Infrared radiation from compact X-ray sources

Observations of the X-ray binaries 4U 0115+634 and A 0535+262 performed in 1981 – 1982 revealed significant IR variabilities of these sources. 4U 0115+634 was observed twice, in the state close to X-ray activity, and in the state of low activity, thus changing from K = 7^m.75 to K > 9^m. A 0535+262 shows the temporal variations ranged from tens minutes to some days. The regular variations of IR brightness are suspected with teh period close to 104 s that corresponds to the period of axial rotation of neutron star in A 0535+262. The results obtained present arguments in favour of hypothesis that IR radiation is generated near the accreting neutron star and is possibly of maser nature.     

Observations of compact X-ray binaries with the Chandra X-ray Observatory

The Chandra X-ray Observatory was launched on July 23, 1999. The first X-ray photons were detected on August 12 of that same year. Subsequently observations with the Observatory, which features sub-arcsecond angular resolution, have revolutionized our understanding of the X-ray emitting sky providing hosts of spectacular energy-resolved images and high-resolution spectra. Here we present a brief overview of Chandra X-ray Observatory observations of compact X-ray binaries.     

X-rays from the first massive black holes

X-ray studies of high-redshift (z > 4) active galaxies have advanced substantially over the past few years, largely due to results from the new generation of X-ray observatories. As of this writing X-ray emission has been detected from nearly 60 high-redshift active galaxies. This paper reviews the observational results and their implications for models of the first massive black holes, and it discusses future prospects for the field.     

X-ray radiation from accreting, magnetized neutron stars

A review is given of recent developments in the theory of emission from a magnetized plasma for accreting neutron star conditions. Some observational data on X-ray pulsars are discussed, and present problems are indicated.     

Challenges in X-ray binary timing: current and future

Millisecond X-ray time variability studies of accreting low-magnetic-field neutron stars and stellar-mass black holes in X-ray binaries probe the motion of matter in regions of strong gravity. In these regions, general relativity (GR) is no longer a small correction to the classical laws of motion, but instead dominates the dynamics: we are studying motion in strongly curved spacetime. Such millisecond X-ray variability studies can therefore provide unique tests of GR in the strong-field regime. The same studies also constrain neutron-star parameters such as stellar mass and radius, and thereby the equation of state (EOS) of ultradense matter. I briefly review the status, and discuss the prospects for mapping out space-time near accreting stellar-mass compact objects, and measuring the EOS of dense matter, through millisecond timing, particularly with an eye towards future missions. The overwhelming consideration for timing sensitivity is collecting area: contrary to most applications, the signal-to-noise ratio for the aperiodic timing phenomena produced by accretion flows increases proportionally with count rate rather than as the square root of it. A 10 times larger instrument turns 1σ effects into 10σ effects (or does as well in 1% of the time). With the Rossi X-ray Timing Explorer (RXTE), using 0.6 m² collecting area, we have found several timing diagnostics from the accretion flow in the strong field region around neutron stars and black holes, as well as signals from neutron star surface hot spots. Combined work between RXTE and the new sensitive X-ray spectrographs onboard Chandra and XMM can already begin to clinch the geometry and physical mechanisms underlying these signals. Future instruments, larger in area by an order of magnitude and in some cases with enhanced spectral capabilities, are expected to turn these diagnostics of GR into true tests of GR. They are also expected to put strong constraints on neutron-star structure, and thereby on the EOS of supranuclear density matter.     

Progress Report on Insight-HXMT: China's First X-ray Astronomy Satellite

Insight-HXMT is China's first X-ray astronomy satellite. It was launched on 15 June 2017 and is currently in service smoothly. Insight-HXMT has been used to scan the Galactic plane repeatedly, making pointing observations to neutron stars and black holes, and monitor the whole sky continuously in the MeV band. Insight-HXMT is also very flexible in making ToO observations, with the response time from about 3 hours to within a day. So far more than 50 refereed publications have been made with data from its observations; many more publications have used the data or results of Insight-HXMT one way or another. The scientific impacts of Insight-HXMT have been growing rapidly since launch. We expect Insight-HXMT to continue to operate for several more years.     

