An X-ray and optical study of the low-mass X-ray source 4U 1556-605.

Abstract

Partly simultaneous EXOSAT and optical observations of the low-mass X-ray binary 4U 1556-605 are presented. X-ray flux variations of relatively small amplitude (about 20 percent) are accompanied by a significant hardening of the X-ray spectrum. This behavior is investigated in the framework of several currently proposed spectral models. The optical brightness varies by about 0.4 mag on a time scale of hours. However, neither strictly periodic modulation nor strong correlation with X-ray intensity is apparent in the observations. During 28 h of uninterrupted X-ray observations, no X-ray burst was detected. Compared to other low-mass X-ray binaries, 4U 1556-605 shows a low intensity Bowen N III 4634,41-A emission line complex, and a possible relation is suggested between metal abundance and X-ray burst behavior in low-mass X-ray binaries.