A. N. Parmar

The X-ray spectral properties of accretion discs in X-ray binaries

On utilise des observations du satellite EXOSAT pour comparer les proprietes spectrales de lemission persistante de sources a sursaut RX, de binaires RX a faible masse et haute luminosite, et de candidats trous noirs galactiques, avec divers modeles demission RX provenant dun disque daccretion entourant un objet compact dans un systeme binaire

Broadband Spectrum of Cygnus X-1 in Two Spectral States with BeppoSAX

We report on the 0.5-200 keV spectral properties of Cyg X-1 observed at different epochs with the Narrow Field Instruments of the BeppoSAX satellite. The source was in its soft state during the first observation of 1996 June. In the second observation of 1996 September, the source had parameters characteristic to its hard state. A soft X-ray excess, a broad Fe Kα line and Compton reflection are clearly detected in both states. The soft-state broadband continuum is well modeled by a disk blackbody (accounting for the soft excess) and Compton upscattering of the disk photons by a hybrid, thermal/nonthermal plasma, probably forming a corona above the disk (also giving rise to the Compton-reflection component). In the hard state, the primary hard X-ray spectrum can be well modeled by Compton upscattering of a weak blackbody emission by a thermal plasma at a temperature of ~60 keV. The soft excess is then explained by thermal Comptonization of the same blackbody emission by another hot plasma cloud characterized by a low value of its Compton parameter. Finally, we find the characteristic ratio of the bolometric flux in the soft state to that in the hard state to be about 3. This value is much more compatible with theories of state transitions than the previously reported (and likely underestimated) value of 1.5.

The transient 42 second X-ray pulsar EXO 2030+375. I: The discovery and the luminosity dependence of the pulse period variations

The discovery of a new transient X-ray pulsar, EXO 2030+375, that is associated with a Be star companion, is reported. The results of a series of EXOSAT observations, where the evolution of the pulsars properties over two orders of magnitude in luminosity was monitored. The dependence of the change in pulsar period on luminosity is measured and compared with that predicted by accretion torque theory. The pulsar properties are also compared with those of other transient X-ray pulsars. 58 refs.

The transient 42 second X-ray pulsar EXO 2030+375. II: The luminosity dependence of the pulse profile

During an outburst from the transient, 42 s, X-ray pulsar EXO 2030+375, marked luminosity-dependent changes in pulsar profile were observed. At high luminosities the pulse profile consisted of a smooth asymmetric main pulse separated by about 180 deg in phase from a small interpulse. As the luminosity decreased by a factor of about 100, the relative strength of the two pulses reversed. During a second outburst, the phase-averaged X-ray light curve was punctuated by a series of flares that repeated about every four hours and involved a factor of six increase in intensity. The observed pulse profile is modeled using a simple geometric model where axially symmetric fan and pencil-beams of emission are emitted from two magnetic poles on a rotating neutron star. The profile shows a marked asymmetry which is modeled by offsetting the magnetic dipole axis from the rotation axis of the neutron star. The modeling indicates that, as the luminosity decreased, the dominant beam changed from a fan beam to a pencil-beam configuration. 27 refs.

A measurement of the broadband spectrum of XTE J1118+480 with BeppoSAX and its astrophysical implications

We report on results of a Target-of-Opportunity observation of the X-ray transient XTE J1118+480 performed on 2000 April 14-15 with the Narrow Field Instruments (0.1-200 keV) of the BeppoSAX satellite. The measured spectrum is a power law with a photon index of ~1.7 modified by an ultrasoft X-ray excess and a high-energy cutoff above ~100 keV. The soft excess is consistent with a blackbody with a temperature of ~40 eV and a low flux, while the cutoff power law is well fitted by thermal Comptonization in a plasma with an electron temperature of ~102 keV and an optical depth of order unity. Consistent with the weakness of the blackbody, Compton reflection is weak. Although the data are consistent with various geometries of the hot and cold phases of the accreting gas, we conclude that a hot accretion disk is the most plausible model. The Eddington ratio implied by recent estimates of the mass and the distance is ~10-3, which may indicate that advection is probably not the dominant cooling mechanism. We finally suggest that the reflecting medium has a low metallicity, consistent with the location of the system in the halo.

