http://hdl.handle.net/2289/101 Search the Channel search http://dspace.rri.res.in:8080/jspui/simple-search http://hdl.handle.net/2289/5347 Title: New measurements of orbital period change in Cygnus X-3<br/><br/>Authors: Singh, N.S.; Naik, S.; Paul, Biswajit; Agrawal, P.C.; Rao, A.R.; Singh, K.Y.<br/><br/>Abstract: The nonlinear nature of the binary ephemeris of Cygnus X-3 indicates either a change in the orbital period oran apsidal motion of the orbit. We have made extended observations of Cygnus X-3 with the Pointed Proportional Counters(PPCs) of the Indian X-ray Astronomy Experiment (IXAE) during 1999 July 3−13 and October 11−14. Using the data fromthese observations and the archival data from ROSAT, ASCA, BeppoSAX and RXTE, we have extended the database for thissource. Adding these new arrival time measurements to the published results, we make a comparison between the variouspossibilities, (a) orbital decay due to mass loss from the system, (b) mass transfer between the stars, and (c) apsidal motion ofthe orbit due to gravitational interaction between the two components. The orbital decay due to mass loss from the companionstar seems to be the most probable scenario<br/><br/>Description: Restricted Access. http://hdl.handle.net/2289/5346 Title: BeppoSAX observations of the X-ray binary pulsar GX 1+4<br/><br/>Authors: Naik, S.; Callanan, P.J.; Paul, Biswajit<br/><br/>Abstract: We present here the timing and spectral properties of BeppoSAX observations of the binary X-ray pulsar GX 1+4 carried out in August 1996, March 1997, and August 2000. In the middle of the August 2000 observation, the source was in a rare low intensity state that lasted for about 30 hours. Though the source does not show pulsations in the soft X-ray band (1.0-5.5 keV) during the extended low state, pulsations are detected in 5.5-10.0 keV energy band of the MECS detector and in hard X-ray energy bands (15-150 keV) of the PDS instrument. Broad-band (1.0-150 keV) pulse averaged spectroscopy reveals that the best-fit model comprises of a Comptonized continuum along with an iron Kα emission line. A strong iron Kβ emission line is detected for the first time in GX 1+4 during the extended low state of 2000 observation. The optical depth and temperature of the Comptonizing plasma are found to be identical during the high and low intensity states whereas the hydrogen column density and the temperature of the seed photons are higher during the low state.<br/><br/>Description: Restricted Access. http://hdl.handle.net/2289/5345 Title: Observations of the transient X-ray pulsar EXO 053109-6609.2 with ASCA, BeppoSAX and XMM-Newton<br/><br/>Authors: Naik, S.; Paul, Biswajit<br/><br/>Abstract: We report timing and spectral properties of the transient X-ray pulsar EXO 053109-6609.2 using observations carried out with ASCA, BeppoSAX, and XMM-Newton observatories. Pulse period measurements of the source show a monotonic spin-up trend since 1996. The pulse profile is found to have a strong luminosity dependence, a single peaked profile at low luminosity that changes to a double peaked profile at high luminosity. The X-ray spectrum is described by a simple power-law model with photon index in the range of 0.2-0.8. A soft excess over the power-law continuum is also detected from XMM-Newton observation.<br/><br/>Description: Restricted Access. http://hdl.handle.net/2289/5344 Title: Spectral evolution of GRS 1915+105 during the regular flares<br/><br/>Authors: Paul, Biswajit; Rao, A.R.<br/><br/>Abstract: We have investigated spectral evolution of GRS 1915+105 during the regular flares by high time resolution spectroscopy. Twenty-four color light-curves were converted into 1 s and 3 s spectra. A spectral model consisting of a disk black-body and a power-law, shows that the flux variations during the regular flares (in 2-20 keV range) can mainly be attributed to changes in the disk black-body emission, with a small increase in the power-law flux. The disk black-body temperature increases gradually with the flux, and is associated with a decrease in the size of the emission region. Two different types of intensity variations, that is spectral-state transitions and regular flares involving a disappearing disk are found to occur simultaneously.<br/><br/>Description: Restricted Access.