thermal, the temperatureof the radiating matter should be of the order K for X-ray photons and greaterfor gamma ray photons. The fundamental physical mechanisms which give rise to highenergy emissions from a thermalised distribution of matter are few, viz. thermal black body radiation, bremsstrahlung, Compton scattering. Soon, however, it was discovered,mainly from the supernova remnants, that non-thermal physical processes also play veryimportant part in these high energy emission. Such physical processes may also involvebremsstrahlung and Compton scattering, in addition to synchrotron emission.

This interplay of thermal / non-thermal emission is best observed in accreting black hole (both stellar mass and super massive) and neutron star systems. Various theoreticalparadigms exist today which attempt to comprehensively explain the accretion phenom-ena in these systems. Shakura & Sunyaev laid the foundation of the rst standard discmodels (now known as SS discs). However these discs were untenable due to instabil-ity arising out of temperature crossing the hydrogen ionization point, and this led to theclassic disc instability paradigm which sought to explain various transitions and variabil-ities in the accretion disc systems. Improved wide band X-ray observational capabilitiesenabled the spectral energy distribution (SED) of these sources to reveal the ubiquitous

vii


12/174

viii Synopsis

presence of power law component extending well into hard X-ray and gamma ray band,along with any soft component (black body, generally multicoloured). Thereafter var-ious paradigms involving hybrid (i.e. both thermal and non-thermal) Comptonization,advection dominated accretion ow (ADAF), Compton reection, bulk motion Comp-tonization, two component accretion ows, etc., with or without magnetic eld, weredeveloped to explain the hard power law extension, the various states of black hole tran-

sitions and their variabilities. Models involving synchrotron emission, Compton self-synchrotron, etc., are also used to explain the high energy emission in these sources.Long term monitoring of the various high energy SEDs and variabilities of these sourcesare needed to devise any comprehensive physical and geometrical picture of the pro-cesses.

Quasars, which fall in the broader classication of radio loud Active Galactic Nu-clei (AGN), were discovered in the radio band of electromagnetic radiation during theera of the very early discovery of X-ray sources. These were subsequently identied tobe accreting supermassive black holes ! "$ #& %( ' of galactic scales with outowsin the form of a jet observable in the radio band by virtue of the physical mechanismof synchrotron emission. Therefore a paradigm of accretion being related to ejectionwas gradually developed, although it was not observable in the radio quiet AGNs. Thediscovery of Galactic X-ray binaries exhibiting (superluminal) radio jets, with both phys-ical and temporal (variability) scale roughly at 6 orders of magnitude less than those of quasars, led to the notion of ubiquitous presence of outow in the form of collimated jetsin accreting black hole systems and low magnetic eld ( )0 " G) neutron stars, lendingthem the terminology of microquasars. The observation of microquasars over the AGNis advantageous for, chiey, two reasons. Firstly, these sources are located within theGalaxy, the astronomical equivalence of our own backyard. And, secondly, the charac-teristic dynamical time scales in the ow of matter are proportional to the black holemass and any variability time scale of hours to days of microquasars correspond to anal-ogous phenomena with duration of hundreds to thousands of years in AGNs, assuming

that the same fundamental physical processes underlie the behaviour of these sources.Therefore monitoring the microquasars for a few days may sample phenomena not pos-sible to observe in quasars. These features led to the current upsurge in the observationalstudy of these sources. The aim of this thesis is to gather together a comprehensive pic-ture of the high energy observational features of Galactic microquasars, with a particularemphasis on the enigmatic source Cygnus X-3, in order to develop a phenomenologicalunderstanding of the fundamental processes and geometrical structure of these systems.Since accretion (inow of matter, generally in the form of discs) and ejection (outow of matter in collimated jets) are closely related, a correlated study of high energy and radio


13/174

ix

emission is presented to provide a coherent picture of the systems.Microquasars, generally black hole candidates (BHCs), mimic, at a much smaller

scale, the main astrophysical attributes of a quasar: general relativistic accretion identi-ed by the X-rays and gamma rays from the surrounding accretion discs, and the specialrelativistic outows in the form of collimated jets with low opening angles ( )1 3 2 5 4 ) ob-served by means of their synchrotron emission. Of the 200 Galactic X-ray binaries cata-

logued so far, about 20 are radio loud, half of which show evidences of radio jets, a fewof them superluminal (eg. GRS 1915+105, GRO J1655-40). These X-ray binary sourceshave some common salient characteristics which may be enumerated as follows:-

Structural characteristics:-

6 They consist of one compact object (generally a black hole candidate) and onenormal star, generally from the main sequence.

6 The compact object accretes matter from the companion, via an accretion disc.The donor may lose mass through Roche lobe overow or via stellar wind. Theextent of the inner disc is a function of time (and probably accretion rate), theexplanation of the variability generally depends on the particular model adopted

to explain the X-ray characteristics.6 The outow of matter takes place via a collimated beam, visible in the radio, at

times infra-red or, arguably, even X-ray. The conical jet has a small opening angle( )7 3 2 4 ) directed perpendicular to the accretion plane. This system may showprecessional movement in some cases.

From the observational point of view, the study of the behavioural pattern of thesesources, in the various electromagnetic bands, may be classied into three different typesof analysis: 1) image analysis, which gives the (extended) spatial information about thesource, 2) temporal analysis, which gives the variability of the source with respect totime, and 3) spectral analysis, which gives the pattern of the emission with respect tothe energy (or frequency / wavelength), and provides the best analytical tool for identi-fying the physical processes giving rise to the emission. Some basic generic patterns of the temporal and spectral characteristics (in the X-ray regime) of the microquasars arehighlighted below:-

Temporal characteristics:-

6 The X-ray light curve may show a variety of diverse variabilities, even for a singlesource, depending on the particular states or transitions among them during theperiod of observation.


14/174

x Synopsis

6 The power density spectra (PDS) shows, typically, a power law dependence witha positive index in the region of 0.01 1 Hz, at spectra for the next decade of frequency range, followed by a power law decay (i.e. negative index) of power inthe 10 100 Hz. The PDS of the neutron stars is generally shifted towards thehigher frequency region by an order of magnitude.

6 Various types of quasi periodic oscillations (QPOs) are observed in most of themicroquasars, prominently in the low-hard state of X-ray emission.

Spectral characteristics:-6 The spectra constitutes of continuum component and line emissions (mostly Fe

K 8 ). The continuum, for a canonical black hole candidate, consists of two com-ponents, a soft thermal (originating from a multi-coloured disc) and a hard non-thermal (generally characterized by a power law). The Fe line is now consideredan essential feature of black hole X-ray spectroscopy.

6 The standard black hole candidates, viz. Cygnus X-1, exhibit two distinctly dif-ferent kinds of behaviour, i) low-hard, with the soft X-ray ux low and spectral

behaviour comparatively harder, and ii) high-soft, with the soft X-ray ux higherand the SED dominated by the softer X-ray. But the individual spectra of thesources may differ dramatically from one another.

While these sources may or may not show radio aring episodes which entail hugeblobs of matter being ejected (superluminally) from the system, recently it was realizedthat non-thermal radio emission is a ubiquitous feature during the quieter phases. Thenon-thermal emission forms a substantial fraction (5%50%) of the energy budget. His-torically, the radio and X-ray studies were done in a disjoint manner for these sources,and the development of the models describing the accretion and ejection took place in-dependently. In the last decade rst efforts were made to create models to treat accretionand ejection in a unied scenario, underlying the physical connection between the two.

