Next Generation Of X-ray Observatories Research Articles

Current results and future prospectives on the XMM–Newton Heritage project CHEX-MATE, and on the non-thermal pressure in galaxy clusters

The Cluster HEritage project with XMM-Newton – Mass Assembly and Thermodynamics at the Endpoint of structure formation (CHEX-MATE) is a Multi-Year Heritage Programme to obtain X-ray observations of a minimally-biased, signal-to-noise-limited sample of galaxy clusters detected by Planck through the Sunyaev–Zeldovich effect. The program aims to study the ultimate products of structure formation in time and mass. On behalf of our large international collaboration, I will summarize the most recent results obtained, highlighting the role of multi-band datasets in resolving the astrophysics of the most massive collapsed halos in the universe and in studying the interplay between hot plasma and dark matter. I will also present some new methods for estimating the non-thermal pressure support in galaxy clusters, and how we can convert it into a measurement of the hydrostatic mass bias, also for a cosmological purpose. These studies will pave the way for using the next generation of X-ray observatories to construct a consistent picture of the formation and composition in mass and energy of galaxy clusters.

Possible observational signatures of supermassive black hole binaries in their Fe Kα line profiles

Here we study the potential observational signatures of supermassive black hole binaries (SMBHBs) in the Fe K$\alpha$ line profiles emitted from the accretion disks around their components. We simulated the Fe K$\alpha$ line emission from the relativistic accretion disks using ray tracing method in Kerr metric. The obtained profiles from the SMBHBs are then compared with those in the case of the single supermassive black holes (SMBHs). We considered two models of the SMBHBs: a model when the secondary SMBH is embedded in the accretion disk around the primary, causing an empty gap in the disk, and a model with clearly separated components, where the accretion disks around both primary and secondary give a significant contribution to the composite Fe K$\alpha$ line emission of a such SMBHB. The obtained results showed that both models of SMBHBs can leave imprints in the form of ripples in the cores of the emitted Fe K$\alpha$ line profiles, which may look like an absorption component in the line profile. However, in the case of the composite line profiles emitted from two accretion disks, these ripples could have much higher amplitudes and strongly depend on orbital phase of the system, while for those emitted from a disk with an empty gap, the corresponding ripples mostly have lower amplitudes and do not vary significantly with orbital phase. The present day X-ray telescopes are not able to detect such signatures in the observed X-ray spectra of SMBHBs. However this will be possible with the next generation of X-ray observatories, which will also enable application of such effects as a tool for studying the properties of these objects.

Thermal and non-thermal X-ray emission from stellar clusters and superbubbles

Massive stellar clusters inject significant mass and energy into their surroundings driving superbubbles, 100–1000 pc diameter shells of swept-up interstellar material that contain a hot ($10^{6}$ K), shock-heated gas. Analytically, the evolution of these objects is usually described as an expanding, pressure-driven bubble. However, in the 90s discrepancies between predictions of the pressure-driven model and X-ray observations of superbubbles were observed. Resolutions to these discrepancies were proposed, mainly through modifications to the model, though it wasn’t until the advent of Chandra, XMM-Newton, and Suzaku that these could be observationally tested, with advances in numerical modelling also providing valuable insight. Here I review X-ray observations of superbubbles and their massive stellar clusters and describe how their thermal and non-thermal X-ray emission have helped the understanding of the physical properties and evolution of these objects. Future prospects with the next generation of X-ray observatories are also discussed.

Measuring masses in low mass X-ray binaries via X-ray spectroscopy: the case of MXB 1659-298

Abstract The determination of fundamental parameters in low-mass X-ray binaries (LMXBs) typically relies on measuring the radial velocity curve of the companion star through optical or near-infrared spectroscopy. It was recently suggested that high resolution X-ray spectroscopy might enable a measurement of the radial velocity curve of the compact object by monitoring the Doppler shifts induced by the orbital motion of the disc wind or the disc atmosphere. We analysed a Chandra-HETG+NuSTAR soft state observation of MXB 1659-298, an eclipsing neutron star LMXB. We measured a radial velocity curve whose phase offset and semi-amplitude are consistent with the primary star. We derived the value for the semi-amplitude of the radial velocity for the compact object K1 = 89 ± 19 km s−1, constrained the mass of the secondary (0.3 M⊙ ≤ M2 ≤ 0.8 M⊙) and the orbital inclination of the binary system (73°≤ i≤77°). These values are consistent with previous estimates from independent methods. Via the same technique, the next generation of X-ray observatories equipped with high spectral resolution instruments (e.g. Athena) will have the potential to measure the radial velocity curve of the primary in high inclination X-ray binaries to an accuracy of a few per cent.

