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GCN Circular 22620

Subject
GRB 180410A: Swift detection of a burst or a Galactic transient.
Date
2018-04-10T08:27:45Z (6 years ago)
From
Kim Page at U.of Leicester <klp5@leicester.ac.uk>
J.D. Gropp (PSU), A. Deich (PSU), P. A. Evans (U Leicester),
C. Gronwall (PSU), J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC) and
K. L. Page (U Leicester) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 08:02:59 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180410A or a Galactic transient (trigger=824063).  
Swift slewed immediately to the source. 
The BAT on-board calculated location is 
RA, Dec 95.981, +12.808 which is 
   RA(J2000) = 06h 23m 56s
   Dec(J2000) = +12d 48' 28"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex structure
structure with a duration of about 100 sec.  The peak count rate
was ~800 counts/sec (15-350 keV), at ~26 sec after the trigger. 

The XRT began observing the field at 08:05:49.8 UT, 170.7 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
95.9570, 12.8110 which is equivalent to:
   RA(J2000)  = 06h 23m 49.68s
   Dec(J2000) = +12d 48' 39.4"
with an uncertainty of 2.1 arcseconds (radius, 90% containment). This
location is 84 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at http://www.swift.ac.uk/sper. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 6.56
x 10^21 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 1.55e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 179 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. The typical 3-sigma upper limit has been about 19.6 mag. 
The 8'x8' region for the list of sources generated on-board covers 100% of the
XRT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the large, but uncertain, extinction expected. 

Given the source's proximity (0.17 degrees) to the Galactic Plane, and
the fact that this is a BAT image trigger, we cannot rule out that
this may be a new Galactic Transient, rather than a GRB. If it is a
transient, we name it Swift J0623.9+1248. 

Burst Advocate for this burst is J.D. Gropp (jdg44 AT psu.edu). 
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.)
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