Aya Kubota

Modelling the behaviour of accretion flows in X-ray binaries

We review how the recent increase in X-ray and radio data from black hole and neutron star binaries can be merged together with theoretical advances to give a coherent picture of the physics of the accretion flow in strong gravity. Both long term X-ray light curves, X-ray spectra, the rapid X-ray variability and the radio jet behaviour are consistent with a model where a standard outer accretion disc is truncated at low luminosities, being replaced by a hot, inner flow which also acts as the launching site of the jet. Decreasing the disc truncation radius leads to softer spectra, as well as higher frequencies (including quasi periodic oscillations, QPOs) in the power spectra, and a faster jet. The collapse of the hot flow when the disc reaches the last stable orbit triggers the dramatic decrease in radio flux, as well as giving a qualitative (and often quantitative) explanation for the major hard–soft spectral transition seen in black holes. The neutron stars are also consistent with the same models, but with an additional component due to their surface, giving implicit evidence for the event horizon in black holes. We review claims of observational data which conflict with this picture, but show that these can also be consistent with the truncated disc model. We also review suggested alternative models for the accretion flow which do not involve a truncated disc. The most successful of these converge on a similar geometry, where there is a transition at some radius larger than the last stable orbit between a standard disc and an inner, jet dominated region, with the X-ray source associated with a mildly relativistic outflow, beamed away from the disc. However, the observed uniformity of properties between black holes at different inclinations suggests that even weak beaming of the X-ray emission may be constrained by the data. After collapse of the hot inner flow, the spectrum in black hole systems can be dominated by the disc emission. Its behaviour is consistent with the existence of a last stable orbit, and such data can be used to estimate the black hole spin. By contrast, these systems can also show very different spectra at these high luminosities, in which the disc spectrum (and probably structure) is strongly distorted by Comptonization. The structure of the accretion flow becomes increasingly uncertain as the luminosity approaches (and exceeds) the Eddington luminosity, though there is growing evidence that winds may play an important role. We stress that these high Eddington fraction flows are key to understanding many disparate and currently very active fields such as ULX, Narrow Line Seyfert 1’s, and the growth of the first black holes in the Early Universe.

The nature of ultraluminous compact X-ray sources in nearby spiral galaxies

Studies were made of ASCA spectra of seven ultraluminous compact X-ray sources in nearby spiral galaxies: M33 X-8, M81 X-6, IC 342 source 1, Dwingeloo 1 X-1, NGC 1313 source B, and two sources in NGC 4565. With the 0.5-10 keV luminosities in the range 1039-1040 ergs s-1, they are thought to represent a class of enigmatic X-ray sources often found in spiral galaxies. For some of them, the ASCA data are newly processed or the published spectra are reanalyzed. For others, the published results are quoted. The ASCA spectra of all seven sources have been described successfully with so-called multicolor disk blackbody emission arising from optically thick standard accretion disks around black holes. Except for the case of M33 X-8, the spectra do not exhibit hard tails. For the source luminosities not to exceed the Eddington limits, the black holes are inferred to have rather high masses, up to ~100 M☉. However, the observed innermost disk temperatures of these objects, Tin = 1.1-1.8 keV, are too high to be compatible with the required high black hole masses, as long as the standard accretion disks around Schwarzschild black holes are assumed. Similarly high disk temperatures are also observed from two Galactic transients with superluminal motions, GRO 1655-40 and GRS 1915+105. The issue of unusually high disk temperature may be explained by the black hole rotation, which makes the disk get closer to the black hole and hence hotter.

Hard X-ray Detector (HXD) on board Suzaku

The Hard X-ray Detector (HXD) on board Suzaku covers a wide energy range from 10 keV to 600 keV by combination of silicon PIN diodes and GSO scintillators. The HXD is designed to achieve an extremely low in-orbit back ground based on a combination of new techniques, including the concept of well-type active shield counter. With an effective area of 142 cm^2 at 20 keV and 273 cm2 at 150 keV, the background level at the sea level reached ~1x10^{-5} cts s^{-1} cm^{-2} keV^{-1} at 30 keV for the PI N diodes, and ~2x10^{-5} cts s^{-1} cm^{-2} keV^{-1} at 100 keV, and ~7x10^{-6} cts s^{-1} cm^{-2} keV^{-1} at 200 keV for the phoswich counter. Tight active shielding of the HXD results in a large array of guard counters surrounding the main detector parts. These anti-coincidence counters, made of ~4 cm thick BGO crystals, have a large effective area for sub-MeV to MeV gamma-rays. They work as an excellent gamma-ray burst monitor with limited angular resolution (~5 degree). The on-board signal-processing system and the data transmitted to the ground are also described.

