xeus x-ray evolving universe spectroscopyemits.sso.esa.int/emits-doc/4964-rd1-xeus_scird.pdfxeus is...

$: XEUS X-RAY EVOLVING UNIVERSE SPECTROSCOPYemits.sso.esa.int/emits-doc/4964-RD1-XEUS_SciRD.pdfXEUS is to investigate the high-redshift Universe. A large fraction of the total baryonic$
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D O C U M E N T

XEUS – X-RAY EVOLVING UNIVERSE SPECTROSCOPY

SCIENCE REQUIREMENTS DOCUMENT

Prepared by/préparé par A.N. Parmar (XEUS Study Scientist) and M.J.L. Turner (Chair, XEUS Science

Definition Team) contribution from XEUS Science Definition Team:

M.J.L. Turner (Chair), M. Arnaud, X. Barcons, J.A.M. Bleeker, G. Hasinger, H. Kunieda, G. Palumbo, A.N. Parmar and T. Takahashi. Ex-Officio: D. Barret, M. Cappi, A. Comastri, E. Costa, J.-W. den Herder, J. Kaastra, K. Makishima, G. Matt and M. Méndez.

Reference/réference SA/05.001/AP/cv issue/édition 4 revision/révision 0 date of issue/date d’édition 31 March 2006 status/état Document type/type de document

Technical Note

Distribution/distribution Public Document

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XEUS Scientific Requirements Issue 4.0

C H A N G E L O G

Reason for change /raison du changement Section/Section

Page/Page

issue/issue

revision/revision

date/date

First issue 1 0 18 February 2005

Revision 1 1 1 27 February 2005

Following Con-X/XEUS meeting and inputs on polarimetry, timing, enrichment, acceleration, and WHIM.

1 2 10 March 2005

Further comments 1 3 11 March 2005

Version for PDD input 2 0 16 March 2005

Updates following XEUS Science Advisory Group Meeting on 18-19 April 2005

2 1 19 April 2005

Following Payload Working Group Meeting of 22 May 2005, Telescope Working Group Meeting of 01 June 2005 and response to memo SCI-A/2005/123/NR

2 2 01 July 2005

Change of baseline launcher to Ariane V following US withdrawal from study

3 0 01 October 2005

Revised following SAG meeting of 14 December 2005 and revised spacecraft configuration with circular optic. Pseudo-grating requirement removed. LFOV and single NFOV defined as core instruments with an assumed focal length of 35 m

4 0 31 March 2006

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The contents of this Science Requirements Document (Sci-RD) are agreed by all the contributors to be the scientific requirements for the XEUS mission against which the Reference Payload will be designed and the mission profile and costs established.

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T A B L E O F C O N T E N T S

1 INTRODUCTION....................................................................................................6

2 SCIENTIFIC REQUIREMENTS .............................................................................8 2.1 Evolution of Large Scale Structure and Nucleosynthesis.................................................................8

2.1.1 Formation, dynamical and chemical evolution of groups and clusters ..................................8 2.1.2 Baryonic composition of the Intergalactic Medium ............................................................11 2.1.3 Enrichment dynamics: inflows, outflows and mergers ........................................................13

2.2 Coeval Growth of Galaxies and Supermassive Black Holes..........................................................15 2.2.1 Birth and growth of supermassive black holes ....................................................................15 2.2.2 Supermassive black hole induced galaxy evolution ............................................................18

2.3 Matter under Extreme Conditions ..................................................................................................20 2.3.1 Gravity in the strong field limit ...........................................................................................20 2.3.2 Equations of state.................................................................................................................23 2.3.3 Acceleration phenomena......................................................................................................26

3 MISSION PERFORMANCE REQUIREMENTS AND GOALS............................29 3.1 Core Instrumentation ......................................................................................................................29 3.2 Additional High Priority Instruments .............................................................................................29 3.1 Mission Requirements and Goals ...................................................................................................30 3.2 Mission Performance Summary .....................................................................................................31 3.3 High-priority goals .........................................................................................................................31 3.4 Mirror Effective Area .....................................................................................................................32

4 ACRONYMS.........................................................................................................33

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1 INTRODUCTION This document presents the science requirements to be satisfied by the XEUS mission, the successor to XMM-Newton, ESA’s current cornerstone X-ray observatory. The baseline design of the XEUS mirror system provides for a far larger collecting area and significantly improved angular resolution compared to XMM-Newton, such that all fields of X-ray astronomy will be transformed by observations made by this observatory. The mission scenario evolved in early 2004 and currently XEUS comprises of the deployment of a large X-ray telescope and separate detector spacecraft formation flying in an L2 halo orbit. In order to scope the key performance parameters, this document summarizes the science requirements of a range of key science investigations as presented in the Cosmic Vision 2015-2025 process. The large throughput and good angular resolution of XEUS will allow the detailed spectral investigation of sources which are too faint for study with the current generation (Chandra, Suzaku and XMM-Newton) of X-ray observatories. One of the main science goals of XEUS is to investigate the high-redshift Universe. A large fraction of the total baryonic matter in the Universe is now known to reside in the X-ray emitting component of clusters and groups and the study of their properties will be another important topic for XEUS. The accretion power onto massive black holes is the dominant component of the total X-ray emission in the Universe, so conversely the ability to trace this evolution will be an important diagnostic of the evolution of black holes and the coeval growth of galaxies with cosmic time. Probing the high-energy emitting regions around collapsed objects provides the best laboratory for testing the physics of matter in extreme gravity environments. In addition to these specific themes, the unprecedented high collecting area will make an enormous impact on studies of nearby objects which have been the mainstay of traditional X-ray astronomy, and therefore we also discuss the detailed spectroscopic, timing and polarimetric investigations of brighter objects that will be addressed by XEUS. Two sorts of imaging instruments are assumed to be part of the payload, one type will be optimised for high-spectral resolution, but with a small field of view (FOV) and the other will have a larger FOV, but poorer spectral resolution. For each of the 3 topics identified in CV2015-2025 a number of sub-topics have been determined, each of which provides the driver for one, or more, of the science requirements to be met by XEUS:

1. Evolution of Large Scale Structure and Nucleosynthesis: a. Formation, dynamical and chemical evolution of groups and clusters. This is the

driver for spectral grasp, the product of the FOV, area, and spectral resolution for the high spectral resolution instruments.

b. Baryonic composition of the Intergalactic Medium. This drives the ultimate spectral response required from the high spectral resolution instruments.

c. Enrichment dynamics, inflows, outflows and mergers. This drives the stability of the absolute spectral calibration of the high spectral resolution instruments.

2. Coeval Growth of Galaxies and Supermassive Black Holes:


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a. Birth and growth of supermassive black holes which drives the overall FOV size and the limiting sensitivity

b. Supermassive black hole induced galaxy evolution which drives the angular resolution requirements.

3. Matter under Extreme Conditions: a. Gravity in the strong field limit with drives the ultimate timing resolution required. b. Equations of State studies which drive the product of collecting area and spectral

response for the high spectral resolution instruments, as well as the polarization performance.

c. Acceleration phenomena which drive the high energy (>10 keV) spectral grasp. In Sect. 2 we present these areas of science investigation and address the requirements that each places on the following performance topics:

1. Effective area 2. Energy range 3. Angular resolution 4. Field of view 5. Spectral resolution 6. Sensitivity 7. Time resolution 8. Count rate capability 9. Polarimetry 10. Observing constraints

The effective area is defined as the area “available for science” and is the product of mirror area, detector efficiency and filter transmission, as appropriate. In Sect. 3 we summarize the science requirements and their drivers to derive a coherent set of performance requirements for the mission.


