X-ray binaries

Based on: Compact Stellar X-Ray Sources', eds. W.H.G. Lewin and M. van der Klis, Cambridge University Press

Tauris & van den Heuvel: arXiv:0303456

Mc Clintock & Remillard: arXiv:0306213

Van der Klis arXiv:0410551, Hasinger & van der Klis 1989 A&A

Psaltis arXiv: arXiv:0410536

Fender+ 2004 arXiv:0409360

Basic facts and discovery• Sco-X1 discovered in one of the first X-

ray observation of the sky (1962)• ~100 bright (Fx>10-10 cgs) X-ray sources

in the Galaxy, most discovered already by Uhuru (1971)

• 1034<LX<1038 erg/s• NS in a binary hypothesis confirmed

soon by discovery of X-ray pulsating emission and regular eclipses (Cen-X3, 1972)

P=4.84 sPo=2.087 days

X-ray pulsars

X-ray pulsars

X-ray pulsars

X-ray pulsars

Masses in binaries

Neutron star masses

X-ray binaries

HMXB, LMXB

X-ray binaries

Accretion and B field

Accretion and B field

Accretion and B fieldThe material coming out from the companion star (blue arrows) is captured by the NS. The particle are deviated from the original trajectory and converge behind the NS. There they collide, loosing their energies and then fall toward the NS. AS they come closer the grav. Field accelerates them to very high energies. In the second panel the NS is surrounded by a strong B fiels, the incoming matter is very hot and cannot penetrate the magnetosphere. The matter move along B lines and continue to accelerate. B lines converge to poles and the particles are there focused, forming an accretion column. The density is high and the collisions frequent. The particles loose energy in form of X-rays. Other particles loose their energy impacting the NS.

Accretion and B field• When a strongly magnetic neutron star accretes plasma from a

companion star or the interstellar medium, its magnetic field becomes dynamically important close to the stellar surface and determines the properties of the accretion flow. The radius at which the effects of the magnetic field dominate all others is called the Alfven radius.

• For thin-disk accretion onto a neutron star, the Alfven radius is defined as the radius at which magnetic stresses remove efficiently the angular momentum of the accreting material

• For a surface magnetic field strength of 1012 G and a mass accretion rate ~Eddington critical rate, the Alfven radius is ∼100 neutron-star radii.

• If the stellar spin frequency is smaller than the orbital frequency of matter at the interaction radius, then the accreting material is forced into corotation with the star and is channeled along field lines onto the magnetic poles. An accretion-powered pulsar is produced

• if the stellar spin frequency is larger than the orbital frequency of matter at the interaction radius, then the material cannot overcome the centrifugal barrier in order to accrete onto the star. Matter eventually escapes the neutron star in the form of a wind. “Propeller” regime

High mass X-ray binaries

HMXB

HMXB

• A compact object can accrete matter from a companion star that does not fill its Roche lobe, if the latter star is losing mass in the form of a stellar wind. For this process to result in a compact star that is a bright X-ray source, the companion star has to be massive (≥ 10 M⊙) in order to drive a strong wind. In this configuration, the optical luminosity of the companion star dominates the total emission from the system and the rate of mass transfer is determined by the strength and speed of the wind and the orbital separation. Such systems are called High-Mass X-ray Binaries.

• ~150 HMXB known, ~30 with good orbital parametes• because neutron stars in HMXBs accrete for a relatively short period of

time, their magnetic fields do not evolve away from their high birth values, and hence these neutron stars appear mostly as accretion-powered pulsars. ~40 pulsating HMXB with P=10-300 sec (0.07s-20min)

• Porb<10days• The lifetimes of HMXBs are determined by the evolution of the high-mass

companions and are short (∼ 105 − 107 yr)• HMXBs are distributed along the galactic plane, as young stellar

populations do

HMXB X-ray spectraThe accretion is disrupted at hundreds NS radii and most matter is funneled into NS poles, on relatively small areas. The average spectrum of persistent HMXB can be approximated by a broken power law:

With =1.2+/-0.2 c~20 keV F~12 keVCold/warm absorption from the star windIron featuresCyclotron features

Cyclotron lines• For neutron-star B fielf of ≃1012 G, the

cyclotron energy on the stellar surface is≃11.6 keV and the continuums pectra are expected to show evidence for harmonically related cyclotron resonances cattering features (or cyclotron lines) in the X-rays.

• Observation of such features was anticipated from the early days of X-ray astronomy and expected to lead to direct measurements B (e.g., Trumper

et al. 1978).