Meanwhile, observational strategies were developed independently to monitor some of these sources in the radio and X-ray at a regular basis to study the long-term behaviouralpatterns in these systems, chiey to observe the transient features of mostly transientand a few persistent sources. The most methodical and consistently regular of thesestrategies were the ones carried out in the radio at the Green Bank Interferometer (GBI)operated by NRAO, and concurrently in the soft and hard X-rays by All Sky Monitoraboard the Rossi X-ray Timing Experiment (RXTE - ASM, 2-12 keV) and the Burst andTransient Sources Experiment aboard the Compton Gamma Ray Observatory (CGRO -BATSE, 20-100 keV), respectively.


15/174

xi

Given the various diverse types of temporal variabilities and spectral characteristicsof the high energy emissions of the different microquasars, along with their differenttypes of radio emissions (from the outow), there was no single consistent picture thatcould provide a general scenario of the disc (accretion) jet (ejection) connection inthese systems. Our investigation commenced with the next logical step of understandingthis connection at a broad scale across the diverse type of sources of this class exhibiting

their characteristic idiosyncratic behaviour, in order to provide a unied, consistent set of observational features with the aim of developing a phenomenological model to unravelthe physical and geometrical structure of these X-ray binary systems. We achieved thisby carrying out a systematic correlation analysis among the radio, soft and hard X-rays,for the sources Cygnus X-3, GRS 1915+105 and Cygnus X-1, using the available datafrom the archives of GBI (2.2 & 8.3 GHz), RXTE-ASM (2-12 keV), CGRO-BATSE(20-100 keV), during the long term steady hard states of these systems. These threepersistent X-ray as well as radio sources were the only ones monitored simultaneouslyby these three observatories. The results of the correlation studies from these sourceswas complemented by the observations reported for GX 339-4 (and also V404 Cyg),scattered in the literature, to provide a qualitative self-consistent picture of the disc-jetconnection, using the Two Component Advection Flow (TCAF) model. In this thesis,our emphasis lies in the detailed multi-band (X-ray and radio) study of the enigmaticbinary system Cygnus X-3, where we provide the complete evolution of the radio aringepisodes of Cygnus X-3 driven by the X-ray spectral states in the system. In addition, wereport the temporal properties of the X-ray emission in this particular binary system andprovide a time scale of anti-correlation between the soft and hard X-rays in the system.

Of all the Galactic microquasars, Cygnus X-3 is one of the brightest in both radioand X-ray bands, but one of the least understood of all binary systems. Located at adistance of 9 kpc in one of the Galactic arms, it exhibits a binary period of 4.8 hours inboth X-ray as well as infra-red bands, while the radio emission doesnt show the binarymodulation. The emission lines of He I and He II in the infra-red band with the absenceof any H lines suggest the presence of dense winds and the companion to be of the Wolf-Rayet type, establishing the system to be a High Mass X-ray binary (HMXRB). Thenature of the compact object of the binary system is still not conclusively ascertained,and a prime motive of undertaking the detailed X-ray spectral study was to glean theobservational features that may pertain to any particular class of compact objects, black hole or neutron star.

To attain this goal we carried out a thorough and comprehensive analysis of the X-ray emission of the source from the complete data set of the the RXTE archives availablepublicly. This satellite observatory combined the dual advantage of the best X-ray tim-


16/174

xii Synopsis

ing capabilities (PCA) along with a very wide band X-ray spectral coverage (PCA andHEXTE). Furthermore, RXTE has an extensive collection of data sets covering the mi-croquasars at a (semi) regular basis. The analysis consisted of, downloading the raw dataof all the observations for this source, reducing and ltering the data in accordance withthe housekeeping les, extraction of lightcurve and the spectra keeping the diverse con-ditions of the source as well as the observatory in consideration, creating the response

matrix for the obtained spectra, analyzing the lightcurves and the spectra using the desig-nated software, and nally, interpreting the analyzed result in conjunction with, chiey,(contemporaneous) radio observations. The background spectra and lightcurves for PCAwere reduced from the model background event les provided by the PCA team, whilethe two HEXTE detectors alternately point on and away from the source, measuring thesource and background ux with a duty cycle of 16 seconds. The PCA background hasa time resolution of 16 seconds, as a result the timing studies above 9 3 @ keV can notbe done at a time scale less than this. All the X-ray spectra are obtained at the timeresolution of 16 seconds.

The long term X-ray spectral variation of this source shows two distinct states, highand low, and correspondingly soft and hard (although the individual spectral model com-ponents are different from a canonical high-soft and low-hard state of stellar mass black hole candidates, characterized chiey by Cygnus X-1). Correspondingly the radio emis-sion can be broadly classied into two types, persistent and aring. The persistent emis-sion predominates the X-ray hard (as well as low) state. In the X-ray soft (as well ashigh) state the persistent radio emission is quenched, interspaced by the aring events,both minor and major. The major radio ares are among the brightest in the Galaxy, oc-curring only during the X-ray high (soft) state. The distinct features of the X-ray spectraof this source are: 1) very high absorption in the soft X-ray regime, probably by the dustand/or halo engulng the binary system, whose most likely origin is in the wind from theWolf-Rayet companion, which obscures the thermal blackbody spectra, 2) above 5 keV,the continuum is complex and a mixture of model components consisting of thermalised

Comptonization and a power law is needed to model the spectra in the low (hard) state,while the high (soft) state, in most cases, shows a very strong multicoloured disc black body emission and a Comptonization component, as a result the unusual hump in the5-30 keV region present in the low (hard) state is absent in this high (soft) state, 3) thehard X-ray tail, which may be characterized by power law, is present in both the low(hard) and high (soft) states, 4) EXOSAT, ASCA & Chandra observations have revealedthree Fe lines (6.4 keV, 6.7 keV & 6.9 keV) along with two absorption edges (7.1 keV &9.1 keV) present in the source, but the resolution of the RXTE-PCA is not good enoughto resolve these to desired accuracy.


17/174

xiii

Detailed analyses of the X-ray spectra show a denite pivoting at about 12-20 keVin the hard state. In the soft state the picture is different with the thermal emission fromthe accretion becoming the prominent feature of the SED, in most cases. There is a de-nite causal relationship between the X-ray spectral evolution and the radio aring events,which may be explained by classifying this state into three sub-states: 1) the radio qui-escent phase, in which the thermal multicoloured disc black body and the Comptonizing

component are equally strong, 2) pre-radio are, during which the Comptonizing com-ponent becomes vanishingly small ( AB 3 2 D C ), suggesting the evacuation of the centralCompton cloud, resulting in the are, which may occur at a time scale of less than a day,3) post-radio are, in which the succession of radio ares, both minor and major, arebrought to an end by a change in the X-ray spectrum, with the spectral shape harden-ing and the thermal disc black body component vanishing, following which the sourcemakes transition into the low (hard) state. Unless this spectral hardening takes place,the succession of ares, minor ones interspaced by a few major ones, continues withthe Comptonizing component remaining signicantly less than the thermal component.Thus, a picture of a complete evolution of the occurrence of radio ares in relation to theX-ray spectra is obtained, for the rst time for this source. These spectral features tilt thebalance of evidence in the favour of the compact object being a black hole, although adenitive statement can only be made after ascertaining the mass function of the system.

To obtain the long term behavioural pattern of this source we carried out a systematiccorrelation analysis among radio (GBI. 2.2 GHz), soft and hard X-ray (RXTE - ASM:2-12 keV & CGRO - BATSE: 20-100 keV) emissions, during the period for which thesethree observatories were simultaneously monitoring the source, as mentioned before.We employ the Spearmans Partial Rank correlation test for the correlation among thethree variables. In the high (soft) state, the long-term correlation results are not verysignicant, due to the complex evolution of the radio as well as X-ray emission, asexplained earlier. In the low (hard) state, the soft X-ray is very strongly correlated to theradio emission, while the hard X-ray is anti-correlated to both soft X-ray as well as radioemission. This interesting correlation result is explained by the pivoting in the spectra,at about 12-20 keV, being correlated to the radio emission.