In-orbit performance of the hard X-ray detector on board Suzaku

The in-orbit performance and calibration of the Hard X-ray Detector (HXD) on board the X-ray astronomy satellite Suzaku are described. Its basic performances, including a wide energy bandpass of 10–600keV, energy resolutions of ∼ 4keV (FWHM) at 40keV and ∼ 11% at 511keV, and a high background rejection efficiency, have been confirmed by extensive in-orbit calibrations. The long-term gains of PIN-Si diodes have been stable within 1% for half a year, and those of scintillators have decreased by 5–20%. The residual non-X-ray background of the HXD is the lowest among past non-imaging hard X-ray instruments in energy ranges of 15–70 and 150–500keV. We provide accurate calibrations of energy responses, angular responses, timing accuracy of the HXD, and relative normalizations to the X-ray CCD cameras using multiple observations of the Crab Nebula.

Discovery of Spectral Transitions from Two Ultraluminous Compact X-Ray Sources in IC 342

Two ASCA observations were made of two ultraluminous compact X-ray sources (ULXs) in the spiral galaxy IC 342. In the 1993 observation, source 2 showed a 0.5-10 keV luminosity of 6 × 1039 ergs s-1 (assuming a distance of 4.0 Mpc) and a hard power-law spectrum of photon index ~1.4. As already reported, source 1 was ~3 times brighter on that occasion and exhibited a soft spectrum represented by a multicolor disk model with an inner-disk temperature of ~1.8 keV. The second observation, made in 2000 February, revealed that source 1 had made a transition into a hard spectral state, while source 2 made a transition into a soft spectral state. The ULXs are therefore inferred to exhibit two distinct spectral states, and they sometimes make transitions between them. These results significantly reinforce the scenario that describes ULXs as mass-accreting black holes.

Disc–corona energetics in the very high state of Galactic black holes

(truncated version) The X-ray spectra of Galactic binary systems in the very high state show both strong disk emission and a strong, steep tail to high energies. We use simultaneous optical-ASCA-RXTE data from the black hole transient XTE J1550-564 as a specific example, and show that these have disc spectra which are significantly lower in temperature than those seen from the same source at the same luminosity when in the disc dominated state. If these give a true picture of the disc then either the disc emissivity has reduced, and/or the disc truncates above the last stable orbit. However, it is often assumed that the tail is produced by Compton scattering, in which case its shape in these spectra requires that the Comptonising region is marginally optically thick (tau~2-3), and covers a large fraction of the inner disc. This will distort our view of the disc. We build a theoretical model of a Comptonising corona over an inner disc, and fit this to the data, but find that it still requires a large increase in inner disc radius for a standard disc emissivity. Instead it seems more probable that the disc emissivity changes in the presence of the corona. We implement the specific inner disc-corona coupling model of Svensson & Zdziarski (1994) and show that this can explain the low temperature/high luminosity disc emission seen in the very high state with only a small increase in radius of the disc. While this inferred disc truncation is probably not significant given the model uncertainties, it is consistent with the low frequency QPO and gives continuity of properties with the low/hard state spectra.

Suzaku Results on Cygnus X-1 in the Low/Hard State

AbstractThe black-hole binary Cygnus X-1 was observed for 17 ks with the Suzaku X-ray observatory in 2005October, while it was in a low/hard state with a 0.7–300 keV luminosity of 4.6×10 37 erg s −1 . The XIS andHXD spectra, spanning 0.7–400 keV, were reproduced successfully incorporating a cool accretion disk anda hot Comptonizing corona. The corona is characterized by an electron temperature of ∼100 keV, and twooptical depths of ∼0.4 and ∼1.5 which account for the harder and softer continua, respectively. The disk hasthe innermost temperature of ∼0.2 keV, and is though to protrude half way into the corona. The disk notonly provides seed photons to the Compton cloud, but also produces a soft spectral excess, a mild reflectionhump, and a weakly broadened iron line. A comparison with the Suzaku data on GRO J1655−40 revealsseveral interesting spectral differences, which can mostly be attributed to inclination effects assuming thatthe disk has a flat geometry while the corona is grossly spherical. An intensity-sorted spectroscopy indicatesthat the continuum becomes less Comptonized when the source flares up on times scales of 1–200 s, whilethe underlying disk remains unchanged.Key words: accretion disks — black hole physics — stars: individual (Cygnus X-1)— X-ray: binaries1. IntroductionLuminous soft X-ray radiation of accreting stellar-massblack holes (BHs) has generally been explained as thermalemission from optically-thick (in particular “standard”)accretion disks (Shakura & Sunyaev 1973; Makishimaet al. 1986; Dotani et al. 1997; Remillard & McClintock2006), which are expected to form around them underrather high accretion rates. In contrast, their hard X-rayproduction process is much less understood, even thoughintense hard X-ray emission characterizes black-hoe bina-ries (BHBs) among varioustypes of compact X-raysourcesin the Milky Way and Magellanic clouds.Indeed, BHBs often emit a major fraction of their ra-diative luminosity in the hard X-ray band, in the formof spectral hard-tail component if they are in so-calledhigh/soft state, or as the entire power-law (hereafter PL)like continua if they are in so-called low/hard state (here-after LHS) which appears under relatively low accretionrates. Furthermore, the hard X-ray emission (partic-ularly in the LHS) involves another interesting aspect,namely the long-known aperiodic variation over a widefrequency range (e.g., Oda et al. 1971; Oda 1977; Nolanet al. 1981; Miyamoto et al. 1991; Pottschmidt et al.2003; Remillard & McClintock 2006). These spectral andtiming studies are not limited to stellar-mass BHs, since a

