GRB 260918A
GCN Circular 45641
Subject
GRB 260918A: SVOM/C-GFT optical upper limits
Date
2026-09-19T08:55:02Z (17 hours ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
Y.Y. Shi (GXU), Z. Kang (CHO), C. Wu , 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), Yun Wang (UCB; PMO), Yehao Cheng(YNU) report on behalf of the SVOM/C-GFT team:
We observed the field of GRB 260918A detected by SVOM/Eclairs (Zhao et al., GCN 45619) with LATIOS on SVOM/C-GFT. Our observations started at 2026-09-18T16:11:08 UTC, ~ 12.39 hr after the trigger.
We obtained follow-up imaging in g, r, and i bands. After preliminary analysis, no credible candidate was detected within the error box of source #1 and #2 provided by EP-FXT (Zou et al., GCN 45632) in our images. We measure the following upper limits calibrated against the Pan-STARRS DR1 catalog, and not corrected for Galactic extinction:
g > 20.5 AB (3-sigma, mid-time 13.56 hr after the trigger).
r > 21.1 AB (3-sigma, mid-time 13.24 hr after the trigger).
i > 20.5 AB (3-sigma, mid-time 12.67 hr after the trigger).
Our non-detection is consistent with the non-detections by Méndez et al. (GCN 45620), Yin et al. (GCN 45622), Li et al. (GCN 45634).
We note that in source #3 position of EP/FXT (Zou et al., GCN 45632), a known optical source (GR*244: Double or Multiple Star) is bright in PS1 DR1; in source #4 position, a known optical source (6dFGS gJ233834.0-090039 : Seyfert 1 Galaxy) is ~0.6 mag brighter than in the catalogue in b band.
We thank the observation assistants C.-L. Guo and Y.-H. 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 arcmin x 21 arcmin 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 45639
Subject
Fermi GBM Sub-Threshold Detection of GRB 260918A
Date
2026-09-18T19:07:04Z (a day ago)
From
Angus Jameson <abj0023@uah.edu>
Via
Web form
A. Jameson (UAH) reports on behalf of the Fermi-GBM Team:
SVOM ECLAIRs detected the GRB 260918A on 2016-09-18 at 03:47:36 UTC (GCN 45619). 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 no candidates.
The GBM Targeted Search [1], the most sensitive coherent search for GRB-like signals in GBM, identified a transient starting approximately 14 seconds after the ECLAIRs best image SNR time of 2026-09-18T03:47:15 UTC. The Targeted Search candidate was found most significantly on the 2.048 s timescale, with a false alarm rate of 1.3e-4 Hz. The Fermi-MET of the start time of this transient is 811396053.676 s. The Targeted Search localization has a spatial association probability of 71.3%, but has large uncertainties due to the weak nature of the transient.
Additionally, the GBM Targeted Search event was found with the highest significance using a "hard" spectrum (Comptonized function with Epeak = 1500 keV, alpha = -0.5) for a GRB.
The Targeted Search data release product for this event can be found here: https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/targeted_search/trigger_report.php?trigger_id=811396040&sample=SVOM_GCNs&hash=e9806eaf891b7de8
[1] Goldstein et al. 2019 arXiv:1903.12597
GCN Circular 45634
Subject
GRB 260918A: SVOM/VT optical upper limits
Date
2026-09-18T15:27:31Z (a day ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
H. L. Li, Y. N. Ma, C. Wu, L. P. Xin, Y. L. Qiu, Z. H. Yao, J. R. Xu, L. J. Chen, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, J. S. Deng, J. Y. Wei (NAOC), J. T. Palmerio (CEA/Irfu), Yun Wang (UCB; PMO) and Yehao Cheng(YNU) report on behalf of the SVOM/VT team.
SVOM/VT performed an automatic slew on the burst GRB 260918A triggered by SVOM/ECLAIRs (sb26091801, Zhao et al., GCN 45619). The observation started at 2026-09-18T03:53:19 UTC, 323 sec in VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
No credible candidates were identified in our single or stacked images within the EP-FXT (Zou et al., GCN 45632) and ECLAIRs (Zhao et al., GCN 45619) error box, compared with the Legacy Survey.
The following measurements are in the AB magnitude and are not corrected for Galactic extinction:
Mid time | Band | Exposure Time | 5-sigma upper limit
8.7 min VT_B 8*50 sec > 22.2 mag
8.7 min VT_R 8*50 sec > 22.1 mag
1.51 hr VT_B 56*50 sec > 23.1 mag
1.51 hr VT_R 55*50 sec > 23.0 mag
Our non-detection is consistent with the reports from SVOM/COLIBRÍ (Méndez et al., GCN 45620) and LCO (Yin et al., GCN 45622).