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2 SCIENTIFIC REQUIREMENTS

2.1 Evolution of Large Scale Structure and Nucleosynthesis

About 96% of the energy density of the Universe exists in the form of Dark Matter and Dark Energy, which govern the structure and evolution of the Universe on the largest possible scales. Clusters of Galaxies are the largest collapsed objects in the Universe. Their formation and evolution is dominated by gravity, i.e. Dark Matter, while their large scale distribution and number density depends on the geometry of the Universe, i.e. Dark Energy. They are filled with hot bary-onic gas, which is enriched with elements by star formation and stellar explosions, and is preferably detected by its high energy radiation. X-ray observations of clusters provide information on the Dark Matter and Dark Energy content of the Universe, on the amplitude of primordial density fluctuations, on the complex physics governing the formation and evolution of structures in the Universe and on the history of metal synthesis. Whilst nearby clusters of galaxies have been studied at great detail with existing X-ray satellites, very little is known about their formation and evolution in the early Universe. In addition, the fate of almost 50% of the baryons in the Universe, believed to reside in warm/hot filamentary structures observable with X-ray absorption spectroscopy, is still a mystery. In order to study the genesis of groups and clusters of galaxies and the Cosmic Web at up to z ~ 2, and the evolution of the physical state and chemical abundances of the intergalactic medium, an X-ray telescope combining a very large collecting power with excellent energy resolution and good spatial resolution is necessary.

2.1.1 Formation, dynamical and chemical evolution of groups and clusters

In the standard hierarchical formation scenario, the first groups of galaxies formed around z = 2. Clusters then formed and grew by the continuous accretion of surrounding matter and through sporadic merger events. X-ray observations provide essential diagnostics on the physics involved, both for the gaseous (the ICM), and Dark Matter (DM) components. Merger events are central to hierarchical theory of cluster formation. These are complex dynamical processes that are driven by gravitation. The detailed physics (e.g., substructure relaxation times, shock heating, particle acceleration, and the influence of large scale environments) and the effects on the cluster properties have to be understood. Also important for the testing of the hierarchical scenario are statistical studies of cluster substructures and their evolution with redshift. Clusters scaling properties are another topic of investigation for XEUS. In the simplest formation model, purely based on gravitation, galaxy clusters (between major merger events) constitute a self-similar population. The internal shape of clusters is universal, independent of mass and redshift and scaling laws relate each physical property to the cluster total mass, M, (or temperature, T) and redshift. A powerful method to test the actual physics governing cluster formation and evolution (both DM and gas) is to compare the statistical properties of an observed cluster population with theoretical predictions. Historically, the first observed deviation was the steepening of the X-ray luminosity to temperature (Lx-T) relation, which indicates that non-


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gravitational processes play a role for the gaseous component. Various processes have been proposed to account for this, such as heating before, or after, collapse (from SNs or AGNs), or radiative cooling. Their relative importance and roles need to be understood. As does the complex physics in cluster cores which is discussed in more detail below and the heavy element abundances which provide diagnostics both on metal synthesis and SN heating of the ICM. The understanding of cooling cores in clusters has dramatically changed with XMM-Newton and Chandra. A major surprise was the lack of very cool gas, inconsistent with standard cooling flow models. High spatial resolution images with Chandra have shown the presence of cavities in the core associated with central AGN activities. Whether both phenomena are connected (AGN heating limiting the cooling) is still unclear. Cluster cores are ideal laboratories for the study of both phenomena. A better understanding of these processes (e.g., using spatially resolved spectroscopy) has profound cosmological implications because they play an important role on larger scales in clusters and also during galaxy formation. There is no evidence for any abundance evolution in clusters out to z ~ 1. In the local Universe the abundance variations with radius can be mapped, as can the abundance of the elements in the cooling cores and in poor clusters. There is a clear increase of abundances towards the centre in cooling flow clusters, probably associated to SN I enrichment by a central dominant galaxy.

A key scientific aim for XEUS is to study the evolution in cluster properties in the 0.5 < z < 2 and 2 < kT < 5 keV domain. These limits are used to define the XEUS scientific requirements for cluster studies. We note that with the current capabilities the redshift range 1 < z < 2 is essentially unexplored. Low-temperature clusters are of particular interest. Non-gravitational effects are most noticeable in low-mass systems. The mass range corresponding to kT > 2 keV is where we see regularity in the local Universe. The redshift range up to z ~ 2 roughly corresponds to the epoch of formation of these clusters. Abundance studies, outside of the cooling core, are practically limited to kT < 5 keV (the equivalent widths, EWs, decrease with increasing kT). Such information is required to recover the contributions of SN I and SN II to the metal-synthesis history.

These studies require observations with both high and low spectral resolution instruments with small and large FOVs, respectively.

2.1.1.1 Effective area

The requirement is an effective area of 5 m2 at 1 keV and 1 m2 (with a goal of 1.5 m2) at 0.2 keV. These are derived from the science requirement of a 5σ detection in 200 ksec of the Mg line in a kT= 2 keV cluster at z = 2. This line is chosen since it is normally well separated from any background lines and provides important diagnostic information. For the expected detector and filter efficiencies these requirements translate into mirror areas of 5 m2 between 0.2 and 1.0 keV.

2.1.1.2 Energy range

An energy range of 0.2–6 keV is required for the high spectral resolution instrument. The low-energy threshold is to detect the O VIII line at z = 2 which will have a redshifted energy of 0.22 keV. The upper threshold of 6 keV is required for temperature estimates. An energy range of 0.2-


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10 keV is required for the low spectral resolution instrument with an extension to 40 keV desirable in order to study hard tails in clusters.

2.1.1.3 Angular resolution

The requirement is at least 5″ HEW in the centre of the FOV. This can decrease to 20″ HEW in the outer regions (radii >2.5′) of the FOV. The goal is 2″ HEW in the centre of the FOV, with a decrease to 10″ HEW in the outer regions. The science requirement is to resolve cluster cores down to kT = 2 keV and at z = 2 and to resolve high-z groups found serendipitously using a larger FOV imager. The higher angular resolution is important to limit source confusion, not only with field point sources, but also with AGN within the clusters.

2.1.1.4 Field of view

The requirement is a diameter of 0.75′ for the high spectral resolution instrument with a high priority goal of 1.7′ diameter. The goal size is derived from the science objective of covering half of the virial radius of clusters (kT < 5 keV) above z = 0.8. XEUS is required to measure α-element abundances beyond z = 0.8. It should be noted that the goal FOV is also suitable for spatially resolved high-resolution spectroscopy of the cooling core of moderate redshift clusters, as well as velocity studies. For the lower spectral resolution instrument the requirement is a diameter of 7′ in order to enable a proper background estimates for clusters down to z = 0.5 and up to kT = 5 keV.

2.1.1.5 Spectral resolution

The requirement is 2 eV FWHM at energies below 2 keV. This originates from the science requirement of being able to separate cluster lines and the sky lines and be photon limited for the α-element lines up to z = 2.

2.1.1.6 Sensitivity

Not a driver. A typical goal is to detect a group with a 0.2-2.0 keV flux of 1.2 10-16 ergs s-1 cm-2.

2.1.1.7 Time resolution

Not a driver.

2.1.1.8 Count rate capability

Not a driver. High z clusters are faint and extended sources.

2.1.1.9 Polarimetry

Not a driver.

2.1.1.10 Observing constraints

Not a driver.


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2.1.2 Baryonic composition of the Intergalactic Medium

Current cosmological models restrict the baryon fraction in the Universe to a few per cent of the total matter and energy content. A large fraction of the atoms, ions and electrons almost certainly reside in the intergalactic medium. Lyman-α clouds (including Damped Lyman-α absorption systems) are seen to dominate the baryon content of the Universe at high redshifts, but at lower redshifts their number density and the subsequent contribution to the baryonic content of the Universe decreases. Simulations of the cosmological evolution of the baryons show that the Lyman-α absorbing gas at temperatures of 104 K undergoes shock heating at lower redshifts and its temperature rises to 105-7 K. The baryons in this warm and hot intergalactic medium (WHIM) consist of 50-60% of the total baryons in the local Universe and are distributed in a tenuous filamentary gas. This gas is very well detected in absorption, rather than emission, using a bright (and featureless) distant source. The limiting EW detectable for a resonance absorption line is ultimately determined by the spectral resolution of the spectrograph (assuming a suitable over-sampling of each energy resolution element) providing that sufficient signal to noise ratio, S/N, is achieved. For a typical S/N=10 spectrum, where each spectral resolution element is sampled by several channels, the weakest absorption line detectable has an EW of the order of the width of one channel. After very long integrations with the Chandra and XMM-Newton grating spectrometers, the weakest absorption lines that can be detected have EWs of 5-10 mÅ using very bright and nearby sources. A powerful telescope equipped with high resolution spectrometers can detect absorption lines, mainly from O VII and O VIII, out to distant redshifts in virtually every line of sight. This will enable the evolution of the ionisation state of the majority of baryons in the Universe to be traced all the way from mostly a cold state (Lyman α clouds at z ~ 2) to a warm and hot state (z ~ 0). It will also provide information on the baryon budget, providing a measure of the fraction that actually forms galaxies. The strongest X-ray forest absorption systems, where transitions from various chemical elements will be detected, can be used to trace the chemical evolution of the Universe. Finally, the most challenging, as well as the most important input in tracing the cosmic evolution of baryons in the WHIM is to map their spatial distribution. The WHIM is predicted to follow a filamentary structure as dictated by the underlying Dark Matter distribution, a fact that can be tested when multiple lines of sight in nearby regions of the sky are available with sufficient statistics. These studies will be performed using the high spectral resolution instruments.


