[vsnet-grb-info 10656] Swift J185003.2-005627: Swift detection of a burst from a new Galactic transient

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  12083
SUBJECT: Swift J185003.2-005627: Swift detection of a burst from a new Galactic transient
DATE:    11/06/25 00:32:47 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

A. P. Beardmore (U Leicester), W. H. Baumgartner (GSFC/UMBC),
J. M. Gelbord (PSU), S. T. Holland (CRESST/USRA/GSFC),
J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL), C. Pagani (U Leicester),
D. M. Palmer (LANL), T. Sakamoto (NASA/UMBC), C. J. Saxton (UCL-MSSL),
M. H. Siegel (PSU) and C. A. Swenson (PSU) report on behalf of the
Swift Team:

At 00:06:07 UT, the Swift Burst Alert Telescope (BAT) triggered and
located an unknown source (trigger=456014).  Swift slewed immediately
to the burst. 
The BAT on-board calculated location is 
RA, Dec 282.551, -0.968 which is 
   RA(J2000) = 18h 50m 12s
   Dec(J2000) = -00d 58' 04"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peaked
structure with a duration of about 2 sec.  The peak count rate
was ~400 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 00:07:42.4 UT, 94.8 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 282.5134, -0.9410 which is equivalent to:
   RA(J2000)  = 18h 50m 03.21s
   Dec(J2000) = -00d 56' 27.5"
with an uncertainty of 5.4 arcseconds (radius, 90% containment). This
location is 166 arcseconds from the BAT onboard position, within the
BAT error circle. No event data are yet available to determine the
column density using X-ray spectroscopy. 

UVOT took a finding chart exposure of 250 seconds with the U filter
starting  263 seconds after the BAT trigger. No credible afterglow
candidate has been  found in the initial data products. The 2.7'x2.7'
sub-image covers 100% of the  XRT error circle. Because of the density
of catalogued stars, further analysis  is required to report an upper
limit for any afterglow in the sub-image.  Data from the list of
sources generated on-board  are not available at this time. 

Given the soft nature of the initial BAT detection
and the proximity to the Galactic plane (7 arcmin)
we believe that this is probably a previously-unknown
Galactic transient, rather than a cosmological GRB,
which we name 'Swift J185003.2-005627'. 

Burst Advocate for this burst is A. P. Beardmore (apb AT star.le.ac.uk). 
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/too.html.)