GRB 250331B

S. B. Cenko (GSFC), C. Gronwall (PSU), R. Gupta (NASA GSFC),
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 04:19:43 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250331B (trigger=1299965).  Swift slewed to the burst
after a brief delay to clear an observing constraint.
The BAT on-board calculated location is 
RA, Dec 71.381, +43.805 which is 
   RA(J2000) = 04h 45m 31s
   Dec(J2000) = +43d 48' 17"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peak
structure with a duration of about 8 sec.  The peak count rate
was ~1400 counts/sec (15-350 keV), at ~2 sec after the trigger.

The XRT began observing the field at 04:24:57.3 UT, 313.9 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 71.37364, 43.81080
which is equivalent to:
   RA(J2000)  = 04h 45m 29.67s
   Dec(J2000) = +43d 48' 38.9"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 28 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 does not constrain the column density.

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 317 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 large, but uncertain, extinction expected.

Burst Advocate for this burst is S. B. Cenko (brad.cenko 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/)

Damien Dornic (CPPM), Margarita Pereyra (UNAM), Jean-Grégoire Ducoin (CPPM), Jean-Luc Atteia (IRAP), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Dalya Akl (AUS), Camila Angulo (UNAM), Sarah Antier (OCA), Rosa L. Becerra (U Roma), Nathaniel R. Butler (ASU), Francis Fortin (IRAP), Ny Avo Rakotondrainibe (LAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM) report:

We imaged the field of Swift/BAT GRB 250331B (Cenko et al., GCN Circ. 39958
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 ) with the DDRAGO wide-field imager on the COLIBRÍ telescope at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir in Mexico.

We started observing at 2025-03-31 04:21:10 UTC (87 s after the trigger) and obtained a total of 100 s exposure in the i filter. The data were coadded with the COLIBRÍ pipeline and analyzed in STDWeb/STDPipe (Karpov 2025), with photometric calibration against Pan-STARRS DR1 and image subtraction against Pan-STARRS DR2. Our photometry is in the AB system and is not corrected for Galactic extinction.

At the XRT localization, we do not find any optical counterpart to a 3-sigma limiting magnitude of:
	i > 20.96
We warmly thank the COLIBRÍ and DDRAGO engineering teams and the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir.

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

Using 824 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 250331B, 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 = 71.37215, +43.81257 which is equivalent
to:

RA (J2000): 04h 45m 29.32s
Dec (J2000): +43d 48' 45.2"

with an uncertainty of 3.6 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.

V.Lipunov, E.Gorbovskoy, N.Tiurina,  P.Balanutsa, , D.Vlasenko, I.Panchenko,
A.Kuznetsov,  G.Antipov,  A.Sankovich, A.Sosnovskij, Yu.Tselik, M.Gulyaev, Ya.Kechin,
V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),
O.Gress, N.Budnev (ISU),
C.Francile,  F. Podesta, R.Podesta, E. Gonzalez  (Observatorio Astronomico Felix Aguilar (OAFA),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity)
D.Buckley (SAAO),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) 

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 250331B ( S. B. Cenko et al., GCN 39958
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 ) errorbox  47556 sec after notice time and 47612 sec after trigger time at 2025-03-31 17:33:15 UT, with upper limit up to  16.9 mag. Observations started at twilight.  The observations began at zenith distance = 85 deg. The sun  altitude  is -13.0 deg. 

The galactic latitude b = -1 deg., longitude l = 161 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2828939

We obtain a following upper limits.  

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

   47702 |         MASTER-SAAO |   C |   180 | 16.5 |        
   47902 |         MASTER-SAAO |   C |   180 | 16.7 |        
   48101 |         MASTER-SAAO |   C |   180 | 16.9 |        
Filter C is a clear (unfiltred) band. 


