Udita Shukla

Colour Evolution of Low-Mas X-ray Binary 4U 1636-536

The work deals with the colour evolution of neutron star low-mass X-ray binary 4U 1636-536 by constructing light curves. 1,476 light curves were extracted corresponding to the energy bands of (2-60) keV, (2.5-5.7) keV and (7.8-16.4) keV. The light curve shows a rough quasi-periodicity of ~ 40-46 days in terms of a clear count rate oscillation. Hardness ratio is found to vary in more or less periodic fashion but being constantly out of phase with the light curve. The hardness-Intensity Diagram has most of the observations clustered in the softer region of the plot corresponding to the banana state whereas, the harder positions or the island state appear as a nearly constant band in the vicinity of 0.5 (Hardness Ratio). Spectra extracted for the energy range (3.0 – 50.0) keV, were fit with a compound model comprising of photo-electric absorption, ‘phabs’, and Blackbody, Gaussian and Comptonizing (high-energy) cut-off. The Gaussian line rises up at ~ 6 keV in the fits. The fit is seen to support the so-called ‘Western’ model (White et al. 1988) comprising of a Comptonizing Accretion Disk Corona (ADC) and the neutron star emitting as a Blackbody. The Blackbody temperature of the neutron star centres around 1 keV to 2 keV, while the (Blackbody) radius broadens as we move from the island to the banana state reaching ~ 1 km, implying that the blackbody emission is from a restricted part of the neutron star.