We report X-ray observations of the Galactic X-ray transient source GRS 1915]105 with the pointed proportional counters of the Indian X-ray Astronomy Experiment (IXAE) onboard the Indian satellite IRS-P3, which show remarkable richness in temporal variability. The observations were carried out on 1997 June 12È29 and August 7È10, in the energy range of 2È18 keV and revealed the presence of very intense X-ray bursts. All the observed bursts have a slow exponential rise, a sharp linear decay, and broadly can be put in two classes : irregular and quasi-regular bursts in one class, and regular bursts in the other. The regular bursts are found to have two distinct timescales and to persist over extended durations. There is a strong correlation between the preceding quiescent time and the burst duration for the quasi-regular and irregular bursts. No such correlation is found for the regular bursts. The ratio of average Ñux during the burst time to the average Ñux during the quiescent phase is high and variable for the quasi-regular and irregular bursts, while it is low and constant for the regular bursts. We present a comprehensive picture of the various types of bursts observed in GRS 1915]105 in the light of the recent theories of advective accretion disks. We suggest that the peculiar bursts that we have seen are characteristic of the change of state of the source. The source can switch back and forth between the low-hard state and the high-soft state near critical accretion rates in a very short timescale, giving rise to the irregular and quasi-regular bursts. The fast timescale for the transition of the state is explained by invoking the appearance and disappearance of the advective disk in its viscous timescale. The periodicity of the regular bursts is explained by matching the viscous timescale with the cooling timescale of the postshock region. A test of the model is presented using the publicly available 13È60 keV RXT E/PCA data for irregular and regular bursts concurrent with our observations. It is found that the 13È60 keV Ñux relative to the 2È13 keV Ñux shows clear evidence for state change between the quiescent phase and the burst phase. The value of this ratio during burst is consistent with the values observed during the high-soft state seen on 1997 August 19, while its value during quiescent phase is consistent with the values observed during the low-hard state seen on 1997 May 8.

Abstract.A new survey for pulsating, chemically peculiar A-and F-type stars in the northern hemisphere has been initiated using the 1-m telescope at Uttar Pradesh State Observatory in Naini Tal. The survey is primarily structured to reveal high-overtone pulsations in chemically peculiar A-F stars, but it is also revealing low-overtone δ Scuti-type pulsations in stars with peculiar colours. This paper describes the scope and methods of the survey. Preliminary observations of 28 candidate stars are presented and the discovery of three new pulsators, HD 12098, HD 13038 and HD 13079, is announced. Null results for the remaining 25 stars are also discussed.

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We report the detection of a series of X-ray dips in the Galactic black hole candidate GRS 1915+105 during 1999 June 6−17 from observations carried out with the Pointed Proportional Counters of the Indian X-ray Astronomy Experiment on board the Indian satellite IRS-P3. The observations were made after the source made a transition from a steady low-hard state to a chaotic state which occuered within a few hours. Dips of about 20−160 seconds duration are observed on most of the days. The X-ray emission outside the dips shows a QPO at ∼4 Hz which has characteristics similar to the ubiquitous 0.5 − 10 Hz QPO seen during the low-hard state of the source. During the onset of dips this QPO is absent and also the energy spectrum is soft and the variability is low compared to the non-dip periods. These features gradually re-appear as the dip recovers. The onset of the occurrence of a large number of such dips followed the start of a huge radio flare of strength 0.48 Jy (at 2.25 GHz). We interpret these dips as the cause for mass ejection due to the evacuation of matter from an accretion disk around the black hole. We propose that a super-position of a large number of such dip events produces a huge radio jet in GRS 1915+105.

Three different types of very intense, quasi-regular X-ray bursts have been observed from the Galactic superluminal X-ray transient source GRS 1915+105 with the Pointed Proportional Counters of the Indian X-ray Astronomy Experiment onboard the Indian satellite IRS-P3. The observations were carried out from 1997 June 12 to June 29 in the energy range of 2−18 keV and revealed the presence of persistent quasi-regular bursts with different structures. Only one of the three types of bursts is regular in occurrence revealing a stable profile over extended durations. The regular bursts have an exponential rise with a time scale of about 7 to 10 s and a sharp linear decay in 2 to 3 s. The X-ray spectrum becomes progressively harder as the burst evolves and it is the hardest near the end of the burst decay. The profile and energetics of the bursts in this black hole candidate source are distinct from both the type I and type II X-ray bursts observed in neutron star sources. We propose that the sharp decay in the observed burst pattern is a signature of the disappearance of matter through the black hole horizon. The regular pattern of the bursts can be produced by material influx into the inner disk due to oscillations in a shock front far away from the compact object.

Abstract.We report results from two observations of the transient binary X-ray pulsar XTE J1946+274 with the Indian X-ray Astronomy Experiment (IXAE) made in September 18-30 1999 and June 28-July 7 2000. The pulsar has a ∼80 days orbital period during which the X-ray intensity varies almost sinusoidally between intensity levels of 5 and 50 mCrab. The mid-time of the two observations with the IXAE are separated by an orbital phase of 0.56 and were in the decaying and rising parts of its orbital modulation, respectively. During both the observations, strong pulsations with 15.8 s period are detected in the 2-6 and 6-18 keV energy bands of IXAE. The pulse profiles in both the observations are double peaked and identical with a similar pulse fractions of ∼30% in the two energy bands. The pulse periods and its local derivatives are obtained from the IXAE observations. The known pulse period history shows small deviations from an otherwise nearly constant spin-up trend of the pulsar since its discovery. If the measured local period derivatives are a sum of a nearly constant spin-up of the pulsar and effect of its orbital motion, it suggests that the orbit of this Be X-ray binary is eccentric. The RXTE-ASM light curve shows a continuous detection of 10 orbital modulation of the source since its discovery.

Pulsation in cool main‐sequence Am stars is a rare phenomenon observed in a few marginal Am stars and only one classical Am star. The chemically peculiar star HD 13079 is shown to be pulsating with a 78‐min period and a peak‐to‐peak B amplitude of 0.02 mag. The Hipparcos parallax, uvbyβ photometry and pulsations together suggest that HD 13079 is an Am star near the zero‐age main sequence, and that it is a fundamental‐mode pulsator on the red edge of the instability strip.

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