Dependence of the orbital modulation of X-rays from 4U 1820-303 on the accretion rate

We report the discovery, using RXTE data, of a dependence of the X-ray orbital modulation depth on the X-ray spectral state in the ultracompact atoll binary 4U 1820-303. This state (measured by us by the position on the X-ray colour-colour diagram) is tightly coupled to the accretion rate, which, in turn, is coupled to the phase of the ∼170-d superorbital cycle of this source. The modulation depth is much stronger in the high-luminosity, so-called banana, state than in the low-luminosity, island, state. We find the X-ray modulation is independent of energy, which rules out bound-free X-ray absorption in an optically thin medium as the cause of the modulation. We also find a significant dependence of the offset phase of the orbital modulation on the spectral state, which favours the model in which the modulation is caused by scattering in hot gas around a bulge at the disc edge, both the size and the position of which vary with the accretion rate. Estimates of the source inclination appear to rule out a model in which the bulge itself occults a part of an accretion disc corona. We calculate the average flux of this source over the course of its superorbital variability (which has the period of ∼170 d), and find it to be fully compatible with the model of accretion due to the angular momentum loss via emission of gravitational radiation. Also, we compare the dates of all X-ray bursts observed from this source by BeppoSAX and RXTE with the RXTE light curve, and find all of them to coincide with deep minima of the flux, confirming previous results based on smaller samples.