TITLE:   GCN CIRCULAR
NUMBER:  22696
SUBJECT: GRB 180510B: Swift detection of a burst with an optical counterpart
DATE:    18/05/10 21:22:21 GMT
FROM:    Boris Sbarufatti at PSU  <bxs60@psu.edu>

A. Tohuvavohu (PSU), A. Deich (PSU), J.D. Gropp (PSU),
J. A. Kennea (PSU), S. J. LaPorte (PSU), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester) and
B. Sbarufatti (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 20:15:42 UT, the Swift Burst Alert Telescope (BAT) triggered and
located possible GRB 180510B (trigger=831816). Due to an observing constraint
Swift did not slew immediately to the burst. 

The BAT on-board calculated location is 
RA, Dec 77.989, -62.335 which is 
   RA(J2000) = 05h 11m 57s
   Dec(J2000) = -62d 20' 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 20 sec.  The peak count rate
was ~800 counts/sec (15-350 keV), at ~5 sec after the trigger. 

The XRT began observing the field at 21:04:52.4 UT, 2950.1 seconds
after the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source located at RA, Dec 77.96962, -62.32356 which
is equivalent to:
   RA(J2000)  = 05h 11m 52.71s
   Dec(J2000) = -62d 19' 24.8"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 52 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 (2.54 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 4.8
(+3.31/-2.67) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 2953 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	05:11:52.64 =  77.96932
  DEC(J2000) = -62:19:26.3  = -62.32396
with a 90%-confidence error radius of about 0.76 arc sec. This position is 4.0
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.25 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.03. 

Burst Advocate for this burst is A. Tohuvavohu (aaronb AT swift.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.)