The key ingredient to study the WHIM is that every line of sight towards a distant AGN can be sampled properly in an observation lasting 100 ksec. The Kα transitions of O VII (0.57 keV) and O VIII (0.65 keV) are the most relevant tracers. A suitable reference background source has a 0.5-4.5 keV flux of 10-13 erg cm-2 s-1 of which there are about 10 sources deg-2 in the sky with a mean AGN redshift is around z ~ 0.7 (and a significant tail to higher z). For a flux of 2 10-14 erg cm-2 s-1 the source density reaches 100 deg-2 and the average redshift is now close to 1.5. Assuming an efficient spectrometer, the S/N ratio for a 5 m2 effective area in the spectral region of interest (0.5-0.2 keV) is of the order of 8-5 per 0.1 eV channel, in the case of a source with a flux of 10-13 erg


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cm-2 s-1 in a 100 ksec integration without any background contribution (the inclusion of which will degrade the S/N only slightly as these are bright sources). Effective areas of 2.5, 1.5 and 1.0 m2 will yield S/Ns per channel of around 5, 4 and 3 at 0.5 keV and of 4, 3, and 2 at 0.2 keV, respectively. Under these circumstances, absorption lines with EWs as weak as 0.4 eV (or 20 mÅ) for a 1 eV energy resolution can be detected. In the linear curve of growth regime, this corresponds to a column density of 3 1015 atom cm-2 of e.g., O VIII. Thus, this implies an effective area of 5 m2

at 1.0 keV in order to study the WHIM in the way envisaged. This translates into a mirror area of 5 m2 between 0.2 and 1.0 keV.


The most relevant energy range is below 1 keV where most of the important resonance absorption lines occur. Since the strongest O transitions occur around 0.5-0.7 keV at z = 0, to trace these out to z = 1.5, energy coverage between 0.2 and 7.0 keV is required, the latter being required to study Fe K absorption.


Angular resolution is not a driver in most cases, as the targets will be bright and confusion will play a secondary role. However, for the strongest systems arising in intra-group or intra-cluster gas, a spatial resolution worse than 5″ HEW may result in resonance absorption lines starting to be filled in by integrated emission from the same gas. Thus a spatial resolution of 5″ HEW is required.


This science does not require a large FOV as the goal is to perform the observations using known, bright targets and so a FOV of 0.75′ diameter is assumed.


This is a key driver as the sensitivity in absorption line EW is directly related to this, for a given S/N ratio. The requirement to detect and characterize the X-ray forest (not just a few strong lines) means that the spectral resolution must be at least 250 at 0.5 keV, i.e., 2 eV FWHM resolution with a goal of 1 eV FWHM. With appropriate sampling and S/N, lines with EWs as low as 0.8 eV (~40 mÅ) can be detected. (Note that this is the XMM-Newton RGS resolution).

2.1.2.6 Sensitivity

Not a driver.


Not a driver.


Not a driver.


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2.1.2.9 Polarimetry

Not a driver.


To sample the WHIM adequately, approximately 50 observations of distant AGN will be required.

2.1.3 Enrichment dynamics: inflows, outflows and mergers

The lack of cool gas in the cooling cores of clusters, as can be deduced from the lack of moderately ionised Fe iron, is a major outstanding problem. In addition, high spatial resolution observations have shown the presence of low-density cavities in the cores of a number of clusters (with a typical size of 10″). Various models have been proposed to explain the heating of the gas, some of which also include a clear spatial component (heating by the central dominant galaxy). Hence, spatially resolved spectroscopy is expected to have a significant impact on our understanding of clusters of galaxies. Elliptical galaxies (for example NGC 4636) probe the same parameter space, but have the advantage of being more compact. Current capabilities include studying the metallicity as function of radial position for the most abundant elements (e.g., Fe, Si, S and O), studies of the differential emission measure of the cluster (showing the lack of cool gas in the central core), and studies of cold fronts in between merging clusters. The aims for XEUS include:

• Spatial analysis of the temperature distribution of the cooling gas and the study of the gas dynamics (e.g., turbulent and bulk velocities).

• Abundance determinations for the most abundant elements in cluster gas. • Abundance determinations of the rare elements.

Study of core-collapsed supernovae (type Ib,c and II) and accretion induced supernovae (type Ia) provide fundamental clues for the study of galaxy enrichment and energy balance. Shock heating of the interstellar medium raises the typical gas temperatures to millions of degrees, consequently the bulk of the matter radiates at X-ray wavelengths, predominantly through X-ray line emission. Hence spatially resolved high resolution X-ray spectroscopy is a major tool for unravelling the enrichment and energizing processes of galaxies, i.e. the temperature and density distribution of the hot gas and the detailed nucleosynthesis processes including the formation of the rare elements. Current capabilities include the classification of the main SN-types and the assessment of the hot plasma physical state in terms of temperatures, equilibration time scales and the metallicity distribution of the most abundant elements. The aims for XEUS include:

• Spatially resolved high spectral resolution analysis of the shock-heated gas dynamics, i.e. bulk velocity distributions and the measurement of turbulent velocity-structures.

• Determination of the spatial structure in the most abundant elements in the SN-ejecta and the circumstellar gas.

• Abundance determination and distribution of the rare elements. In recent years the potential importance of AGN outflows on the growth of super-massive black holes, enrichment of the intergalactic medium, evolution of the host galaxy, magnetization of


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cluster and galactic gas, and the luminosity function of AGN has been widely recognized. Approximately half of the nearby AGN show the presence of a warm, photo-ionised wind. Due to the large number of ions that produce X-ray absorption lines and edges, the diagnostic power of the AGN winds is extremely large. Current knowledge is limited to the properties of warm absorbers (abundances and ionisation stage) in a handful of bright AGN. Many problems remain unanswered, such as the role of outflows in more distant AGN, the location (accretion disk or torus induced) and mass flux through the outflow. The aims for XEUS include:

• Study of the location and geometry of the wind through reverberation mapping • Determination of the dynamics and mass/momentum flux of the outflow • Abundance determination in the nuclei of galaxies and study of the enrichment by AGN

outflows • Study of the evolution and population of photo-ionised outflows in a large sample as a

function of redshift, luminosity and galactic environment. These studies will require observations with both low and high spectral resolution instruments.


The driver here is to determine the abundances of rare elements. For point sources this requires an effective area of 5 m2 at 1 keV.


The energy range should be at least 0.2–10 keV with low spectral resolution. For the high spectral resolution studies an energy range of at least 0.2–6 keV is required.


A spatial resolution of 5″ HEW is required to study the gas temperature distribution in nearby clusters (z < 0.05). A spatial resolution of 2″ would increase the range to z ~ 0.15.


Whereas some science can be performed with expected energy resolution of large FOV instruments, the study of dynamics requires high spectral resolution (2 eV at 2 keV) over an extended FOV of at least 0.75′ diameter. Mosaicing with high spectral resolution instruments may be necessary for some targets.


