GCN Circular 33527

J.D. Gropp (PSU), S. Dichiara (PSU),
R. A. J. Eyles-Ferris (U Leicester), N. P. M. Kuin (UCL-MSSL),
F. E. Marshall (NASA/GSFC), M. J. Moss (GWU),
K. L. Page (U Leicester), D. M. Palmer (LANL) and M. H. Siegel (PSU)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 14:54:48 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 230328B (trigger=1162001).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 290.986, +80.016 which is 
   RA(J2000) = 19h 23m 57s
   Dec(J2000) = +80d 00' 58"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex GRB
structure with a duration of about 20 sec, followed by a bright flaring
afterglow visible below 25 keV for at least 200 s.  The peak count rate
was ~4375 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 14:56:38.8 UT, 110.3 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source located at RA, Dec 291.00418, 80.00951 which
is equivalent to:
   RA(J2000)  = 19h 24m 01.00s
   Dec(J2000) = +80d 00' 34.2"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 25 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 https://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 (6.28 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.6
(+1.76/-1.56) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 114 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	19:24:01.84 = 291.00765
  DEC(J2000) = +80:00:34.5  =  80.00959
with a 90%-confidence error radius of about 0.62 arc sec. This position is 0.7
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.22 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.070. 

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/)