[vsnet-grb-info 27442] GRB 210222B: Swift detection of a burst with an optical counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  29545
SUBJECT: GRB 210222B: Swift detection of a burst with an optical counterpart
DATE:    21/02/22 23:02:05 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

J.D. Gropp (PSU), J. A. Kennea (PSU), N. J. Klingler (PSU),
N. P. M. Kuin (UCL-MSSL), F. E. Marshall (NASA/GSFC),
M. J. Moss (GWU), D. M. Palmer (LANL) and B. Sbarufatti (PSU) report
on behalf of the Neil Gehrels Swift Observatory Team:

At 22:37:26 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210222B (trigger=1034325).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 154.621, -14.898 which is 
   RA(J2000) = 10h 18m 29s
   Dec(J2000) = -14d 53' 52"
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 ~1250 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 22:39:02.1 UT, 95.9 seconds after
the BAT trigger. Using promptly downlinked data we find an X-ray source
with an enhanced position: RA, Dec 154.6062, -14.9316 which is
equivalent to:
   RA(J2000)  = 10h 18m 25.50s
   Dec(J2000) = -14d 55' 53.7"
with an uncertainty of 2.1 arcseconds (radius, 90% containment). This
location is 131 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 (8.87 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 3
(+1.87/-1.66) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of nominal 150.000 seconds with the White
filter starting 100 seconds after the BAT trigger. There is a candidate
afterglow in the list of sources generated on-board at
  RA(J2000)  =	10:18:25.45 = 154.60606
  DEC(J2000) = -14:55:55.7  = -14.93215
with a 90%-confidence error radius of about 1.10 arc sec. This position is 2.0
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.65. No correction has been made for the expected extinction corresponding to
E(B-V) of 0.101. 

This source lies within the current (Sector 35) field-of-view of TESS camera 1. 

We note that this source is 32 arcseconds from the known ROSAT All-Sky 
X-ray Survey source 1RXS J101827.6-145604.  This is outside of the nominal 
joint error radius and so we believe that the ROSAT source is a 
coincidental alignment unrelated to the new GRB and its optical counterpart.