To study turbulence in cluster gas an energy resolution of 6 eV at 6 keV is required with a goal of 3 eV at 6 keV. For the study of the dynamics in the cooling flow a similar resolution (1000) is desired around the Fe-L complex at 1 keV (goal). For the desired study of the abundance of rare elements an energy resolution of 2 eV at 2 keV is necessary. This would enable the determination of abundances of rare elements using L and K shell transitions for the more abundant elements such as Na, Al, P, Cl, Ca and Ni. With such resolution line shifts in the order of a few 100 km s-1


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can be detected for weaker lines and turbulent velocities between a few 100 and thousand km s-1 for the weaker lines (at 5σ confidence).

2.1.3.6 Sensitivity

Not a driver.


Not a driver.


Not a driver

2.1.3.9 Polarimetry

Not a driver


Not a driver.

2.2 Coeval Growth of Galaxies and Supermassive Black Holes

The first stars and galaxies formed where gravity overpowered the pressure of the ambient baryons. Ultimately, gravity dominated and the first stellar mass black holes were formed, very likely in gamma-ray burst explosions. Supermassive black holes can grow in cataclysmic feeding events. The highest redshift accreting black holes known are around z = 6.5. The WMAP studies of the microwave background show that the first light must have ionised the universe already as early as z = 10-20. The fact that practically all galaxy bulges in the local Universe contain supermassive black holes, with a tight relation between black hole mass and the stellar velocity dispersion, indi-cates a co-existence and co-evolution of stars and central black holes early in the universe. Supermassive black holes must thus be an important constituent of the evolving universe. Only recently has the importance of feedback of stellar explosions and accreting black holes into the intergalactic and interstellar medium, and thus their role for star and galaxy formation been realized. The study of the birth and growth of supermassive black holes at z ~ 10 requires an unprecedented combination of large spectral throughput, high angular resolving power and large field of view in the X-ray regime matching those of future optical and radio telescopes.

2.2.1 Birth and growth of supermassive black holes

The XEUS requirement is, to detect and to study X-ray emitting black holes out to z = 10 and to investigate their nature. These objects could be either continuously accreting black holes in their quasar growth phase, or, as discovered recently, mature black holes which are tidally capturing, disrupting and consuming individual stars. In either case, their X-ray luminosity should be >1042.5 erg s-1, in order to discriminate them from star forming emitters. Assuming that 10% of the bolometric luminosity is emitted in X-rays and that these objects accrete at the Eddington limit,


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this corresponds to a black hole mass of 3 106 M . At z = 8, such a black hole would have an X-ray flux of 4 10-18 erg cm-2 s-1 (assuming standard WMAP cosmology: H0 = 70 km s-1 Mpc-1, λ0 = 0.73 and a flat Universe). In order to study the overall properties of such objects the overall spectral shape, the amount of absorption and the properties of any Fe lines need to be measured. This latter topic – the study of Fe lines from distant X-ray sources is one of the key XEUS scientific aims, as the line energy provides the red-shift which allows the object’s luminosity and distance to be determined. For the less luminous objects, JWST and ALMA observations will be needed to determine redshifts. XMM-Newton observations of the Lockman Hole field, in combination with extensive optical identifications of the AGN population, has revealed strong, relativistically broadened, fluorescent Fe lines in the average rest-frame spectra of type-1 and -2 AGN. A Laor line profile with an inner disk radius smaller than the last stable orbit of a Schwarzschild black hole provides the best-fit to the observed line profiles. This indicates that the average supermassive black hole has significant spin. A key scientific aim for XEUS is to measure the precise shape of these relativistically broadened lines to determine the black hole spin. Another key scientific aim of XEUS is to study the evolution of AGN with cosmic time. XEUS will do this by studying how the mean source luminosity, Fe abundance, Fe line properties (and in particular the black hole spin-rate) evolve with cosmic time. Finally, the bulk of the energy density of the Cosmic X-ray Background (CXB) is in the range 30-40 keV, where imaging X-ray telescopes are just being developed and only a small fraction has been resolved so far. By the time XEUS will be operational, even the most sensitive observations in the hard X-ray range will have probably resolved <10% of the CXB. Population synthesis models suggest that the background peak is produced by relatively local, low luminosity, and heavily obscured Seyfert 2 galaxies. Such objects may lurk undetected in almost all of the nearby galaxies. To complete the census of black hole growth, XEUS is required to resolve the bulk of the energy density of the CXB around 40 keV. These studies require observations with low spectral resolution and a large FOV.


The XEUS requirement is to be able to detect the first massive black holes which have 0.2-10 keV fluxes of 4 10-18 erg cm-2 s-1. Simulations show that the redshift of an object at z ~ 8 which is a factor 30 more luminous (1.2 10-16 erg cm-2 s-1) can be determined directly through measuring its Fe line energy. This sensitivity will allow sufficient numbers of such objects to be studied over the likely lifetime of the mission. This means that low-resolution spectroscopic investigations will be possible in deep field exposures. We have derived the effective area needed to meet this requirement. Assuming a 5 m2 effective area at 1 keV, such a source would produce 2.5 10-4 count s-1 in the 0.1-10 keV energy range. The main background components at these fluxes are the diffuse galactic X-ray (foreground) emission below 1 keV which varies by a factor 2 across the sky and the particle-induced internal background at higher energies (the extragalactic background is assumed to be almost completely resolved at these flux levels). We assumed mirror PSFs of 2″ and 5″ HEW. The detection of a source with a flux of 4 10-18 erg cm-2 s-1 at 4σ confidence requires 100,


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160, 270 and 640 ksec for 2″ low; 2″ high, 5″ low and 5″ high FOV/background combinations, respectively. The overall performance with a 5 m2 effective area is therefore adequate for the detection of high-redshift black holes, provided the PSF is <5″ HEW and low foreground regions of sky are chosen. We therefore assume that an effective area below 2 keV of 5 m2 is the requirement. At 0.2 keV the effective area should be 1.0 m2 in order to study high-redshift absorption and Fe line emission. The goal of resolving the hard X-ray background and characterizing the broad-band continua of AGN up to a redshift of 3 requires an effective area of 0.1 m2 at both 15 and 40 keV. These specifications translate into a mirror area of 5 m2 between 0.1 and 2.0 keV and 0.1 m2 at both 15 and 40 keV.


At high redshift, the requirement is to extend the energy coverage down to 0.1 keV to detect moderate absorbing columns as low as 1021 cm-2, as they are being routinely found in 10% of the type 1 AGN, as well as to trace the continuum below relativistically smeared Fe-Kα lines from redshift z = 10. The upper energy range requirement is 10 keV with a goal of 40 keV to resolve the hard X-ray background and to allow the population of nearby, heavily absorbed, and Compton-thick sources to be properly studied.


To resolve the most distant, fainter sources and to provide the necessary sensitivity, a minimum of 5″ HEW angular resolution is necessary. In this case, effects of source confusion will set in for exposure times longer than 200 ksec. An angular resolution of 2″ is the goal for studying sources with fluxes <10-17 erg cm-2 s-1, where we will study the most distant black holes. In order to resolve the X-ray background near its peak energy (which is assumed to be produced by massive black holes), where there are 10,000 sources per square degree are expected, an angular resolution of 10″ FWHM is required at 40 keV to avoid confusion.


X-ray luminous objects at high redshift are very rare. The likelihood of having a z > 5 object in a 5′ x 5′ image is around unity or less which sets the requirement for FOV. A larger FOV is needed in order to detect more objects and the goal 7′ diameter FOV would provide significantly more sources.


These faint objects are at the limit of detectability; therefore high spectral resolution is not a requirement. However, since the observations will be background limited by the diffuse galactic emission, which is of thermal origin, sensitivity can be improved by removing the energy bins containing the most prominent background lines. Therefore CCD-type energy resolution of 150 eV FWHM at 6 keV is required. At 40 keV an energy resolution of 1 keV FWHM is required.


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2.2.1.6 Sensitivity

To be able to detect an object with a 0.2-10 keV flux of 4 10-18 erg cm-2 s-1 is the requirement.


Not a driver.


Not a driver.

