GX 17+2: X-ray spectral and timing behaviour of a bursting Z source

Abstract

We investigated the properties of the Z pattern of the low-mass X-ray binary GX 17+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, using all EXOSAT ME data. The Z pattern in the CD does not move to within 2 per cent between observations separated by 1 day to 1.5 years. The power spectra and the characteristics of the Z pattern both support the division of the Z sources into two sub-groups, GX 17+2 being in the group together with Sco X-1 and GX 349+2. It has been noted previously that the maximum frequency of the horizontal branch quasi-periodic oscillations in GX 17+2 is lower than for Cyg X-2, GX 5-1 and GX 340+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 GX 17+2. We also report the detection of two new bursts in the EXOSAT data of GX 17+2, which we interpret as type I bursts. No regular pulsations are observed during these two bursts nor during the bursts previously detected in GX 17+2 with EXOSAT, with upper limits to the pulsed amplitude of 1.5 per cent for pulsations with a frequency below 512 Hz. Three of the four bursts occurred in the normal branch, while one occurred in the lower part of the flaring branch.