Rapid optical and X-ray timing observations of GX 339−4: multicomponent optical variability in the low/hard state

A rapid timing analysis of Very Large Telescope (VLT)/ULTRACAM (optical) and RXTE (X- ray) observations of the Galactic black hole binary GX 339−4 in the low/hard, post-outburst state of 2007 June is presented. The optical light curves in the r � ,gand ufilters show slow (∼20 s) quasi-periodic variability. Upon this is superposed fast flaring activity on times approaching the best time resolution probed (∼50 ms inrandg � ) and with maximum strengths of more than twice the local mean. Power spectral analysis over ∼0.004-10 Hz is presented, and shows that although the average optical variability amplitude is lower than that in X-rays, the peak variability power emerges at a higher Fourier frequency in the optical. Energetically, we measure a large optical versus X-ray flux ratio, higher than that seen on previous occasions when the source was fully jet dominated. Such a large ratio cannot be easily explained with a disc alone. Studying the optical-X-ray cross-spectrum in Fourier space shows a markedly different behaviour above and below ∼0.2 Hz. The peak of the coherence function above this threshold is associated with a short optical time lag with respect to X-rays, also seen as the dominant feature in the time-domain cross-correlation at ≈150 ms. The rms energy spectrum of these fast variations is best described by distinct physical components over the optical and X-ray regimes, and also suggests a maximal irradiated disc fraction of 20 per cent around 5000 A. If the constant time delay is due to propagation of fluctuations to (or within) the jet, this is the clearest optical evidence to date of the location of this component. The low-frequency quasi-periodic oscillation is seen in the optical but not in X-rays, and is associated with a low coherence. Evidence of reprocessing emerges at the lowest Fourier frequencies, with optical lags at ∼10 s and strong coherence in the blue ufilter. Consistent with this, simultaneous optical spectroscopy also shows the Bowen fluorescence blend, though its emission location is

Improvements of the astro-E2 hard X-ray detector (HXD-II)

We summarize significant improvements which have been achieved in the development of Astro-E2 Hard X-ray Detector (HXD-II). An expanded energy range and better energy resolution have been achieved from progresses in device materials and redesigning of the front-end electronics. An improved estimation for the detector background in orbit has also been conducted based upon results from our proton irradiation experiment. The sensitivity of HXD-II can be expected to reach an order of 10/sup -6/ [cs/sup -1/ keV/sup -1/ cm/sup -2/].

Spectral variability of ultraluminous compact X-ray sources in nearby spiral galaxies

Using the X-ray data taken with ASCA, a detailed analysis was made of intensity and spectral variations of three ultraluminous extragalactic compact X-ray sources (ULXs): IC 342 source 1, M81 X-6, and NGC 1313 source B, all exhibiting X-ray luminosity in the range from 2 × 1039 to 1.5 × 1040 ergs s-1. As already reported, IC 342 source 1 showed short-term X-ray intensity variability by a factor of 2.0 on a typical timescale of 10 ks. M81 X-6 varied by a factor of 1.6 across seven observations spanning 3 yr, while NGC 1313 source B varied by a factor of 2.5 between two observations conducted in 1993 July and 1995 November. The ASCA spectra of these sources acquired on these occasions were all described successfully as optically thick emission from standard accretion disks around black holes. This confirms previous ASCA works that explained ULXs as mass-accreting massive black hole binaries. In all three sources, the disk color temperature was uncomfortably high at Tin = 1.0-2.0 keV and was found to vary in proportion to the square root of the source flux. The apparent accretion disk radius is hence inferred to change as inversely proportional to Tin. This suggests a significant effect of advection in the accretion disk. However, even taking this effect fully into account, the too high values of Tin of ULXs cannot be explained. Further invoking the rapid black hole rotation may give a solution to this issue.