2.2.1.9 Polarimetry

Not a driver.


Instantaneously available sky coverage in not an issue, but the ability to meet exposure duration goals for a deep field in the same season requires that any region of sky should be visible for >500 ksec once per six month interval.

2.2.2 Supermassive black hole induced galaxy evolution

Much of the emphasis in recent years in extragalactic astrophysics has been to investigate the origin of the correlation between galaxy host velocity dispersions and the masses of the black holes in the centre of the galaxies. An anti-hierarchical evolution has been found in black hole accretion and star formation, with massive objects being created in the early universe and lower mass objects much later. Another key observational area is investigating the X-ray scaling relation of the gas luminosity, or the entropy K, versus the X-ray temperature, in groups and clusters of galaxies (Sect. 2.1.3). There is increasing (both observational and theoretical) evidence that galaxy mergers and energy feedback from AGN play a major role in regulating the fate of structure formation, and star formation, but we are still far from a detailed observational understanding of this scenario. There is strong evidence in AGNs of matter flowing outward as well as being accreted. As well as relativistic jets in radio-loud AGNs and outflowing systems in some broad-line AGNs, X-rays have been probing warm outflowing gas with both low (warm absorber) and high (massive outflows) ionisation and densities. Recent XMM-Newton and Chandra grating and CCD measurements have detected blueshifted absorption lines in radio-quiet AGNs, indicating the existence of massive, high velocity (100 to 50000 km s-1) outflows. These are particularly interesting because they may be transporting most of the mechanical energy emerging from black holes. Outflows may thus be the key "ingredient" to link the properties of the nuclear black holes to that of the surrounding host galaxies, and provide the extra heating necessary to explain the observed scaling relations between X-ray luminosity and temperature from rich to poor clusters of galaxies and groups. Most of supermassive black holes residing at the centre of galaxies are nowadays inactive. It is extremely important to search for past activity as this will help to better understand the interplay between black holes and host galaxies. The supermassive black hole in our own Galaxy is quiet at


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the moment (<10-10 its Eddington luminosity). However, at a projected distance of 100 parsecs from the black hole, there is a giant molecular cloud, Sgr B2, which in X-rays has a pure reflection spectrum. It is therefore likely that a few hundreds years ago the galactic centre was a low luminosity AGN. Polarimetric measurements can confirm this hypothesis and measure the true distance between the galactic centre and Sgr B2 as reflected radiation should be highly polarized, with a polarization angle perpendicular to the projected line connecting the two objects. This topic will require observations with high and low spectral resolution instruments.


The requirements here are very similar to those presented in Sect. 2.1.3 and an effective area of 5 m2 at 1 keV is required.


An energy range of 0.1–10 keV is required, with an extension to 40 keV desirable for the low spectral resolution instrument. For the high spectral resolution instrument, the energy range required is 0.2–7 keV.


In order to study the out-flowing material in the vicinity of AGNs and the X-ray morphology of binary mergers, a spatial resolution of at least 5″ HEW is required with a goal of 2″ HEW. Achievement of this goal would significantly enhance the science return from this topic.


A FOV of 5′ x 5′ is required with the spectral resolution expected from the larger FOV instruments, and 0.5′ x 0.5′ with high spectral resolution instruments.


The requirements here are very similar to those of Sect. 2.1.3 and a spectral resolution of 6 eV at 6 keV is required with a goal of 3 eV at 6 keV.

2.2.2.6 Sensitivity

Not a driver.


Not a driver.


Not a driver.


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2.2.2.9 Polarimetry

To study the radiation from the black hole at the centre of the Galaxy reflected by the molecular cloud Sgr B2, a minimum detectable polarization (MDP) of 10% should be detectable at 3σ confidence in a 0.1 mCrab source in a 100 ksec observation.


Not a driver.

2.3 Matter under Extreme Conditions

Black holes play a key role in the evolution of galaxies and ultimately in the star formation history, but they also distort the shape of space in their vicinity. X-rays resulting from accretion of matter onto a compact object (a supermassive or stellar mass black hole, or neutron star) probe the motion of matter in strongly curved space time, in which general relativity is no longer a small correction to the classical laws of motion. This is a regime in which fundamental predictions of general rela-tivity are still to be tested, such as the existence of an event horizon for black holes, or the dragging of inertial frames. These predictions cannot be confirmed using weak field measurements from Earth orbiting gravity probes. High throughput timing, hard X-ray spectroscopic and polarimetric measurements, the energy and time dependence of the polarization angle of the accretion disk emission, all probe the strong field region and constrain the physical parameters of the compact object (mass and spin). Deviations from general relativity in the strong field limit can be studied in a way complementary to measurements by gravitational wave detectors such as LISA. In addition, black holes and neutron stars provide a unique laboratory for studying matter under strong gravity, while neutron stars also allow the study of matter in the presence of extreme magnetic fields and at supra-nuclear densities. The structure of a neutron star is set by the nuclear equation of state, whose determination is one of the priorities of physics today. The composition of a neutron star can vary from neutrons and protons to hyperons - particles that contain strange quarks - and possibly even free quarks. X-rays emerging from the strongly curved space time of neutron stars encode information on the mass and radius of the compact object, hence the equation of state, EOS, of matter at supra-nuclear density. A large area, high spectral and high time resolution X-ray telescope is required to constrain physics in the strong field and high density limit.

2.3.1 Gravity in the strong field limit

Black holes play a key role in the evolution of galaxies and ultimately in the star formation history, but their main characteristic is that they strongly distort the shape of space-time in their vicinity. X-rays resulting from the accretion of matter onto a compact object (a supermassive or stellar-mass black hole, or neutron star) probe the motion of matter in strongly curved space time. This is a regime in which fundamental predictions of General Relativity are to be tested, such as the existence of an event horizon for black holes, an innermost stable circular orbit, or the strong drag-ging of inertial frames, also known as Lense-Thirring precession. The high-frequency phenomena discovered by the Rossi X-ray Timing Explorer (R-XTE), the so-called kilo-Hz Quasi-periodic Oscillations (QPOs) seen from accreting black holes and neutron stars, have triggered much


page 21

interest because, for the first time, variability on the dynamical timescale of the accretion flow originating from a region in which strong gravity must play a central role was revealed. The effects of strong gravity can be observed through the distorted Fe lines seen from accreting black holes and it is possible to discriminate between Kerr and Schwarzschild black holes by the difference in their Fe line profiles. Studies of MCG-6-30-15 have shown the presence of a relativistically distorted Fe line consistent with that expected from a maximally rotating (Kerr) black hole. Such investigations will allow the regions close to a black hole be studied by measuring the smallest radius at which emission occurs – a sensitive measure of the black hole spin-rate. This requires that the inner radius be measured to a precision of ±0.5 GM/c2. These studies do not provide drivers for the effective area at 1 keV (but do at 7 keV), or energy resolution for XEUS, but place constraints on the amount of pile-up (and hence distortion of the spectral continuum) for moderately bright sources. X-rays from AGN and galactic black hole systems are expected to be polarized due to Compton scattering. As accretion is believed to occur via a disc, the resulting axis symmetric geometry implies that, in the Newtonian case, the polarization is either parallel to, or perpendicular to, the disc plane. This is no longer true in General Relativity, where the bending of the photons' geodesics results in a rotation of the polarization angle. The effect is significant only in the strong field regime. In galactic systems, where X-rays are mostly produces by thermal disc emission, the rotation is energy dependent, because both the temperature and the rotation of the polarization angle increase with decreasing radii. In AGN, where X-rays are due to inverse Compton of disc photons, the rotation is expected to be time dependent, as time variability likely reflects variations of the distance of the emitting region from the black hole. These observations do not need high spectral resolution, or a large FOV, although some of the sources may be observed with a high-spectral resolution instrument to provide desirable extra information.


The effective area needed to discriminate between Schwarzschild and Kerr black holes was investigated using the Fe line observed from MCG-6-30-15 and using the Laor model in XSPEC. An integration interval of 2 104 s was used – longer intervals may be affected by changes in the underlying continuum. Using the ASCA line parameters indicates that the minimum emitting radius could be determined to ±0.5 GM/c2, for an effective (and hence mirror) area of 2 m2 at 7 keV. This would provide a sensitive measure of the black hole spin-rate, and hence probe GR.


