[vsnet-grb-info 20978] GRB 180805B: Swift detection of a burst

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  23076
SUBJECT: GRB 180805B: Swift detection of a burst
DATE:    18/08/05 13:18:15 GMT
FROM:    Boris Sbarufatti at PSU  <bxs60 at psu.edu>

P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), N. P. M. Kuin (UCL-MSSL),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC),
A. Melandri (INAF-OAB) and B. Sbarufatti (PSU) report on behalf of the
Neil Gehrels Swift Observatory Team:

At 13:02:36 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180805B (trigger=851855).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 25.813, -17.503 which is 
   RA(J2000) = 01h 43m 15s
   Dec(J2000) = -17d 30' 11"
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 1.5 sec.  The peak count rate
was ~4000 counts/sec (15-350 keV), at ~0.1 sec after the trigger. 

The XRT began observing the field at 13:03:53.2 UT, 76.8 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
25.7814, -17.4926 which is equivalent to:
   RA(J2000)  = 01h 43m 07.55s
   Dec(J2000) = -17d 29' 33.2"
with an uncertainty of 2.1 arcseconds (radius, 90% containment). This
location is 114 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 (1.63 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.1
(+2.89/-2.44) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 1.14e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 84 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 expected extinction corresponding to E(B-V) of
0.02. 

Burst Advocate for this burst is P. D'Avanzo (paolo.davanzo AT brera.inaf.it). 
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.)