GRB 260305A
GCN Circular 44408
Subject
GRB 260305A: likely detection by Fermi-LAT and the LAT gamma-ray position
Date
2026-04-25T12:00:14Z (6 days ago)
From
yixing@shao.ac.cn
Via
Web form
Yi Xing, Wenfei Yu (SHAO, CAS)
We report a likely detection of high-energy emission from GRB 260305A using Fermi-LAT data with our photon-cluster analysis method, which was initially applied to the search for gamma-ray flares from FRB 20240114A (ATel #16630; Xing et al. arXiv:2411.06996) and has successfully recovered approximately half of the previously reported LAT-detected GRBs with >1 GeV photons (doi:10.3847/1538-4357/ab1d4e).
The GRB was discovered by Fermi-GBM (trigger 794371303.18619 / 260305112; GCN 43920). We found five LAT photons arriving within a 27.0 s interval starting at 2026-03-05 02:41:52.640 UTC (T0 + 14.45 s), with the interval ending just before the end of the GBM T90 interval (T90 start = T0 + 0.768 s; T90 = 41.473 s; Fermi GBM Burst Catalog). The highest-energy photon in the cluster arrived at T0 + 33.42 s and has a photon energy of 1.6 GeV. Using LAT data from T0 to T0 + 100 s, we performed standard likelihood analysis and obtained a best-fit position at RA, Dec = 218.73, 35.49 (J2000), with a 68% containment statistical error radius of 0.22 deg, much smaller than the 2.4 deg statistical uncertainty reported in the GBM final real-time localization (GCN 43920). This best-fit position is consistent with, though near the edge of, the GBM localization region. The estimated photon flux and photon index above 100 MeV over the 100-s interval are (9.95 +/- 5.18) x 10^-6 ph cm^-2 s^-1 and -1.80 +/- 0.38, respectively, with a TS of 23.7.
GCN Circular 43921
Subject
Fermi GRB 260305A: Global MASTER-Net observations report
Date
2026-03-05T03:00:46Z (2 months ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
email
V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko,
G.Antipov, A.Sankovich, Yu.Tselik, 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.Sosnovskij (CrAO),
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 [1] located in South Africa (South African Astronomical Observatory) started inspect of the Fermi GRB 260305A ( Fermi GBM team, GCN 43920) errorbox 55 sec after notice time and 87 sec after trigger time at 2026-03-05 02:43:05 UT, with upper limit up to 18.6 mag. The observations began at zenith distance = 70 deg. The sun altitude is -22.7 deg.
The galactic latitude b = 68 deg., longitude l = 57 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=3169780
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
98 | 2026-03-05 02:43:05 | MASTER-SAAO | (14h 27m 04.56s , +37d 55m 52.3s) | C | 20 | 17.2 |
98 | 2026-03-05 02:43:05 | MASTER-SAAO | (14h 24m 55.60s , +38d 11m 54.7s) | C | 20 | 17.0 |
217 | 2026-03-05 02:44:55 | MASTER-SAAO | (14h 12m 08.18s , +34d 56m 29.5s) | C | 40 | 17.4 |
272 | 2026-03-05 02:44:55 | MASTER-SAAO | (14h 12m 08.18s , +34d 56m 29.4s) | C | 150 | 18.6 |
217 | 2026-03-05 02:44:55 | MASTER-SAAO | (14h 14m 11.70s , +34d 40m 22.7s) | C | 40 | 17.6 |
272 | 2026-03-05 02:44:55 | MASTER-SAAO | (14h 14m 11.71s , +34d 40m 22.8s) | C | 150 | 18.4 |
281 | 2026-03-05 02:45:54 | MASTER-SAAO | (14h 14m 11.94s , +34d 40m 26.1s) | C | 50 | 17.8 |
281 | 2026-03-05 02:45:54 | MASTER-SAAO | (14h 12m 08.41s , +34d 56m 34.1s) | C | 50 | 17.5 |
355 | 2026-03-05 02:47:03 | MASTER-SAAO | (14h 12m 08.69s , +34d 56m 39.2s) | C | 60 | 17.7 |
355 | 2026-03-05 02:47:03 | MASTER-SAAO | (14h 14m 12.16s , +34d 40m 30.0s) | C | 60 | 17.8 |
625 | 2026-03-05 02:51:33 | MASTER-SAAO | (14h 24m 26.43s , +33d 57m 36.0s) | C | 60 | 17.8 |
625 | 2026-03-05 02:51:33 | MASTER-SAAO | (14h 22m 22.92s , +34d 13m 45.5s) | C | 60 | 17.5 |
704 | 2026-03-05 02:52:52 | MASTER-SAAO | (14h 24m 26.68s , +33d 57m 40.9s) | C | 60 | 17.8 |
704 | 2026-03-05 02:52:52 | MASTER-SAAO | (14h 22m 23.24s , +34d 13m 52.3s) | C | 60 | 17.7 |
784 | 2026-03-05 02:54:11 | MASTER-SAAO | (14h 22m 23.58s , +34d 13m 57.5s) | C | 60 | 17.7 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
[1] - V.M. Lipunov, V.G. Kornilov, E.S. Gorbovskoy, N.A. Tiurina & A.S.Kuznetsov, 2023, Astronomical Robotic Networks and Operative Multichanel Astrophysics, Lomonosov MSU PRESS, 591pp.
http : // www.pereplet.ru/lipunov/625.html
GCN Circular 43920
Subject
GRB 260305A: Fermi GBM Final Real-time Localization
Date
2026-03-05T02:52:09Z (2 months ago)
From
Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply@GIOC.nsstc.nasa.gov>
Via
email
The Fermi GBM team reports the detection of a likely LONG GRB
At 02:41:38 UT on 5 Mar 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260305A (trigger 794371303.18619 / 260305112).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 216.5, Dec = 34.0 (J2000 degrees, equivalent to J2000 14h 26m, 34d 00'), with a statistical uncertainty of 2.4 degrees.
The angle from the Fermi LAT boresight is 49.0 degrees.
The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260305112/quicklook/glg_skymap_all_bn260305112.png
The HEALPix FITS file, including the estimated localization systematic, can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260305112/quicklook/glg_healpix_all_bn260305112.fit
The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260305112/quicklook/glg_lc_medres34_bn260305112.gif