The low-energy cut off is not a problem here since most previous works have ignored the spectrum below 3 keV due to its complexity. Hence 0.5 keV is taken as the requirement. A high-energy response to 15 keV is required and an extension to 40 keV is desirable to accurately model the reflection component.


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Point source studies of bright sources so angular resolution is not a driver.


Generally studying known point sources, so as long as the FOV is significantly larger than the mirror PSF this will be sufficient.


To study the structure of the broad Fe line a resolution of 150 eV FWHM is desired. In order to separate narrow emission line components, or the effects of warm absorbers, simultaneous high-spectral resolution data are desirable.

2.3.1.6 Sensitivity

Sensitivity to faint sources is not an issue, since this topic is detailed spectroscopy of bright sources.


Not a driver, 10 sec would be sufficient.


A bright AGN (20 mCrab) source should be observable with <1% distortion of the spectrum due to pile-up. This requirement derives from the need to accurately characterize the continuum so that the properties of the broad Fe-lines can be reliably obtained with the spectral resolution expected from the larger FOV instruments. Since a Crab-like spectrum gives 4 105 count s-1 with an (assumed) 5 m2 effective area, such an AGN will give 8 103 count s-1. The high spectral resolution instrument desired to characterise any narrow spectral components, should be capable of handling similar sources (8 103 count s-1) without significant loss of spectral resolution and an effective detector dead-time of <10% assuming count rates per pixel can be limited to about 150 count pixel-1 s-1.

2.3.1.9 Polarimetry

The amount of polarization expected from galactic black holes and AGN is model dependent. A realistic goal for XEUS is to detect polarization degrees of 2% at 3σ confidence from a bright (10 mCrab) source in a 100 ksec observation.


At least 1000 sec, but continuous (100 ksec) observations are highly desirable. A region of sky should be observable for at least 2 weeks every 6 month observing season.


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2.3.2 Equations of state

Black holes and neutron stars provide a unique laboratory for studying matter under strong gravity. Neutron stars offer also the possibility to study matter under extreme magnetic fields and at supra nuclear density. In the interior of neutron stars density can reach up to 10 times the nuclear density. Besides neutrons, protons, electrons and muons, at those high densities the physics of strong nuclear interactions predicts the existence of other more exotic particles, like strangeness-bearing baryons, pions and kaon condensates, or even deconfined quarks. Neutron stars offer the only possibility, since the Big Bang, to find those particles in nature. Since the structure of neutron stars is set by the, yet unknown, equation of state, EOS, of nucleonic matter at these high densities, measurements of neutron star masses and radii can provide information not only on the physical makeup of neutron stars, but also on the nature of the interactions between the particles that constitute them. Very accurate mass measurements of neutron stars in binary systems have not set useful constraints on the EOS (<M> 1.35 M ). Fits to the continuum X-ray spectra of isolated neutron stars and X-ray bursters favour radii smaller than 6–8 km. However, there is strong evidence that the X-ray emission only comes from a fraction of the neutron star surface. Since information on the mass and radius of the neutron star is encoded in the emission from hot (T ≥106 K) gas deep in the neutron star gravitational potential, the most efficient way forward to address this issue is a combination of large effective area, with high-spectral or high-time resolution in the X-ray band. Addressing first spectral studies: Because of the strong gravitational field, spectral lines formed in the neutron star atmosphere are redshifted by an amount that is a function of the star’s mass-to-radius ratio, M/R. At the very high densities encountered in neutron star atmospheres, spectral lines are pressure broadened (Stark effect) by an amount that is a function of M/R2. Measurements of both these effects provide M and R separately.

X-ray bursts offer the most promising environment to observe these redshifted/broadened absorption lines. X-ray bursts are short (10–1000 s duration) events in which mass accreted onto the surface of the neutron star from its companion is unstably burned via a thermonuclear flash that progressively engulfs the whole surface of the neutron star. The fresh accreting material reflects the chemical composition of the donor star, while the thermonuclear reactions on the neutron star surface changes the chemical composition of this material. There is circumstantial evidence for the presence of lines in X-ray burst spectra taken with XMM-Newton. However, the evidence is undermined by the fact that long burst intervals and several burst spectra had to be co-added to improve the statistics, which implies that complex dynamical effects on the surface of the neutron star during bursts complicate the interpretation. The spectroscopic aims for XEUS are to:

• Detect and study the properties of X-ray lines during a single burst with sufficient accuracy to constrain the M/R and the M/R2 ratios independently for a handful of selected systems.

• Study the evolution of the physical conditions at the surface during a single burst for a few neutron stars.


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Timing studies provide an alternative, and highly complementary, way to study the EOS of neutron stars: Most models for kilo-Hz QPOs involve orbital motion in the strong-field region. If the orbital interpretation is correct, then direct constraints on the fundamental parameters of compact stars (e.g. black-hole spin) can be derived. Furthermore, for neutron stars, this directly sets constraints on their masses and radii, hence on the EOS of dense matter at supra-nuclear density. Evidence for strong-field Lense-Thirring precession has been claimed, as has been the signature of the innermost stable circular orbit (ISCO) around a couple of accreting neutron stars. Similarly, epicyclic motion, a unique feature of Einstein’s gravity, is currently implied in the most recent models of high frequency QPOs detected from black holes. The pairs of QPOs detected from a handful of stellar-mass black holes are constant and show a 3:2 frequency ratio. This has been interpreted as being due to a resonance between the vertical and radial epicyclic frequencies. Under this assumption, for black holes of dynamically estimated mass, the spin can be estimated and all results point to maximally rotating Kerr black holes. Although disputed, these results and interpretations clearly demonstrate the power of timing observations for probing strong-field effects, which has become in the last few years, a very complementary tool to X-ray spectroscopy. Studies of the polarization properties of the X-ray emission from the surfaces of magnetised neutron stars will provide additional insights into the emission and reprocessing regions, and the magnetic field strength and geometry complementing the information obtained through timing and spectroscopic studies. Because R-XTE is sensitivity limited, it is generally agreed that only the strongest high-frequency signals have been seen, whereas most models predict that there should be weaker sidebands signals. A much more sensitive timing instrument, combining moderate resolution spectroscopy, would allow the validity of the orbital and epicyclic interpretation of the signals to be investigated by independently measuring the radius and frequency though time-resolved spectroscopy of the oscillations on their coherence times (e.g., waveform fitting). Detection of weaker signals will allow different QPO models to be discriminated and the detection of QPOs at higher frequencies, making possible searches for the frequency ceiling predicted at the ISCO, and setting more stringent constraints on the EOS. A QPO frequency as high as 1800 Hz would be large enough to exclude all standard nucleonic or hybrid quark matter EOS, leaving only strange stars. For both timing and spectroscopy the instantaneous effective area is the key driver. For the spectroscopic studies this needs to be combined with high spectral resolution. For the timing studies this needs to be combined with a detector with high count-rate capability and accurate time resolution. These studies will use both a dedicated timing instrument and a high spectral resolution instrument.


For timing studies, the significance of a QPO detection scales directly with the source count rate. Therefore a significant improvement in the sensitivity of timing studies with respect to R-XTE


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requires a 2 m2 effective area at 7 keV with a goal of 3 m2. Note that an area of 3 m2 at 7 keV would yield approximately 10 times more effective area than the PCA on board R-XTE at 3 keV. The goal area of 3 m2 at 7 keV will allow the detection of QPOs with amplitudes as low as 0.4% rms in 1000 s. The same area will enable the detection of 5 Hz broad QPOs at 4σ confidence down to their coherence timescales (<0.1 s). This will enable detailed waveform fitting, opening the way to study entirely new phenomena such as Doppler shifts in the modulated signal. In order to study harder QPOs effective (and hence mirror) areas of 1.0 m2 at 10 keV and 0.1 m2 at 15 keV are desired. For high resolution spectral studies, a typical X-ray burst reaching the Eddington luminosity (~1038 erg s-1) at the peak and at the galactic center distance, an effective area of 5 m2 at 1 keV would allow a 3σ characterization of absorption lines similar to the ones implied by the XMM-Newton measurements in a 30 s exposure. This would enable the study of the evolution of the surface properties over the duration of a single X-ray burst for the most luminous sources.