The short term temporal properties of the X-ray emission of Cygnus X-3 is perhapsthe least studied aspect of this source, also among all other microquasars this sourcesX-ray temporal properties are least explored. The principal reason for this is the fact thatthe binary modulation of 4.8 hours is too strong and smothers any other variability pat-tern that may emerge out of any analysis in the study of the shorter time scale variability.Hence, our rst step was to correct for the binary modulation using the binary templatefor a quadratic ephemeris which is amazingly stable for more than two decades of obser-


18/174

xiv Synopsis

vation. The long term RXTE-ASM monitoring data, containing all sorts of variabilitiesincluding the period of major radio ares, folded by the template shows very meagreresidue, proving its applicability to correct for the binary modulation. The latest tem-plate was obtained from the ROSAT, ASCA, BeppoSAX, RXTE and IXAE pointing andmonitoring, which was used to correct for the binary modulation in the ux variabilityin the light curves obtained at all the different X-ray energy bands, after making ap-

propriate scaling adjustments, for all RXTE-PCA pointed observations analyzed in thisthesis. The binary correction using this template is very good at the rising and fallingphase, highlighting the small variations which were otherwise smothered by the binarymodulation of the ephemeris, whereas at the peak the smooth nature of the template isnot always successful to correct the generally random uctuations in the emission. Thecorrelation among the radio, soft and hard X-ray emissions remains the same after thecorrection for the binary modulation, and detailed tests show that the anti-correlationtime-scale between the soft and hard X-rays, due to the pivoting of the spectra, is lessthan a day.

The power density spectra of this source has a feature distinct from its counterparts,the shifting of the spectra towards low frequency. In this pattern of temporal variability it

resembles less like other Galactic microquasars and bit more like the massive AGNs witha central massive black hole. One may reconcile the absence of power in the high fre-quency regime to the reprocessing of the X-ray photons in the dust and/or halo engulngthe system, reducing the amplitude of the fast X-ray variability.

The most interesting result of the X-ray timing properties reported of the source, byus, is the anti-correlated time lag of 400900 seconds between the soft and hard X-raysin the low (hard) state of the system. Unless corrected for the binary modulation, thisdelay is not observable. Also, this delay is observed only in the hard (low) states, the nonaring soft (high) state doesnt show any such delay. This anti-correlated delay betweenthe soft and hard X-ray ux, with the hard X-ray lagging, provides the dynamical timescale of the pivoting of the spectra, in the hard state. It is noteworthy that from the

long term correlation among the monitoring data in the radio, soft and hard X-ray bandswe had predicted an anti-correlation time scale shorter than a day. This is the rst suchobservation for this source and will provide stringent constraints on the accretion modelsfor microquasars in general.

The generalization of the X-ray:radio correlation, during the low (hard) state, fordifferent types of microquasars, was done by repeating the correlation study of the longterm radio(GBI, 2.2 GHz), soft and hard X-ray (RXTE - ASM: 2-12 keV & CGRO -BATSE: 20-100 keV) monitoring data of two more sources with apparently diverse be-havioural pattern, viz. GRS 1915+105 & Cygnus X-1, and collating the existing results


19/174

xv

of another black hole candidate GX 339-4. It was successfully demonstrated that allthese three sources, plus Cygnus X-3, show a very similar behaviour during the low(hard) state, i.e. pivoting of the X-ray spectra correlated to the radio emission, with theradio emission being higher in the comparative softer state (within the bounds of the hardstate). The only difference lies in the pivoting energy of the individual sources. GRS1915+105 has a pivot point between 20-30 keV, Cygnus X-1 between 50-100 keV and

GX 339-4 has it atEG F

keV. We also established that the pivot point moved furtherinto the hard X-ray / low gamma ray regime as the intrinsic luminosity of the sourcesin the soft X-ray band weakened. Furthermore, continuing the X-ray:radio associationbeyond the X-ray state transition it is revealed that the radio emission is quenched, tovarying degrees, for all the sources (during the non-aring periods). Therefore, for therst time, a uniform behavioural pattern was found of the radio emission and correlatedX-ray spectral emission evolution, encompassing various microquasars with apparentlydifferent characteristics. Most remarkably, all the sources (along with another black holecandidate, V404 Cygni), were found to show a linear monotonic increase of radio emis-sion with the soft X-ray ux, spanning a 5 orders of magnitude variation in the intrinsicluminosities, with the radio emission suppressed for the intrinsically weaker X-ray emit-ters (during the non-aring state). Arguing that radiation in both the bands, radio aswell as X-ray, are unlikely to originate from a single mechanism (like synchrotron emis-sion), we invoked the TCAF model to explain the accretion-ejection behaviour in thesesystems.

According to the TCAF model, the Compton scattered X-rays in a black hole sourceoriginate from a region close to the compact object, conned within the CentrifugalBoundary Layer (CENBOL). The X-ray spectral shape in various states of the sourceessentially depends on the location of the CENBOL. At low accretion rates, the CEN-BOL is far away from the compact object and the X-ray spectrum is dominated by athermal (-non thermal)-Compton spectrum, originating from the high temperature re-gion within the CENBOL. In the transition state, the CENBOL comes closer to the

compact object and the CENBOL can sometimes give rise to radial shocks, causing in-tense quasi-periodic oscillations. In the high state, the increased accretion rate producescopious photons in the accretion disc which cool the Compton region, giving rise to veryintense disc blackbody emission along with bulk motion Comptonization (a power-lawin hard X-rays with a photon index of 2.5). The outow rate is found to be a mono-tonic function of the compression ratio of the gas at the shock region. In this scenario,at low accretion rates, the CENBOL is far away from the compact object, a weak shock can form with low compression ratio, giving low and steady outow. If this outowgives rise to radio emission, one can expect a relation between the radio emission and


20/174

xvi Synopsis

the X-ray emission. In this state (off state to low-hard state), an increased accretionrate increases the overall amount of energy available to the Comptonizing region andhence increasing the X-ray emission. The CENBOL location would be pushed inward,increasing the compression ratio (and hence increasing the radio emission) and also canincrease the temperature and optical depth of the Comptonizing region, thus giving riseto a pivoting behavior at hard X-rays as seen in Cygnus X-1 and GX 339-4. At increased

accretion rate, the CENBOL can come closer to the compact region, giving the spectraland radio properties as seen in GRS 1915+105 and Cygnus X-3. For a given accretionrate the compression ratio, after reaching a critical value (with the shock region comingcorrespondingly closer to the event horizon), causes the source to transit into the high-soft state state, for which the radio emission is progressively suppressed, clearly seen inCygnus X-3, GRS 1915+105, Cygnus X-1 and GX 339-4.

During the aring states, the TCAF model predicts ejection of the central Comptoncloud resulting in the radio ares, with the size of the ejected blob determining the uxin the radio band. This model further predicts the absence of radio ares during periodsdominated by bulk motion Comptonization, when the infalling matter falls directly intothe event horizon due to its bulk motion, resulting in quenched radio emission. Theobservational evolution of the X-ray spectra with the radio aring of Cygnus X-3 maybe explained by transition between these two states, during the high (as well as soft)state.

Thus, in this thesis we provide a comprehensive phenomenological modeling of theclass of Galactic X-ray binary system exhibiting jets, called microquasars, with veryspecial emphasis on the enigmatic source, Cygnus X-3. The future works should en-tail a detailed quantitative modeling of the complete behavioural pattern of the sourceCygnus X-3. Then one has to reconcile the qualitative as well as quantitative features of the accretionejection mechanism in these systems with other cosmic sources showingaccretion as well ejection at diverse physical scales, namely the AGNs at one and theYSOs (Young Stellar Objects) on the other.

