Thursday, June 20, 2013

1306.4365 (Dipankar Maitra et al.)

Swift Monitoring of 4U 1957+11: Constraints on Binary Parameters and Nature of the Accretor    [PDF]

Dipankar Maitra, Jon M. Miller, Mark T. Reynolds, Rubens Reis, Mike Nowak
We present the results of a uniform spectral analysis of Swift/XRT observations of the X-ray binary system 4U 1957+11. This includes 26 observations of the source made between MJD 54282 - 55890 (2007 July 01 - 2011 November 25). All 26 spectra are predominantly thermal, and can be modeled well with emission from an accretion disk around a black hole. Assuming a black hole accretor, our analysis strongly prefers a rapidly rotating hole with near maximal prograde spin. For an accretor mass of 10 M_sun, orbital inclination of 75 degrees, and source distance of 10 kpc, the 90% confidence level lower limit on the value of the black hole's spin parameter (a*) is 0.997. Also, our models favor an orbital inclination of ~75 degrees and a distance of >5 kpc, both of which are consistent with, and strengthen results from, previous optical and X-ray studies of the source. Distances less than 5 kpc are unlikely and not only ruled out based on our analysis but also from other independent observations. Based on model-derived bolometric luminosities, we require the source distance to be >10 kpc if the black hole's mass is >10 M_sun. If the hole's mass is <10 M_sun, then the distance could be in the range of 5-10 kpc.
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