Intermediate mass X-ray binaries

Low Mass X-ray Binary providesObservational Evidence of NS

Structure

Neutron star primary

Evolved red dwarf secondary

Accretion disk

Roche point

LMXB: properties

• 150 known LMXB (2001): – 130 in the Galaxy, – 13 in globular clusters, – 2 in LMC

• 63 are X-ray bursters • 75 transient (not always observable) • 11 with a black hole (& 8 possible candidates)• Typical luminosity 1036-1038 erg/s• Soft X-ray spectra • Accretion process: Roche-lobe overflow • Orbital periods: from 11 minutes to 17 days

Formation of LMXB

• Direct: Birth as binary system – More massive star ⇒compact object

• Less massive star fills Roche radius ⇒mass-transfer ⇒LMXB

• Capture: – Birth of more massive star alone ⇒ compact object – Close encounter ⇒capture of second star– High star density ⇒happens almost only in globular

clusters

Transients LMXB• Fraction of transients among the BH

systems is > than the fraction of transients among NS systems and their outbursts are typically longer and rarer.

• BH transients in quiescence are significantly fainter than NS transients.

• These differences are caused by the different mass ratios of the members of the binary systems between the two populations as well as by the presence of an event horizon in BH systems.

The prevailing model of transient sources is based on the disk instability model of illuminated accretion disks (van Paradijs 1996; King+ 1996): accretion flows that extend to large radii ( > 109 − 1010 cm) from the compact object have T< 104 K, at which the anomalous opacity related to the ionization of H renders them susceptible to a thermal instability. At the off-cycle of the instability, material piles up at the outer edges of the accretion disk with very little mass accreted by the central object: quiescent phase. When the disk becomes unstable, the accretion flow evolves towards the central object at the viscous timescale, and the system becomes a bright X-ray source in outburst.

Bursts from LMXB

EXO0748-676

circumstellar material

origin of X-ray bursts

Gravitationally Redshifted Neutron Star Absorption Lines

• XMM-Newton found red-shifted X-ray absorption features • Cottam et al. (2002, Nature, 420, 51):

- observed 28 X-ray bursts from EXO 0748-676ISM

ISM

z = 0.35

z = 0.35

z = 0.35

• Fe XXVI & Fe XXV (n = 2 – 3) and O VIII (n = 1 – 2) transitions with z = 0.35

• Red plot shows: - source continuum - absorption features from circumstellar gas

• Note: z = (and = (1 – 2GM/c2r)-1/2

X-ray absorption lines

quiescence

low-ionizationcircumstellar

absorber

redshifted, highly ionized gas

z = 0.35 due to NSgravity suggests:M = 1.4 – 1.8 M

R = 9 – 12 km

High T bustsFe XXVI(T > 1.2 keV)

Low T burstsFe XXV & O VIII(T < 1.2 keV)

Bursts from LMXB• Two Types of bursts: • Type I: thermonuclear explosion of He on the neutron star The

material that is accreted on the surface of a weakly-magnetic neutron star may be compressed to densities and temperatures for which the thermonuclear burning of helium is unstable. The ignition of helium results in a rapid (∼ 1 s) increase in the X-ray luminosity of the neutron star, followed by a slower (∼tens of seconds) decay that reflects the cooling of the surface layers that ignited. During bursts coherent oscillations of the observed X-ray fluxes are often detected. In bursts from two ultra-compact millisecond pulsars, in which the spin frequencies of the stars are known, the asymptotic values of the burst oscillation frequencies are nearly equal to the spin frequencies of the NS

• Type II: instabilities of accretion flow onto the neutron star

Spectral and timing propertiesX-ray timing properties are correlated with X-ray spectral states. Source states are qualitatively different, recurring patterns of spectral and timing characteristics. They arise from qualitatively different inner flow configurations.

Spectral and timing properties: QPOs

Spectral and timing properties• Z sources on time scales of hours to a day or so trace

out roughly Z shaped tracks (Fig. 2.4c) in CD/HIDs consisting of three branches connected end-to-end and called horizontal branch, normal branch and flaring branch (HB, NB, FB). kHz QPOs and a15-60Hz QPO called HBO occur on the HB and upper NB, an ∼6Hz QPO called NBO on the lower NB, and mostly power-law noise <1Hz on the FB

• At high Lx atoll sources trace out a well-defined, curved banana branch in the CD/HIDs

LMXB spectra

• For weak (<109 G) B fields the accretion disk may touch or come close to the NS surface and the accreting matter is distributed over large areas.

• No pulsations• Partially Comptonized spectrum

mmsec pulsars• millisecond radio pulsars were most

often found in binaries with evolved, low-mass white dwarf companions (Bhattacharya & van den Heuvel 1991), which were thought to be the descendents of LMXBs.

• The discovery, with RXTE, of highly coherent pulsations in the X-ray fluxes of LMXBs during thermonuclear X-ray bursts (Strohmayer et al. 1996) provided the then strongest evidence for the presence of neutron stars with millisecond spin periods in LMXBs.

• However, the first bona fide millisecond, accretion powered pulsar was discovered only in 1998, in a transient ultracompact binary SAX J1808.4−3658

Black hole binaries

BH binaries

BH binaries• Found in HMXB, LMXB.

