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GCN Circular 22318

Subject
GRB 180111A: Swift detection of a burst with an optical counterpart
Date
2018-01-11T17:12:29Z (6 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
A. Y. Lien (GSFC/UMBC), A. P. Beardmore (U Leicester),
A. D'Ai (INAF-IASFPA), V. D'Elia (ASDC), A. Deich (PSU),
S. W. K Emery (UCL-MSSL), J.D. Gropp (PSU), J. A. Kennea (PSU),
N. P. M. Kuin (UCL-MSSL), K. L. Page (U Leicester),
D. M. Palmer (LANL), M. H. Siegel (PSU), A. Tohuvavohu (PSU) and
T. N. Ukwatta (LANL) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 16:42:06 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180111A (trigger=804692).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 149.841, +48.265 which is 
   RA(J2000) = 09h 59m 22s
   Dec(J2000) = +48d 15' 53"
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 80 sec.  The peak count rate
was ~11000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 16:43:34.1 UT, 87.8 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 149.77814,
48.26769 which is equivalent to:
   RA(J2000)  = 09h 59m 06.75s
   Dec(J2000) = +48d 16' 03.7"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 150 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 in excess of the Galactic value (1.08 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 5.7
(+3.91/-3.29) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 5.71e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White
filter  starting 97 seconds after the BAT trigger. An afterglow
candidate has  been found in the initial data products at RA=9:59:7.0
Dec=+48:16:03.3 which is RA=149.77938, Dec=48.26758 deg (J2000)  
matching the XRT position. This source lies in the wings of a bright 
star and we will provide a magnitude after further ground processing. 

Burst Advocate for this burst is A. Y. Lien (amy.y.lien 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/too.html.)
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