EXOSAT observations of GX 304-1 in an X-ray of state.

Abstract

Three EXOSAT observations of the Be/neutron star X-ray binary system GX 304-1 in July/August 1984 around the time of an expected periodic outburst occurring at 132.5 d intervals showed no detectable flux of X-rays. The derived upper limits of the flux in the low and medium energy range (0.03 - 2.4 keV and 2 - 6 keV) are both 3×10^-12erg cm^-2s^-1. These upper limits are a factor of 25 below the quiescent flux level measured from GX 304-1 reported from other satellites, indicating that the source is in an "off" state that is different from the previous quiescent states. An interpretation of this state is discussed in view of optical observations, indicating that the shell of the Be system has been diminishing since 1978.