– 3 persistent (Cyg X-1, LMC X-3, LMC X-1)– many LMXB X-ray Novae (A0620-00, from 50

Crabs to 1uCrab!).

BH binaries light curves

BH binaries transients• 6 X-ray novae detected by Rossi-XTE

ASM• U 1543-47: clean example of a

classic light curve with an e-folding decay time of ≈ 14 days.

• XTE J1859+226: another classic light curve that does show a secondary maximum (at about 75 days after discovery). Note the intense variability near the primary maximum.

• XTE J1118+480: One of five X-ray novae that remained in a hard state throughout the outburst and failed to reach the HS state. Note the prominent precursor peak.

• GRO J1655-40:double peaked profile During the first maximum strong flaring and intense non-thermal emission (VH state).

• XTE J1550-564: The complex profile includes two dominant peaks

BH binaries high/soft state

• High accretion rates.• Geometrically thin, optically

thick disk, Tmax~107K, 1 keV X-rays

• Multicolor disk model, estimate rin from normalization, T, inclination and distance

• Weak variability, f-1, no or weak QPO

BH binaries low/hard state• Lower accretion rates, a few% of

Eddington• Hard, non-thermal power law

component ( ∼1.7)• steep cut- off near 100 keV • Comptonization of soft photons by

a hot optically thin plasma. Disk is faint or undetected.

• presence of a compact and quasi-steady radio jet (first in GRS1915, then Cyg X-1 and others). Flat radio spectral index

• Strong variability

BHB quiescent state• BHB spends most of its life in this

state, L-1030.5 - 1033.5 ergs/s, 10-8 outburst L!!

• L/Ledd ~10-8

• Hard spectrum, =1.5-2.1• Quiescent state may be just an

extremely low state• In the quiescent state the disk is

truncated at some larger radius and the interior volume is filled with a hot (Te ∼ 100 keV) advection dominated accretion flow or ADAF. Most of the energy released via viscous dissipation remains in the accreting gas rather than being radiated away (as in a thin disk). The bulk of the energy is advected with the flow and it is lost in the BH. Radiative efficiency <0.1-1%.

BH binaries very high state• Both disk and power law component

present, both with a luminosity >0.1 LEdd

• Steep power law component, =2.5 up to 1MeV: Compton scattering in a non-thermal corona

• QPOs in both disk and power law component in the range 0.1-30Hz, both LFQPO and HFQPO. Persistent. Organized emission region.

• LFQPO<<Keplerian f. BH 10 M⊙, an orbital frequency near 3 Hz coincides with a disk radius near 100 Rg , while the expected radius for maximum X-ray emission 1-10 Rg. Disk oscillations, spiral waves.

• HFQPO: often commensurate frequencies. Resonance phenomenon of GR oscillations.

• Explosive formation of radio jets: the instability that causes impulsive jets is somehow associated with the VHS state

HFQPOs

BH binaries spectral states1. the high/soft (HS) state, a

high intensity state dominated by thermal emission from an accretion disk;

2. the low/hard (LH) state, a low intensity state dominated by power law emission and rapid variability;

3. the quiescent state an extraordinarily faint state also dominated by power law emission;

4. the very high (VH) state;

5. the intermediate state

Jets and radio emission in BHB• Relativistic, superluminal jets. • Non-thermal, polarized radio spectra,

indicating shock-accelerated e- emitting synchrotron

• Very clear correlation between the presence of jets and the X-ray spectral state of the accretion flows. Jets appear when the X-ray spectra of the sources indicate emission from hot electrons (∼ 100 keV)

• The mechanism responsible for the heating of electrons in the accretion flow may be related to the formation of an outflow, as is the case both for magnetically active accretion disks

Jets, disks and spectral states

Jets, disks and spectral states

• i low state steady jet Ljet ∝ LX0.5

• ii motion nearly vertical. After a peak motionnearly horizontal to the left, Source move in theVHS/IS. Jet persist.• iii source approaches the jet line betweenJet producing and jet free states. Velocity increases. Propagation of an internal shock. • iv source is in the soft state and no jet is produced.

Refill of disk. • The thin disk extend close to the BH. Following phase iv

sources drop in intensity to reach the canonical LS.• Inner disk is ejected resulting in a disappearence of

the inner disk, transition to LS, jet launch.

Relativistic iron lines• The first broad Fe Kα line observed for either a BHB or an AGN was

reported in the spectrum of Cyg X-1 based on EXOSAT data. This result that inspired Fabian et al. (1989) to investigate the production of such a line in the near vicinity of a Schwarzschild BH, a result that was later generalized by Laor (1991) to include the Kerr metric.

• Beppo-SAX discovered relativistic lines in several BHB: SAXJ1711+3808, XTEJ1909+094,GRS1915+105, V4641Sgr

• XMM and Chandra: CCD and gratings

In many cases ISCO consistent with non-spinning BHDetection of “smeared edges”