- GCN Circular #43940
F. Magnani, A. Foisseau (CPPM), M. Brunet, O. Godet (IRAP), report on behalf of the SVOM mission team:
At 2026-03-07T13:45:26 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 260307B (SVOM burst-id sb26030704).
The following trigger information was received on the ground with low latency by the SVOM VHF Alert Network.
The burst was detected both by the Count-Rate Trigger (CRT) and the Image Trigger (IMT), which produced a sequence of 4 alerts. IMT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 9.92 in the [8-120] keV energy band over a time window of 40.96 seconds starting at 2026-03-07T13:45:06.
The localization of the best alert is R.A., Dec. 155.5760, -21.0620 degrees (J2000) with a 90% confidence level (C.L.) radius of 8.01 arcmin (including systematic error of 2 arcmin added in quadrature).
SVOM slewed to the burst.
SVOM/MXT began observing the field at 2026-03-07T13:48:16 UTC, 170 seconds after T0. Using onboard processed data we found an uncatalogued X-ray source located at R.A., Dec. 155.5216, -20.9972 degrees:
R.A. (J2000) = 10h22m05.19s
Dec. (J2000) = -20d59m49.81s
with a 90% C.L. radius of 43.83 arcseconds.
This location is 4.94 arcminutes from the ECLAIRs onboard position. This position may be improved as more data is received.
No optical observation could be performed by SVOM/VT for the time being.
The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. SVOM/ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. SVOM/GRM was developed by the Institute of High Energy Physics (IHEP) of CAS. SVOM/MXT was developed jointly by CNES, CEA-IRFU, CNRS-IJCLab, University of Leicester, MPE.
The Burst Advocate (BA) on shift for this alert is Francesco Magnani: magnani@cppm.in2p3.fr.
Please contact the BA by email if you require additional information.
- GCN Circular #43941
Z. Kang(CHO), C. Wu (NAOC), L.P. Xin, X.H. Han, P.P. Zhang, X.M. Lu (NAOC), Z.W. Li, Y. Lv (CHO), R.S. Zhang, Y.J. Xiao, Y.L. Qiu, J. Wang, J.S. Deng, L. Huang, J.Y. Wei (NAOC), F. Mag-nani, A. Foisseau (CPPM) report on behalf of the SVOM/C-GFT team:
We observed the field of GRB 260307B detected by SVOM/ECLAIRs (Magnani et al. GCN 43940) with LATIOS on SVOM/C-GFT. Observations started at 2026-03-07T13:47:15 UTC, ~109 seconds after the trigger.
We obtained i-band follow-up imaging. The target had a very low elevation. After preliminary analysis, no credible candidate was detected within the error box provided by SVOM/MXT (Magnani et al., GCN 43940) in our images.
The 5-sigma upper limits are:
i > 19.7 mag (AB) (5-sigma, mid-time 7.7 min after the trigger)
The photometry was calibrated against the UCAC4 catalogue, and no correction for Galactic dust extinction was applied. Further observations and analyses are ongoing.
We thank the observation assistants Chun-Lei Guo and Ying-Huai Hao at Jilin observatory for their excellent support.
The Chinese Ground Follow-up Telescope (C-GFT) for the SVOM mission is located at Jilin Station, Changchun Observatory, National Astronomical Observatories, CAS. It features two instruments: (1) CATCH at the Cassegrain focus with a 21 arcsec x 21 arcsec FOV for simultaneous g/r/i-band imaging, and (2) LATIOS, a 4k x 4k CMOS camera at the prime focus with a 1.28 deg x 1.28 deg FOV that images in g, r, and i bands via filter switching.
- GCN Circular #43942
J. T. Palmerio (CEA/Irfu), H. L. Li, L. P. Xin, Y. N. Ma, Y. L. Qiu, C. Wu (NAOC), A. Saccardi (CEA/Irfu), F. Magnani, A. Foisseau (CPPM)
on behalf of the SVOM/VT team.
After the trigger by SVOM/ECLAIRs at 2026-03-07T13:45:26 UTC (T0), SVOM performed an automatic slew on the burst location (Magnani et al., GCN 43940). SVOM/VT began observing the field at 2026-03-07T13:51:31, 364.31 seconds after T0, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
From a preliminary analysis of the 1-bit subimage and source list downloaded via VHF network, at least one credible candidate is identified, the details of which are presented below.
