GRB 250312B

The Fermi GBM team reports the detection of a likely LONG GRB

"At 20:36:56.64 UT on 12 March 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250312B (trigger 763504621/250312859).

The on-ground calculated location, using the Fermi GBM trigger data,
is RA = 180.86, Dec = -50.96 (J2000 degrees, equivalent to J2000 12h 3m, -50d 57'),
with a statistical uncertainty of 4.87 degrees.

The angle from the Fermi LAT boresight is 105 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250312859/quicklook/glg_skymap_all_bn250312859.png

The HEALPix FITS file, including the estimated localization systematic, can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250312859/quicklook/glg_healpix_all_bn250312859.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250312859/quicklook/glg_lc_medres34_bn250312859.gif"

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) started inspect of the Fermi GRB 250312B ( Fermi GBM team, GCN 39692
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 ) errorbox  1617 sec after notice time and 1650 sec after trigger time at 2025-03-12 21:04:27 UT, with upper limit up to  18.4 mag. The observations began at zenith distance = 25 deg. The sun  altitude  is -47.1 deg. 

The galactic latitude b = 11 deg., longitude l = 296 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2810182

We obtain a following upper limits.  

Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

    1681 | 2025-03-12 21:04:27 |         MASTER-SAAO | (11h 29m 04.39s , -48d 18m 30.7s) |   C |    60 | 18.2 |        
    3030 | 2025-03-12 21:26:57 |         MASTER-SAAO | (11h 29m 01.89s , -48d 17m 08.7s) |   C |    60 | 18.2 |        
    3110 | 2025-03-12 21:28:16 |         MASTER-SAAO | (12h 02m 31.64s , -51d 04m 13.5s) |   C |    60 | 18.0 |        
    3170 | 2025-03-12 21:28:16 |         MASTER-SAAO | (12h 02m 31.64s , -51d 04m 13.5s) |   C |   180 | 18.4 |  Coadd 
    3189 | 2025-03-12 21:29:35 |         MASTER-SAAO | (12h 02m 31.65s , -51d 04m 13.4s) |   C |    60 | 17.9 |        
    3268 | 2025-03-12 21:30:54 |         MASTER-SAAO | (12h 02m 31.60s , -51d 04m 13.0s) |   C |    60 | 17.9 |        
    3347 | 2025-03-12 21:32:13 |         MASTER-SAAO | (12h 02m 31.51s , -51d 04m 12.3s) |   C |    60 | 17.9 |        
Filter C is a clear (unfiltred) band. 


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

James DeLaunay (PSU), Samuele Ronchini (PSU), Aaron Tohuvavohu (Caltech), Gayathri Raman (PSU), Jamie A. Kennea (PSU), Tyler Parsotan (NASA GSFC) report: 

Swift/BAT did not localize GRB 250312B onboard (T0: 2025-03-12T20:36:56.64 UTC, Fermi trig 763504621) 

The Fermi notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; Tohuvavohu et al. 2020, ApJ, 900, 1). 

Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground. 

The BAT likelihood search, NITRATES (DeLaunay + Tohuvavohu 2022, ApJ, 941, 169), performed on the temporal window [T0-20 s, T0+20 s], detects the burst with a sqrt(TS) of 12.4 in a 8.192 s analysis time bin, starting at T0 - 2.048 s. 

Using the NITRATES analysis, parameter estimation was performed to obtain the localization of this burst in the form of a HEALPIX Multi-Order Coverage (MOC) skymap. This localization accounts for both statistical and systematic errors. More details in the creation and calibration of these maps will soon be published (DeLaunay et al. 2025. in prep)

The 90% credible area is 10,136 deg2 and the 50% credible area is 2,853 deg2.
The integrated probability inside the coded field of view is 1%. 

The NITRATES skymap is consistent with the Fermi localization reported in the final position notice (GCN 39692
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 ). The combined Fermi/GBM+NITRATES 90% credible area is 1,014 deg2 and the 50% credible area is 223 deg2.


A plot of the probability skymap can be viewed here:

[skymap_plot](https://guano.swift.psu.edu/trigger_report?id=763504651/#:~:text=Probability%20Skymap)

The probability skymap and joint skymap files can be downloaded from the links here

[skymap_fits_file](https://guano.swift.psu.edu/files/763504651/0_n_PROBMAP)

[joint_skymap_fits_file](https://guano.swift.psu.edu/files/763504651/0_n_JOINTMAP)

Instructions on how to read and manipulate this map can be found here:

https://guano.swift.psu.edu/documentation

More details about this burst can be found on the trigger report page here:

https://guano.swift.psu.edu/trigger_report?id=763504651

GUANO is a fully autonomous, extremely low latency, spacecraft commanding pipeline designed for targeted recovery of BAT event mode data around the times of compelling astrophysical events to enable more sensitive GRB searches. 

