GRB 260316A — All Circulars

Francis Fortin (IRAP), Diego López-Cámara (UNAM), Jean-Grégoire Ducoin (CPPM), Antonio de Ugarte Postigo (LAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Missimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM) and Benjamin Schneider (LAM) report:

We imaged the field of the GRB 260316A / EP260316a (Jiang et al., GCN Circ. 44027, GCN Circ. 44034) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-03-18 07:20:28 to 09:50:38 UTC (from 42.8 to 45.3 hours after the trigger) and obtained 111 minutes of simultaneous exposure in the r and z filters.

The data were reduced and coadded with the COLIBRÍ pipeline and analysed with STDWeb/STDPipe (Karpov 2025) and the COLIBRÍ ASU pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.

We still detect the optical counterpart candidate that we previously reported in Fortin et al., GCN Circ. 44037. Template subtraction from 2026-03-17 to 2026-03-18 shows no significant residuals between the two epochs, and therefore does not allow us to conclude on the potential variability of this source.


We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM.

COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico.

H. L. Li, L. P. Xin, Y. N. Ma, C. Wu, Z. H. Yao, Y. L. Qiu, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. R. Xu, J. S. Deng, J. Y. Wei (NAOC), J. T. Palmerio (CEA/Irfu) report on behalf of the SVOM/VT team.

SVOM/VT performed ToO observation of the field of EP260316a detected by Einstein Probe (Jiang et al., GCN 44027, GCN 44034), and GRB 260316A with the sub-threshold detection by Fermi (Ravasio et al., GCN 44029). The observation started at 2026-03-16T22:31:53 UTC, i.e., about 9.98 hours post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously.

No uncatalogued optical source was detected in our stacked images. The 3-sigma upper limits are:

Mid_time         Band        Exposure Time     Magnitude (AB) 
10.35 hour       VT_R           37*70 sec       > 23.2  mag
10.35 hour       VT_B           39*70 sec       > 23.3  mag

The faint optical candidate reported by COLIBRÍ (Fortin et al., GCN 44037) was not detected in our stacked images.

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. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS.

Harsha K. H. (IUCAA), M. Tembhurnikar (IUCAA), S. Salunke (IUCAA), A. Arya (IITB), A. Goyal (IITB), G. Waratkar (Caltech/IITB), A. Vibhute (IUCAA), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:

Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al., 2021, JApA, 42, 73) showed the detection of a GRB 260316B which was also detected by Fermi GBM (Fermi GBM Team, GCN Circ. 44031), Calet (Trigger Num. 1457716347), and SVOM (Trigger Num. sb26031602).

The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve peaks at 2026-03-16 17:12:53.75 UTC. The measured peak count rate associated with the burst is 524 (+158, -53) counts/s above the background in the combined data of all quadrants, with a total of 818 (+139, -151) counts. The local mean background count rate was 306 (+5, -8) counts/s. Using cumulative rates, we measure a T90 of 4.9 (+1.7, -2.5) s. 

The source was also faintly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range.

CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project.

CZTI GRB detections are reported regularly on the payload site at:
http://astrosat.iucaa.in/czti/?q=grb

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

At 17:12:52 UT on 16 Mar 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260316B (trigger 795373977.681026 / 260316717).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 305.5, Dec = -79.3 (J2000 degrees, equivalent to J2000 20h 22m, -79d 17'), with a statistical uncertainty of 3.2 degrees.

The angle from the Fermi LAT boresight is 81.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260316717/quicklook/glg_skymap_all_bn260316717.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/bn260316717/quicklook/glg_healpix_all_bn260316717.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260316717/quicklook/glg_lc_medres34_bn260316717.gif

M. E. Ravasio (ICE-CSIC and Radboud Univ.), P.G. Jonker (Radboud Univ.) 
and 
E. Burns (LSU), R. Hamburg (USRA), and P. Veres (UAH)

report on behalf of the Fermi-GBM Team:

Fermi-GBM had full spatial and temporal coverage of the transient EP260316a detected by EP-WXT (EP Team GCN 44027). There was no Fermi-GBM onboard trigger around the corrected EP starting time at T0 = 2026-03-16T12:33:13 UTC.

The GBM Targeted Search [1], the most sensitive, coherent search for GRB-like signals, was run in the time interval [-50;+500] s from T0, seeking signals between 64 ms and 32.768 s in duration. A faint transient signal was found most significantly at ~T0-3.6 s on a 4 s timescale, with a false alarm rate of 6.5e-05 Hz, although there is evidence of an earlier start of the emission at T0-20 s. The localisation is consistent with the EP one, with a spatial association probability of 98.8%. 
Among the three spectral templates tested, the transient was best-fit with a“soft” spectral template  (Band function with Epeak = 70 keV, alpha = -1.9, beta = -3.7) for a GRB.

[1] Goldstein et al. 2019 arXiv:1903.12597

M. E. Ravasio (ICE-CSIC and Radboud Univ.), P.G. Jonker (Radboud Univ.) 
and 
E. Burns (LSU), R. Hamburg (USRA), and P. Veres (UAH)

report on behalf of the Fermi-GBM Team:

Fermi-GBM had full spatial and temporal coverage of the transient EP260316a detected by EP-WXT (EP Team GCN 44027). There was no Fermi-GBM onboard trigger around the corrected EP starting time at T0 = 2026-03-16T12:33:13 UTC.

The GBM Targeted Search [1], the most sensitive, coherent search for GRB-like signals, was run in the time interval [-50;+500] s from T0, seeking signals between 64 ms and 32.768 s in duration. A faint transient signal was found most significantly at ~T0-3.6 s on a 4 s timescale, with a false alarm rate of 6.5e-05 Hz, although there is evidence of an earlier start of the emission at T0-20 s. The localisation is consistent with the EP one, with a spatial association probability of 98.8%. 
Among the three spectral templates tested, the transient was best-fit with a“soft” spectral template  (Band function with Epeak = 70 keV, alpha = -1.9, beta = -3.7) for a GRB.

[1] Goldstein et al. 2019 arXiv:1903.12597