Ken Ebisawa

X-ray variability of GX 339-4 in its very high states

GX 339-4 was observed with the large area counters (LAC) onboard Ginga in its very high state, where the X-ray intensity was about a factor of 2-3 larger than its high state and it showed very rapid variations on time scales of less than several minutes, which had not been observed earlier in the high state of this source. The X-ray energy spectrum was very soft; it consisted of a low-energy component and a high-energy tail

The MAXI Mission on the ISS: Science and Instruments for Monitoring All-Sky X-Ray Images

The Monitor of All-sky X-ray Image (MAXI) mission is the first astronomical payload to be installed on the Japanese Experiment Module — Exposed Facility (JEM-EF or Kibo-EF) on the International Space Station. It has two types of X-ray slit cameras with wide FOVs and two kinds of X-ray detectors consisting of gas proportional counters covering the energy range of 2 to 30 keV and X-ray CCDs covering the energy range of 0.5 to 12 keV. MAXI will be more powerful than any previous X-ray All Sky Monitor payloads, being able to monitor hundreds of Active Galactic Nuclei. A realistic simulation under optimal observation conditions suggests that MAXI will provide all-sky images of X-ray sources of � 20 mCrab (� 7 � 10 � 10 erg cm � 2 s � 1 in the energy band of 2–30 keV) from observations during one ISS orbit (90 min), � 4.5 mCrab for one day, and � 2 mCrab for one week. The final detectability of MAXI could be � 0.2 mCrab for two years, which is comparable to the source confusion limit of the MAXI field of view (FOV). The MAXI objectives are: (1) to alert the community to X-ray novae and transient X-ray sources, (2) to monitor long-term variabilities of X-ray sources, (3) to stimulate multi-wavelength observations of variable objects, (4) to create unbiased X-ray source cataloges, and (5) to observe diffuse cosmic X-ray emissions, especially with better energy resolution for soft X-rays down to 0.5 keV.

A Simple Comptonization Model

We present an empirical model of Comptonization for fitting the spectra of X-ray binaries. This model, named SIMPL, has been developed as a package implemented in XSPEC. With only two free parameters, SIMPL is competitive as the simplest model of Compton scattering. Unlike the pervasive standard power-law model, SIMPL incorporates the basic features of Compton scattering of soft photons by energetic coronal electrons. Using a simulated spectrum, we demonstrate that SIMPL closely matches the behavior of physical Comptonization models that consider the effects of optical depth, coronal electron temperature, and geometry. We present fits to RXTE spectra of the black hole transient H1743-322 and a BeppoSAX spectrum of LMC X-3 using both SIMPL and the standard power-law model. A comparison of the results shows that SIMPL gives equally good fits, while elim- inating the troublesome divergence of the standard power-law model at low energies. SIMPL is completely flexible and can be used self-consistently with any seed spectrum of photons. We show an example of how SIMPL—unlike the standard power law—teamed up with DISKBB (the standard model of disk accretion) provides a uniform disk normalization that is unaffected by moderate Comptonization.

A DETERMINATION OF THE SPIN OF THE BLACK HOLE PRIMARY IN LMC X-1

The first extragalactic X-ray binary, LMC X-1, was discovered in 1969. In the 1980s, its compact primary was established as the fourth dynamical black hole candidate. Recently, we published accurate values for the mass of the black hole and the orbital inclination angle of the binary system. Building on these results, we have analyzed 53 X-ray spectra obtained by RXTE and, using a selected sample of 18 of these spectra, we have determined the dimensionless spin parameter of the black hole to be a{sub *} = 0.92{sup +0.05}{sub -0.07}. This result takes into account all sources of observational and model-parameter uncertainties. The standard deviation around the mean value of a{sub *} for these 18 X-ray spectra, which were obtained over a span of several years, is only {delta}a{sub *} = 0.02. When we consider our complete sample of 53 RXTE spectra, we find a somewhat higher value of the spin parameter and a larger standard deviation. Finally, we show that our results based on RXTE data are confirmed by our analyses of selected X-ray spectra obtained by the XMM-Newton, BeppoSAX, and Ginga missions.

Spectral variations of LMC X-3 observed with Ginga

The prime black hole candidate LMC X-3 was observed over three years with the Ginga satellite, and a characteristic spectral variation was found accompanying the periodic intensity variation of about 198 (or possibly about 99) days (Cowley et al., 1991). The energy spectrum of LMC X-3 consists of the soft, thermal component and the hard, power-law component, which are respectively dominant below and above about 9 keV. The soft component, which carries most of the X-ray intensity, shows a clear correlation between the intensity and the hardness, while the hard component varies independently of the soft component. It was found that the spectral variation of the soft component is well described by an optically thick accretion disk model with a remarkably constant innermost radius and variable mass accretion rate. The constancy of the innermost radius suggests it is related to the mass of the central object.

