IS X1957+11 A BLACK-HOLE CANDIDATE

The X-ray source X1957 + 11 has an unusually soft X-ray spectrum, very similar to the X-ray spectra of Galactic black hole candidates. We re port on a recent Ginga observation of X1957 + 11, the first X-ray obse rvation since HEAO-1, and find significant spectral variability, on ti me-scales of hours, in the sense that the spectrum becomes harder as t he luminosity increases. We model the spectrum with a two-component mo del: a soft component associated with the disc emission and a hard com ponent which is best described by a power law. The spectral variabilit y is predominantly due to changes in the hard component while the soft component remains relatively stable. Recent results suggest that Gala ctic black hole candidates might be distinguished from X-ray binary sy stems in which the compact object is a neutron star by their X-ray spe ctral signatures alone. We discuss the extent to which this may be tru e and compare the results of our spectral analysis for X1957 + 11 with those for other low-mass X-ray binary systems (LMXBs) and black hole candidates, and argue that the compact object in X1957 + 11 is likely to be a neutron star.