Indicators of black-hole mass and Eddington accretion ratio from QSO X-ray and UV spectra

The evolution of luminous QSOs is linked to the evolution of massive galaxies. We know this because the relic black-holes found locally have masses dependent on the properties of the host galaxy’s bulge. An important way to explore this evolution would be to measure dependences of black-hole masses and Eddington accretion ratios over a range of redshifts, i.e., with cosmological age. For low redshift QSOs (and their lower luminosity Seyfert galaxy counterparts) it has been possible to infer black-hole masses from the luminosities and velocity dispersions of their host-galaxy bulges. These masses agree with those virial black-hole masses calculated from the Doppler widths of the broad Hβ emission lines. The latter method can then be extended to more distant and luminous QSOs, up to redshifts of 0.6 with ground-based optical observations. We discuss ways to extend these explorations to higher redshifts – up to 3 using the widths of QSOs’ broad UV emission lines, and in principle, and to redshifts near 4 from ground-based infrared observations of rest-frame Hβ at 2.5 μm. We discuss the possibility of investigating the accretion history of the higher redshift QSOs using measures of Eddington accretion ratio – the soft X-ray spectral index and the eigenvectors of Principal Components Analyses of QSOs’ UV emission-line spectra.     

Comptonization of low-frequency radiation in accretion disks: Angular distribution and polarization of hard X-ray radiation

We present the results of computation for angular distribution and polarization of radiation for the set of the disk Thomson optical depth values.     

Large light X-ray optics: basic ideas and concepts

One of the main guidelines for future X-ray astronomy projects like, e.g., XEUS (ESA) and Generation-X (NASA) is to utilize grazing-incidence focusing optics with extremely large telescopes (several tens of m² at 1 keV), with a dramatic increase in collecting area of about two order of magnitude compared to the current X-ray telescopes. In order to avoid the problem of the source's confusion limit at low fluxes, the angular resolution required for these optics should be superb (a few arcsec at most). The enormous mirror dimensions together with the high imaging performances give rise to a number of manufacturing problems. It is basically impossible to realize so large mirrors from closed Wolter I shells which benefit from high mechanical stiffness. Instead the mirrors need to be formed as rectangular segments and a series of them will be assembled in a petal. Taking into account the realistic load capabilities of space launchers, to be able to put in orbit so large mirror modules the mass/geometric-area ratio of the optics should be very small. Finally, with a so large optics mass it would be very difficult to provide the electric power for an optics thermal active control, able to maintain the mirrors at the usual temperature of 20 °C. Therefore, very likely, the optics will instead operate in extreme thermal conditions, with the mirror temperature oscillating between −30 and −40 °C, that tends to exclude the epoxy replication approach (the mismatch between the CTE of the substrate and that of the resin would cause prohibitively large deformations of the mirror surface profiles). From these considerations light weight materials with high thermal–mechanical properties such as glass or ceramics become attractive to realize the mirrors of future Xray telescopes. In this paper, we will discuss a segments manufacturing method based on Borofloat^TM glass. A series of finite element analysis concerning different aspects of the production, testing and integration of the optics are also presented as well.     

Supersoft X-ray sources

More than 100 supersoft X-ray sources (SSS) are reported in 20 external galaxies, the Magellanic Clouds (MCs) and our Galaxy. The effective temperatures of the brighter SSS are 20–100 eV. SSS with luminosities below ≈3 × 10³⁸ erg s⁻¹ are consistent with accreting white dwarfs (WDs) with steady nuclear burning or post-novae. Optical identifications exist for SSS in our Galaxy and the MCs (including orbital period determinations) and for SSS in M31 (with novae and symbiotic stars, SySs). High resolution X-ray spectra of the brightest SSS in our Galaxy and the MCs reveal the existence of spectral features due to high gravity WDs. Timing studies in X-rays (combined with the optical) of the stable nuclear burning phase in steady nuclear burning sources and in post-novae allow to constrain the mass accretion rate onto and the mass of the nuclear burning WD. The nature of a few SSS with luminosities 10³⁹ erg s⁻¹ remains unclear.     

Relativistic astrophysics explorer

The great success of the Rossi X-Ray Timing Explorer (RXTE) has shown that X-ray timing is an excellent tool for the study of strong gravitational fields and the measurement of fundamental physical properties of black holes and neutron stars. Here, we describe a next-generation X-ray timing mission, the Relativistic Astrophysics Explorer (RAE), designed to fit within the envelope of a medium-sized mission. The instruments will be a narrow-field X-ray detector array with an area of 6 m² equal to 10 times that of RXTE and a wide-field X-ray monitor. We describe the science made possible with this mission, the design of the instruments, and results on prototype large-area X-ray detectors.     

