GCN Circular 40613

T. M. Parsotan (GSFC), R. Brivio (INAF-OAB), R. Gupta (NASA GSFC),
M. J. Moss (GSFC), D. M. Palmer (LANL) and C. Salvaggio (INAF-OAB)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 01:10:23 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250603A (trigger=1320335).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 129.918, -39.268 which is 
   RA(J2000) = 08h 39m 40s
   Dec(J2000) = -39d 16' 05"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of about 18 sec with a possible short 
spike at T0+16 sec.  The peak count rate was ~1,300 counts/sec 
(15-350 keV), at ~1 sec after the trigger.

The XRT began observing the field at 01:12:04.7 UT, 101.7 seconds after
the BAT trigger. No source was detected in the 2.5-s promptly available
image. We are waiting for the full dataset to detect and localise the
XRT counterpart.

Although the initial 2.5s of XRT data do not show a clear X-ray
afterglow, the high significance of the BAT image (7.82 sigma)
gives us confidence that this is an astrophysical source.
Further analysis awaits the full downlinked dataset.

We note the presence of Fermi-GBM event at ~T+10 minutes
(GBM trigger: 770606412) with a position consistent (to its
25° error radius) with this source location.

Given the location of the BAT detection at 1.42° from the Galactic
plane, persistent flux over 10 minutes would suggest a Galactic transient.
If so, we would name it Swift J0839.7-3916 .

Burst Advocate for this burst is T. M. Parsotan (tyler.parsotan AT nasa.gov).
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/)