Chandra/RXTE Observations of 4U1957+11: Ostensibly the Most Rapidly Spinning Galactic Black Hole

Details Chandra/RXTE Observations of 4U1957+11: Ostensibly the Most Rapidly Spinning Galactic Black Hole Chandra/RXTE Observations of 4U1957+11: Ostensibly the Most Rapidly Spinning Galactic Black Hole

Mar 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008head...10.1404n&link_type=abstract

American Astronomical Society, HEAD meeting #10, #14.04

Astronomy and Astrophysics

Astronomy

1

Scientific paper

We present simultaneous Chandra-High Energy Transmission Gratings and Rossi X-ray Timing Explorer observations of a `soft state' of the black hole candidate 4U1957+11. These data are an excellent test of modern disk atmosphere models that include the effects of black hole spin. The HETG data show that a fitted disk model implies a highly inclined disk around a low mass black hole rapidly rotating with a* 1. We show, however, that pure Schwarzschild black hole models describe the data extremely well, albeit with large disk atmosphere ``color-correction'' factors. ``Standard'' color-correction factors are attained if one additionally incorporates mild Comptonization. We conclude that even with this extremely well-measured, nearly pure disk spectrum, it is very difficult to uniquely determine spin. We then present a series of 48 RXTE observations taken over the span of several years and at different brightness/hardness levels. These data prefer a spin of a* 1, even when including a mild Comptonization component; however, they also show evolution of the disk atmosphere color-correction factors. If the rapid spin models with standard atmosphere color-correction factors of 1.7 are to be believed, then 4U1957+11 can range from a 3 solar mass black hole at 10 kpc with a* 0.83 to a 16 solar mass black hole at 22 kpc with a* 1, with the latter being statistically preferred at high formal significance.

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