GRB 210318A

E. Troja (NASA/GSFC/UMCP), C. Gronwall (PSU), J.D. Gropp (PSU),
N. J. Klingler (PSU), H. A. Krimm (NSF), A. Y. Lien (GSFC/UMBC),
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 03:54:31 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210318A (trigger=1037339).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 33.930, -37.789 which is 
   RA(J2000) = 02h 15m 43s
   Dec(J2000) = -37d 47' 21"
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 30 sec.  The peak count rate
was ~700 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 03:56:35.2 UT, 123.5 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 33.9263, -37.7772 which
is equivalent to:
   RA(J2000)  = 02h 15m 42.32s
   Dec(J2000) = -37d 46' 38.1"
with an uncertainty of 3.2 arcseconds (radius, 90% containment). This
location is 43 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.  We
cannot determine whether the source is fading at the present time. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 1.88
x 10^20 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 126 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
0.016. 

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

V. Lipunov, E. Gorbovskoy, V.Kornilov, N.Tyurina, P.Balanutsa, A.Kuznetsov, F.Balakin, 
V.Vladimirov, D. Vlasenko, I.Gorbunov, D.Zimnukhov, V.Senik, T.Pogrosheva,
D.Kuvshinov,  D. Cheryasov
(Lomonosov Moscow State University, SAI, Physics Department),

R. Podesta, C.Lopez, F. Podesta, C.Francile 
(Observatorio Astronomico Felix Aguilar OAFA),

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

D. Buckley 
(South African Astronomical Observatory),

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

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

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




MASTER-SAAO robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in South Africa (South African Astronomical Observatory) was pointed to the Swift GRB 210318A ( E. Troja et al., GCN 29662
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 ) errorbox  50452 sec after notice time and 50501 sec after trigger time at 2021-03-18 17:56:12 UT, with upper limit up to  19.7 mag. Observations started at twilight.  The observations began at zenith distance = 58 deg. The sun  altitude  is -14.2 deg. 

The galactic latitude b = -69 deg., longitude l = 248 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=1570922

We obtain a following upper limits.  

Tmid-T0  |          Site       |Filt.| Expt. | Limit| Comment
_________|_____________________|_____|_______|______|________

   50591 |         MASTER-SAAO |   C |   180 | 19.7 |        
Filter C is a clear (unfiltred) band. 


The observation and reduction will continue. 
The message may be cited.

J.P. Osborne, A.P. Beardmore, P.A. Evans and M.R. Goad (U. Leicester) 
report on behalf of the Swift-XRT team.

Using 1075 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 210318A, we find an astrometrically corrected X-ray
position (using the XRT-UVOT alignment and matching UVOT field sources
to the USNO-B1 catalogue): RA, Dec = 33.92621, -37.77693 which is equivalent
to:

RA (J2000): 02h 15m 42.29s
Dec (J2000): -37d 46' 36.9"

with an uncertainty of 2.4 arcsec (radius, 90% confidence).

This position may be improved as more data are received. The latest
position can be viewed at http://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).

This circular was automatically generated, and is an official product of the
Swift-XRT team.

B. Sbarufatti (PSU), J.P. Osborne (U. Leicester), K.L. Page (U.
Leicester), A.P. Beardmore (U. Leicester), P. D'Avanzo (INAF-OAB), M.G.
Bernardini (INAF-OAB), E. Ambrosi  (INAF-IASFPA) , D.N. Burrows (PSU),
J. D. Gropp (PSU) and E. Troja report on behalf of the Swift-XRT team:

We have analysed 3.1 ks of XRT data for GRB 210318A (Troja et al. GCN
Circ. 29662
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 ), from 107 s to 45.6 ks after the  BAT trigger. The data
comprise 9 s in Windowed Timing (WT) mode (taken while Swift was
slewing), with the remainder in Photon Counting (PC) mode. The enhanced
XRT position for this burst was given by Osborne et al. (GCN Circ.
29670
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 ).

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.09 (+0.27, -0.29).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.8 (+0.6, -0.5). The
best-fitting absorption column is  1.4 (+1.9, -1.2) x 10^21 cm^-2,
consistent with the Galactic value of 1.9 x 10^20 cm^-2 (Willingale et
al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum  is 3.8 x 10^-11 (4.6 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.4 (+1.9, -1.2) x 10^21 cm^-2
Galactic foreground: 1.9 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.8 (+0.6, -0.5)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01037339.

This circular is an official product of the Swift-XRT team.

T. Sakamoto (AGU), S. D. Barthelmy (GSFC), H. A. Krimm (NSF),
S. Laha (GSFC/UMBC), A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC),
D. M. Palmer (LANL), M. Stamatikos (OSU), E. Troja (NASA/GSFC, UMCP),
T. N. Ukwatta (LANL) (i.e. the Swift-BAT team):

Using the data set from T-60 to T+243 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 210318A (trigger #1037339)
(Troja, et al., GCN Circ. 29662
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 ).  The BAT ground-calculated position is
RA, Dec = 33.892, -37.755 deg which is 
  RA(J2000)  =  02h 15m 34.0s 
  Dec(J2000) = -37d 45' 17.7" 
with an uncertainty of 2.9 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 75%.

The mask-weighted light curves shows a single broad FRED-shaped peak with
some structure.  The emission starts at ~T10 sec, peaks at T+5 sec, and 
falls to background by T+50 sec.  T90 (15-350 keV) is 48.00 +- 22.63 sec 
(estimated error including systematics).

The time-averaged spectrum from T-12.37 to T+35.63 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.48 +- 0.33.  The fluence in the 15-150 keV band is 6.4 +- 1.3 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+11.13 sec in the 15-150 keV band
is 0.3 +- 0.3 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level. 

The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/1037339/BA/

M. H. Siegel (PSU) and E. Troja (NASA/GSFC/UMCP)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 210318A
127 s after the BAT trigger (Troja et al., GCN Circ. 29662
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 ).
No optical afterglow consistent with the XRT position
(Troja et al. GCN Circ. 29662
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 )
is detected in the initial UVOT exposures.
Preliminary 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first
finding chart (FC) exposure and subsequent exposures are:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag

white_FC           127          277          147         >20.5
u_FC               285          535          246         >19.9
white              127         1188          353         >21.0
v                  614         1064           58         >18.6
b                  541         1163           58         >19.3
u                  285          535          246         >19.9

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.017 in the direction of the burst
(Schlegel et al. 1998).