Transient X-ray pulsar GRO J2058+42 is in outburst

ATel #1516; H. A. Krimm (CRESST/GSFC/USRA), S. D. Barthelmy (GSFC), W. Baumgartner (CRESST/GSFC/UMBC), J. Cummings (CRESST/GSFC/UMBC), E. Fenimore (LANL), N. Gehrels (GSFC), C. B. Markwardt (CRESST/GSFC/UMD), D. Palmer (LANL), A. Parsons (GSFC), T. Sakamoto (CRESTT/GSFC/UMBC), G. Sato (GSFC/ISAS), G. Skinner (CRESST/GSFC/UMBC), M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC)
on 10 May 2008; 21:05 UT
Credential Certification: Hans A. Krimm (Hans.Krimm@nasa.gov)

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The high-mass X-ray binary and pulsar GRO J2058+42 (also identified as CXOU J205847.5+414637) has entered a new period of activity. The Swift/BAT hard X-ray transient monitor detected an increase in 15-50 keV flux from the source beginning on May 2, 2008 (MJD 54588), when it was found to be at 0.003 ± 0.001 count/s/cm2 (13 mCrab). The level remained roughly steady until May 8, 2008, when it rose to 0.005 ± 0.001 count/s/cm2 (23 mcrab). A 2 ksec target of opportunity observation with Swift was made on May 9, 2008, beginning at 14:32:51 UT. The Swift/XRT found a bright point source at

RA = 20:58:47.301 (314.697087 deg) dec = +41:46:37.342 (41.777039 deg) (J2000)

with an error of 4 arc seconds (statistical plus systematic). This location is 2.2 arc seconds from the reported position of CXOU J205847.5+414637. The XRT found an average count rate of 0.645 ± 0.018 counts/s (0.3-10.0 keV), and a fit to an absorbed power law model with photon index 0.98 ± 0.16, nH = 0.9 ± 0.2 X 1022 cm-2, and flux of (1.45 ± 0.09) X 10-10 erg/cm2/s (chi-sq/dof = 71.1/57). This source was also detected in the Swift/UVOT at V=14.671 ± 0.006.

The BAT flux is well below the 300 mcrab level reported by CGRO/BATSE in 1995 (Wilson, Finger and Harmon, ApJ, 499, 820 (1998)), but consistent with the weaker 15 mcrab outbursts that were reported to follow the giant outburst and with the RXTE/ASM measurements from 1996 to 2002 (C.A. Wilson et al, ApJ, 622, 1024 (2005)).