The required energy range for QPO timing studies is 1 to 15 keV with a high-energy extension to 40 keV desirable to study harder QPOs and for accurate modelling of the continuum spectrum to study the Fe line and reflection. For red-shifted line studies during X-ray bursts using a high spectral resolution instrument, an energy range of 0.2 – 7 keV is required.


Point source studies of bright sources so angular resolution is not a driver.


Not a driver.


For the timing studies, a moderate energy resolution of 200 eV FWHM at 6 keV is required to allow e.g., phase-resolved spectroscopy of the Fe line on the QPO phases. We note that a 200 eV FWHM is 5 times better than the energy resolution of proportional counters such as the PCA, and is sufficient to cleanly sample broad Fe lines. For spectroscopic studies, for masses between 1 and 2 M and radii between 10 and 20 km, the Stark broadening is ∆E ~ 10 eV. The effects of the neutron star spin and relativistic light bending would also broaden the detected lines. Hence, a resolution of 5 eV at 2 keV is required.

2.3.2.6 Sensitivity

Sensitivity to faint sources is not an issue, since this topic is a detailed study of bright sources.


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The required time resolution of the instrument is 10 µsec, as we are interested in high-frequency phenomena (>1000 Hz).


For timing studies, with a maximum flux of 5 times the Crab (around the mean peak flux of bright transients) yields a maximum count rate of 2 106 s-1, which should be handled with a deadtime of <10% in the timing instrument. Typical count rates for burst peaks corresponding to the Eddington luminosity of a 1.4 M object at the galactic center (8 kpc) are 3 105 s-1 in the spectroscopic instruments. These high count rates would last for 1–5 s (the duration of typical burst peaks), and will be 10–100 less before, or after, the burst peaks.

2.3.2.9 Polarimetry

The requirement is to detect, at 3σ confidence, a 2% degree of polarization in a 10 mCrab source during a 10 ksec observation (this is to allow 10 phase bins in a 100 ksec pulsar observation).


Continuous (50 ksec) observations are highly desirable, with a minimum continuous observation duration of 1000 s. A single source could be observable for at least 2 weeks during any one observing season. This corresponds approximately to a range of observing angles with respect to the solar vector of ±5o from the nominal. Such a narrow constraint is likely to severely impact the mission planning efficiency and a goal is to extend this constraint to ±15o (the same as for XMM-Newton). In addition, a goal is to start observing a Target of Opportunity anywhere on the visible region of sky within 1 day of the decision being made to change targets.

2.3.3 Acceleration phenomena

Our objectives to study diffuse hard X-ray sources, particularly those with non-thermal nature, are threefold. The first is to obtain a much improved knowledge of the physics of particle acceleration, including the origin of cosmic rays, mechanisms and energy sources in various acceleration sites, feedback effects from the accelerated particles to their energy sources, the relation between the attainable electron and ion energies, and the contribution of energetic particles to the overall cosmic energy budget. In clusters of galaxies, energy flow is an important topic to investigate from the potential and kinetic energy of visible and Dark Matter to thermal and non-thermal components. The second objective is to explore the vast ``discovery space'' associated with these sources, where our knowledge is still embryonic compared to the much better understanding of thermal X-ray emitters. The final objective is to tackle a fundamental science area of the 21st century, that is, the physics of energy non-equipartition and evolution away from equilibrium; this behavior is manifested in a very clean manner by the cosmic non-thermal phenomena. With the capability summarized below, XEUS would enable a sensitive search for non-thermal signals from a large number of clusters and groups of galaxies and to examine whether these


page 27

objects are the source of the most energetic cosmic rays. Similarly, we would be able to examine essentially every nearby galaxy for possible diffuse hard X-ray emission, as is seen from the Milky Way. Then, we can for the first time investigate the relation between the hot interstellar media and relativistic particles contained therein; we can then examine an emerging scenario that a certain fraction of thermal particles are heated into a supra-thermal population, which in turn provides the genuine non-thermal population in the form of a runaway tail. Through anticipated detections of the inverse-Compton hard X-rays from a large number of synchrotron-emitting lobes of radio galaxies, we may clarify whether the suggested dominance of the particle energy density over that of the magnetic field is a general truth. This topic requires imaging observations at energies above 15 keV – presumably using a dedicated hard X-ray instrument.


A goal effective area of 0.1 m2 at 15 and 40 keV together with the high spatial sensitivity will provide a significantly improved hard X-ray sensitivity than will be achieved by forthcoming missions such as NeXT and SIMBOL-X.


In order to study purely non-thermal phenomena without being hampered by thermal contamination, a significantly better imaging spectroscopic capability than XMM-Newton up to energies of 15 keV is required. In order to study the Compton reflection peak an extension to 40 keV is desirable.


The spatial resolution necessary to distinguish diffuse components from point sources in clusters of galaxies and to avoid source confusion is 15″ HEW with a goal of 10″ HEW.


The requirement is 5′ x 5′.


Spectral resolution is not a priority and a resolution of 1 keV FWHM at 40 keV is the requirement.

2.3.3.6 Sensitivity

Not a driver, but to be optimised through background reduction and other techniques.


Not a driver.


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Not a driver.

2.3.3.9 Polarimetry

To study the polarization properties of acceleration mechanisms a polarization degree of 2% should be detectable at 3σ confidence in a 2 mCrab source in a 100 ksec observation.


Not a driver.


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3 MISSION PERFORMANCE REQUIREMENTS AND GOALS In this section, the mission performance requirements and goals are summarized. There are two sets of energy resolutions given in this document: those expected from future generation of high spectral resolution instruments of a few eV, and those expected from instruments which provide larger FOVs and spectral resolutions of around 100 eV. LFOV and NFOV refer to larger and smaller FOV instruments; HXC is a hard X-ray camera, XPOL an X-ray polarimeter, and HTRS a high time resolution spectrometer. It should be noted that:

• There is no requirement to provide the LFOV and NFOV capabilities simultaneously, although this would be welcomed, should it be possible.

• There is no requirement to provide a hard X-ray (>10 keV) extension simultaneously with the high-spectral resolution capability, although this would be welcomed should it be possible.

There is no requirement to provide the HTRS or XPOL capabilities simultaneously with data from any of the other instruments. These instruments may therefore be placed elsewhere in the focal plane than the main imaging instruments (see ESA SP-1273).

3.1 Core Instrumentation

The core instruments are the LFOV and one of the two NFOV.

3.2 Additional High Priority Instruments

The HTRS, HXC, second NFOV and XPOL are additional high priority instruments that are necessary to fulfil the science objectives presented in the XEUS Cosmic Vision proposal. They should be considered for XEUS should sufficient resources be available.


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3.1 Mission Requirements and Goals

Summary of the scientific requirements (and goals) of each of the sub-topics from the Cosmic Vision proposal.

Topic Effective area (m2)

Energy range(KeV)

Ang. res. (″ HEW)

Instrument FOV Diameter

(')

Spec. res. (eV, FWHM)

Point source det. sens.

