A Very Energetic X-Ray Burst from 4U 2129+11 in M15

Abstract We report on a Ginga observation of the globular-cluster source 4U 2129+11 in M15, during which we detected a very energetic X-ray burst. This burst showed a precursor, separated from the main event by ~ 6 s, which indicates that strong expansion of the neutron star photosphere occurred. During the first 30 s of the main part of the burst a series of slow radius oscillations were observed, whose termination may signify the transition from an outflowing neutron star wind to a static extended (but very slowly shrinking) envelope. From a time-resolved spectral analysis of the burst data we find evidence for deviations of the burst spectra from a blackbody, which cannot be described in terms of a spectral hardening alone. We show that, in principle, the results of the analysis may be used to derive information on the mass and radius of the neutron star, as well as on the anisotropy of the burst emission (as long as this remains the same during the burst). The applicability of this method is presently limited by uncertainties in the interpretation of the burst spectra.