TITLE:   GCN CIRCULAR
NUMBER:  40782
SUBJECT: GRB 250612A / Swift J1643.6−3854: Swift-BAT refined analysis
DATE:    25/06/18 22:43:10 GMT
FROM:    Rahul Gupta at NASA GSFC <rahul.gupta@nasa.gov>

H. A. Krimm (NSF), S. D. Barthelmy (GSFC),
R. Gupta (GSFC), S. Laha (GSFC/UMBC),
A. Y. Lien (U Tampa), C. B. Markwardt (GSFC),
M. J. Moss (GSFC), D. M. Palmer (LANL),
T. Parsotan (GSFC), D. Sadaula (GSFC/UMBC),
T. Sakamoto (AGU), M. H. Siegel (PSU)
(i.e. the Swift-BAT team):
 
Using the data set from T-240 to T+962 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 250612A (trigger #1323295) (Siegel, et al., GCN Circ. 40694).  
The BAT ground-calculated position is
RA, Dec = 250.873, -38.948 deg which is 
   RA(J2000)  =  16h 43m 29.6s 
   Dec(J2000) = -38d 56' 53.9" 
with an uncertainty of 4.3 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 64%.
 
The BAT mask-weighted light curve showed a complex structure with a duration of about 300 sec. 
T90 (15-350 keV) is 274.17 +- 66.54 sec (estimated error including systematics).
 
The time-averaged spectrum from T-0.58 to T+291.52 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
2.55 +- 0.43.  The fluence in the 15-150 keV band is 1.0 +- 0.2 x 10^-06 erg/cm2.
The 1-sec peak photon flux measured from T+0.62 sec in the 15-150 keV band
is 0.9 +- 0.2 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level. 

Given the BAT detection at a Galactic latitude of 4.6 deg and the absence of a detected X-ray afterglow in XRT observations ~ 17 hours post-trigger, a Galactic origin of this source remains plausible. If confirmed as a new Galactic transient, it would be designated Swift J1643.6−3854.

The results of the batgrbproduct analysis are available at
https://swift.gsfc.nasa.gov/results/batgrbcat/BAT_refined_circular/1323295