Testing the Kerr metric with the iron line and the KRZ parametrization

The spacetime geometry around astrophysical black holes is supposed to be well approximated by the Kerr metric, but deviations from the Kerr solution are predicted in a number of scenarios involving new physics. Broad iron Kα lines are commonly observed in the X-ray spectrum of black holes and originate by X-ray fluorescence of the inner accretion disk. The profile of the iron line is sensitively affected by the spacetime geometry in the strong gravity region and can be used to test the Kerr black hole hypothesis. In this paper, we extend previous work in the literature. In particular: i) as test-metric, we employ the parametrization recently proposed by Konoplya, Rezzolla, and Zhidenko, which has a number of subtle advantages with respect to the existing approaches; ii) we perform simulations with specific X-ray missions, and we consider NuSTAR as a prototype of current observational facilities and eXTP as an example of the next generation of X-ray observatories. We find a significant difference between the constraining power of NuSTAR and eXTP. With NuSTAR, it is difficult or impossible to constrain deviations from the Kerr metric. With eXTP, in most cases we can obtain quite stringent constraints (modulo we have the correct astrophysical model).

Cold fronts: probes of plasma astrophysics in galaxy clusters

The most massive baryonic component of galaxy clusters is the ‘intracluster medium’ (ICM), a diffuse, hot, weakly magnetized plasma that is most easily observed in the X-ray band. Despite being observed for decades, the macroscopic transport properties of the ICM are still not well constrained. A path to determine macroscopic ICM properties opened up with the discovery of ‘cold fronts’. These were observed as sharp discontinuities in surface brightness and temperature in the ICM, with the property that the denser side of the discontinuity is the colder one. The high spatial resolution of the Chandra X-ray Observatory revealed two puzzles about cold fronts. First, they should be subject to Kelvin–Helmholtz instabilities, yet in many cases they appear relatively smooth and undisturbed. Second, the width of the interface between the two gas phases is typically narrower than the mean free path of the particles in the plasma, indicating negligible thermal conduction. It was thus realized that these special characteristics of cold fronts may be used to probe the properties of the cluster plasma. In this review, we will discuss the recent simulations of cold fronts in galaxy clusters, focusing on those which have attempted to use these features to constrain ICM physics. In particular, we will examine the effects of magnetic fields, viscosity, and thermal conductivity on the stability properties and long-term evolution of cold fronts. We conclude with a discussion on what important questions remain unanswered, and the future role of simulations and the next generation of X-ray observatories.

Magnetically confined wind shocks in X-rays – A review

A subset (∼10%) of massive stars present strong, globally ordered (mostly dipolar) magnetic fields. The trapping and channeling of their stellar winds in closed magnetic loops leads to magnetically confined wind shocks (MCWS), with pre-shock flow speeds that are some fraction of the wind terminal speed. These shocks generate hot plasma, a source of X-rays. In the last decade, several developments took place, notably the determination of the hot plasma properties for a large sample of objects using XMM and Chandra, as well as fully self-consistent MHD modeling and the identification of shock retreat effects in weak winds. Despite a few exceptions, the combination of magnetic confinement, shock retreat and rotation effects seems to be able to account for X-ray emission in massive OB stars. Here we review these new observational and theoretical aspects of this X-ray emission and envisage some perspectives for the next generation of X-ray observatories.

Fe xxv line profiles in colliding wind binaries

Strong wind–wind collisions in massive binaries generate a very hot plasma that frequently produces a moderately strong iron line. The morphology of this line depends upon the properties of the wind interaction zone and its orientation with respect to the line of sight. As the binary components revolve around their common centre of mass, the line profiles are thus expected to vary. With the advent of the next generation of X-ray observatories (Astro-H, Athena) that will offer high-resolution spectroscopy above 6 keV, it will become possible to exploit these changes as the most sensitive probe of the inner parts of the colliding wind interaction. Using a simple prescription of the wind–wind interaction in an early-type binary, we have generated synthetic line profiles for a number of configurations and orbital phases. These profiles can help constrain the properties of the stellar winds in such binary systems.

