EXOSAT observations of the ultra-soft X-ray binary 4U 1957+11.

Abstract

We present results from the analysis of the two EXOSAT observations of the low mass X-ray binary (LMXRB) 4U 1957+11. The 1-20keV spectrum of the source is best fitted by a power law model with exponential cutoff, that provides an approximation of a thermal Comptonisation spectrum. The cutoff energy (~2keV), as well as the X-ray colours, are intermediate between those of ultrasoft sources containing black hole candidates (BHCs) and those of soft LMXRBs containing an old accreting neutron star. We find no evidence for a black body spectral component. During the 1985 observation the source flux was ~10% higher than in the 1983 observation, and the ME spectra provide evidence for a high energy tail (up to ~16 keV), which strengthens the similarity with the spectra of high state BHCs. However the source luminosity was only ~5x10^36^ergs/s. During the 1985 observation the GSPC spectra revealed the presence of a 100eV EW iron K-line emission feature with a centroid energy of 7.06+/-0.09keV; this value is among the highest measured by EXOSAT. The ME light curves showed only a low-amplitude variability on timescales of a few hours. No evidence for a periodic modulation with periods between 0.016 and 13000s was found.