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RXTE Observations of GRO J1008-57

ATel #1304; J. Wilms (Dr. Karl Remeis-Observatory Bamberg), K. Pottschmidt (CRESST-UMBC/GSFC), C. Graemer (Remeis-Observatory), S. Roth (Remeis-Observatory), V. McBride (Southampton), P. Kretschmar (ESAC-ESA), I. Kreykenbohm (ISDC / IAA Tuebingen), S. Suchy (CASS, UCSD), R. E. Rothschild (CASS, UCSD), I. Caballero (IAA Tuebingen), G. Schoenherr (AIP), R. Staubert (IAA Tuebingen)
on 24 Nov 2007; 21:47 UT
Credential Certification: Joern Wilms (j.wilms@sternwarte.uni-erlangen.de)

Subjects: X-ray, Request for Observations, Binary, Neutron Star, Transient

Referred to by ATel #: 1619, 4577, 4759, 7490

Following the Swift/BAT announcement of a new strong outburst of the Be transient GRO J1008-57 (Krimm et al., ATEL #1298), the source was observed twice with RXTE, on 2007 November 22, 01:20UT (MJD 54426.001) for 560 seconds, and on 2007 November 23, 19:23UT (MJD 54427.808), for 2850 seconds.

Between the observations, the 2-10 keV flux rose from 2.09×10-9 erg cm-2 s-1 (104mCrab) to 2.47×10-9 erg cm-2 s-1 (125mCrab). The source was detected out to 70keV with the HEXTE-B detector, with a 15-50 keV source flux of 3.45×10-9 erg cm-2 s-1 (280mCrab) in the second observation.

GRO J1008-57 shows strong pulsations with a period of 93.75±0.20 seconds on 2007 November 23. This result indicates that the pulse period has remained constant since the previous outbursts reported by Shrader et al. (1999, ApJ 512, 920-928) and by Coe et al. (2007, MNRAS 378, 1427-1433). The pulse profile is consistent with that seen during previous outbursts, i.e., a double pulse in the 2-20keV PCA band and a single pulse profile in the HEXTE band.

The source spectrum softened slightly between the two observations. In the second observation, the phase averaged joint PCA-HEXTE data can be described by an absorbed (NH=(4.6±0.3)×1022 cm-2) and exponentially cutoff power law (photon index 1.06±0.03, folding energy 19.9±0.8keV, 90% confidence intervals). A weak iron line is present with an equivalent width of 47eV. The less than optimal reduced Χ2 of 2.3 (91dof) is mainly due to residuals around 5keV.

RXTE and Swift will continue to monitor GRO J1008-57 with pointed observations. Measurements in other wavebands are encouraged.