GRB 180221A

M. Stamatikos (OSU/NASA/GSFC), J. A. Kennea (PSU),
H. A. Krimm (NSF/USRA), A. Y. Lien (GSFC/UMBC),
K. L. Page (U Leicester), D. M. Palmer (LANL), M. H. Siegel (PSU) and
A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 06:30:44 UT, the Swift Burst Alert Telescope (BAT) triggered and
located possible GRB 180221A (trigger=811074).  Swift did not slew 
immediately due to an observational constraint. 
The BAT on-board calculated location is 
RA, Dec 326.772, -72.868 which is 
   RA(J2000) = 21h 47m 05s
   Dec(J2000) = -72d 52' 05"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of about 20 sec.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~2 sec after the trigger. 

Due to an observing constraint, Swift will not slew until T0+43.6
minutes. There will be no XRT or UVOT data until this time. 

Due to the marginal significance of this event (7.01 sigma)
and the proximity of Swift to the SAA at the time of the
trigger, we cannot confirm that this is an actual GRB. 
Observations with XRT will help to determine the
nature of this event. 

Burst Advocate for this burst is M. Stamatikos (Michael.Stamatikos-1 AT nasa.gov). 
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.)

E.Gorbovskoy, V. Lipunov, N.Tyurina, V.Kornilov, D.Zimnukhov, V.Vladimirov, 
A.Krylov, I.Gorbunov, P.Balanutsa, A.Kuznetsov, V.Chazov, D. Kuvshinov
(Lomonosov Moscow State University, SAI),

R.Podesta, C.Lopez, F. Podesta
(Observatorio Astronomico Felix Aguilar (OAFA),National University of San Juan, 
Argentina),

H. Levato, C. Saffe
(Instituto de Ciencias Astronomicas,de la Tierra y del Espacio (ICATE), San 
Juan, Argentina),

D.Buckley, S. Potter
(South African Astronomical Observatory),

R. Rebolo, M. Serra
(The Instituto de Astrofisica de Canarias),

O.Gres,  N.M.Budnev
(Irkutsk State University),

A. Tlatov, V.Senik, D. Dormidontov
(Kislovodsk Solar Station of the Pulkovo Observatory)

V.Yurkov, A.Gabovich, Yu.Sergienko
(Blagoveschensk Educational State University, Blagoveschensk),


MASTER-OAFA  robotic telescope (MASTER-Net: http://observ.pereplet.ru) located 
in Argentina was pointed to the Swift possible GRB 180221A (Stamatikos et al GCN 22427
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 )
  119 sec after notice time and 257 sec after trigger time at 2018-02-21 06:35:01 UT.
On our first (50s exposure)  set we didn't find optical transient  within 
SWIFT-BAT error-box (RA, Dec 326.772, -72.868, radius 3arcmin).

The 5-sigma upper limit has been about 18.3  mag at single (50 sec) and 
19.5  at coadd (310 sec) images.

The message may be cited.

====================================================================
The observations were made on
zenit distance 73 degrees,
The Sun  altitude  is -40.6 degree.
Dist. to Moon: 92.17 (Moon coord. 35.522966 d, 8.630816 d)

A. P. Beardmore (U. Leicester), C. B. Markwardt (GSFC), M. H. Siegel (PSU),
S. D. Barthelmy (GSFC), S. B. Cenko (GSFC), J. R. Cummings (CPI),
P. A. Evans (U Leicester), J. A. Kennea (PSU), H. A. Krimm (NSF/USRA),
A. Y. Lien (GSFC/UMBC), D. M. Palmer (LANL), T. Sakamoto (AGU),
M. Stamatikos (OSU), T. N. Ukwatta (LANL)
report on behalf of the Swift team:

Using the data set from the recent telemetry downlink, we report further
analysis of the possible GRB 180221A (trigger #811074)
(Stamatikos et al., GCN Circ. 22427
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 ).

Using the BAT data from T-239 to T+963 sec,
the BAT ground-calculated position is
RA, Dec = 326.787, -72.891 deg which is
   RA(J2000)  =  21h 47m 08.9s
   Dec(J2000) = -72d 53' 27.3"
with an uncertainty of 2.9 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 31%.

The BAT mask-weighted light curve shows a hint of a weak pulse that starts
at ~ T+2 s and ends at ~T+15 s. However, the mask-weighted light curve also
suggests a negative average count rate for the background interval (~T-200 s
to ~T0), which indicates incorrect background estimation that is likely caused
by the high count rates around the SAA. When performing the analysis without
subtracting the background image, the image significance of the pulse in the
15-350 keV drops to ~ 3.5 sigma, which is consistent with noise fluctuation.

Because of the uncertainty in the background estimation, it is difficult to
confirm the nature of the source with BAT data alone.

Swift-XRT observed the field of the possible GRB 180221A (BAT trigger
811074) in Photon Counting mode for 5.73 ks from 3.1 ks to 16.4 ks
after the trigger. We detect a candidate counterpart within the BAT
error circle with an enhanced XRT position (using the XRT-UVOT
alignment and matching UVOT field sources to the USNO-B1 catalogue) of
RA, Dec (J2000) = 326.77306, -72.86947, which is equivalent to:

RA (J2000):  21h 47m 05.53s
Dec(J2000): -72d 52' 10.1"

with an uncertainty of 2.2 arcsec (radius, 90% confidence).

Although faint, with a peak rate of 0.035 c/s (over the 0.3-10.0 keV
energy band), the source appears to be fading with a power-law decay
slope alpha = 0.57 +/- 0.31.

A spectrum formed from the PC mode data over this interval can be
fitted with an absorbed power-law with a photon index 1.8 (+0.5 -0.4)
and column density of 4.4(+14.2 -4.4)E20 cm^-2, in addition to the
Galactic column density of 3.98e20 cm^-2 in the direction of the
source. The observed 0.3-10 keV flux is 7.8E-13 erg cm^-2 s^-1.

The Swift/UVOT began settled observations of the field of the possible
GRB 180221A 2950 s after the BAT trigger.  No optical afterglow consistent
with the XRT position is detected in the initial UVOT exposures nor is any
other optical transient detected down to a 3-sigma upper limit of 21.4 in
the white filter.

Further Swift observations are planned to determine if the
source fades further.

The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/811074/BA/