GRB 260503A

Xing-Hao Luo, Chen-Wei Wang, Shao-Lin Xiong (IHEP) report on behalf of GECAM team:

GECAM-B was triggered in-flight by GRB 260503A, at 2026-05-03T20:40:18.050 UTC (denoted as T0), which is also detected by Fermi/GBM (Fermi GBM team, GCN #44451
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 ).

According to the GECAM-B light curves in about 70-6000 keV, this burst mainly consists of multiple pulses with a duration (T90) of 21 +4/-3 s.

The GECAM-B light curve can be found here:
https://www.bursthub.cn//admin/static/gecambgrb260503A.png

Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) mission originally consists of two micro-satellites (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).

SVOM/GRM team: Xing-Hao Luo, Chen-Wei Wang, Yue Huang, Shi-Jie Zheng, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)

SVOM/ECLAIRs team: Marius Brunet (IRAP)

Report on behalf of the SVOM team:

SVOM/GRM was triggered in-flight by GRB 260503A (SVOM trigger reference: sb26050309) at 2026-05-03T20:40:20.000 UTC (T0), which is also detected by Fermi/GBM (Fermi GBM team, GCN #44451
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 ).

With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of multiple pulses with a T90 of 21 +7/-3 s in the 15-5000 keV band.

The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb260503A.png

In addition, the position of this burst, as determined by Fermi/GBM (RA=287.1 , DEC=-2.0, with a statistical uncertainty of 2.6 degrees, Fermi GBM team, GCN #44451
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 ), is located at about 87 degrees form the SVOM optical axis, which is outside the ECLAIRs field of view.

With this localization, the time-averaged spectrum from T0-3 to T0+17 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.12 +0.17/-0.16 and the cutoff energy, parameterized as Epeak, is 128 +21/-15 keV. The event fluence (10-1000keV) in this time interval is (6.65 +0.53/-0.46)E-06 erg/cm^2.

The localization of GRB 260503A in the 'Amati' relation diagram is shown at: 
https://www.bursthub.cn//admin/static/svgrb260503A_amati.png

The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. GRM is developed by the Institute of High Energy Physics (IHEP) of CAS.

The SVOM point of contact for this burst is: Xing-Hao Luo (luoxh@ihep.ac.cn)

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

At 20:40:17 UT on 3 May 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260503A (trigger 799533622.581348 / 260503861).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 287.1, Dec = -2.0 (J2000 degrees, equivalent to J2000 19h 08m, -2d 00'), with a statistical uncertainty of 2.6 degrees.

The angle from the Fermi LAT boresight is 30.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260503861/quicklook/glg_skymap_all_bn260503861.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/bn260503861/quicklook/glg_healpix_all_bn260503861.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260503861/quicklook/glg_lc_medres34_bn260503861.gif