(erg cm-2 s-1)

Time res.(s)

Count rate capability

Polarimetry MDP at 3σ conf 100 ks

Observing constraints

Sub-topic

requires Evolution of large Scale Structure and Nucleosynthesis Formation, dynamical and chemical evol. of groups and clusters

1.0 @ 0.2 keV 1.5 @ 0.2 keV (goal)

5 @ 1 keV

0.2-6 NFOV0.2–10 LFOV

0.2-40 (goal)

5 2 (goal)

7 (LFOV) 0.75 (NFOV)1.7 Ø (high

priority goal)

2 eV @ <2 keV

N/A N/A N/A N/A N/A NFOV LFOV

Baryonic composition of the IGM (WHIM)

1.0 @ 0.2 keV 5 @ 1 keV

0.2 – 7 5 0.75 2 eV @ 500 eV 1 eV @ 500 eV (goal)

N/A N/A N/A N/A N/A NFOV

Enrichment dynamics

5 @ 1 keV 0.2 – 6

5 2 (goal)

5 (LFOV) 0.75 (NFOV)

1 eV @ 1 keV (goal)2 eV @ 2 keV (goal)

6 eV @ 6 keV 3 eV @ 6 keV (goal)

N/A N/A N/A N/A N/A NFOV LFOV

Coeval Growth of Galaxies and Supermassive Black holes Birth and growth of supermassive black holes

1.0 @ 0.2 keV 5 @ 1 keV

1 @ 10 keV, 0.1 @ 15 & 40 keV (goals)

0.1 – 10 0.1-40 (goal)

5 2 (goal)

10 @ 40 keV

5 (LFOV) 7 (goal)

150 eV @ 6 keV 1 keV @ 40 keV (goal)

4 10-18 (0.2-10.0 keV;

4σ)

N/A N/A N/A >500 ksec visibility once per 6-month observing season

LFOV HXC

Supermassive black hole induced galaxy evolution

5 @ 1 keV 0.1 – 10 0.1-40 (goal)

5 2 (goal)

0.75 (NVOV) 5 (LFOV)

6 eV @ 6 keV 3 eV @ 6 keV (goal)

N/A N/A N/A 10% MDP 0.1 mCrab

N/A NFOV LFOV

Matter Under Extreme Conditions

Gravity in the strong field limit

2 @ 7 keV 1 @ 10 keV (goal)

0.5 – 15 0.5-40 (goal)

N/A N/A 150 eV @ 6 keV N/A 10 8 103 s <1% pileup LFOV. <10% pileup

NFOV

2% MDP 10 mCrab. 3σ

conf.

103 s (105 s goal) continuous observ. >2 weeks/season

LFOV HXC

Equations of State 5 @ 1 keV

2 @ 7 keV 1 @ 10 keV (goal)

0.1 @ 15 keV (goal)

0.2 – 7 1 – 15 HTRS1 – 40 (goal)

N/A N/A 5 eV @ 2 keV 200 eV @ 6 keV

N/A 10-5 (high pri.

goal)

2 106 s-1 (with <10% deadtime) HTRS

2% MDP 10 mCrab

(10 ksec). 3σ conf.

103 s (5 104 s goal) continuous observ. ToO <1 day (goal). ±5o (±15o goal) range of Sun angles

HTRS NFOV

Acceleration phenomena 0.1 @ 15 keV (goal) 0.1 @ 40 keV (goal)

1 – 15 1 – 40 (goal)

10 @ 40 keV(goal)

5 x 5 1 keV @ 40 keV (goal) N/A N/A N/A 2% MDP 2 mCrab. 3σ

conf.

N/A HXC


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3.2 Mission Performance Summary

The table below lists the XEUS scientific requirements (and goals), and the science sub-topic from which each requirement is derived. Parameter Requirement Science Drivers Effective area (m2) 1.0 @ 0.2 keV

1.5 @ 0.2 keV (goal) 5 @ 1 keV 2 @ 7 keV 3 @ 7 keV (goal) 1 @ 10 keV (goal) 0.1 @ 15 keV (goal) 0.1 @ 40 keV (goal)

WHIM, early black holes, clusters WHIM, early black holes, clusters Clusters, WHIM, early black holes EOS, gravity in strong fields EOS EOS, acceleration, early black holes Acceleration, early black holes, EOS Acceleration, early black holes, EOS

Energy range (keV) 0.1 – 15 0.1 – 40 (goal)

Birth and growth of black holes Acceleration, clusters

Angular resolution (″) (below 10 KeV)

5 2 (goal) 10 @ 40 keV

Clusters, early black holes, WHIM Clusters, early black holes Early black holes (HXC)

Field of view (′) 7 diameter (WFI) 0.75 diameter (NFI) 1.7 diameter (NFI high priority goal)5 x 5 (>10 keV; goal)

Clusters. early black holes (LFOV) Clusters, enrichment, galaxy evolution Clusters (NFOV) Acceleration (HXC)

Spectral resolution (eV) (FWHM)

2.0 @ 0.5 keV 1.0 @ 0.5 keV (goal) 2.0 @ <2 keV 6.0 @ 6 keV 3.0 @ 6 keV (goal) 150 @ 6 keV 1000 @ 40 keV (goal)

WHIM WHIM Clusters Enrichment, induced galaxy evolution Enrichment, induced galaxy evolution Early black holes (LFOV) Early black holes (HXC)

Point source detection sensitivity (erg cm-2 s-1)

4 10-18 (0.2-10.0 keV; at 4σ confidence)

Early black holes

Time Resolution (s) 10-5 (high priority goal) EOS studies (HTRS) Count rate capability (s-1) 2 106 EOS studies (HTRS) Polarimetry (MDP, 3σ confidence).

2% for 10 mCrab in 10 ksec EOS studies

Observing constraints >2 weeks visibility each 6 month observing season. ToO response in <1 day (goal). 103 s (5 104 s goal) continuous obs. ±5o (±15o goal) range of Sun angles

EOS studies, EOS studies EOS studies, strong gravity EOS studies

3.3 High-priority goals

The following are deemed to be high-priority scientific goals:

1. The FOV requirement of 0.75′ diameter is sufficient for Fe line spectroscopy of nearby clusters by mosaicing. However, it is of crucial importance to investigate whether the high-


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spectral resolution FOV can be extended up to a diameter of 1.7′ in order to be able to measure the 0.2 – 6 keV properties of distant clusters.

2. The inclusion of the HXC and HTRS instruments in the payload so that all the high-priority

goals of the XEUS Cosmic Vision proposal can be met by the studied payload.

3.4 Mirror Effective Area

As stated in Sect 1.0, the effective areas given in this document are those available for scientific investigation and therefore include detection efficiencies and filter transmissions (if applicable). The specified effective areas are given either at particular energies, or over a range in energies in the case of e.g., of Fe line studies. Although the XEUS science goals only provide requirements on the total area available for science, the specified effective areas have been converted into mirror areas using:

Energy (keV)

Detector efficiency * Filter Transmission

0.2 0.2 1.0 1.0 2.0 1.0 7.0 1.0 10 1.0 15 1.0 40 1.0

A goal for the mirror system is to provide an effective area that varies smoothly between the requirement values (see figure below). This may be understood as maintaining the mirror areas integrated between 2 and 6 keV and between 6 and 10 keV to within 50% of the requirements in any 0.5 keV interval (2–6 keV) or 1 keV interval (6–10 keV).

XEUS Mirror Area versus Energy

0.01

0.1

1

10

0.1 1 10 100

Energy (keV)

Area

(m2 )


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4 ACRONYMS AGN Active galactic nucleus ALMA Atacama Large Millimetre Array CXB Cosmic X-ray background DM Dark Matter EOS Equation of state eV Electron Volt EW Equivalent width FOV Field of view FWHM Full-width at half-maximum GR General Relativity HEW Half-energy width HTRS High Time Resolution Spectrometer HXR Hard X-ray Camera ISCO Innermost stable circular orbit ICM Intra cluster medium IGM Intergalactic medium ISM Interstellar medium JWST James Webb Space Telescope KeV Kilo-electron Volt ΛCDM Lambda Cold Dark Matter (cosmology) LFOV Large FOV instrument MDP Minimum detectable degree of polarization NeXT New X-ray Telescope mission NFOV Narrow FOV instrument N/A Not applicable PCA Proportional Counter Array (instrument on R-XTE) PSF Point Spread Function QPO Quasi-periodic oscillations RGS Reflection Grating Spectrometer (instrument on XMM-Newton) R-XTE Rossi X-ray Timing Explorer satellite SN Supernova ToO Target of opportunity WHIM Warm/hot intergalactic medium WMAP Wilkinson Microwave Anisotropy Probe XPOL X-ray Polarimeter instrument

xeus x-ray evolving universe spectroscopyemits.sso.esa.int/emits-doc/4964-rd1-xeus_scird.pdfxeus is...

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