T. Takeshima

Ginga observations of Centaurus X-3

Attention is given to a series of observations of Centaurus X-3 made with Ginga on March 22-24, 1989, over a complete orbital cycle. The data set includes a preeclipse dip, the eclipse ingress and egress, the eclipse itself, and a phase of high steady emission after egress. The mideclipse ephemeris obtained yields an estimate of the rate of change of the orbital period of -(1.738 +/- 0.004) x 10 exp 6/yr, confirming the rate of change reported by Kelley et al. (1983) and improving the precision by a factor of 20. The pulse profiles obtained with Ginga during the high postegress phase are double-peaked at low energies, and show a reversal of phase of 180 deg. No pulsations are detected during the eclipse and at low energies during the preeclipse dip, indicating that X-ray emission at these phases is due to scattering by extended matter. The energy of the iron emission line varies between 6.5 and 6.7 keV during the observation, while its equivalent width varies between 0.18 and 1.5 keV.

Neon Line Emission in the X-Ray Spectrum of the Pulsar 4U 1626–67

Using the ASCA Observatory we have discovered a strong emission-line complex centered on 1 keV in the X-ray spectrum of the 7 s pulsar 4U 1626-67. The strongest line is identified with hydrogen-like Ne Lyα at 1.008 keV. The strength of the neon emission compared to the expected iron L complex implies a large neon overabundance relative to iron. 4U 1626-67 is a low-mass X-ray binary (LMXRB) system with an orbital period of 2485 s, and is unusual because it is one of a few LMXRB to contain an X-ray pulsar. A hydrogen burning companion star was suggested to explain the X-ray and optical characteristics. Neon is a by-product of helium burning and its overabundance in this system may indicate instead that the companion star is burning (or has burned) helium.

Accurate Position of SGR 1900+14 by Bursts and Changes in Pulse Period and Folded Pulse Profile with ASCA

The Advanced Satellite for Cosmology and Astrophysics (ASCA) observed the soft gamma repeater SGR 1900+14 on 1998 April 30-May 1 and discovered a pulsar with a period of 5.1589715(8) s from the known X-ray source of RX J190714.2+0919.3. Four months later, on September 16-17, ASCA observed SGR 1900+14 again just after the giant burst on 1998 August 27. Comparing the observations in September with those in April, there are several changes in characteristics. The pulse period changed to 5.160295(3) s, and thus the long-term period derivative is 1.1 × 10-10 s s-1. This strongly supports a magnetar model. The folded pulse profile in 2-10 keV largely changed from three peaks in April to one simple peak, while the steady intensity increased by a factor of 2. Finally, we successfully determined the accurate location of SGR 1900+14 by the bursts with an accuracy of 15 in diameter.

Rossi X-Ray Timing Explorer Observations of the X-Ray Pulsar XTE J1855–026: A Possible New Supergiant System

A new X-ray source, XTE J1855-026, was discovered during Rossi X-Ray Timing Explorer (RXTE) scans along the Galactic plane. The source shows pulsations at a period of 361 s and also modulation at a period of 6.1 days, which we interpret as the orbital period of the system. The X-ray spectrum above ~3 keV can be fitted with an absorbed power-law model with a high-energy cutoff and an iron emission line at approximately 6.4 keV. We interpret these results as indicating that XTE J1855-026 is likely to consist of a neutron star accreting from the wind of an O or B supergiant primary. A less likely interpretation is that XTE J1855-026 is instead a Be/neutron star binary, in which case it would have the shortest known orbital period for such a system.

Dust-grain scattering of X-rays observed during the lunar occultation of a transient X-ray source near the Galactic center

Extended X-ray emission surrounding point X-ray sources has been detected in the energy band 1-10 keV during lunar occultation observations of the Galactic center region. These extended X-rays are most likely due to X-ray scattering by interstellar dust grains. The spatial size and the intensity of the extended emission around the transient X-ray source GS 1741.2-2859/1741.6-2849 have been studied extensively. The spatial size is consistent with the typical grain size of about 0.06 micron. The intensity is used to obtain the energy dependence of the scattering optical depth to the source, which suggests the existence of iron in the grains. The ratio of the iron column density contained in the grains to the hydrogen column density of the neutral gas is roughly consistent with the cosmic abundance of iron. 30 refs.

