Timing and spectral studies of neutron star X-Ray binaries

Abstract

This thesis concerns with the timing and spectral studies of X-ray binaries that contain neutron star as a compact object. We have studied both high and low magnetic field neutron stars in binary systems. In the case of high magnetic field neutron star binary systems, one of the important manifesta- tions of the interaction between the accretion disk around a neutron star and its magnetic field is the accretion torque on to the neutron star. We reviewed the pulse profile evolution of the unique accretion powered X-ray pulsar 4U 1626–67 over the last 40 years since its discovery. This pulsar showed two distinct eras of steady spin-up separated by a steady spin-down episode for about 18 years. Using data from different observatories active during each phase of spin- up and spin-down we have established a clear correlation between the accretion torque and its pulse profile. Pulse profiles of many accretion powered pulsars show an interesting feature known as dips. We performed a very detailed pulse profile evolution studies of an X-ray pulsar LMC X–4, using data from two observatories XMM-Newton and RXTE. Using the long observations containing both flares and persistent emission we have estimated the timescales required for the formation of accretion stream that caused dip in the pulse profiles of LMC X–4, after the accretion region and the beaming etc is disturbed during flares in this system. Pulse phase resolved spectroscopy is an important tool to probe into the geometry of emission region. We have performed pulse phase resolved spectroscopy of an X-ray pulsar 4U 1626–67 in order to investigate the correlation between the features in the pulse profiles with Neon and Oxygen emission lines in its spectrum. We found the line fluxes to have pulse phase depen-dence, making 4U 1626–67 only the second pulsar after Her X–1 to show such variability. The O VII line at 0.568 keV from 4U 1626–67 varied by a factor of ∼ 4, stronger than the contin- uum variability. Flux variability may appear due to variable illumination of the accretion disk or more likely, a warp like structure in the accretion disk. For the case of low magnetic field neutron stars sudden X-ray bursts has been observed in nearly 80 Low mass X-ray binaries (LMXBs). These are known as Thermonuclear X-ray bursts. We have carried out a study of temperature evolution during these X-ray bursts in LMXBs using broad band data from two instruments onboard BeppoSAX, the MECS and the PDS. However, instead of applying the standard technique of time resolved spectroscopy, we have determined the temperature in small time intervals using the ratio of count rates in the two instruments assuming blackbody nature of burst emission and different interstellar absorption for different sources. Data from a total of twelve observations of six sources were analysed during which 22 bursts were detected. We have obtained temperatures as high as ∼3.0 keV, even when there is no evidence of photospheric radius expansion. These high temperatures were observed in the sources within different broadband spectral states (soft and hard).