Chapter-wise organization of the thesis:-

6 Chapter 1 gives introduction to astronomical high energy phenomena, with em-phasis on accretion discs as sources of high energy emission. It further discussesthe cosmic black hole systems: stellar mass and supermassive, and introduces theconcept of microquasars and their importance, laying down the aim of this thesis.

6 Chapter 2 deals with X-ray detectors and techniques of instrumentation as wellas radio astronomy observatories. The modern X-ray observatory RXTE and its


21/174

xvii

data analysis techniques is illustrated in detail, this being the principal observa-tory whose data is analyzed in these thesis. Other high energy observatories, viz.CGRO is also mentioned. A brief introduction to radio astronomical observato-ries, namely GBI and GMRT are provided.

6 Chapter 3 starts with general introduction to Cygnus X-3. Thenceforth it pro-vides the detailed spectral analysis and features of this source, providing an X-rayspectral behavioural evolution during the radio ares.

6 Chapter 4 demonstrates the temporal properties of Cygnus X-3. Here the longterm radio : X-ray correlation tests and results are presented. Further, a recipe isprovided for the correction of X-ray binary modulation with the given ephemeris.Thereafter the power density spectra and time lag between soft and hard X-rayemissions is obtained.

6 Chapter 5 provides the generalized accretion - ejection mechanism of micro-quasars by extending the long term radio : X-ray monitoring to other persistentsources. Uniform behaviour of X-ray spectral shape with radio emission and uni-versal correlation is obtained in both low (hard) as well as -non aring- high(soft)states.

6 Chapter 6 describes the TCAF model, elucidating a qualitative phenomenologicalexplanation of our multi-band data analyses results, providing a uniform pictureof the disc - jet connection in the microquasars.

6 Chapter 7 summarizes the results and the phenomenological models and outlinesfuture directions for further study of the accretion - ejection mechanism.


22/174

Chapter 1

Introduction

An X-ray is a quantum of electromagnetic radiation with an energy, to an order of magni-tude approximation, some 1000 times greater than that of optical photons. Traditionally,the soft X-ray band is dened as the energy range 0.5 12 keV (corresponding to wave-length of 25 1

), the hard X-ray extends to 50 keV and the energy range beyondit till a few MeV is regarded as soft H -rays, although this classication is not very strin-gent. High energy astronomy pertains to the observation of the sky in this regime of theelectromagnetic spectrum. The study of cosmic sources at these high energies of X-raysand H -rays began only in the early 1960s, after the serendipitous discovery of the lowmass X-ray binary (LMXRB) Sco X-1, which houses a neutron star (the compact object) and a low mass optical companion in the main sequence. These early observationswere made by rocket ights, which provided only a few minutes of data. This eld of astronomy matured with the advent of use of balloon ights and the rst generation X-ray astronomy satellites during the 1970s. Post 1980s the enormous advancement andprogress in the computational and technological capabilities completely revolutionizedthis branch of astronomy so much so that currently high energy astronomy is, perhaps,the most happening branch of astronomy and astrophysics.

1.1 High energy physical processes and phenomena

Simple extrapolation from the optical regime suggests that, considering a thermal originof the physical processes giving rise to these high energy X-ray & H -ray, the temperatureof the radiating matter should be of the order of I D K for X-ray photons and greaterfor the H -ray emission. The fundamental physical mechanisms which give rise to highenergy emissions from a thermalised distribution of matter are few, viz. thermal black

1


23/174

2 Chapter 1. Introduction

body radiation, bremsstrahlung, Compton scattering. Soon, however, it was discovered,mainly from the supernova remnants, that non-thermal physical processes also play veryimportant part in these high energy emission. Such physical processes may also involvebremsstrahlung and Compton scattering, in addition to synchrotron emission.

Black body radiation. The spectrum of thermalised emission of a black body is given

by (see, for eg. Rybicki and Lightman 1979)

P Q

S RT ' UW V

X

Y

X

@ `

V

a

Qc b df e

g

(1.1)

where Ri h temperature of the body,V

h frequency of the radiation and ` , p ,Y

h Plank constant, Boltzmann constant and velocity of light, respectively. Atemperature of K will render most of the radiation in the X-ray band.

Thermal bremsstrahlung. Free-free emission occurs with the interaction (accelera-tion) of energetic electron with the near stationary nuclei, with a fractional depo-sition of the energy into the electromagnetic radiation. For a hot thermal plasmathe spectrum is given by

P

V

' qs r

Xc tu tw vy x

v

p RVI

RT ' (1.2)

where R h temperature,t v

h electron density, ri h atomic no. of the con-cerned nuclei,

t

h space density of nuclei in laboratory frame andVI

R ' h

the Gaunt factor which gives the correction due to the collision parameters afterintegrating over velocity.

Synchrotron radiation. The process of radiation due to acceleration of charged par-

ticles, normally electrons, in a region containing magnetic eld is known as syn-chrotron radiation process for relativistic velocities of the electrons (in the non-relativistic limit the process is known as cyclotron emission).

P

V

'( q

x

V

f

(1.3)

where 1 h magnetic eld and h spectral index when the spectrum is a power-law of electron energy distribution, given by

t

'j qk m lo n .


24/174

1.1. High energy physical processes and phenomena 3

Comptonization. The scattering of an energetic photon with a stationary electron isknown as Compton scattering, where fractional energy is transferred to the elec-tron. The inverse of this process involves transfer of energy from energetic elec-tronto a low energy photon (inverse Compton scattering), and it is a very importantphysical mechanism generating high energy radiation in cosmic sources. The in-teraction between ensembles of photons and electrons is known as Comptoniza-

tion. The evolution of the photon and electron energy distributions subject toComptonization is governed by the Kompaneets equation (Kompaneets 1957, alsosee Rybicki and Lightman 1979)

5

U

x

p R

Y

X X

x

z &

X

( {

(1.4)

where} |

vf ~

e

Y

' h time measured in units of mean time between scat-terrings. Eq. 1.4 may be expressed in terms of the Compton parameter in anintegrated form

5

U

X

0

x

z &

X

{

(1.5)

where U `V

p R

v

, U ( p Rv v

Y

X

'

~

e

t vf o

and is the occupation numberwhich is dened as the number of particles or photons per state. If the energydensity of isotropic radiation in the frequency interval

V

toV V

isQ

, then thenumber density of photons is

Q

V

`

V

and the occupation number is given by

V

'( U

Q

Y

`

V

(1.6)

The value of the Compton parameter determines the type of Comptonization i.e.saturated or unsaturated. General solutions of the Kompaneets equation need to beobtained numerically, although it is possible to nd analytic solutions, under someapproximations. There have been many such attempts in this direction (e.g. seeSunyaev and Titarchuk 1980, 1985, Lightman and Zdziarski 1987, Titarchuk 1994,Poutanen and Svensson 1996, etc.) and accordingly many different Comptoniza-tion models, which are valid under various sets of approximations, are availablefor modeling the X-ray spectra from cosmic sources.

X-ray line emission. Excitation of the lowest-shell electrons (K,L, etc.) into the un-bound state results in emission of radiation in the X-ray band when the emptylevels are lled by transition form outer shells. However, in order to excite theselow shell electrons the excitation energy need to be very high as well, which is


25/174


only possible in environment with extremely high temperature or with extremelyhigh density high energy photon elds. Thus, the X-ray line spectroscopy providesus a powerful method for plasma diagnostics under extreme conditions.