First results from a next-generation off-plane X-ray diffraction grating

Future NASA X-ray spectroscopy missions will require high throughput, high resolution grating spectrometers. Off-plane reflection gratings are capable of meeting the performance requirements needed to realize the scientific goals of these missions. We have identified a novel grating fabrication method that utilizes common lithographic and microfabrication techniques to produce the high fidelity groove profile necessary to achieve this performance. Application of this process has produced an initial pre-master that exhibits a radial (variable line spacing along the groove dimension), high density (>6000 grooves/mm), laminar profile. This pre-master has been tested for diffraction efficiency at the BESSY II synchrotron light facility and diffracts up to 55% of incident light into usable spectral orders. Furthermore, tests of spectral resolving power show that these gratings are capable of obtaining resolutions well above 1300 ($\lambda/\Delta\lambda$) with limitations due to the test apparatus, not the gratings. Obtaining these results has provided confidence that this fabrication process is capable of producing off-plane reflection gratings for the next generation of X-ray observatories.

Weighing super-massive black holes with narrow Fe Kα line

It has been suggested that the narrow cores of the Fe Kα emission lines in Active Galactic Nuclei (AGNs) are likely produced in the torus, the inner radius of which can be measured by observing the lag time between the V and K band flux variations. In this paper, we compare the virial products of the infrared time lags, and the narrow Fe Kα widths for 10 type 1 AGNs, with the black hole masses from other techniques. We found the narrow Fe Kα line width is in average 2.6 +0.9−0.4 times broader than expected, assuming an isotropic velocity distribution of the torus at the distance measured by the infrared lags. We propose the thick disk model of the torus may explain the observed larger line width. Another possibility is the contamination by emission from the broad line region or the outer accretion disk. Alternatively, the narrow iron line might originate from the inner most part of the obscuring torus within the sublimation radius, while the infrared emission may be from the outer cooler part. We note the correlations between the black hole masses based on this new technique and those based on other known techniques are statistically insignificant. We argue that this could be attributed to the small sample size and the very large uncertainties in the measurements of iron K line widths. The next generation of X-ray observatories could help verify the origin of the narrow iron Kα line and the reliability of this new technique.

Solutions without a maximum mass limit of the general relativistic field equations for neutron stars

It has been suggested that the narrow cores of the Fe K$\alpha$ emission lines in Active Galactic Nuclei (AGNs) are likely produced in the torus, the inner radius of which can be measured by observing the lag time between the $V$ and $K$ band flux variations. In this paper we compare the virial products of the infrared time lags and the narrow Fe K$\alpha$ widths for 10 type 1 AGNs with the black hole masses from other techniques. We find the narrow Fe K$\alpha$ line width is in average 2.6$^{+0.9}_{-0.4}$ times broader than expected assuming an isotropic velocity distribution of the torus at the distance measured by the infrared lags. We propose the thick disk model of the torus could explain the observed larger line width. Another possibility is the contamination by emission from the broad line region or the outer accretion disk. Alternatively, the narrow iron line might originate from the inner most part of the obscuring torus within the sublimation radius, while the infrared emission from outer cooler part. We note the correlation between the black hole masses based on this new technique and those based on other known techniques is statistically insignificant. We argue that this could be attributed to the small sample size and the very large uncertainties in the measurements of iron K line widths. The next generation of X-ray observatories could help verify the origin of the narrow iron K$\alpha$ line and the reliability of this new technique.