LOBSTER-ISS: an imaging x-ray all-sky monitor for the International Space Station

We describe the design of Lobster-ISS, an X-ray imaging all-sky monitor (ASM) to be flown as an attached payload on the International Space Station. Lobster-ISS is the subject of an ESA Phase-A study which will begin in December 2001. With an instantaneous field of view 162 x 22.5 degrees, Lobster-ISS will map almost the complete sky every 90 minute ISS orbit, generating a confusion-limited catalogue of ~250,000 sources every 2 months. Lobster-ISS will use focusing microchannel plate optics and imaging gas proportional micro-well detectors; work is currently underway to improve the MCP optics and to develop proportional counter windows with enhanced transmission and negligible rates of gas leakage, thus improving instrument throughput and reducing mass. Lobster-ISS provides an order of magnitude improvement in the sensitivity of X-ray ASMs, and will, for the first time, provide continuous monitoring of the sky in the soft X-ray region (0.1-3.5 keV). Lobster-ISS provides long term monitoring of all classes of variable X-ray source, and an essential alert facility, with rapid detection of transient X-ray sources such as Gamma-Ray Burst afterglows being relayed to contemporary pointed X-ray observatories. The mission, with a nominal lifetime of 3 years, is scheduled for launch on the Shuttle c.2009.

The discovery of 0.2 Hz quasi-periodic oscillations in the X-ray flux of the transient 42 second pulsar EXO 2030+375

The discovery of about 0.2 Hz quasi-periodic oscillations (QPOs) in the 1-15 keV X-ray flux of the transient 42 s pulsar EXO 2030 + 375 is reported. The detection of QPOs in the power spectra of this pulsar was made possible by the application of a newly developed technique to decouple the power spectrum peaks. QPOs were detected during an outburst, when the source was close to its maximum observed luminosity. Upper limits to the QPO strength during the rest of the outburst are consistent with the mean value observed of about 3.5 percent rms. During the outburst the QPOs centroid frequency decreased by about 12 percent, as the luminosity decreased by about 25 percent. The observed QPOs strength, frequency, and frequency-luminosity dependence are all in remarkable agreement with the predictions of the beat frequency model for QPOs and provide the first quantitative confirmation of it. Simple models in which the QPOs signal is produced close to the magnetospheric boundary are ruled out as only about 0.2 percent of the total accretion luminosity can be liberated at this distance from the neutron star surface. 31 refs.

The discovery of 4.4 second X-ray pulsations from the rapidly variable X-ray transient V0332 + 53

On a observe trois sursauts de V 0332+53 avec lObservatoire EXOSAT, entre novembre 1983 et janvier 1984. En plus de la variabilite rapide de type Cyg X-1, la source presente des pulsations stables de periode 4,4 s. Les variations Doppler de la periode de pulsation montrent que le pulsar est sur une orbite de 34,25 jours, dexcentricite 0,31. Les variations rapides de V 0332+53 pourraient resulter dinstabilites dans la magnetosphere dune etoile a neutron a accretion spherique

Bimodal quasi-oscillatory and spectral behavior in Scorpius X-1

X-ray observations were made to follow up the discovery of quasi-periodic oscillations (QPO) in Sco X-1. The QPO is observed in the quiescent state, and between flares in the active state. When the source is quiescent, the QPO frequency is 6 Hz and anticorrelated with intensity; in the active state, the frequency ranges from 10 to 20 Hz and is strongly correlated with intensity. At high intensities the QPO dissolves into continuum noise. The QPO rms amplitude decreases with count rate, and increases with photon energy. There are two modes of spectral behavior, with a one-to-one correspondence to the two modes of QPO: the spectral hardness ratio varies more steeply with intensity when the source shows 6 Hz QPO, than for 10 to 20 Hz QPO. Transitions between QPO modes, which occur when the flux is at its lowest levels, correspond to transitions between hardness ratio modes. The QPO frequency changes continuously through the transitions, which last a few hundred seconds.

Discovery of 39 hour periodic dips in the X-ray intensity of XB 1254 - 690

The low-mass X-ray binary XB 1254 - 690 has been discovered to show irregular dips in X-ray intensity that recur with a period of 3.9 hr consistent with the optical period derived independently. The spectral changes observed during dipping are complex and can be successfully described using a two-component model. One of these components may be X-rays scattered into the line of sight by an accretion disk corona. In contrast to other X-ray dipping sources for which the metallicity has been measured, the abundance of the material responsible for the dips is close to that of cosmic material. 29 references.