GRB 250331C

S. B. Cenko (GSFC), C. Gronwall (PSU), 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 04:37:40 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250331C (trigger=1299967).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 80.993, +33.029 which is 
   RA(J2000) = 05h 23m 58s
   Dec(J2000) = +33d 01' 43"
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 25 sec.  The peak count rate
was ~800 counts/sec (15-350 keV), at ~1 sec after the trigger.

The XRT began observing the field at 04:39:51.5 UT, 130.9 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 80.95740, 33.06310 which
is equivalent to:
   RA(J2000)  = 05h 23m 49.78s
   Dec(J2000) = +33d 03' 47.2"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 163 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 consistent with the Galactic value of 6.49
x 10^21 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 1.04e-09 erg cm^-2 s^-1 (0.2-10
keV).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 139 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 none of
the XRT error circle. 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/)

P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:

Using 1.6 ks of promptly downlinked XRT event data for GRB 250331C, we
find an enhanced XRT position of the afterglow: RA, Dec: 80.95640,
33.06317 which is equivalent to:
   RA (J2000)  = 05 23 49.54
   Dec (J2000) = +33 03 47.4
with an uncertainty of 1.9 arcseconds (radius, 90% confidence).
Analysis of the promptly available data is online at
http://www.swift.ac.uk/sper/1299967.

Position enhancement is described by Goad et al. (2007, A&A, 476, 1401)
and Evans et al. (2009, MNRAS, 397, 1177).

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

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 250331C (Cenko et al., GCN Circ. 39959
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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 04:43:50 UTC (370 s after the trigger) and obtained a total of 720 s of exposure time 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 (Evans et al. 39960), we do not find any optical counterpart to a 3-sigma limiting magnitude of:
	i > 22.26
	
We warmly thank the COLIBRÍ and DDRAGO engineering teams and the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir.

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

Using 1624 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 250331C, 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 = 80.95619, +33.06347 which is equivalent
to:

RA (J2000): 05h 23m 49.49s
Dec (J2000): +33d 03' 48.5"

with an uncertainty of 2.2 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 250331C ( S. B. Cenko et al., GCN 39959
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 ) errorbox  48174 sec after notice time and 48531 sec after trigger time at 2025-03-31 18:06:32 UT, with upper limit up to  18.0 mag. The observations began at zenith distance = 75 deg. The sun  altitude  is -20.0 deg. 

The galactic latitude b = -1 deg., longitude l = 174 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2828956

We obtain a following upper limits.  

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

   48622 |         MASTER-SAAO |   C |   180 | 17.9 |        
   48821 |         MASTER-SAAO |   C |   180 | 18.0 |        
Filter C is a clear (unfiltred) band. 


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

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

We have analysed 3.9 ks of XRT data for GRB 250331C, from 137 s to 62.7
ks after the  BAT trigger. The data comprise 95 s in Windowed Timing
(WT) mode with the remainder in Photon Counting (PC) mode. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=2.81 (+0.11, -0.10).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.11 (+0.11, -0.08). The
best-fitting absorption column is  consistent with the Galactic value
of 6.5 x 10^21 cm^-2 (Willingale et al. 2013). The PC mode spectrum has
a photon index of 1.65 (+0.32, -0.30) and a best-fitting absorption
column of 9.0 (+3.4, -2.5) x 10^21 cm^-2. The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum  is 5.9 x 10^-11 (8.6 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     9.0 (+3.4, -2.5) x 10^21 cm^-2
Galactic foreground: 6.5 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.65 (+0.32, -0.30)

If the light curve continues to decay with a power-law decay index of
2.81, the count rate at T+24 hours will be 3.2 x 10^-7 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.9 x
10^-17 (2.8 x 10^-17) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01299967.

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 250331C
140 s after the BAT trigger (Cenko et al., GCN Circ. 39959
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 ).
No optical afterglow consistent with the XRT position (Beardmore et al., 
GCN Circ. 39977
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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           140          289          147         >21.1
u_FC               297          547          246         >19.9
white              140         1717          411         >22.3
v                  628         1767          136         >19.1
b                  553         1863          133         >20.1
u                  297         1841          363         >20.1
w1                 676         1817          136         >19.1
m2                 652         1792          136         >19.8
w2                 603         1743          136         >20.0

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.921 in the direction of the burst
(Schlegel et al. 1998).

Giovanni Calapai at Calapai Astronomical Observatory, Massa S. Giorgio, (Messina) Italy 
Member of: GRB/UAI Gamma Ray Burst Section of Unione Astrofili Italiani. 

Report:

We imaged the field of GRB 250331C detected by Swift Burst Alert Telescope (BAT) (Cenko et al. GCN 39959
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 ),
with the 11 inches Schmidt-Cassegrain (Celestron 11) telescope F/D=6,3. 

The observations were started at 2025-03-31 20:59 UT (approximately 16.37 hours after burst) stacking a set of unfiltered CCD image. The observations were made with modest sky conditions and the target low on the horizon. 

We co-added 30 exposures of 60 sec each.

Start T0+      End T0+     CR lim
16.37 hour    16.92 hour    18.7

We did not found any optical counterpart in the error box of the XRTcandidate.
Evans GCN 39960
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Magnitudes were estimated with the PanSTARRS cat. and 
are not corrected for galactic dust extinction.

Our observations are consistent with other already reported Dornic et al. (GCN 39963
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 ), Lipunov et al. (GCN 39981
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 ), Siegel et al. (GCN 40003
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 ).

The message may be cited.

A. Y. Lien (U Tampa), S. D. Barthelmy (GSFC),
S. B. Cenko (GSFC), R. Gupta (GSFC),
H. A. Krimm (NSF), S. Laha (GSFC/UMBC),
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-239 to T+963 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 250331C (trigger #1299967)
(Cenko, et al., GCN Circ. 39959
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 ).  The BAT ground-calculated position is
RA, Dec = 80.963, 33.068 deg which is
   RA(J2000)  =  05h 23m 51.0s
   Dec(J2000) = +33d 04' 03.4"
with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 100%.

The mask-weighted light curve shows a dim pulse of emission with some possible
complex structure. The T90 (15-350 keV) is 70.10 +- 11.70 sec (estimated error
including systematics).

The time-averaged spectrum from T-12.59 to T+66.67 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.88 +- 0.23.  The fluence in the 15-150 keV band is 6.6 +- 1.0 x 10^-07 erg/cm2.
The 1-sec peak photon flux measured from T-3.19 sec in the 15-150 keV band
is 0.3 +- 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/1299967