An EXOSAT detection of the transient X-ray pulsar GRO J1008-57

Mathematics – Probability

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Accretion Disks, Pulsars, X Ray Binaries, X Ray Spectra, Astronomical Models, Exosat Satellite, Gamma Ray Observatory, Light Curve, Orbital Mechanics, Rosat Mission, Satellite-Borne Instruments, Stellar Luminosity

Scientific paper

The hard X-ray transient, GRO J1008-57, was discovered during a bright outburst in 1993 July by the Burst and Transient Source Experiment (BATSE) instrument on board the Compton Gamme-Ray Observatory (CGRO). There are no published reports of previous emission from this 93.6 sec X-ray pulsar. Recent optical results have suggested a Be star as the companion. A search of the EXOSAT archives shows that an ME observation centered on the star HD 88661 includes GRO J1008-57 within the field of view. The characteristics of the medium-energy detection including a hard spectrum and pulsed emission at 91.36 sec (with chance probability of 3 x 10-4) indicates that EXOSAT detected GRO J1008-57 rather than the field star. The estimated flux is 2.4 x 10-11 ergs/cm sq/sec for a luminosity of 1.1 x 1034 (D/2 kpc)2 ergs/sec. The X-ray spectrum is hard (photon index approximately equals 1.2) and highly absorbed (NH approximately equals 0.7 x 1022). The detection of this transient suggests that there is a pulse shape dependence on luminosity, a spectral shape independent of luminosity, a large period derivative of P-dot/P = 2.3 x 10-3/yr, and a dynamic range of at least 100 in Lchi. We discuss GRO J1008-57 data in the context of wind-accretion models for this new member of the Be class of X-ray binaries.

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