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

T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), J.A. Kennea (PSU), D.N.
Burrows (PSU), M. A. Williams (PSU), A.P. Beardmore (U. Leicester),
J.P. Osborne (U. Leicester), M.G. Bernardini (INAF-OAB) and P.A. Evans
report on behalf of the Swift-XRT team:

We have analysed 5.5 ks of XRT data for GRB 250331B, from 321 s to 35.1
ks after the  BAT trigger. The data are entirely in Photon Counting
(PC) mode. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.11 (+0.17, -0.14).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.2 (+0.9, -0.8). The
best-fitting absorption column is  2.1 (+1.5, -1.1) x 10^22 cm^-2, in
excess of the Galactic value of 6.6 x 10^21 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 5.7 x 10^-11 (1.6 x 10^-10) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.1 (+1.5, -1.1) x 10^22 cm^-2
Galactic foreground: 6.6 x 10^21 cm^-2
Excess significance: 2.2 sigma
Photon index:	     2.2 (+0.9, -0.8)

If the light curve continues to decay with a power-law decay index of
1.11, the count rate at T+24 hours will be 7.9 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.5 x
10^-14 (1.2 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01299965.

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

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

We have analysed 5.5 ks of XRT data for GRB 250331B, from 321 s to 35.1
ks after the  BAT trigger. The data are entirely in Photon Counting
(PC) mode. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.11 (+0.17, -0.14).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.2 (+0.9, -0.8). The
best-fitting absorption column is  2.1 (+1.5, -1.1) x 10^22 cm^-2, in
excess of the Galactic value of 6.6 x 10^21 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 5.7 x 10^-11 (1.6 x 10^-10) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.1 (+1.5, -1.1) x 10^22 cm^-2
Galactic foreground: 6.6 x 10^21 cm^-2
Excess significance: 2.2 sigma
Photon index:	     2.2 (+0.9, -0.8)

If the light curve continues to decay with a power-law decay index of
1.11, the count rate at T+24 hours will be 7.9 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.5 x
10^-14 (1.2 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01299965.

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

M. H. Siegel (PSU) and S. B. Cenko (GSFC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250331B
317 s after the BAT trigger (Cenko et al., GCN Circ. 39958
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 ).
No optical afterglow consistent with the XRT position (Osborne et al., GCN
Circ. 39974
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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           317          467          147         >19.5
white              317         1020          334         >19.9
v                  474         1070           78         >18.3
b                  572          764           39         >18.3
u                  547         1144           58         >18.6
w1                 523         1120           58         >18.0
m2                 670          690           19         >17.7
w2                 621         1046           58         >18.4

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.979 in the direction of the burst
(Schlegel et al. 1998).

S. Laha (GSFC/UMBC), S. D. Barthelmy (GSFC),
S. B. Cenko (GSFC),  R. Gupta (GSFC),
H. A. Krimm (NSF), A. Y. Lien (U Tampa),
C. B. Markwardt (GSFC), M. J. Moss (GSFC),
D. M. Palmer (LANL), T. Parsotan (GSFC),
D. Sadaula (GSFC/UMBC), T. Sakamoto (AGU)
(i.e. the Swift-BAT team):

Using the data set from T-240 to T+842 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 250331B (trigger #1299965)
(Cenko, et al., GCN Circ. 39958
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 ).  The BAT ground-calculated position is
RA, Dec = 71.377, 43.794 deg which is
   RA(J2000)  =  04h 45m 30.4s
   Dec(J2000) = +43d 47' 39.1"
with an uncertainty of 2.6 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 55%.

The mask-weighted light curve displays a single dim pulse.
The T90 (15-350 keV) is 3.95 +- 0.77 sec (estimated error including systematics).

The time-averaged spectrum from T-0.72 to T+3.76 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
2.61 +- 0.38.  The fluence in the 15-150 keV band is 1.5 +- 0.3 x 10^-07 erg/cm2.
The 1-sec peak photon flux measured from T+1.28 sec in the 15-150 keV band
is 0.9 +- 0.2 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level.
 
The results of the batgrbproduct analysis are available at
https://swift.gsfc.nasa.gov/results/batgrbcat/BAT_refined_circular/1299965