1.1.1 Accretion disc as a source of high energy emission

The most common physical phenomenon that gives rise to emission in the X-ray band isaccretion of matter onto compact objects. This process involves accumulation of diffusegas or matter onto the compact object under the inuence of gravity, and is expected tobe responsible for the observed properties of a wide range of X-ray sources from X-raybinaries to Active Galactic Nuclei (AGNs) (Frank et al. 1992).

A mass being accreted to a body of mass # and radius will lose potentialenergy , which, if converted to electromagnetic radiation, will cause the system tohave a luminosity of

U

#

U

@

Y

XI 3

Uk

Y

X

(1.7)

where $ U @ # YX

h Schwarzschild radius, and U X 3 ' gives the efciencyof the accretion process. Thus the efciency of energy conversion due to accretion sim-ply depends upon the compactness of the accreting body. For a white dwarf star with

# U #& % , 3 U Fm & m and 2 m m U Fm & I l . Correspondinglyfor 1 # % neutron star with R = 15 km, Ui 1 , and for black holes ranges from0.06 (Schwarzschild blackhole) to 0.426 (maximally rotating Kerr blackhole). Thus,black holes, particularly, maximally rotating ones, are the most powerful energy sourcesin the Universe and accretion is the process by which the energy is released. Despitethis high efciency of emission of electromagnetic energy, the balance between the out-wardly directed radiation pressure (obtained from Thomson cross-section) and inwardlydirected gravitational pressure limits the luminosity to a limiting value, called Eddingtonluminosity , which is given by

j

U

@

$

n

Y

~

eU F

x

#

# %

(1.8)

It should be noted that this is derived assuming spherically symmetric geometry. Itis possible to exceed the Eddington limit by adopting a different geometry, but not by alarge factor. Further, the limit applies to steady-state situations and in none steady-statesituations like supernova explosions the Eddington limit can be exceeded by a largemargin.

1 the efciency for nuclear burning in neutron stars is only $ f c


26/174

1.2. Black hole sources: stellar mass and supermassive 5

Thus observation of accretion of matter onto compact objects provides a unique op-portunity to investigate the most powerful energy sources in the Universe. Two suchclasses of objects exist in the universe known to us:-

1. Active Galactic Nuclei. These are supermassive blackholes, with the mass of compact object # 1 l "$ # % , at the centre of galaxies. The blackhole accretes

mass from the interstellar medium around it, disrupting the stellar structures andconsuming the matter within the envelope of its horizon.

2. Galactic X-ray binaries. These binary system have stars with a compact object,neutron stars or blackhole candidates, accreting matter from the companion star,which is normally in the main sequence.

1.2 Black hole sources: stellar mass and supermassive

Since the efciency of conversion of gravitational energy into electromagnetic radiationis more for blackholes, as compared to neutron stars, they provide a unique opportunity

to understand the behaviour of matter under extreme physical conditions. Also, the phe-nomena of accretion makes the black hole visible to observers from a distance, whichotherwise doesnt allow any particle, including light photons, to escape and thus renderthe direct observations of these sources impossible. Therefore these sources offer theopportunity to experimentally verify the general theory of relativity directly, and henceput to test the underlying fundamental principles of our understanding of the physicalstructure of the universe.

It is presently established that blackholes exist in two classes, supermassive andstellar mass blackholes. 2 As mentioned above, the accretion phenomenon is ubiqui-tously observed in Active Galactic Nuclei (henceforth referred to as AGNs) as well asthe Galactic X-ray blackhole binary systems. The more interesting observational fea-

ture seen in these sources is that accreted matter is more often than not ejected out inthe form of a jet perpendicular to the accretion disc. In fact, the observations, in theradio band, of outowing conical jets from the core of the galaxies, often exhibiting su-perluminal (with apparent velocity greater than that of light) expansion, were reportedbefore the phenomenon of accretion was inferred from the observational studies, forthe extra-Galactic sources. Quasars, a sub-class of AGNs, were discovered in the radioband of electromagnetic radiation during the era of the very early discovery of X-ray

2 recently there are growing arguments regarding a third kind, the intermediate mass blackhole, but adiscussion on this topic is beyond the purview of this thesis


27/174


sources. These were subsequently identied to be accreting supermassive black holesT s " #& %( ' of galactic scales with outows in the form of a jet observable in the

radio band by virtue of the physical mechanism of synchrotron emission. Thereforea paradigm of accretion being related to ejection was gradually developed, although itwas not observable in the radio quiet AGNs. The discovery of Galactic X-ray binariesexhibiting (superluminal) radio jets, with both physical and temporal (variability) scale

roughly at 6 orders of magnitude less than those of quasars, led to the notion of ubiq-uitous presence of outow in the form of collimated jets in accreting blackhole systemsand low magnetic eld ( )i " G) neutron stars, lending them the terminology of mi-croquasars. There are obvious advantages of observing microquasars (over quasars) inorder to understand their physical and geometrical structures:-

1. Microquasars are located within the Galaxy, the astronomical equivalence of ourown backyard. Their proximity enables the study of both the lobes of the out-owing jet more feasible and practical. The measurement of both the componentsas well as evolution of the ux and size ratio of the two components are essen-tial to attempt a detailed modeling of the jets. Further, the improved accuracy of the measurement of the jet properties provide more accurate values of the basicparameters of such astrophysical structures.

2. The characteristic dynamical time scales in the ow of matter are proportional tothe black hole mass and any variability time scale of hours to days of microquasarscorrespond to analogous phenomena with duration of hundreds to thousands of years in AGNs, assuming that the same fundamental physical processes underliethe behaviour of these sources. Therefore monitoring the microquasars for a fewdays may sample phenomena not possible to observe in quasars.

Over the last few years of intense observations one disadvantage has become glaring inthe case of the microquasars:-

1. The dynamical time scales of the fast variability, milli-second variation for micro-quasars and correspondingly s for AGNs, do not provide enough photon countfor any meaningful statistical analysis, because the present day detectors do notpossess the capability to capture enough photons necessary during the small timespan of the very fast variability.

Despite this one deciency, in the recent past there has been tremendous upsurge in theobservational study of the microquasars, and understandably so.


28/174

1.2. Black hole sources: stellar mass and supermassive 7

Fig. 1.1: The similarities between quasars and micro-quasars. Both systems contain similar ba-sic ingredients viz. (1) a central black hole, (2) an accretion disk and (3) collimated jets of relativistic particles. The difference between them lies in the mass scale. Micro-quasars have stellar mass blackhole of the order of a few solar mass and the jets travelto a distance of a few light years whereas in quasars the black hole mass is supermas-sive of the order of - j - $ solar mass and the jet can travel to a distance of a fewmillion light years. This gure is taken from Mirabel and Rodriguez (1998)


29/174


1.3 Microquasars: general properties and behaviour

Microquasars, generally black hole candidates (BHCs), mimic, at a much smaller scale,the main astrophysical attributes of a quasar: general relativistic accretion identiedby the X-rays and gamma rays from the surrounding accretion discs, and the specialrelativistic outows in the form of collimated jets with low opening angles (

15 )

observed by means of their synchrotron emission. Of the 200 Galactic X-ray binariescatalogued so far, about 20 are radio loud, half of which show evidences of radio jets,a few of them superluminal (eg. GRS 1915+105, GRO J1655-40). These X-ray binarysources have some common salient characteristics which may be enumerated as follows:-

Structural characteristics:-

6 They consist of one compact object (generally a black hole candidate) and onenormal star, generally from the main sequence.

6 The compact object accretes matter from the companion, via an accretion disc.The donor may lose mass through Roche lobe overow or via stellar wind. Theextent of the inner disc is a function of time (and probably accretion rate), theexplanation of the variability generally depends on the particular model adoptedto explain the X-ray characteristics.

