The bright and hot disk of the LMXB 4U 1957+11
Abstract
4U 1957+11 is a low mass X-ray binary (LMXB) but, opposed to most other LMXBs, it is in a persistently bright accreting state. It shows one of the cleanest examples of a disk spectrum in the X-ray band, with an extremely small inner disk radius and a disk temperature in the 1.3-1.8 keV range and therefore among the highest detected. The low absorption towards 4U 1957+11 makes disk studies especially rewarding. The spectral shape, together with the low short-term variability, hints towards it being the most rapidly spinning galactic LMXB as long as its mass is over 3 solar masses at a distance greater than 10 kpc. Optical modulation has, however, been used to argue that the mass cannot be greater than 3 solar masses, favoring a neutron star interpretation.We reassess the historical RXTE data, using recent revisions of the RXTE calibration. We use these observations, along with an assessment of all sky monitor lightcurves, to place more recent pointed observations in their proper context of the overall behavior of the source. These pointed observations include a coordinated XMM + NuSTAR + HST/COS campaign that allows us to probe disk models over a wider bandpass than previously possibly. In particular we can asses the underlying spectral correction factor (the ratio of color to thermal temperature) and test whether the spin results and thus the black hole interpretation of the source can be upheld when taking the UV signature of the disk into account.
- Publication:
-
42nd COSPAR Scientific Assembly
- Pub Date:
- July 2018
- Bibcode:
- 2018cosp...42E1288G