Super-eddington luminosity in the bursting pulsar GRO J1744-28. GRANAT/WATCH results

Abstract

We present the results of GRANAT/WATCH observations of the BATSE bursting pulsar GRO J1744-28 in January – March 1996, during an intense outburst of this transient. The observations started in mid-January, exactly at the peak of the source light curve. High, ∼ 3.7 Crab, photon flux measured in these first observations in the energy band 8 – 20 keV indicates that the bolometric persistent X-ray luminosity of GRO J1744-28 was at that moment reaching 10³⁹ erg/s, a value that exceeds the Eddington luminosity for a neutron star by a factor of ∼ 5. This estimate is made assuming that the source is at the distance of the Galactic Center (8.5 kpc). 70 bursts have been detected from GRO J1744-28 by WATCH, during which the luminosity of the source increased 7 to 22 times relative to the persistent level, reaching over 10⁴⁰ (D/8.5 kpc)² erg/s, thus exceeding the neutron star Eddington luminosity by two orders of magnitude. All of the bursts have similar profiles characterized by FWHM ∼ 4 sec. The frequency of bursts also remained constant through the observations, 41±5 day⁻¹ (after correction for the observations’ windowing). The WATCH flux measurements are completely unaffected by any dead-time or pile-up effects, therefore the luminosity estimates we have obtained are quite reliable. We believe that the thin-wall column geometry of the accretion flow near the neutron star magnetic poles outlined by Basko & Sunyaev (1976) makes it possible for the GRO J1744-28 luminosity to overcome the Eddington limitation.