VT_ID 245:
This candidate was flagged as an uncatalogued fading source.
The position of this candidate is R.A., Dec. 155.5129, -21.0067 degrees, corresponding to:
R.A. (J2000) = 10h22m03.1s
Dec. (J2000) = -21d00m24.1s
with an uncertainty of 0.5 arcsec.
This location is within the R90 uncertainty region of the SVOM/MXT onboard localization.
The source was detected only in VT_R and faded beyond detection in the third sequence.
The candidate's magnitudes are:
| date-obs (UTC) | mid-time | exposure | band | mag(AB) |
| ------------------- | -------- | -------- | ---- | ---------- |
| 2026-03-07T13:51:31 | 6.1 min | 6*50 sec | VT_R | 21.3 ± 0.1 |
| 2026-03-07T13:56:31 | 11.1 min | 6*50 sec | VT_R | 21.8 ± 0.1 |
Magnitudes were not corrected for Galactic extinction.
The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe.
SVOM/VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS.
- GCN Circular #43943
V. Vijaykumar, A. Khade, V. Swain, S. Patil, A.P. Saikia, T. Mohan, V. Bhalerao (IITB), G.C. Anupama, S. Barway (IIA) and K. Angail (IAO) report on behalf of the GIT team:
We observed the field of GRB260307B detected by SVOM/ECLAIRs (SVOM, GCN 43940), with 0.7m GROWTH-India Telescope (GIT). We started the observation at 2026-03-07 15:51:15 (UTC), i.e., 2.1 hours after the trigger, and obtained 5x300 exposures in the r' filter. We did not detect any transient in our image. The photometric upper limit is as follows:
| MJD (mid) | Filter | tmid-t0 (hours) | Exposure Time (sec) | Upper limit (AB) |
| ------------ | ------ | --------------- | ------------------- | ---------------- |
| 61106.6694 | r' | 2.26 | 5x300 | 20.4 |
The measurement is calibrated against PanSTARRS DR1 (Chambers et al., 2016) and not corrected for Galactic extinction.
Our magnitude is consistent with other optical observations (Kang. et al., GCN 43941, Palmerio. et al., GCN 43942).
The GROWTH India Telescope (GIT; Kumar et al. 2022) is a 70-cm telescope with a 0.7-degree field of view, set up by the Indian Institute of Astrophysics (IIA) and the Indian Institute of Technology Bombay (IITB) with funding from DST-SERB and IUSSTF. It is located at the Indian Astronomical Observatory (Hanle), operated by IIA. We acknowledge funding by the IITB alumni batch of 1994, which partially supports the operations of the telescope. Telescope technical details are available at https://sites.google.com/view/growthindia/.
- GCN Circular #43945
Y. N. Ma, H. L. Li, Z. H. Yao, Y. L. Qiu, C. Wu, L. P. Xin, X. H. Han, Y. Xu, J. Wang, 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. Palmerio (CEA), F. Magnani, A. Foisseau (CPPM) report on behalf of the SVOM/VT team.
SVOM/VT performed an automatic slew on the burst GRB 260307B (sb26030704) triggered by SVOM/ECLAIRs (Magnani et al., GCN 43940). SVOM/VT began observing the field at 2026-03-07T13:50:41 UTC, 315.5 seconds after T0, in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously.
After quick analyzing the X band data avaliable, the VT-VHF candidate (Palmerio et al., GCN 43942) was not confirmed in either single or stacked X band images. Therefore, we conclude that the VT-VHF candidate is not a real source.
Also, no any uncatalogued sources are detected within the error range of MXT (Magnani et al., GCN 43940) down to ~22.5 AB magnitude at the mid time of 10.25 minutes in both VT_B and VT_R.
Considering the bright X-ray detection by MXT and non-detection by VT with auto slew, the GRB is an optically dark burst or a high-redshift candidate. Near infrared observations are encouraged.
Further data analysis is ongoing.
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.