6 The outow of matter takes place via a collimated beam, visible in the radio, attimes in the infra-red or, arguably, even in the X-ray. The conical jet has a smallopening angle (

15 ) directed perpendicular to the accretion plane. This systemmay show precessional movement in some cases.

From the observational point of view, the study of the behavioural pattern of thesesources, in the various electromagnetic bands, may be classied into three different types

of analysis: 1) image analysis, which gives the (extended) spatial information about thesource, 2) temporal analysis, which gives the variability of the source with respect totime, and 3) spectral analysis, which gives the pattern of the emission with respect to theenergy (or frequency / wavelength), and provides the best analytical tool for identifyingthe physical processes giving rise to the emission. The spectral analysis identies the in-dividual physical mechanism which gives rise the electromagnetic emission. A completeunderstanding requires the combined understanding of the all the three analysis. Somebasic generic patterns of the temporal and spectral characteristics (in the X-ray regime)of the microquasars are highlighted below:-


30/174

1.3. Microquasars: general properties and behaviour 9

X-ray temporal characteristics:-6 The X-ray light curve may show a variety of diverse variabilities, even for a single

source, depending on the particular states or transitions among them during theperiod of observation.

6 The power density spectra (PDS) shows, typically, a power law dependence with

a positive index in the region of

0.01 1 Hz, at spectra for the next decade of frequency range, followed by a power law decay (i.e. negative index) of power inthe 10 100 Hz range. The PDS of the neutron stars is generally shifted towardsthe higher frequency region by an order of magnitude.

6 Various types of quasi periodic oscillations (QPOs) are observed in most of themicroquasars, prominently in the low-hard state of X-ray emission.

X-ray spectral characteristics:-6 The spectra constitutes of continuum component and line emissions (mostly Fe

K 8 ). The continuum, for a canonical black hole candidate, consists of two com-

ponents, a soft thermal (originating from a multi-coloured disc) and a hard non-thermal (generally characterized by a power law). The Fe line is now consideredan essential feature of black hole X-ray spectroscopy.

6 The standard black hole candidates, viz. Cygnus X-1, exhibit two distinctly dif-ferent kinds of behaviour, i) low-hard, with the soft X-ray ux low and spectrumcomparatively harder, and ii) high-soft, with the soft X-ray ux higher and thespectrum dominated by the softer X-ray. But the individual spectra of the sourcesmay differ dramatically from one another.

1.3.1 Transient X-ray blackhole binaries

Most of the Galactic blackhole candidates are transient in nature, which implies thatthe source is not continuously detectable by the X-ray astronomical detectors given thecurrent sensitivities of these instruments (with improving sensitivity it is increasinglypossible to probe into the off state of X-ray emission of these sources). The iden-tication of the optical counterparts during the off state of the X-ray emission hasrecognized many new blackhole candidates by virtue of the mass function of the binarysystem which is given by

# '( U

@

U

#

u

#

# '

X (1.9)


31/174


Fig. 1.2: The long term monitoring of transient X-ray binary blackhole candidates by RXTE ASM. These sources are also the Galactic superluminal sources.

where # & # h masses of the companion star and the compact object respectively,

h inclination angle of the orbit of the binary system,

h period of the binary systemand

h velocity amplitude obtained from the Doppler shifts of the spectral lines of thecompanion due to the orbital motion. These sources are normally Galactic sources thatbrandish superluminal motion, in the form of outowing jets. Normally these superlu-minal expansions occur during the X-ray on state. The X-ray bright state may persistfrom less than a day to more than a decade (see Figure 1.2).

1.3.2 Persistent sources: canonical states of X-ray emission

These sources, in obvious contrast to the transient ones, never go off in the X-rayband. Always visible (Figure 1.3), they present the opportunity to study the X-rayemission behaviour in the various states in which they may exist, evolving from one toanother in a cyclic process. There are very few persistent established Galactic blackholecandidates, whose mass function is known, viz. Cygnus X-1, GX 339-4. As evidentfrom Figure 1.3, they exhibit prolonged episodes of high and soft states, whose spectralshapes are intrinsically different and hence are classied into different states (Tanaka


32/174

1.3. Microquasars: general properties and behaviour 11

Fig. 1.3: The long term monitoring of persistent X-ray binaries by RXTE ASM. Cygnus X-1(top panel) is an arche-typical blackhole candidate, and Cygnus X-3 (bottom panel).

and Lewin 1995), the high-soft and low-hard states. The sufx soft and hardare added to emphasize the X-ray spectral shape during the concerned states. Withdetailed studies of different Galactic BHCs the presence of other states like intermediateand very-high states have emerged. In general the spectra is described by two continuumcomponents, 1) soft thermal component believed to originate from a multi-coloured disc,and 2) hard non-thermal component characterized by a power law. Figure 1.4 shows thewide band X-ray/ H -ray spectra of Cygnus X-1 in three different canonical states whichclearly demonstrate the differences in the spectral properties of these X-ray states.

High-soft state. In this state the soft X-ray ux is higher and the spectral energy dis-tribution (SED) is dominated by the softer X-ray ux below 10 keV, i.e. of the twocontinuum components describing the spectra the multicoloured disc black body com-ponent dominates the SED, which most likely originates in the inner regions of the ac-cretion disc. The powerlaw component is, at times, regarded as the blackhole accretionsignature (Tanaka 2000). The temporal characteristic feature of this state is the strongaperiodic variability over a wide range of time scales from milliseconds to minutes re-ected in a typical power-law shape of the power density spectrum.


33/174


Soft

HardIntermediate

Fig. 1.4: The various canonical states of Galactic blackhole candidates, characterized by thearche-typical BHC Cygnus X-1. The gure obtained from Gierli nski et al. (1999)

Low-hard state. In this state the soft X-ray ux is low and spectral behaviour compar-atively harder, i.e. the powerlaw component dominates the SED. This powerlaw com-ponent is commonly regarded as a signature of Comptonization process in the system.The temporal behaviour is characterized by the presence of quasi-periodic oscillations(QPO).

Other states. During the intermediate state the shape of the SED is intermediate be-tween the high-soft and low-hard states described by the two continuum components of the canonical states, while the lightcurve shows the presence of QPOs. The very-high

state has the SED with a shape similar to the intermediate state but with total luminosityhigher than that of the high-soft state. For completeness sake one may also mention theoff state, during which the transient sources are not detectable.

1.4 Accretion in X-ray binaries

Accretion, by denition, means accumulation of diffuse gas or matter onto some object under the inuence of gravity. The importance of this phenomenon as a source of


34/174

1.4. Accretion in X-ray binaries 13

Fig. 1.5: The two types of accretion process in X-ray binaries. Left panel: accretion via stellarwinds, more common in HMXBs. Right panel: accretion via Roche- lobe overow,predominant in the LMXBs.

highly energetic emission was realized with the discovery of X-ray binary systems andsubsequently applied to interpret the properties of cataclysmic variables and AGNs.

The Newtonian dynamics states that a test particle with initial velocity fallingonto a massive object due to gravitational attraction (conserving angular momentum) isdescribed by

X

`

X

X

@ #

UB

X

(1.10)

where U

X

h specic angular momentum , and

are polar coordinates. Hencefor a non-zero ` the particle can never reach Uk . Using Einsteins general relativistictreatment the dynamics of the same particle in the Schwarzschild metric is given by

X

`

X

X

@ # @ # `

X

Y

X

U1

X

g $ '

Y

X

(1.11)

where ` & are constants of motion, UG where is proper time, while ` and

X

! $ ' are constants ` and X

(in Newtonian case). Due to the term lX

theparticle is allowed to fall to U if ` )i @ Y . Hence, relativistically the effect of gravity is increased close enough to UW overcoming the centrifugal barrier. Also,there is a last stable orbit (Schwarzschild metric) with radius Us F D beyond which thecircular orbits cease to exist and the particle spirals rapidly towards the singularity (seeLongair 1994, for a detailed treatment).

