GRB 250312A

A. Melandri (INAF-OAR), V. D'Elia (SSDC & INAF-OAR),
J. J. DeLaunay (PSU), S. Dichiara (PSU), R. Gupta (NASA GSFC),
J. A. Kennea (PSU), M. J. Moss (GSFC), K. L. Page (U Leicester),
D. M. Palmer (LANL), B. Sbarufatti (INAF-OAB) and M. A. Williams (PSU)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 13:24:17 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250312A (trigger=1294765).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 29.510, -42.404 which is 
   RA(J2000) = 01h 58m 02s
   Dec(J2000) = -42d 24' 15"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  Only the first 8 seconds of the BAT light
curve are immediately available.  This limited data shows a complex peak
structure that extends beyond the T+8s cut-off. The peak count rate
was ~1800 counts/sec (15-350 keV), at ~0 sec after the trigger.

The XRT began observing the field at 13:26:33.0 UT, 135.4 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
29.51911, -42.39599 which is equivalent to:
   RA(J2000)  = 01h 58m 04.59s
   Dec(J2000) = -42d 23' 45.6"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 37 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 in excess of the Galactic value (1.79 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 3
(+2.65/-2.31) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 7.29e-09 erg cm^-2 s^-1 (0.2-10
keV).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 427 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
0.017.

Burst Advocate for this burst is A. Melandri (andrea.melandri AT inaf.it).
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, 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 250312A ( A. Melandri et al., GCN 39686
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 ) errorbox  15966 sec after notice time and 16003 sec after trigger time at 2025-03-12 17:51:00 UT, with upper limit up to  18.2 mag. Observations started at twilight.  The observations began at zenith distance = 54 deg. The sun  altitude  is -11.5 deg. 

The galactic latitude b = -69 deg., longitude l = 264 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2809737

We obtain a following upper limits.  

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

   16093 |         MASTER-SAAO |   C |   180 | 17.1 |        
   16292 |         MASTER-SAAO |   C |   180 | 18.2 |        
Filter C is a clear (unfiltred) band. 


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

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

Using 1241 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 250312A, 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 = 29.51954, -42.39605 which is equivalent
to:

RA (J2000): 01h 58m 4.69s
Dec (J2000): -42d 23' 45.8"

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.

D.N. Burrows (PSU), M. A. Williams (PSU), J.P. Osborne (U. Leicester),
K.L. Page (U. Leicester), A. D'Ai (INAF-IASFPA), A. Melandri
(INAF-OAR), T. Sbarrato (INAF-OAB), S. Dichiara (PSU) and P.A. Evans
report on behalf of the Swift-XRT team:

We have analysed 8.3 ks of XRT data for GRB 250312A, from 385 s to 34.7
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=0.51 (+/-0.06).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.80 (+0.22, -0.21). The
best-fitting absorption column is  1.3 (+0.7, -0.6) x 10^21 cm^-2, in
excess of the Galactic value of 1.8 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 3.9 x 10^-11 (4.7 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.3 (+0.7, -0.6) x 10^21 cm^-2
Galactic foreground: 1.8 x 10^20 cm^-2
Excess significance: 3.0 sigma
Photon index:	     1.80 (+0.22, -0.21)

If the light curve continues to decay with a power-law decay index of
0.51, the count rate at T+24 hours will be 0.022 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 8.5 x
10^-13 (1.0 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/01294765.

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

M. H.Siegel (PSU) and A. Melandri (INAF-OAR) on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250312A 464s after the BAT trigger (Melandri et al., GCN Circ. 39686
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 ). No optical afterglow consistent with the enhanced XRT position (Evans et al., GCN Circ. 39693
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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)         464         1563          165            >20.47
white              743         1563          223            >20.50
b                  718         1539           38            >18.84
b                 6529        33943         1081            >20.52
u                 6324        23538          403            >19.80
uvw1             21814        22713          885            >19.99
uvm2             10515        16904         1012            >19.85
uvw2              6939        28312         1082            >20.24

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.015 in the direction of the burst (Schlegel et al. 1998).

Jia-Cong Liu, Shao-Lin Xiong, Peng Zhang, Yan-Qiu Zhang,  Chen-Wei Wang (IHEP) report on behalf of the GECAM team:

GECAM-A detected a long burst, GRB 250312A, at 2025-03-12T13:24:17 UTC (denoted as T0), which was also observed by Swift/BAT (A. Melandri et al. 2025, GCN 39686
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 ). 

According to the GECAM-A light curves in about 50-200 keV, this burst mainly consists of a pulse with a T90 of about 5.0 ± 0.2 s.

The GECAM light curve can be found here:
https://www.bursthub.cn//admin/static/gecamgrb250312A.png

We note that these results are preliminary. Refined analysis will be reported.

Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) mission originally consists of two microsatellites (GECAM-A and GECAM-B) launched in Dec. 2020. 
As the third member of GECAM constellation, GECAM-C was launched onboard SATech-01 experimental satellite in July 2022. GECAM mission is funded by the Chinese Academy of Sciences (CAS).