X-ray variability of GRS 1915+105 during the low-hard state observed with the Indian X-ray astronomy experiment (IXAE)

The galactic superluminal transient X-ray source GRS 1915+105 was observed with the pointed proportional counters (PPCs) onboard the Indian satellite IRS-P3 during 1996 July 23-27. We report here details of the behavior of this source during the relatively quiet and low luminosity state. Large intensity variations by a factor of 2 to 3, generally seen in black-hole candidates, are observed at a time scale of 100 ms to few seconds. No significant variation is detected over larger time scale of minute or more. The intensity variations are described as sum of shots in the light curve, and the number distribution of the shots are found to be exponential function of the fluence and duration of the shots. The cross correlation spectrum between 6-18 keV and 2-6 keV X-rays is found to have asymmetry signifying a delay of the hard X-rays by about 0.2 to 0.4 sec. This supports the idea of hard X-rays being generated by Compton up-scattering from high energy clouds near the source of soft X-rays. Very strong and narrow quasi periodic oscillations in the frequency range 0.62 to 0.82 Hz are observed. We discuss about a model which explains a gradual change in the QPO frequencies with corresponding changes in the mass accretion rate of the disk.Comment: 14 pages including 6 figures. To appear in Astronomy and Astrophysics Supplement Serie

Multi-Site Observations of the DAV White Dwarf R 548

The pulsating DA white dwarf R 548 was observed for 46 h in October 1993 in an eight-site campaign. New peaks near the known doublets in the Fourier transform are found

Asteroseismology of the DOV Star PG 1159-035 with the Whole Earth Telescope

We report the results from 264.1 hr of nearly continuous time-series photometry on the pulsating pre-white dwarf star (DOV) PG 1159-035. The high-resolution power spectrum of this data set is dominated by power in the range from roughly 1000 to 2600 μHz (1000 s to 385 s periods). This power is completely resolved into 125 individual frequencies; we have identified 101 of them with specific, quantized pulsation modes, and the rest are completely consistent with such modal assignment. The luminosity variations are therefore certainly the result of g-mode pulsations. Although the amplitudes of some of the peaks exhibit significant variations on time scales of a year or so, the underlying frequency structure of the pulsations is stable over much longer intervals. With the help of existing linear theory we use these identifications to determine, or strongly constrain, many of the fundamental physical parameters describing this star. We find its mass to be 0.586 M⊙, its rotation period 1.38 days, its magnetic field less than 6000 G, its pulsation and rotation axes to be aligned, and its outer layers to be compositionally stratified. With straightforward extensions of existing theory it may be possible to determine uniquely from this data set all of the parameters necessary to construct a quantitative model of its interior. These observations also reveal several interesting phenomena that challenge the current theory of nonradial pulsations, and may require substantial new developments to describe them

Asteroseismology of the dov star PG 1159-035 with the Whole Earth Telescope

We report the results from 264.1 hr of nearly continuous time-series photometry on the pulsating pre-white dwarf star (DOV) PG 1159-035. The high-resolution power spectrum of this data set is dominated by power in the range from roughly 1000 to 2600 μHz (1000 s to 385 s periods). This power is completely resolved into 125 individual frequencies; we have identified 101 of them with specific, quantized pulsation modes, and the rest are completely consistent with such moda! assignment. The luminosity variations are therefore certainly the result of g-mode pulsations. Although the amplitudes of some of the peaks exhibit significant variations on time scales of a year or so, the underlying frequency structure of the pulsations is stable over much longer intervals. With the help of existing linear theory we use these identifications to determine, or strongly constrain, many of the fundamental physical parameters describing this star. We find its mass to be 0.586 Mʘ , its rotation period 1.38 days, its magnetic field less than 6000 G, its pulsation and rotation axes to be aligned, and its outer layers to be compositionally stratified. With straightforward extensions of existing theory it may be pos­ sible to determine uniquely from this data set ali of the parameters necessary to construct a quantitative model of its interior. These observations also reveal severa! interesting phenomena that challenge the current theory of nonradial pulsations, and may require substantial new developments to describe them