GCN Circular 27886

M. G. Bernardini (INAF-OAB), P. D'Avanzo (INAF-OAB),
J. A. Kennea (PSU), N. J. Klingler (PSU), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB),
K. L. Page (U Leicester), D. M. Palmer (LANL) and
T. Sbarrato (INAF-OAB) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 11:18:41 UT, the Swift Burst Alert Telescope (BAT) triggered and
located 1RXS J1708���4009 (trigger=975998).  Swift did not slew
to this previously-known source. 
The BAT on-board calculated location is 
RA, Dec 257.195, -40.127 which is 
   RA(J2000) = 17h 08m 47s
   Dec(J2000) = -40d 07' 36"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed an impulsive spike
structure with a tail out to T+4 seconds, followed by some additional 
weaker emission until T+15 s.  The peak count rate was ~14000 counts/sec 
(15-350 keV), at ~0 sec after the trigger. 

This is the first Soft Gamma Repeater (SGR)-like burst
reported from this Anomalous X-ray Pulsar (magnetar). 

BAT has not previously detected this source in on-board imaging.  
There have been no previous  bursts reported in the literature 
by other instruments (Pons and Rea, 2012, ApJ, 750, L6). 

Follow-up XRT observations are planned.