Accretion ow is essentially of two types, 1) Bondi type ow, which has (quasi)spherical geometry with angular momentum A

, and 2) disc type ow, where the


35/174


inow is attened into a disc with high angular momentum, E

, where

h theangular momentum at minimum stable orbit F . Even for the rst case it can be shownthat for a spherically symmetric infalling cloud with axisymmetric rotation the infallinggas passes through a shock at the equator dissipating the kinetic energy perpendicularto the equatorial plane, resulting (in most cases) in the formation of a disc (Hartmann1998). This implies that even if the accreting matter is falling at an arbitrary angle with

respect to the general rotation axis, the velocity components parallel to the rotation axisgets canceled and the matter eventually settles down in the plane perpendicular to therotation axis forming an accretion disk. In X-ray binaries, the accretion of materialhappens in two modes (Figure 1.5):-

6 Wind Accretion. More common in the High Mass X-ray Binary systems (HMXB),the companion loses mass in the form of stellar wind. The material of the windwithin the capture radius , U

X

, where # h mass of X-ray emittingcompact object, and h velocity of wind relative to the # , is trapped by thegravitational potential and accreted in.

6 Roche-lobe accretion. Predominant in Low Mass X-ray Binary system (LMXB),the companion expands to ll the Roche-lobe and matter ows through the innerLagrangian point to enter the gravitational pit of the compact object. The conser-vation of angular momentum ensures the formation of an accretion disc (Frank et al. 1992). In this case the efciency of the disc formation is denitely higher.

The necessary conditions for the accretion to occur are two-fold, rstly, energy needsto be dissipated as the matter falls into the gravitational well, and secondly, the angularmomentum needs to be transferred outwards. Without fullling these two conditions,the matter will remain in a circular ring (keplerian stable orbit) around the compactobject. The theoretical difculty lies in accounting for a physical mechanism of angularmomentum transfer as, the only obvious agent available, the ordinary uid viscosityis far too weak to be a signicant factor. Shakura and Sunyaev (1973) took the rst

step in developing a phenomenological understanding of the process by introducing ananomalous internal stress (vertically averaged along the disc axis), given by

U 8

(1.12)

where

h vertically averaged pressure, 8 h normalized proportionality constant, suchthat Ak 8 A0 . This idea perpetuated the concept of anomalous viscous stress, denedby the parameter 8 , as the agent necessary to transfer the momentum and energy. 3 A

3 Sunyaev later commented that he was not absolutely convinced about the introduction of the param-eter. Little did they realize the forthcoming impact of this


36/174


commonly used modication of the Shakura-Sunyaev (SS) prescription is to assume akinematic viscosity of the form

V

8

Y$ f

8

Y

X

, where YX

h the ver-tically averaged sound speed (

is vertically averaged density, U ' h verticalhalf-thickness of disc, and

h angular velocity of test particle circular orbits (Kep-lerian angular velocity). When inserted into the standard viscous form of the stress, thisgives

U

V

5

U 8

5

(1.13)

Here

U

'( h actual angular velocity in the ow, which may differ from

if theow is not completely geometrically thin. If

T

, then within factors of order of unity, i.e. the disc is geometrically thin equation 1.13 gives the same stress as equation1.12, with the only difference that the stress here depends explicitly on the shear of theow, just like an ordinary viscous ow (for detailed explanation see Longair 1994, Frank et al. 1992, Blaes 2003). The conditions for the validity of the thin disc are that 1) at anyradius , the vertical height , 2) rotation velocity I the sound speed Y3 withinthe disc (i.e. internal pressure gradients should not inate the disc). In such cases theaccretion ow is extremely turbulent with the Reynolds number W

X

.

Carrying on with the recipe of Shakura and Sunyaev (1973) for the thin disc, theluminosity and the emission spectra of the disc are given by (Longair 1994)

U0

x

@ U

#&

@

(1.14)

P

V

' q

V

b

b

c I g $ '

l

b

(1.15)

where h inner most radius of the disc,j

and

h the boundaries of the mul-ticoloured disc considered and U `

V

p R ' . The integral of equation 1.15 is denite,hence

P

V

' q

V

b

between the frequencies corresponding toj

and

radii. Atfrequencies less than that corresponding to the temperature of the disk at

, the spec-

trum has a shape of the Rayleigh-Jeans spectrum i.e.P

V

' q

V

X

whereas at frequenciesgreater than that corresponding to the temperature at

, the spectrum has a shape of the high frequency tail of the blackbody spectrum i.e.

P

V

'j q

a

l

Q b df e

.The optically thick SS discs (or 8 discs) became the hallmark of the accretion disc

theory because of the rather robust spectral predictions. The predicted emission fromthese disc should consist of a sum of blackbody spectra, peaking at a maximum tempera-ture of k

keV for near Eddington accretion rates or at F keV for an accretion rateof $ C Eddington, onto a typical blackhole ( D # % ). These discs, however, weresoon discovered to be unstable, at low mass accretion rate, due to instability induced


37/174


by the transition of neutral hydrogen to ionized gas (gas pressure dominated regime)causing opacity variation in the solution (see Figure 1.6). At the temperature when thehydrogen ionizes, the (Wiens) photons are absorbed and the energy is not allowed toescape, heating up the disc which causes a runaway heating until most of the hydrogenis ionized. Thus, introduction of instability due to the temperature crossing hydrogenionization at any point in the disc causes the whole disc to become unstable. This led to

the classic disc instability paradigm, which was successful in explaining the dwarf-novaetype of outbursts in white-dwarf binaries. In the case of neutron stars and blackhole bi-naries, the X-ray irradiation contributes to the hydrogen ionization and hence controlsthe disc evolution. At higher mass accretion rates (the radiation pressure dominatedregime, i.e. total pressure q0 R ) instability is introduced as a small increase in temper-ature causes large increase in heating rate, and hence to further increase in temperature.This runaway heating stabilizes only when the time scale for the radiation to diffuse outis longer than the accretion time scale, i.e. the photons are advected into the blackhole(slim disc optically thick accretion: Abramowicz et al. 1988). The spectra of theseslim discs differ from the SS discs as the energy generated in the innermost radii of thedisc is preferentially advected (Watarai and Mineshige 2001), but the disc structure atsuch high mass accretion rate is uncertain.

Alternative solutions to the SS disc models predict a truncated disc with an opticallythin, X-ray hot accretion ow in the inner regions. The rst such a solution was givenby Shapiro-Lightman-Eardley model (SLE: Shapiro et al. 1976) of accretion ow (seeFigure 1.6), where the electrons cool by radiation while the protons cool by Coulombcollisions, and hence the ow is intrinsically a two temperature plasma. 4 In the SLEow the electrons radiate most of the gravitational energy through Comptonization of photons from the outer disc. The other popular solution of the optically thin accretionow also assumes that the gravitational energy is given mainly to the protons, while theelectrons are heated via Coulomb interaction, forming, again, essentially a two temper-

ature plasma; but in this case the protons carry most of the energy inside the blackholehorizon, resulting in an advection dominated accretion ow (ADAF: Narayan and Yi1995). The ADAF solution is supposed to be more stable than the SLE solution. Achief drawback in these models is that the proton temperature approaches the virial tem-perature, hence the pressure support becomes important and the ow no longer remainsgeometrically thin. Another model with the truncated disc geometry is the two com-ponent accretion ow model (TCAF) (Chakrabarti 1996 a ), which assumes a thermal

4o , with maximal temperature possibly as high as

(

K


38/174


S L E

A D A F

log (vertical optical depth through the disk)

Hydrogen ionizationinstability

Radiation pressureinstability

a d v e c t

i o n

o p t i c a

l l y t h

i c k

l o g m .