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

Application of a general relativistic accretion disk model to LMC X-1, LMC X-3, X1608−522, and X1636−536

The persistent X-ray spectra of black hole candidates and X-ray bursters are compared with a model spectrum of an accretion disk around either a black hole or a neutron star. The model takes account of general relativity and assumes that the accretion disk is geometrically thin and in a steady state. This general relativistic model is compared with other conventional model spectra that have been used to fit observations so far. It is shown that the general relativistic model, in shape, is quite similar to the multicolor disk model and the Sunyaev-Titarchuk Comptonization model. These conventional models are used to perform spectral fitting analysis on the black hole candidates LMC X-1 and LMC X-3 and on the X-ray bursters X1608 - 522 and X1636 - 536 using the general relativistic model. As a result, the lower limit of the masses of LMC X-1 and LMC X-3 and the probable masses of X1608 - 522 and X1636 - 536 are obtained. The former masses are larger than the theoretical upper limit of the neutron star mass, 2.5 solar masses, and the latter masses are consistent with the canonical neutron star mass, 1.4 solar mass. 38 refs.

White dwarf masses in intermediate polars observed with the Suzaku satellite

Context. White dwarfs (WDs) in cataclysmic variables (CVs) are important experimental laboratories where the electron degeneracy is taking place on a macroscopic scale. Magnetic CVs increase in number especially in the hard X-ray band (10 keV) thanks to sensitive hard X-ray missions. Aims. From X-ray spectroscopy, we estimate the masses of nearby WDs in moderately-magnetized CVs, or Intermediate Polars (IPs). Methods. Using the Suzaku satellite, we aquired wide-band spectra of 17 IPs, covering 3−50 keV. An accretion column model of Suleimanov et al. (2005, A&A, 435, 191) and an optically-thin thermal emission code were used to construct a spectral emission model of IPs with resolved Fe emission lines. By simultaneously fitting the Fe line complex and the hard X-ray continuum of individual spectra, the shock temperature and the WD mass were determined with a better accuracy than in previous studies.

Do Ultraluminous X-Ray Sources Really Contain Intermediate-Mass Black Holes?

An open question remains whether Ultraluminous X-ray Sources (ULXs) really contain intermediate-mass black holes (IMBHs). We carefully investigated the XMM-Newton EPIC spectra of four ULXs that were claimed to be strong candidates of IMBHs by several authors. Wefirst triedfitting by the standard spectral model of disk blackbody (DBB) + power-law (PL), finding good fits to all of the data, in agreement with others. We, however, found that the PL component dominates the DBB component at ∼ 0.3 to 10keV. Thus, the black hole parameters derived solely from the minor DBB component are questionable. Next, we tried to fit the same data by the “p-free disk model” without the PL component, assuming an effective temperature profile of Teff ∝ r −p ,w herer is the disk radius. Interestingly, in spite of one less free-model parameter, we obtained similarly good fits with much higher innermost disk temperatures, 1.8 <k Tin < 3.2keV. More importantly, we obtained p ∼ 0.5, just the value predicted by the slim (super-critical) disk theory, rather than p =0 .75 that is expected from the standard disk model. The estimated black hole masses from the p-free disk model are much smaller; M 40 M� . Furthermore, we applied a more sophisticated slim-disk model by Kawaguchi (2003, ApJ, 593, 69), and obtained good fits with roughly consistent black hole masses. We thus conclude that the central engines of these ULXs are super-critical accretion flows to stellar-mass black holes.

X-RAY EVOLUTION OF PULSAR WIND NEBULAE

During the search for counterparts of very high energy gamma-ray sources, we serendipitously discovered large, extended, low surface brightness emission from pulsar wind nebulae (PWNe) around pulsars with the ages up to ~100 kyr, a discovery made possible by the low and stable background of the Suzaku X-ray satellite. A systematic study of a sample of eight of these PWNe, together with Chandra data sets, has revealed that the nebulae keep expanding up to ~100 kyr, although the timescale of the synchrotron X-ray emission is only ~60 yr for typical magnetic fields of 100 μG. Our result suggests that the accelerated electrons up to ~80 TeV can escape from the PWNe without losing most energies. Moreover, in order to explain the observed correlation between the X-ray size and the pulsar spin-down age, the magnetic field strength in the PWNe must decrease with time.