2010 (IPP)

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Associated spectral and temporal state transition of the bright ULX NGC 1313 X-1
(2010-05-21) Dewangan, Gulab Chand; et al.
Stellar mass black hole X-ray binaries exhibit X-ray spectral states which also have distinct and characteristic temporal properties. These states are believed to correspond to diﬀerent accretion disc geometries. We present analysis of two XMM-Newton observations of the Ultra-Luminous X-ray source (ULX) NGC 1313 X-1, which reveal that the system was in two diﬀerent spectral states. While spectral variations have been observed in this source before, this data provides clear evidence that the spectral states also have distinct temporal properties. With a count rate of ~ 1.5 counts/s and a fractional variability amplitude of ~ 15%, the ULX was in a high ﬂux and strongly variable state in March 2006. In October 2006, the count rate of the ULX had reduced by a factor of ~ 2 and the spectral shape was distinctly diﬀerent with the presence of a soft component. No strong variability was detected during this low ﬂux state with an upper limit on the amplitude < 3%. Moreover, the spectral properties of the two states implies that the accretion disc geometry was diﬀerent for them. The low ﬂux state is consistent with a model where a standard accretion disc is truncated at a ra- dius of ~ 17 Schwarzschild radius around a ~ 200M⊙ black hole. The inner hot region Comptonizes photons from the outer disc to give the primary spectral component. The spectrum of the high ﬂux state is not compatible with such a geometry. Instead, it is consistent with a model where a hot corona covers a cold accretion disc and Comptonizes the disc photons. The variability as a function of energy is also shown to be consistent with the corona model. Despite these broad analogies with Galactic black hole systems, the spectral nature of the ULX is distinct in having a lower temperature (~ 2 keV) of the hot Comptonizing plasma and higher optical depth (~ 15) than what is observed for the Galactic ones.
CCD photometric study of the late type contact binary EK comae berenices
(2010-04-26) Deb, Sukanta; Singh, Harinder P.; Seshadri, T. R.; et al.
We present CCD photometric observations of the W UMa type contact binary EK Comae Berenices using the 2 metre telescope of IUCAA Girawali Observatory, India. The star was classiﬁed as a W UMa type binary of subtype-W by Samec et al. [20]. The new V band photometric observations of the star reveal that shape of the light curve has changed signiﬁcantly from the one observed by Samec et al. [20]. A detailed analysis of the light curve obtained from the high-precision CCD photometric observations of the star indicates that EK Comae Berenices is not a W-type but an A-type totally eclipsing W UMa contact binary. The photometric mass ratio is determined to be 0.349 ± 0.005. A temperature diﬀerence of ∆T = 141 ± 10 K between the components and an orbital inclination of i[°] = 89.800 ± 0.075 were obtained for the binary system. Absolute values of masses, radii and luminosities are estimated by means of the standard mass-luminosity relation for zero age main-sequence stars. The star shows O’Connell eﬀect, asymmetries in the light curve shape around the primary and secondary maximum. The observed O’Connell eﬀect is explained by the presence of a hot spot on the primary component.