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Swift detection of an outburst from SGR 1935+2154

ATel #7123; P. D'Avanzo (INAF-OAB), D. N. Burrows (PSU), N. Gehrels (NASA/GSFC), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), F. E. Marshall (NASA/GSFC), T. Sakamoto (AGU) and B. Sbarufatti (INAF-OAB/PSU) report on behalf of the Swift Team:
on 22 Feb 2015; 14:56 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Paolo D'Avanzo (paolo.davanzo@brera.inaf.it)

Subjects: Optical, X-ray, Gamma Ray, Soft Gamma-ray Repeater

Referred to by ATel #: 7138

At 12:16:26 UT and 12:31:11 UT on 2015 February 22, the Swift Burst Alert Telescope (BAT) triggered twice on an outburst from SGR 1935+2154 (triggers=632158, 632159). Swift slewed immediately to the location. The BAT on-board calculated location is RA, Dec 293.724, +21.888 which is
RA(J2000) = 19h 34m 54s
Dec(J2000) = +21d 53' 16"
with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve for the first trigger shows a single-peaked structure with a duration of about 0.2 sec. The peak count rate was ~3200 counts/sec (15-350 keV), at ~0 sec after the trigger. The second light curve also shows a single-peaked structure with a duration of about 0.2 sec. The peak count rate was ~11,000 counts/sec (15-350 keV), at ~0 sec after the second trigger.

The XRT began observing the field at 12:17:48.1 UT, 82.0 seconds after the first BAT trigger. Using promptly downlinked data we find an X-ray source with an enhanced position: RA, Dec 293.7315, 21.8973 which is equivalent to:
RA(J2000) = 19h 34m 55.57s
Dec(J2000) = +21d 53' 50.4"
with an uncertainty of 2.3 arcseconds (radius, 90% containment). This location is 42 arcseconds from the BAT onboard position, within the BAT error circle. This position is consistent with the XRT position reported for this source in Cummings et al. (ATel #6294). A power-law fit to a spectrum formed from promptly downlinked X-ray event data gives a column density consistent with the Galactic value of 1.27 x 10^22 cm^-2 (Willingale et al. 2013, MNRAS, 431, 394).

UVOT took a finding chart exposure of 150 seconds with the White filter starting 86 seconds after the BAT trigger. No credible optical counterpart has been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. No correction has been made for the large, but uncertain extinction expected.

This source was discovered by Swift in July 5, 2014 (Cummings et al, ATel #6294). This is the first activity since that time.