The X‐Ray Spectral Properties of SCUBA Galaxies

Deep SCUBA surveys have uncovered a large population of massive submillimeter-emitting galaxies (SMGs; f850μm 4 mJy) at z 1. Although it is generally believed that these galaxies host intense star formation activity, there is growing evidence that a substantial fraction also harbor an active galactic nucleus (AGN; i.e., an accreting super-massive black hole [SMBH]). We present here possibly the strongest evidence for this viewpoint to date: the combination of ultradeep X-ray observations (the 2 Ms Chandra Deep Field-North) and deep Keck spectroscopic data of SMGs with radio counterparts. We find that the majority (≈75%) of these radio-selected spectroscopically identified SMGs host AGN activity; the other ≈25% have X-ray properties consistent with star formation (X-ray-derived star formation rates of ≈1300-2700 M☉ yr-1). The AGNs have properties generally consistent with those of nearby luminous AGNs (Γ ≈ 1.8 ± 0.5, NH ≈ 1020-1024 cm-2, and LX ≈ 1043-1044.5 ergs s-1), and the majority (≈80%) are heavily obscured (NH 1023 cm-2). We construct composite rest-frame 2-20 keV spectra for three different obscuration classes [NH 5 × 1023 cm-2], which reveal features not seen in the individual X-ray spectra. An ≈1 keV equivalent width Fe Kα emission line is seen in the composite X-ray spectrum of the most heavily obscured AGNs, suggesting Compton-thick or near Compton-thick absorption. Even taking into account the effects of absorption, we find that the average X-ray to far-IR luminosity ratio of the AGN-classified SMGs (LX/LFIR = 0.004) is approximately 1 order of magnitude below that found for typical quasars. This result suggests that intense star formation activity (of order ≈1000 M☉ yr-1) dominates the bolometric output of these SMGs. However, we also explore the possibility that the X-ray to far-IR luminosity ratio of the AGN components is intrinsically less than that found for typical quasars and postulate that some SMGs may be AGN dominated. We investigate the implications of our results for the growth of massive black holes, discuss the prospects for deeper X-ray observations, and explore the scientific potential offered by the next generation of X-ray observatories.

X‐Ray Line Tomography of AGN‐induced Motion in Clusters of Galaxies

The thermal broadening of emission lines of heavy ions is small enough such that Doppler shifts due to bulk motions may be detected with the next generation of X-ray observatories. This opens up the possibility to study gas velocities in the intra-cluster medium. Here we study the effect of bulk motions induced by a central active galactic nucleus (AGN) on the emission lines around the FeXXV complex. We have modelled the evolution of AGN-induced bubbles in a realistic cosmological framework and studied the resulting FeXXV line profiles. We found that in clusters with AGN feedback, motions induced by the inflation of bubbles and their buoyant rise lead to distinct features in the iron emission lines that are detectable with a spectral resolution of about 10 eV. These observations will help to determine the mechanical energy that resides in the bubbles and thereby the kinetic luminosity of the AGN.

XMM‐NewtonReveals the Quasar Outflow in PG 1115+080

We report on an observation of the Broad Absorption Line (BAL) quasar PG 1115+080 performed with the XMM-Newton observatory. Spectral analysis reveals the second case of a relativistic X-ray absorbing outflow in a BAL quasar. The first case was revealed in a recent observation of APM 08279+5255 with the Chandra X-ray Observatory. The spectrum of PG1115+080 indicates the presence of complex low-energy absorption in the 0.2-0.6 keV observed energy band and high-energy absorption in the 2-5 keV observed energy band. The high-energy absorption is best modeled by two Gaussian absorption lines with rest-frame energies of 7.4 keV and 9.5 keV. Assuming that these two lines are produced by resonant absorption due to Fe xxv Kalpha, we infer that the X-ray absorbers are outflowing with velocities of ~0.10c and ~0.34c, respectively. We have detected significant variability of the energies and widths of the X-ray BALs in PG 1115+080 and APM 08279+5255 over timescales of 19 and 1.8 weeks (proper-time), respectively. The BAL variability observed from APM 08279+5255 supports our earlier conclusion that these absorbers are most likely launched at relatively small radii of ~ 10^(16)(M_BH/M_8)^(1/2)cm. A comparison of the ionization properties and column densities of the low-energy and high-energy absorbers indicates that these absorbers are likely distinct; however, higher spectral resolution is needed to confirm this result. Finally, we comment on prospects for constraining the kinematic and ionization properties of these X-ray BALs with the next generation of X-ray observatories.

