X-ray bursts at extreme mass accretion rates from GX 17+2

Abstract

(abridged version) We report on ten type I X-ray bursts from GX 17+2 in data obtained with the RXTE/PCA in 1996--2000. Three bursts were short in duration (~10 s), whereas the others lasted for ~6-25 min. Five of the long bursts showed evidence for radius expansion of the neutron star photosphere. No correlations o f the burst properties with respect to the persistent X-ray spectral properties are seen, suggesting no correlation with inferred persistent mass accretion rate. The presence of short bursts in GX 17+2 (and similar bright X-ray sources) is not accounted for in the current X-ray bursts theories at the high mass accretion rates encountered in these sources. We find that in contrast to previous suggestions the persistent black-body emission does NOT arise from the same site as the burst emission. The black-body component of the persistent emission is consistent with arising in an expanded boundary layer, as indicated by recent theoretical work. The total persistent flux just before and after the radius expansion bursts is inferred to be up to a factor of 2 higher than the net peak flux of the burst. If both the burst and persistent emission are radiated isotropically, this would imply that the persistent emission is up to a factor of 2 higher than the Eddington luminosity. This is unlikely and we suggest that the persistent luminosity is close to the Eddington luminosity and that the burst emission is (highly) anisotropic. Assuming that the net burst peak fluxes equal the Eddington limit, applying standard burst parameters (1.4 M_sun neutron star, cosmic composition, electron scattering opacity appropriate for high temperatures), and taking into account gravitational redshift and spectral hardening, we derive a distance to GX 17+2 of ~8 kpc, with an uncertainty of up to ~30%.