- GCN Circular #43946
G. Corcoran (UCD), A. Martin-Carrillo (UCD), A. Saccardi (CEA/Irfu), D. B. Malesani (DAWN/NBI and Radboud), S. Bijavara Seshashayana (NOT and Malmo Univ.), report on behalf of a larger collaboration:
We observed the field of the SVOM GRB 260307B (Magnani et al., GCN 43940) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC instrument. Observations consisted of 6x200 s in the SDSS-z band and started at 22:29:21 UT on March 7 2026 (about 8.7 hr after the SVOM trigger). The sky conditions were modest, with mediocre and variable seeing (1.6" in our stack), and the imaging sequence had to be interrupted. The mid-time of our combined observation was 9.46 hr after the SVOM trigger.
Within the SVOM/MXT localisation region reported by Magnani et al. (GCN 43940), we detect a faint source which visually is brighter than its archival counterpart in the Legacy survey. Its coordinates are:
RA (J2000) = 10:22:02.65
Dec (J2000) = -21:00:02.6
with an estimated error of ~0.5".
In our stacked images we measure a magnitude (AB) of:
z = 22.47 +/- 0.33
This magnitude is calibrated using nearby objects from the Pan-STARRS catalog and is not corrected for Galactic extinction. Photometry of this object on the Legacy survey images yields z = 23.27 +/- 0.22 AB. The source was thus brighter in the NOT images at the 2-sigma level (by 0.8 +/- 0.4 mag).
Given the low significance of the detection, we cannot securely claim the counterpart identification, and we invite to double-check the presence of this candidate in other data sets.
- GCN Circular #43947
Guowang Du, Weikang Lin, Xueling Du, Donglin Gao, Xueteng Zhang, Yu Pan, Xingzhu Zou, Xinlei Chen, Brajesh Kumar, Yuan Fang, Jinghua Zhang, Xufeng Zhu, Tao Wang, Xinzhong Er, Yuanpei Yang, Xiangkun Liu, Xiaowei Liu (SWIFAR, YNU) report on behalf of the Mephisto Team:
The 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University, located at the Lijiang Observatory, was triggered at 2026-03-07T13:48:27 (~181 seconds after the SVOM/ECLAIRs trigger) to observe the field of SVOM GRB 260307B (Magnani et al., GCN 43940). A set of simultaneous multi-band (ugi and vrz) images was collected. In our stacked frames, no optical candidate was detected within the SVOM/ECLAIRs localization and at the position reported by Corcoran et al. (GCN 43946). The upper limits (3 sigma) are listed below, which are consistent with the non-detection previously reported in Kang. et al., GCN 43941, Palmerio. et al., GCN 43942, Vijaykumar et al., GCN 43943 and Ma et al., GCN 43945.
-----------------------------------------------------------------------------
Start_Time(UT) | Band | Exp(s) | LimMag (AB)
-----------------------------------------------------------------------------
2026-03-07T13:48:27 | u | 45.0*7,300.0*2 | >21.67
2026-03-07T13:51:28 | v | 45.0*8,300.0*2 | >21.94
2026-03-07T13:48:29 | g | 45.0*6,300.0*2 | >22.23
2026-03-07T13:51:30 | r | 45.0*8,300.0*2 | >22.54
2026-03-07T13:48:29 | i | 45.0*6,300.0*2 | >22.15
2026-03-07T13:51:30 | z | 45.0*8,300.0*2 | >21.31
-----------------------------------------------------------------------------
Mephisto (Multi-channel Photometric Survey Telescope) is a 1.6-m wide-field multi-channel telescope, the first of its type in the world, capable of imaging the same field of view in three optical bands simultaneously. The facility is operated by the South-Western Institute for Astronomy Research (SWIFAR), Yunnan University. It provides real-time, high-quality colors of stellar objects. The Mephisto mosaic cameras were installed in October 2025. The first light was achieved in all three channels on 10 October 2025, and presently, these are under the commissioning phase. All the data have been reduced by the Mephisto data processing pipeline. We note that the current data-processing pipeline is still at a preliminary stage, with flux calibration precision of ~2% in the u and v bands, ~1% in the g and r bands, and better than 1% in the i and z bands.
