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)

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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 (N_H=(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.