Kazumi Asai

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.

DISCOVERY OF AN IRON K ABSORPTION LINE IN THE LOW-MASS X-RAY BINARY GX 13 1

We report the discovery of an iron K absorption-line feature with ASCA in the spectra of the low-mass X-ray binary GX 13+1. The absorption line is detected at 7.0 keV with an equivalent width of 35 ± 8 eV, which is attributed to resonant scattering of the Kα line from Fe XXVI ions in the line of sight whose column density is larger than ~1018 cm-2. The detection of such a spectral feature from a neutron star binary rules out the hypothesis that it is unique to the Galactic superluminal jet sources. From the similarity of the line parameters to those of the superluminal sources, we infer that we view the X-ray emission through a highly ionized plasma in outer parts of the accretion disk with a relatively high inclination angle. This picture is consistent with the universal presence of such plasmas in X-ray binaries accreting through disks.

ASCA Observations of GRO J1744–28

We report the ASCA results of the bursting X-ray pulsar GRO J1744-28, which was observed in 1996 February and 1997 March. The source flux in the 2-10 keV band was 2.0 × 10-8 ergs s-1 cm2 in 1996 and 5.0 × 10-9 ergs s-1 cm2 in 1997. We detected 12 and 17 type II bursts during the two observations, with mean bursting intervals of about 27 min and 37 min. Each burst is followed by an intensity dip with the depleted flux depending on the burst fluence. The energy spectra are approximated by an absorbed power law with additional structure around 6-7 keV. The constant absorption column, (5-6) × 1022 cm-2, independent of the observation dates and emission phases (persistent, burst, and dip) is interpreted as an interstellar absorption. The source may be actually located near the Galactic center, at a distance of 8.5 kpc. The structure in the energy spectrum at 6-7 keV is most probably due to iron and may be reproduced by a disk line model with additional broadening mechanism.

A PROPELLER-EFFECT INTERPRETATION OF MAXI/GSC LIGHT CURVES OF 4U 1608–52 AND Aql X-1 AND APPLICATION TO XTE J1701–462

We present the luminosity dwell-time distributions during the hard states of two low-mass X-ray binaries containing a neutron star (NS), 4U 1608?52 and Aql X-1, observed with MAXI/GSC. The luminosity distributions show a steep cutoff on the low-luminosity side at ~1.0 ? 1036?erg?s?1 in both sources. The cutoff implies a rapid luminosity decrease in their outburst decay phases and this decrease can be interpreted as being due to the propeller effect. We estimate the surface magnetic field of 4U 1608?52 to be (0.5-1.6) ? 108?G and Aql X-1 to be (0.6-1.9) ? 108?G from the cutoff luminosity and apply the same propeller mechanism to the similar rapid luminosity decrease observed in the transient Z source, XTE J1701?462, with RXTE/ASM. Assuming that the spin period of the NS is on the order of milliseconds, the observed cutoff luminosity implies a surface magnetic field on the order of 109?G.

The Decrease in X-Ray Flux of the Accretion-powered Millisecond Pulsar SAX J1808.4–3658 in Quiescence Detected by ASCA

The accretion-powered millisecond X-ray pulsar SAX J1808.4-3658 was observed in quiescence with ASCA in 1999 September. We detected a dim X-ray source in the Solid-State Imaging Spectrometer (SIS) data at the position consistent with SAX J1808.4-3658. The source count rate was (1.1 ± 0.4) × 10-3 counts s-1 (0.5-5 keV) for a single SIS, which corresponds to (3 ± 1) × 10-14 ergs s-1 cm-2 if a power-law energy spectrum of photon index 2 with low-energy absorption corresponding to a hydrogen column density of 1.3 × 1021 cm-2 is assumed. The statistical quality of the data was insufficient to constrain the energy spectrum or to detect the 401 Hz coherent pulsation. We compare the data with the BeppoSAX observation also made during the quiescent state and find that the X-ray flux measured by ASCA is at least a factor of 4 smaller than that measured by BeppoSAX. We discuss the possible X-ray emission mechanisms that could explain the flux change, including the radio pulsar and the radio pulsar shock emission.

