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

D. J. Macomb, C. R. Shrader, A. B. Schultz

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6 Scopus citations

Abstract

The hard X-ray transient, GRO J1008-57, was discovered during a bright outburst in 1993 July by the BATSE instrument on board the Compton Gamma-Ray Observatory. There are no published reports of previous emission from this 93.6 s 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 s (with chance probability of 3 ×10-4) indicates that EXOSAT detected GRO J1008-57 rather than the field star. The estimated flux is 2.4 ×10-11 ergs cm-2 s-1 for a luminosity of 1.1 ×1034(D/2 kpc)2 ergs s-1. The X-ray spectrum is hard (photon index ≈1.2) and highly absorbed (NH ≈0.7 ×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/̇P = 2.3 ×10-3 yr-1, and a dynamic range of at least 100 in Lx. We discuss GRO J1008-57 data in the context of windaccretion models for this new member of the Be class of X-ray binaries.

Original languageEnglish
Pages (from-to)845-850
Number of pages6
JournalAstrophysical Journal
Volume437
Issue number2
DOIs
StatePublished - 20 Dec 1994

Keywords

  • Pulsars: individual (GRO J1008-57)
  • X-rays: stars

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