ASCA Observations of the Jet Source XTE J1748–288

XTE J1748-288 is a new X-ray transient with a one-sided radio jet. It was observed with ASCA on 1998 September 6 and 26, 100 days after the onset of the radio X-ray outburst. The spectra were modeled with an attenuated power-law model, and the absorbed 2-6 keV flux was 4.6 × 10-11 and 2.2 × 10-12 ergs s-1 cm-2 on 1998 September 6 and 26, respectively. The light curve shows that the steady exponential decay, with an e-folding time of 14 days, lasted over 100 days and 4 orders of magnitude from the peak of the outburst. The celestial region including the source was observed with ASCA on 1993 October 1 and 1994 September 22, years before the discovery. During those periods, the flux was 10-13 ergs s-1 cm-2, below the ASCA detection limit. The jet blob colliding with the environmental matter is probably not the X-ray source, although the emission mechanism is not determined. A possible detection of a K line from highly ionized iron is discussed.

On the Nature of AX J2049.6+2939/AX J2050.0+2914

AX J2049.6+2939 is a compact X-ray source in the vicinity of the southern blow-up region of the Cygnus Loop supernova remnant. This source was the brightest X-ray source inside the Cygnus Loop observed during the ASCA survey project. The X-ray spectrum was well fitted by a power-law function with a photon index of -2.1 ± 0.1. Short-term timing analysis was performed, and no coherent pulsation was found. Follow-up observations with ASCA have revealed a large variation in X-ray intensity by a factor of 50, whereas the spectral shape did not change within the statistical uncertainties. In the second ASCA observation, we found another X-ray source, AX J2050.0+2941, to the northeast of AX J2049.6+2939. During the three ASCA observations, the X-ray intensity of AX J2050.0+2941 varied by a factor of 4. No coherent pulsations could be found for AX J2050.0+2941. We have performed optical photometric and spectroscopic observations in the vicinity of AX J2049.6+2939 at KPNO. As a result, all objects brighter than a B-band magnitude of 22 in the error box can be identified with normal stars. Combined with the X-ray results and the fact that there are no radio counterparts, AX J2049.6+2939 is not likely to be either an ordinary rotation-powered pulsar or an active galactic nucleus (AGN). The nature of AX J2049.6+2939 is still unclear, and further observations over a wide energy band are strongly required. As to AX J2050.0+2941, the long-term X-ray variability and the radio counterpart suggests that it is an AGN.

A turbulent model of time variability in X-ray binary pulsars

It is found that the observed power spectral density of the aperiodic time variability of the X-ray pulsar can be approximated by a power-law spectrum which changes its slope at the pulsation frequency (pulsar rotation frequency). The indices of the observed spectra are, respectively, 1.4-2.0 above the pulsation frequency and 0-1.0 below the pulsation frequency. By studying the density fluctuation spectra with inverse cascades in magnetohydrodynamic turbulence, we find that the observed X-ray spectra can be interpreted as the density fluctuations in the accreting plasmas. It is shown that the structure of either disk-fed or wind-fed pulsars can be determined by comparing the observed X-ray spectra with the theoretical density fluctuation spectra of twoor three-dimensional magnetohydrodynamic turbulence, respectively

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.

Calibrations of imaging gas scintillation proportional counters on ASTRO-D

The fourth Japanese X-ray astronomy satellite, ASTRO-D, was launched successfully by the Institute of Space and Astronautical Science on February 20, 1993 and was named ASCA. Two of the focal plane detectors are imaging gas scintillation proportional counters (Gas Imaging Spectrometer:GIS). The GIS sensors performed the energy resolution of 8% FWHM at 6 keV, and position resolution of 0.5 mm FWHM on-board, which confirmed their ultimate capability as gas counters. The non-Xray background counting rate was approximately 6 X 10-4 c/s/cm2/keV in the energy range of 2 - 10 keV, which was as low as that achieved by the Ginga instrument. The scientific results obtained by the GIS sensors are also presented.