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

Saptarshi’s visit to different Nakshatras:

Summary In several ancient Indian texts a mention is made of the movement of the Saptarshi constellation (Big Bear or Big Dipper) in the sky, visiting each nakshatras for 100 years. Saptarshi is said to visit a nakshatra if the nakshatra is in the middle of the stars in the first part of Saptarshi. Since astronomical objects except planets are more or less stationary in the sky, this is generally considered a fanciful statement devoid of astronomical meaning. We show that this may not be so. We show that the visit of Saptarshi to different nakshatras may be a very significant astronomical observation. The transition is not a constant of time since it depends on the proximity of the Saptarshi to the North Pole, which changes due to Earth"s precession and relative sizes of different nakshatras. We show that since 8000 BC, Saptarshi has visited 5 different nakshatras and for one of them, the transition happening in the span of roughly 100 years. We show that this interpretation allows dating of this belief which is consistent with other evidences of the Harappan civilisation and date different Saptarshi Era with calendar dates

X-ray continuum and line emission of the Seyfert galaxy MCG - 5-23-16

We report observations of X-ray continum and line emission from the highly absorbed and variable Seyfert galaxy MCG-5-23-16, using the low-energy and the medium-energy detectors of the Exosat Observatory.The galaxy was observed in 1983 December and 1984 May. The X-ray intensity of the source is observed to vary by ~25% in the hard (2-10 keV) and by ~40 percent in the soft (0.1-2.0 keV) energy bands over a period of 6 months. The 0.1-10 keV X-ray spectrum is adequately explained by a simple power law. The measured energy index lies in the range 0.42-0.89 and shows a dependence on the intensity. Line emission at 6.4 keV is detected in the 1984 observations when the X-ray spectrum is relatively harder. The equivalent width of the line is measured to be 346 ± 146 eV. The presence of line emission is also indicated in the 1983 data. The line-of-sight absorption is much higher than the 21-cm value, suggesting a very high obscuration of the nuclear source. The present observations are well explained by the reflection of X-rays by cold material in an accretion disk around the nuclear source. It is suggested that as the absorption(due to the disk) of the direct component increases with time, so does the amount of the reflected component, thus hardening the spectrum and also producing more line emission