Super-Eddington X-ray luminosity of the bursting pulsar GRO J1744-28: WATCW/Granat observations

The results of WATCH/Granat X-ray observations of the bursting pulsar GRO J1744-28, carried out between January and March 1996 juring a lengthy burst from this transient source, are presented. The main goal of these observations was to reliably determine the flux from the source during and between bursts, which was made possible by the ability of the WATCH instrument to measure very high photon fluxes without any influence of inhibiting dead-time effects. At the maximum light recorded in mid-January, the 8-20-keV flux from the source between bursts reached similar to 3.7 Crab. If we assume that GRO J1744-28 is at the same distance as the Galactic center and emits quasi-isotropically, then the X-ray (2-100 keV) luminosity of the source was at that time similar to 10(39) erg s(-1), i.e., more than a factor of 5 greater than the Eddington luminosity for a neutron star. Seventy bursts were detected from GRO J1744-28; at their maxima, the luminosity of the source exceeded its luminosity between the bursts by a factor of 7-22, reaching similar to 10(40) erg s(-1). All of the bursts had similar time histories and essentially the same duration of similar to 4 s (FWHM). The burst rate was, on the average, 41 +/- 5 events per day (given the exposure time). The detection of super-Eddington luminosity of GRO J1744-28 can be explained in terms of the Basko-Sunyaev model of a hollow accretion column in a strong magnetic field.