GCN Circular 29663

E. Troja (NASA/GSFC, UMCP), K. L. Page (U. Leicester), A. Lien (GSFC/UMBC),
M. Siegel (PSU), and P. Evans (U. Leicester) report on behalf of the Neil
Gehrels Swift Observatory Team:

At 03:08:33 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210318B (trigger=1037335).  Swift slewed immediately to the
burst.
The BAT on-board calculated location is
RA, Dec 264.227, -35.819 which is
   RA(J2000) = 17h 36m 55s
   Dec(J2000) = -35d 49' 08"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty).  The BAT light curve showed several overlapping
pulses with a duration of about 20 sec.  The peak count rate
was ~22000 counts/sec (15-350 keV), at ~3 sec after the trigger.

We have analysed 1.4 ks of XRT data for GRB 210318B, from 52 s to 1.5
ks after the  BAT trigger. The data comprise 584 s in Windowed Timing
(WT) mode (the first 9 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. The refined XRT position is RA,
Dec = 264.2328, -35.8220 which is equivalent to:

RA (J2000): 17 36 55.88
Dec(J2000): -35 49 19.2

with an uncertainty of 3.5 arcsec (radius, 90% confidence).

The light curve can be modelled with an initial power-law decay with an
index of alpha=0.71 (+0.03, -0.04), followed by a break at T+564 s to
an alpha of 1.23 (+0.18, -0.24).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index  of 1.78 (+/-0.09). The
best-fitting absorption column is  1.43 (+/-0.13) x 10^22 cm^-2, in
excess of the Galactic value of 7.1 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.69 (+0.27, -0.25)
and a best-fitting absorption column of 1.6 (+/-0.4) x 10^22 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 6.5 x 10^-11 (1.1 x 10^-10) erg cm^-2
count^-1.

A summary of the PC-mode spectrum is thus:
Total column:        1.6 (+/-0.4) x 10^22 cm^-2
Galactic foreground: 7.1 x 10^21 cm^-2
Excess significance: 4.0 sigma
Photon index:        1.69 (+0.27, -0.25)

If the light curve continues to decay with a power-law decay index of
1.23, the count rate at T+24 hours will be 0.011 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 6.9 x
10^-13 (1.1 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01037335.

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 57 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 with a 3-sigma upper limit of 20.1 mag.
No correction has been made for the large, but uncertain, extinction
expected.

Due to a temporary TDRSS outage, no automatic GCN notices or alerts were
promptly distributed.

Burst Advocate for this burst is E. Troja (eleonora@umd.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/)