Discovery of a New Soft X-Ray Excess in the Spectrum of EXO 0748–676

In this Letter, we present the Advanced Satellite for Cosmology and Astrophysics data from EXO 0748-676 that indicate that a previously unnoted soft X-ray excess exists in the spectrum of this source. We have modeled the intensity-selected spectra with typical candidate low-mass X-ray binary spectral models. As expected, the best performing model was the generalized thermal (GT) model and an additional component that is thought to arise from the obscuration of the X-ray radiator by dipping material within the system. The GT model, however, has an unacceptable value of χ (=3.16). We find that a previously undetected soft component exists (0.4-1.5 keV) in all intensity-selected data, which is the cause of the large value of χ in our GT model fits. We modeled the new soft excess by adding an additional component to the GT model. A variety of soft component models were examined, and the best fitting was a steep power law with one or two local components (either edges or Gaussian lines), with the number of local components needed being dependent on the intensity-selected spectrum being fit. An edge feature and power-law model best fitted the quiescent soft spectral component, while Gaussian-line and power-law models best fitted the soft spectral component in all dipping spectra. We evaluate these results in terms of the expected physical geometry of this system.

X-Ray Energy Spectra of the Supersoft X-Ray Sources CAL 87 and RX J0925.7–4758 Observed with ASCA

We report observation results of the supersoft X-ray sources CAL 87 and RX J0925.7-4758 with the X-ray CCD cameras (Solid-State Imaging Spectrometers [SISs]) on board ASCA. Because of the superior energy resolution of the SIS (ΔE/E ~ 10% at 1 keV) relative to previous instruments, we could study detailed X-ray spectral structures of these sources for the first time. We have applied theoretical spectral models to CAL 87 and constrained the white dwarf mass and intrinsic luminosity as 0.8-1.2 M☉ and 4 × 1037-1.2 × 1038 ergs s-1, respectively. However, we have found the observed luminosity is an order of magnitude smaller than the theoretical estimate, which indicates that the white dwarf is permanently blocked by the accretion disk, and we are observing a scattering emission by a fully ionized accretion disk corona (ADC) whose column density is ~1.5 × 1023 cm-2. Through simulation we have shown that the orbital eclipse can be explained by the ADC model, such that a part of the extended X-ray emission from the ADC is blocked by the companion star filling its Roche lobe. We have found that very high surface gravity and temperature, ~1010 cm s-2 and ~100 eV, respectively, as well as a strong absorption edge at ~1.02 keV, are required to explain the X-ray energy spectrum of RX J0925.7-4758. These values are only possible for an extremely heavy white dwarf near the Chandrasekhar limit. Although the supersoft source luminosity should be ~1038 ergs s-1 at the Chandrasekhar limit, the observed luminosity of RX J0925.7-4758 is nearly 2 orders of magnitude smaller, even assuming an extreme distance of ~10 kpc. To explain the luminosity discrepancy, we propose a model in which very thick matter that was previously ejected from the system, as a form of jets, intervenes the line of sight and reduces the luminosity significantly because of Thomson scattering.

ASCA OBSERVATION OF THE SUPERSOFT X-RAY SOURCE CAL 87

We have investigated the energy spectrum and the orbital modulation of the supersoft X-ray source CAL 87 using the ASCA satellite. We have detected a deep absorption edge at 0.85+ 0.02−0.01 keV superposed on a continuum that can be approximated with an absorbed blackbody (kT=0.05+ 0.06−0.01 keV). The edge structure may be produced by the highly ionized species of O VIII and O VII. Such deep edge structures from highly ionized heavy elements are expected for X-ray emission from a white dwarf with an envelope of stable hydrogen burning and strongly supports the model that a white dwarf with a large mass accretion rate is involved in the supersoft X-ray sources. Epoch-folding analysis reveals the presence of a partial X-ray eclipse with broad wings at orbital phase 0; the eclipse covers a quarter of the orbital phase and the X-ray flux decreases down to ~40% of the out-of-eclipse level. We discuss the possible cause of the partial eclipse, including the extended emission region due to the accretion disk corona, and the X-ray scattering site formed by the gas escaping from the binary system, where the gas is highly ionized by the large X-ray luminosity from the white dwarf.

Is the changing orbital period of EXO 0748-676 evidence for a triple system?

We present eclipse timing measurements of the low-mass X-ray binary (LMxB) EXO 0748-676 made with ASCA. These measurements, when combined with previous results, are consistent with the hypothesis that eclipse times are modulated by a third body. The parameters of the orbit of the postulated third body are now apparently well constrained and will allow further observations to effectively test the hypothesis that EXO 0748-676 is a triple system. The derived orbital parameters are such that, unless the inclination angle is very low, a third body in the system must have a mass less than the lower limit for a main-sequence star.

The discovery of pulsed iron line emission from Centaurus X-3

We present the first discovery of pulsed iron line emission from an X-ray binary, namely Cen X-3. Compared with the continuum pulsations, the iron line pulsations are shallow (50 percent change in amplitude), smeared (the profile is a single-peaked sinusoid) and phase-shifted (by about half a cycle). We also discuss the constraints on the origin of the line imposed by this discovery and by other observations.