Rossi X-ray Timing Explorer Observations of the Low-mass X-ray Binary 4U 1608 -522 in the Upper-banana State
AstrophysJ_738_62.pdf 1.02 MB
stars: individual (4U 1608-522)
To investigate the physics of mass accretion onto weakly magnetized neutron stars (NSs), 95 archival Rossi X-Ray Timing Explorer data sets of an atoll source 4U 1608-522, acquired over 1996-2004 in the so-called upper-banana state, were analyzed. The object meantime exhibited 3-30 keV luminosity in the range of less than or similar to 10(35)-4 x 10(37) erg s(-1), assuming a distance of 3.6 kpc. The 3-30 keV Proportional Counter Array spectra, produced one from each data set, were represented successfully with a combination of a soft and a hard component, the presence of which was revealed in a model-independent manner by studying spectral variations among the observations. The soft component is expressed by the so-called multi-color disk model with a temperature of similar to 1.8 keV, and is attributed to the emission from an optically thick standard accretion disk. The hard component is a blackbody (BB) emission with a temperature of similar to 2.7 keV, thought to be emitted from the NS surface. As the total luminosity increases, a continuous decrease is observed in the ratio of the BB luminosity to that of the disk component. This property suggests that it gradually becomes difficult for the matter flowing through the accretion disk to reach the NS surface, presumably forming outflows driven by the increased radiation pressure. On timescales of hours to days, the overall source variability was found to be controlled by two independent variables: the mass accretion rate and the innermost disk radius, which changes both physically and artificially.
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