4U 1957+11: Low mass, maximal Kerr black hole, or neutron star?
Abstract
4U 1957+11 is one of the few black hole candidates that is in both a low mass X-ray binary and in a persistently bright accreting state. It exhibits all the hallmarks of a soft/accretion disk dominated spectrum, with a very high inner disk temperature and extremely small inner radius. These together argue for 4U 1957+11 being the most rapidly spinning black hole in our Galaxy, so long as its mass is > 3 solar masses and distance is greater than 10 kpc. Conversely, it has been argued that its optical lightcurve modulation cannot permit a mass greater than 3 solar masses, and favors a neutron star. Here we reassess the historical RXTE data, using recent revisions of the RXTE calibration. We use these observations, along with an assessment of the RXTE All Sky Monitor lightcurve and the MAXI lightcurve, to place more recent pointed observations in their proper context of the overall behavior of the source. Finally, we describe a set of observations that were performed simultaneously with HST-COS, XMM-Newton, and NuSTAR to critically assess whether or not 4U 1957+11 truly is a persistently accreting, low mass, Kerr black hole residing in the halo of our Galaxy.
- Publication:
-
The X-ray Universe 2017
- Pub Date:
- October 2017
- Bibcode:
- 2017xru..conf..160N