Fig. 1.6: The multiple solutions to the accretion ow equations, plotted as a function of (massaccretion rate) vs. (optical depth) of the uid ow in the disc. The right hand solutionis the SS disc modied by advective cooling in the highest mass accretion rates and by

atomic opacities at lowest mass accretion rates. The left hand solution is the opticallythin ows, the ADAF and SLE solutions. (The gure obtained from Done (2002)

multicoloured outer disc and a hot inner sub-Keplerian ow separated by a centrifugallybounded layer (CENBOL), with the presence of bulk-motion Comptonization in the in-ner Comptonizing cloud (Chakrabarti and Titarchuk 1995, 1996). This TCAF modelwill be discussed in more detail in chapter 6.

1.4.1 Unication of hydrodynamic solutions of accretion ow

Of the four physically distinct sequences of accretion discs thermal equilibria that exist

locally (Figure 1.7), two correspond to 8 A 8

' , and other two correspond to 8 E8

' , where 8 h implies critical viscosity, which depends strongly on the radiusand weakly on the mass of the central accreting object. Chen et al. (1996) state a valueof 8 0 while Bjornsson et al. (1996) nd 8 E after treating the microphysicsand inner boundary conditions more accurately. The two low viscosity ow sequencesof models are further differentiated by the optical opacity ( ) of the ow, optically thinor thick, while the two high viscosity sequences by whether advection is negligible ordominant (quantied by the ratio of the advected energy,

, to that of radiated energy

). These four sequences, which are effectively a highlight of the local classication


39/174


Fig. 1.7: The four types of accretion ows labelled with their cooling mechanisms. The so-

lutions are plotted as a function of accretion rate (

) against the vertically averagedsurface mass density ( ). (The gure is taken from Bjornsson et al. (1996))

at a xed radius of the accretion disc, are as follows:-

Type (1): 8 A 8 s EB

varies: This S shaped sequence has threebranches: 1) lower h gas pressure dominated, radiatively cooled, optically thick classical SS disc with modied (multi-coloured disc) blackbody radiation, wherethe bend (i.e. instability) is due to strong opacity dependence on temperature(the characteristic of dwarf novae), for higher accretion rate the opacity is deter-mined by electronscattering and the branch is both thermally and viscously stable;2) middle h optically thick, radiation pressure dominated (and cooled), but ther-mally and viscously unstable classical SS disc (physically unviable); 3) upper hoptically thick, radiation pressure supported, advection cooled, thermally & vis-cously stable, slim accretion disc branch.

Type (2): 8 E 8 varies,

: This shaped sequence has threebranches: 1) right h identical (with higher viscosity) to the lower branch of type1; 2) upper h thermally and viscously unstable discs with

and 1 , the solution is obtained phenomenologically for the latter parameter value;3) left h optically thin and very hot SLE solutions with protons having much


40/174


higher energies than electrons, where radiative processes include bremsstrahlung,synchrotron & Comptonization.

Type (3): 8 A 8 i A

varies: This E shaped sequence has twobranches: lower h SLE models; upper h gas pressure dominated, very hotADAF discs (with very low radiative frequency), where the spectra are dominatedby Comptonization of bremsstrahlung and synchrotron soft photons and pairs aresuppressed by strong advective cooling.

Type (4): 8W E 8 varies,

E : This straight line sequence has twobranches: 1) lower h ADAF solution, similar to upper branch of type 3; 2) upper h copious electron positronpair production in hot, effectively optically thinplasma cloud where two conditions are met i) temperature of plasma is sufcientlyhigh, p R

v

Y

X

and ii) optical depth to photon-photon interactions is suf-ciently large. The radiation eld may include bremsstrahlung, Comptonization of internally generated bremsstrahlung photons, as well as from a hybrid populationof electron positron pair population (Coppi 1992).

1.4.2 Hard X-ray emission models

Wide-band X-ray spectroscopy reveals the presence of hard X-rays, in all spectral states,as a generic feature of accretion onto a blackhole and this emission cannot be explainedby the standard SS disc models. The only physical phenomenon that can account foremission in this energy band is magnetic reconnection, while the physical mechanismmost easily and commonly used to explain the wide band spectra is Comptonization(successive Compton scatterings of soft photons). A very brief outline of the attempts toexplain the hard X-ray emission models is as follows:-

6 MHD dynamo disc viscosity. For an SS disc, to produce the hard X-rays a largeamount of gravitational energy has to be dissipated in optically thin medium, andan obvious candidate (perhaps the only one) is the magnetic ares above the disc,generated by the Balbus-Hawley MHD dynamo responsible for the disc viscosity(Balbus and Hawley 1991). Buoyancy causes the magnetic eld loops to rise to thesurface of the disc and hence they may reconnect in regions of fairly low particledensity.

6 Comptonization from hybrid thermal/non-thermal pair plasma. This model in-volves the seed soft (X-ray) photons getting Comptonized by a cloud of hot elec-tron or electron positron pair plasma (Coppi 1992). The thermal population of


41/174


plasma assumes a given temperature, Rv

and a Thomson optical depth, e

. Thedeviation of the electronpopulation in the corona from the Maxwellian may be ex-plained by a hybrid thermal /non-thermal population of (pair) plasmas (Poutanenand Coppi 1998). Selected electron from a thermal distribution are accelerated torelativistic energies (Zdziarski 2000), probably in the reconnections (for a reviewsee Poutanen 1998).

6 Compton reection. This implies photo-electric absorption and Compton down-scattering of hard radiation from the disc which may be cold (White et al. 1988,Lightman and White 1988, George and Fabian 1991, Magdziarz and Zdziarski1995) or ionized (Done et al. 1992) or may switch between the two states (Zycki et al. 1998). These reprocessed photons may again act as seed for the Comptonizingcorona. The energy balance of the cold and hot phases determine the temperatureand the shape of the emerging spectrum (Haardt and Maraschi 1991, 1993, Sternet al. 1995, Poutanen and Svensson 1996).

6 Bulk motion Comptonization. Accretion ow (passing through a shock) forms aquasi-spherical inow, the free-falling electrons near the horizon has large bulk motion Comptonizes the seed photon from the optically thick, cold, geometri-cally thin, accretion disk, giving rise to the power law spectrum (Chakrabarti andTitarchuk 1995, 1996). This model will be discussed in more detail in conjunctionwith the TCAF model in chapter 6.

1.4.3 Geometrical structure of the accretion system

There are various proposed geometrical structures dening the accretion ow into thecompact object (blackhole event horizon) and the Comptonizing corona which is theorigin of the high energy tail in the spectra:-

6 A hot (magnetic) slab-corona sandwiching the cold accretion disc. In extremecases of complete dissipation of energy the spectra resemble the Seyfert galaxies(Haardt and Maraschi 1991, 1993). The predicted steep spectra is not in accor-dance with the observations (Poutanen 1998, Done 2001).

6 Patchy static magnetic corona above the disc. For the cold disc the predicted spec-trum is harder, but the reection albedo expected is generally too small for neutraldisc (Gierlinski et al. 1997). Also, the predicted anisotropic break is not reallyobserved. For the ionized disc, the formalism of accretion disc is complicatedwith repercussions on the Compton reected spectra (Ross et al. 1999, Don

a thesis on microquasars

Documents