GX17+2: X-ray spectral and timing behaviour of a bursting Z source

Abstract

We have investigated the properties of the Z pattern of the low-mass X-ray binary GX17+2 in the X-ray colour-colour diagrams (CDs) and hardness-intensity diagrams (HIDs), and of the power spectra as a function of position in the Z. We used all EXOSAT ME data on GX17+2, a total of six observations during 1983-86. We find that the Z pattern in the CD does not move to within ~2 per cent between observations separated by ~1 d to ~1.5 yr. The power spectra and the characteristics of the Z pattern both support the division of the Z sources into two sub-groups, GX17+2 being in the group with ScoX-1 and GX349+2. It has been noted previously that the maximum frequency of the horizontal branch quasi-periodic oscillations in GX17+2 is lower than for CygX-2, GX5-1 and GX340+0. We suggest that an asymmetry in the magnetic field giving rise to polar caps with different emission characteristics may be the origin of this. This may also explain the occurrence of bursts in GX17+2. We also report the detection of two new bursts in the EXOSAT data of GX17+2, which we interpret as type I bursts. No regular pulsations are observed during these two bursts, or during the bursts previously detected in GX17+2 with EXOSAT, with upper limits to the pulsed amplitude of ~1.5 per cent for pulsations with a frequency below 512Hz. Three of the four bursts occurred in the normal branch, while one occurred in the lower part of the flaring branch. The Poisson level in power spectra of data processed by the on-board computer of EXOSAT is found to deviate from that expected from simple variable dead-time effects in GX17+2 and also in several other sources. We present corrections to the usually employed relations between count rates observed with EXOSAT and the Poisson level.