- GCN Circular #43948
R. Sonawane (IISER TVM), reports on behalf of the Fermi-GBM Team:
SVOM/ECLAIRs detected the GRB 260307B on 2026-03-07 at 13:45:26 UTC (Magnani et al. 2026, GCN 43940). There was no Fermi-GBM onboard trigger around this event time. An automated, blind search for gamma-ray bursts below the onboard triggering threshold in Fermi-GBM identified a candidate.
The GBM Targeted Search [1], the most sensitive coherent search for GRB-like signals in GBM, identified a transient starting at 2026-03-07T13:45:30.6 (UTC), most significantly on the 32 s timescale, with a false alarm rate of 2.5e-04 Hz. The Fermi-MET of this transient is 794583935.8 s. The Targeted Search localization is found to be spatially consistent with the SVOM/ECLAIRs localization.
Additionally, the GBM Targeted Search event was found with the highest significance using a
"soft" spectrum (Band function with Epeak = 70 keV, alpha = -1.9, beta = -3.7) for a GRB.
[1] Goldstein et al. 2019 arXiv:1903.12597
- GCN Circular #43953
M. Brunet, L. Bouchet (IRAP), F. Magnani, A. Foisseau (CPPM), report on behalf of the SVOM/ECLAIRs team.
Using the event-by-event data downloaded through the X-band ground station, we report further analysis of SVOM/ECLAIRs observations of GRB 260307B (SVOM burst-id sb26030704 – GCN 43940, trigger time T0 = 2026-03-07T13:45:26 UTC), which was also detected by Fermi/GBM (GCN 43948).
The burst that triggered ECLAIRs shows a single peak lightcurve lasting for around 41 s prior to the slew, followed by a featureless emission after the slew. The burst duration is estimated to be around 580 s in the 4-120 keV energy band through imaging.
The time-averaged spectrum before the slew (from T0-24 s to T0+17 s) in the energy range 5-120 keV is best fitted by a powerlaw model with a photon index of -1.29 +/-0.10. With this model, the 4-120 keV fluence is (9.4 +0.5/-1.4)e-7 erg/cm^2 and the 4-120 keV photon flux is 0.54 +0.02/-0.06 ph/cm^2/s.
The time-averaged spectrum after the slew (from T0+151 s to T0+560 s) in the energy range 5-120 keV is best fitted by a powerlaw model with an exponential cutoff. The photon index is -0.96 +0.37/-0.33 and the peak energy is 36 +38/-14 keV. With this model, the 4-120 keV fluence is (1.4 +0.1/-0.7)e-6 erg/cm^2 and the 4-120 keV photon flux is 0.12 +0.01/-0.06 ph/cm^2/s.
Using the best fit parameter from the spectrum before the slew to fit the spectrum after the slew does not give a good fit (chi²/dof: 21.1/12), which implies the presence of a spectral evolution with time.
All the quoted errors are at the 68% confidence level.
The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC.
The SVOM/ECLAIRs point of contact for this burst is: Marius Brunet (IRAP) (marius.brunet at utoulouse.fr)
- GCN Circular #44133
S. I. Chastain (TTU), G. E. Anderson (CSIRO), S. Ryder (Macquarie), A. Gulati (USyd), A. J. van der Horst (GWU), J. K. Leung (UofT/HUJI), and L. Rhodes (TSI/McGill) on behalf of the ATCA PanRadio GRB collaboration:
We observed the long GRB 260307B detected by SVOM (SVOM Team, GCN 43940) with the Australia Telescope Compact Array (ATCA) 0.9, 4.8, and 11 days post-burst.
No radio source was detected within the SVOM MXT localization (SVOM Team, GRB43940). At 8 GHz we find 3-sigma upper limits of 36, 33, and 30 microJy/beam, at 0.9, 4.8, and 11 days post-burst respectively.
We thank the CSIRO Space and Astronomy staff for supporting these observations.
We acknowledge the Gomeroi people as the traditional owners of the Observatory site. The Australia Telescope Compact Array is part of the Australia Telescope National Facility (https://ror.org/05qajvd42) which is funded by the Australian Government for operation as a National Facility managed by CSIRO.
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