Swift J1822.3-1606: A Probable New SGR in Ground Analysis of BAT Data

ATel #3488; J. R. Cummings (GSFC/UMBC), D. Burrows (PSU), S. Campana (INAF-OAB), J. A. Kennea (PSU), H. A. Krimm (GSFC/USRA), D. M. Palmer (LANL), T. Sakamoto (NASA/UMBC), S. Zane (MSSL-UCL) on behalf of the Swift Team
on 15 Jul 2011; 17:36 UT
Credential Certification: Jay R Cummings (jayc@milkyway.gsfc.nasa.gov)

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At 2011-07-14 at 12:47:47.1 UTC, Swift-BAT triggered (#457261) on a previously unknown source, Swift J1822.3-1606. This was at the same time as Fermi-GBM trigger #332340476. Only a subthreshold source was detected onboard. There were two subsequent rate increases of similar size, probably from the same source at about T+26 sec and T+308 sec, the latter also causing a rate trigger with no significant source found onboard (#457263). There were also 2 other rate triggers within the same pointing period (about 1650 seconds long), but no significant flux was observed from the source at these times. Each pulse was about 0.005 +- 0.001 sec long. All the emission was < 100 keV. Because of the repeated soft and short rate increases, and the location of the source near the Galactic plane, we tentatively classify this object as a new Soft Gamma-ray Repeater.

Combining BAT pulse-height events from the two rate triggers yields a highly significant source detection at RA, Dec 275.595, -16.100, which is:

RA (J2000) 18h 22m 22.8s

Dec (J2000) -16d 05' 59"

with an estimated error of 2.3 arcmin (estimated radius, 90% containment).

There was also a rate trigger (#457253) on a longer (~0.1 sec), larger, and harder event that was out of the BAT field of view at T-10754 sec. SWIFT J1822.3-1606 was above the horizon and out of the BAT FOV, and we cannot determine if the event was associated with the source or not.

The Swift/BAT hard X-ray transient archive was examined to search for previous emission from SWIFT J1822.3-1606 in daily averages. No significant excess was found back to 2011 June 15 with an average 1-sigma error in the 15-50 keV band of 0.0016 ct/s/cm2 (~7 mCrab). The history of Swift/BAT on-board source detections was examined and we found no previous rate-triggered or image- triggered detections of the source.

We have detailed data for two rate increases. A powerlaw fit to the combined spectrum has a photon index of 3.7 +- 0.5. A blackbody fit to the combined spectrum has a marginally better chisquared with a temperature of 4.9 +- 0.5 keV. The combined fluence of the two rate increases for which we have data, using the blackbody model, was (2.0 +- 0.4) x 10^-7 ergs/cm^2

A Swift TOO has been approved, but the source is Moon constrained until at least 17:34 UT on July 15. An observation is planned soon after the source comes out of constraint.