Ginga observations of XI820 — 303 in the globular cluster NGC 6624

Abstract

We present the first spectral observations obtained by Ginga of the low-mass X-ray binary source X1820-303 located in the globular cluster NGC6624. Two twocomponent spectral models are found to provide good fits to the data: a bremsstrahlung component plus a blackbody, or a power law with exponential cut-off plus a blackbody. In both cases, inclusion of an iron line at 6.7 keV improves the fits. After considering the importance of Comptonization for the spectral and temporal behaviour, we select the power law plus blackbody model. This model then provides simple explanations for many of the observed features present in our data. The model requires a scattering cloud surrounding the neutron star, with a Thomson optical depth of~ 7. By studying the variation in the amphtude of the 685-s binary period with source luminosity, it is possible to gain some insight into the source geometry. This reveals that our line of sight to X1820 - 303 is close to some ‘critical’ angle. We suggest that gravitationally induced shocks in the rim of the accretion disc cause the height of the rim to vary with azimuthal angle, resulting in the quasi-sinusoidal X-ray orbital modulations. Superposed on this are random ‘dips’ which result from chaotic, timevarying structures in the disc which, principally, only cover the blackbody component of the spectrum.