A live reporting of Swift/BAT event data recovered by GUANO can be found at: 
https://www.swift.psu.edu/guano/

L. Scotton (UAH) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 20:36:56.64 UT on 12 March 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250312B (trigger 763504621/250312859),
which was also localized by Swift/BAT-GUANO (J. DeLaunay et al. 2025, GCN 39708
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 ).
The Fermi GBM on-ground location was given in GCN 39692
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  and it is consistent 
with the Swift/BAT-GUANO localization skymap.

The angle from the Fermi LAT boresight is 105 degrees.

The GBM light curve consists of single emission with two over lapping pulses, 
with a duration (T90) of about 5.4 s (50-300 keV). The time-averaged spectrum 
from T0-1.0 to T0+5.5 s is best fit by a power law function with an exponential 
high-energy cutoff. The power law index is -1.1 +/- 0.1 and the cutoff energy,
parameterized as Epeak, is 260 +/- 50 keV.

The event fluence (10-1000 keV) in this time interval is
(2.7 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+0.003 s in the 10-1000 keV band is 4.4 +/- 0.3 ph/s/cm^2.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog:
https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html

For Fermi GBM data and info, please visit the official Fermi GBM Support Page:
https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/"

M. Dafcikova, J. Ripa, M. Kolar (Masaryk U.), A. Pal (Konkoly Observatory), N. Werner (Masaryk U.), M. Ohno, H. Takahashi (Hiroshima U.), L. Meszaros, B. Csak (Konkoly Observatory), N. Husarikova, F. Munz , M. Topinka, M. Duriskova, L. Szakszonova, J.-P. Breuer, F. Hroch (Masaryk U.), T. Urbanec, M. Kasal,  A. Povalac (Brno U. of Technology), J. Hudec, J. Kapus, M. Frajt (Spacemanic s.r.o), R. Laszlo, M. Koleda (Needronix s.r.o), M. Smelko, P. Hanak, P. Lipovsky (Technical U. of Kosice), G. Galgoczi (Wigner Research Center/Eotvos U.), Y. Uchida, H. Poon, H. Matake (Hiroshima U.), N. Uchida (ISAS/JAXA), T. Bozoki (Eotvos U.), G. Dalya (Eotvos U.), T. Enoto (Kyoto U.), Zs. Frei (Eotvos U.), G. Friss (Eotvos U.), Y. Fukazawa, K. Hirose (Hiroshima U.), S. Hisadomi (Nagoya U.), Y. Ichinohe (Rikkyo U.), K. Kapas (Eotvos U.), L. L. Kiss (Konkoly Observatory),  T. Mizuno (Hiroshima U.), K. Nakazawa (Nagoya U.), H. Odaka (Univ of Tokyo), J. Takatsy (Eotvos U.), K. Torigoe (Hiroshima U.), N. Kogiso, M. Yoneyama (Osaka Metropolitan U.), M. Moritaki (U. Tokyo), T. Kano (U. Michigan) -- the GRBAlpha collaboration.

The long-duration GRB 250312B (Fermi/GBM detection: GCN 39692
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 ; Swift/BAT-GUANO detection: GCN 39708
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 ) was observed by the GRBAlpha 1U CubeSat (Pal et al. 2023, A&A, 677, 40; https://ui.adsabs.harvard.edu/abs/2023A%26A...677A..40P/abstract).

The detection was confirmed at the peak time 2025-03-12 20:36:57.4 UTC. The T90 duration measured by GRBAlpha is 7.0 s and the overall significance during T90 reaches 10 sigma.

The light curve obtained by GRBAlpha is available here: https://grbalpha.konkoly.hu/static/share/GRB250312B_GCN.pdf

All GRBAlpha detections are listed at: https://monoceros.physics.muni.cz/hea/GRBAlpha/
GRBAlpha, launched on 2021 March 22, is a demonstration mission for a future CubeSat constellation (Werner et al. Proc. SPIE 2018). The detector of GRBAlpha consists of a 75 x 75 x 5 mm3 CsI scintillator read out by a SiPM array, covering the energy range from ~50 keV to ~1000 keV. To increase the duty cycle and the downlink rate, the upgrade of the on-board data acquisition software stack is in progress. The ground segment is also supported by the radio amateur community and it takes advantage of the SatNOGS network for increased data downlink volume.