Exploratory [ITAL]CHANDRA[/ITAL][ITAL]Chandra[/ITAL] Observations of the Highest-Redshift Quasars: X-Rays from the Dawn of the Modern Universe

We report exploratory Chandra observations of 14 high-redshift (z=4.06-5.27), optically selected quasars. Ten of these quasars are detected, increasing the number of z>4 X-ray detected quasars by 71%. Our detections include four of the five highest-redshift X-ray detected quasars to date, among them SDSSp J021043.17-001818.4, the highest-redshift (z=4.77) radio-loud quasar detected in the X-ray band. The four undetected objects are the Broad Absorption Line quasars SDSSp J112956.10-014212.4 and SDSSp 160501.21-011220.0, the weak emission-line quasar SDSSp J153259.96-003944.1, and the quasar PSS 1435+3057. A comparison of the quasars' spectral energy distributions (by means of the optical-to-X-ray spectral index, alpha_ox) with those of lower-redshift samples indicates that the Chandra quasars are X-ray fainter by a factor of approx 2. X-ray faintness could be associated with the presence of large amounts of gas in the primeval galaxies harboring these high-redshift quasars, as suggested by recent studies conducted on z>4 quasars in other bands. Using the current Chandra data, predictions for the next generation of X-ray observatories, Constellation-X and Xeus, are also provided.

Development of MOS CCDs for the next generation of X-ray observatories

The new millennium will see the launch of the next generation of X-ray observatories. As well as more specialised cryogenic detectors, observatories such as XEUS and Constellation-X will require large arrays of medium spectral resolution detectors, covering large focal planes with a good response over the 0.1–15 keV energy range. The MOS CCD that has been developed for the XMM, ASCA, ASTRO-E and AXAF observatories, with its proven record as an imaging X-ray spectrometer, will be the ideal detector for this role. In this paper we present the state-of-the-art performance being achieved using the XMM/EPIC CCDs and consider further developments that will be build upon existing technologies to ensure the CCD will meet the needs of the future observatories. In particular we discuss: comparisons of CCD surface design, i.e. back illuminated versus open electrode versus thinned electrode: improving high-energy quantum efficiency via the use of fully depleted or extreme depleted silicon: faster readout (>1 MHz) with multiple low-noise amplifiers (<5 electrons rms) and further enhancement of the radiation hardness of CCDs, including 3-D modelling of the charge transfer process.

Transition edge sensors for X-ray astronomy

Investigations are underway for new detector technologies for the next generation of X-ray observatories. Several groups around the world are concentrating on superconducting transition edge sensors as a technology which may deliver the spectral performance required for these future missions. The rapid development of the TES indicates that the desired 2 eV FWHM resolution at 1 keV may be achievable. However, the technology is still in its infancy and substantial technical challenges lie ahead. Our current emphasis is on the reliability and longevity of the films comprising the sensor, while the same time demonstrating, and improving, spectral resolution. Here we demonstrate Al–Ag films with tunable transition temperature accurate to 5 mK, with transition width of <2 mK, and α∼400. We discuss the reliability of films and both diffusion and oxidation mechanisms are present which lead to ageing of both the normal resistance and the transition temperature.

Thermal X-Ray Line Emission from Accreting Black Holes

We present model X-ray spectra of accreting black holes with hot advection-dominated accretion flows, paying attention to thermal emission lines from the hot plasma. We discuss the emission-line diagnostics that are potentially observable with the next generation of X-ray observatories, emphasizing the possibilities for determining the outer radius of the hot plasma, the presence of a wind, and the relative importance of photoionization. We present example spectra of V404 Cyg in quiescence, the Galactic center black hole Sagittarius A*, and the nucleus of M87.

ENDPOINTS OF STELLAR EVOLUTION : X-RAY SURVEYS OF THE LOCAL GROUP.

ABSTRACT The galaxies of the Local Group offer a variety of settings in which to study galactic source populations unhampered by local interstellar-absorption and distance ambiguities. A review of the subject of Local Group X-ray astronomy from the first discovery of a source in the Large Magellanic Cloud 15 years ago to the most recent analysis of the Einstein Observatory data (in which over 200 X-ray sources were detected in Local Group members) is presented. Following a detailed presentation of the latest results, two examples of the constraints these data impose on stellar and galactic evolution are discussed: study of the distribution of properties for the 30 detected supernova remnants in the Large Magellanic Cloud, and a comparison of the populations of classical X-ray binaries in M31, M33, the Magellanic Clouds, and the Galaxy. A prospectus for future research, including a discussion of the contributions which could be made by the next generation of X-ray observatories, is provided.