MIRAX: a Brazilian X-ray astronomy satellite mission

We describe the “Monitor e Imageador de Raios-X” (MIRAX), an X-ray astronomy satellite mission proposed by the high-energy astrophysics group at the National Institute for Space Research (INPE) in Brazil to the Brazilian Space Agency. MIRAX is an international collaboration that includes, besides INPE, the University of California San Diego, the University of Tübingen in Germany, the Massachusetts Institute of Technology and the Space Research Organization Netherlands. The payload of MIRAX will consist of two identical hard X-ray cameras (10–200 keV) and one soft X-ray camera (2–28 keV), both with angular resolution of 5–7^′. The basic objective of MIRAX is to carry out continuous broadband imaging spectroscopy observations of a large source sample (9 months/yr) in the central Galactic plane region. This will allow the detection, localization, possible identification, and spectral/temporal study of the entire history of transient phenomena to be carried out in one single mission. MIRAX will have sensitivities of 5 mCrab/day in the 2–10 keV band (2 times better than the All Sky Monitor on Rossi X-ray Timing Explorer) and 2.6 mCrab/day in the 10–100 keV band (40 times better than the Earth Occultation technique of the Burst and Transient Source Experiment on the Compton Gamma-Ray Observatory). The MIRAX spacecraft will weigh about 200 kg and is expected to be launched in a low-altitude (600 km) circular equatorial orbit around 2007/2008.     

Hard X-ray focusing optics up to 80 keV for the future missions

X-ray telescopes have been providing high sensitivity X-ray observations in numerous missions. For X-ray telescopes in the future, one of the key technologies is to expand the energy band beyond 10 keV. We designed depth-graded multilayer, so-called supermirrors, for a hard X-ray telescope in the energy band up to 40 keV using lightweight thin-foil optics. They were successfully flown in a balloon flight and obtained a hard X-ray image of Cyg X-1 in the 20–40 keV band. Now supermirrors are promising to realize a hard X-ray telescope. We have estimated the performance of a hard X-ray telescope using a platinum–carbon supermirror for future satellite missions, such as NeXT (Japan) and XEUS (Europe). According to calculations, they will have a significant effective area up to 80 keV, and their effective areas will be more than 280 cm² even at 60 keV. Limiting sensitivity will be down to 1.7 × 10⁻¹³ erg cm⁻² s⁻¹ in the 10–80 keV band at a 100 ks observation. In this paper, we present the results of the balloon experiment with the first supermirror flown and projected effective areas of hard X-ray telescopes and action items for future missions.     

Superclustering of galaxies and X-ray sources

An attempt is made to compare optical properties of Abell clusters with X-ray sources.     

Evolution of X-ray binaries: Achievements and advances

Three recent developments in the field of formation and evolution of neutron stars and black holes in binaries are addressed:

• The finding that there is a class of neutron stars, formed in interacting binaries, that do not receive kick velocities in their birth events. This finding is particularly important for our understanding of the formation – and formation rates – of double neutron stars. It is argued that these low-kick neutron stars, which tend to have low masses, are formed by a different physical mechanism than the neutron stars that receive large kick velocities at birth.

• The occurrence of velocity kicks in the formation events of stellar black holes.

• The nature of the companions of millisecond X-ray pulsars.

Overview of QPOs in neutron-star low-mass X-ray binaries

Some aspects of the rapid X-ray variability of low magnetic-field neutron stars in low-mass X-ray binaries are briefly summarized.     

Future X-ray astronomy missions of Japan

Japanese future space programs for high energy astrophysics are presented. The Astro-E2 mission which is the recovery mission of the lost Astro-E has been approved and now scheduled to be put in orbit in early 2005. The design of the whole spacecraft remains the same as that of Astro-E, except for some improvements in the scientific instruments. In spite of the five years of delay, Astro-E2 is still powerful and timely X-ray mission, because of the high energy resolution spectroscopy (FWHM 6 eV in 0.3–10 keV) and high-sensitivity wide-band spectroscopy (0.3–600 keV). The NeXT (New X-ray Telescope) mission, which we propose to have around 2010, succeeds and extends the science which Astro-E2 will open. It will carry five or six sets of X-ray telescopes which utilize super-mirror technology to enable hard X-ray imaging up to 60–80 keV. In mid-2010s, we would participate in the European XEUS mission, which explores the early (z>5) “hot” universe.     

Fluid instabilities around accreting X-ray sources

Some aspects of fluid instabilities occurring in the magnetospheres of accreting neutron stars are discussed. It is pointed out that (i) in the absence of strong differential rotation, the accreting plasma should be drawn out into spiralling, sheet-like structures, resulting in efficient mixing between the two media; (ii) the Rayleigh-Taylor instability also acts to limit the X-ray luminosity in super-critical sources; and (iii) magnetic shear has a strong stabilizing influence on Kelvin-Helmholtz modes, and its presence may allow substantial amounts of material to be supported around the magnetosphere.     

The infra-red background generated by pregalactic stars and their remnants

We examine whether the IR background reported by Matsumoto (1983) could have been generated by stars or black holes in the pregalactic era, z=10100.