K. Kasturirangan

Ionospheric effects of transient celestial X-ray and gamma-ray events

The paper presents the results on investigations of the ionospheric effects arising out of the transient celestial events at X-ray and gamma-ray energies such as that from X-ray novae and cosmic gamma-ray bursts. The VLF data recorded at Ahmedabad corresponding to the 164 kHz transmission from Tashkent are examined to see whether there are any observable effects time coincident with the transit of the X-ray novae Cen X-4 and Cet X-2 around their intensity maxima. Besides, the VLF data have also been examined in relation to the cosmic gamma-ray bursts detected by Vela, IMP-6 and OSO-7 satellites.Theoretical computations are carried out for estimating electron density enhancements using the available data on intensities and energy spectra for these events. Further, the observational resutls are explained in terms of these theoretical calculations.

Balloon observations of fast intensity fluctuations and flare-like enhancements of X-ray emission from Cygnus X-1

The paper presents the results of the investigation on the short term X-ray emission characteristics of Cyg X-1 in the 20–150 keV range. The observations were made with balloon-borne oriented scintillation telescope and the flights were conducted from Hyderabad, India. The source was tracked over a duration of 1 hr that enabled detailed observations with time resolution of the order of 1 min. Fluctuations in the intensity of Cyg X-1 with time scales of the order of minutes have been detected besides short-term flare-like enhancements. The spectral characteristics of the flare emission features are discussed and their relationship to the phase of the binary is examined.

The intermediate polar PQ Gem-status of WET observations of 1996

A summary of the results on PQ Gem based on WET observations conducted in February 1996 is presented. The 13.9 min spin period of the white dwarf in the system and the orbital side-band at 14.5 min have been detected unambiguously. Searches for other periods in the system gave negative results.

FURTHER PHOTOMETRIC OBSERVATIONS OF THE CATACLYSMIC VARIABLE PG 1012-029

We report time series photometric observations of PG 1012-029, conducted during 1990 and 1991. Light curves of three consecutive eclipses recorded in a single night are presented. The enhanced pre-eclipse hump in our data provides the evidence for a bright hot spot in the sytem. The orbital period derived by us is not significantly different from the value reported by Penning et al.,(1984) value. Fourier analysis of the data did not reveal any short-term periodicites in the system.

1H0857-242 During Outburst

We report preliminary analysis of the photometric observations of 1H0857-242 during its latest outburst in April, 1992. The star was around visual mag 13 when it was observed in outburst and faded at the rate of 0.1 mag/day. The orbital modulation at the known period of 114 min was clearly visible even during outburst. Fourier analysis of the light curve showed a broad peak around 52 min. which is the suspected rotational period of the white dwarf in the system. The average period of the dwarf nova-type outburst of the system is around a month.