Swift confirmation of outburst from Swift J1922.7-1716

ATel #3567; J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), K. Yamaoka (AGU), S. Nakahira (RIKEN) and D. N. Burrows (PSU)
on 17 Aug 2011; 22:11 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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At 04:20 UT on August 13th, 2011, Swift began a target-of-opportunity observation of Swift J1922.7-1716, a transient reported to be in outburst by MAXI (Nakahira et al., ATEL #3548). Analyzing ~1ks of XRT data, we find the source is strongly detected at ~3.5 XRT counts/s. This corresponds to a flux of 1.2 +/- 0.6 x 10^-10 erg/s/cm^-2 (0.3 - 10 keV, uncorrected for absorption). The spectrum is well fit by an absorbed power-law model, with N_H = 1.6 (+3.6/-1.6) x 10²¹ cm^-2, and photon index = 2.21 (+1.15/-0.86). If the source is a BHC binary system as suggested by Falanga et al. (2006), then the it was in the hard state at the time of observation, we find no evidence of a thermal component in the spectrum, although we note that the observation is relatively short, and many of the spectral counts were removed to correct for pile-up.

This source brightness compares well to previous outburst brightness seen in Swift/XRT observations of this source taken in 2005 and 2006 (e.g. Falanga et al., 2006). Although no observations of this source have been taken by Swift/XRT since November 14th, 2006, we note that observations of Swift J1922.7-1716 taken between October 30th 2006 and November 13th 2006 appear to show that the source had entered quiescence after a prolonged outburst. This detection by XRT should therefore be considered confirmation of the MAXI detected outburst of Swift J1922.7-1716.

The recent outburst of Swift J1922.7-1716 is also detected in the Swift/BAT hard X-ray transient monitor. When integrating 8-day intervals, we find that the source was detected starting approximately July 17, 2011. It had a mean rate of 0.0015 +/- 0.0003 ct/s/cm² (15-50 keV) or ~7 mCrab through roughly Aug 9 and has dropped in flux somewhat since that time. This rate is comparable to the average BAT rate for this source in 2005.