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 45632
Subject
GRB 260918A: EP-FXT follow-up observation
Date
2026-09-18T13:55:36Z (a day ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Z.-C. Zou (NJU), B. Zhang, J. P. Feng (USTC), G. Y. Zhao (QTNU), W. D. Zhang (NAO,CAS) on behalf of the Einstein Probe (EP) team:
EP-FXT performed a follow-up observation of the SVOM/ECLAIRs-detected burst GRB 260918A (SVOM/sb26091801; Zhao et al., GCN 45619) starting at 2026-09-18T05:52:20 UTC, approximately 2 hours after the SVOM trigger, with a total exposure time of 3004 s. Four uncataloged sources are detected within the ECLAIRs error circle. Preliminary analysis on these sources is automatically conducted, and the details are listed as follows.
EPF_J233857.5-085504
RA (J2000): 354.7406
Dec (J2000): -8.9179
Flux: 1.3736 x 10^-13 erg/s/cm2 (observed, 0.5-10 keV)
Flux_err: 2.7412 x 10^-14 erg/s/cm2 (1 sigma)
EPF_J233915.3-084941
RA (J2000): 354.8139
Dec (J2000): -8.8279
Flux: 7.3429 x 10^-14 erg/s/cm2 (observed, 0.5-10 keV)
Flux_err: 2.0415 x 10^-14 erg/s/cm2 (1 sigma)
EPF_J233943.4-085235
RA (J2000): 354.9304
Dec (J2000): -8.8773
Flux: 4.3974 x 10^-13 erg/s/cm2 (observed, 0.5-10 keV)
Flux_err: 6.4445 x 10^-14 erg/s/cm2 (1 sigma)
EPF_J233833.8-090039
RA (J2000): 354.6402
Dec (J2000): -9.0102
Flux: 1.533 x 10^-13 erg/s/cm2 (observed, 0.5-10 keV)
Flux_err: 3.6492 x 10^-14 erg/s/cm2 (1 sigma)
Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).
GCN Circular 45622
Subject
GRB 260918A: LCO optical upper limits
Date
2026-09-18T06:24:38Z (2 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
S. J. Yin, Y. X. Guan, Y.Y. Shi (GXU), A. Saccardi (CEA/Irfu), C. Wu, L.P. Xin (NAOC), Y. Wang (UCB; PMO), Y. Cheng (YNU), report on behalf of the SVOM mission team:
We observed the field of GRB 260918A (SVOM burst-id sb26091801) detected by SVOM (Zhao et al., GCN 45619) with the LCO 1m telescope at Teide Observatory equipped with the Sinistro instrument.
Our observation started on 2026-09-18 at 04:00:15.482 UT (about 12.3 min after the trigger) and we obtained 3x200 s exposures in the SDSS r and 3x200 s exposures in the Pan-STARRS z filters.
In the stacked image, we do not detect any new source within the SVOM/ECLAIRs error box, which is consistent with Méndez et al. (GCN 45620). We measure the following upper limits calibrated against the Pan-STARRS DR1 catalog, and not corrected for Galactic extinction:
r > 22.1 AB (5-sigma, mid-time 17.76 min after the trigger);
z > 20.3 AB (5-sigma, mid-time 25.92 min after the trigger).
This project is funded by the SVOM collaboration.
GCN Circular 45620
Subject
GRB 260918A: SVOM/COLIBRÍ (FM-GFT) optical upper limits
Date
2026-09-18T05:29:47Z (2 days ago)
From
enriquemm@astro.unam.mx
Via
Web form
Enrique Moreno Méndez (UNAM), Marion Guelfand (CPPM), Noémie Globus (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (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), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García-García (UNAM), Ramandeep Gill (UNAM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), Alan M. Watson (UNAM) and Yun Wang (UCB, PMO) report:
We imaged the field of the SVOM GRB 260918A (Zhao et al., GCN Circ. 45619) using the DDRAGO wide-field imager on the SVOM/COLIBRÍ (FM-GFT) telescope. We observed from 2026-09-18
03:49:13 to 05:05:28 UTC (from 0.03 to 1.32 hours after the trigger) and obtained 76 minutes of exposure in the r and z filters.
The data were reduced and coadded with the 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.
In the stacked image, we do not detect any credible new source at the ECLAIRs source position (Zhao et al., GCN Circ. 45619) down to the following 5-sigma limit:
r > 23.46,
z > 22.47.
Further observations are ongoing.
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.
GCN Circular 45619
Subject
GRB 260918A: SVOM detection of a burst
Date
2026-09-18T04:30:49Z (2 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
Donghua Zhao (NAOC), Clara Plasse (HKU), Yun Wang (UCB; PMO), Yehao Cheng(YNU), Diego Götz (CEA) report on behalf of the SVOM mission team:
At 2026-09-18T03:47:56 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 260918A (SVOM burst-id sb26091801).
The following trigger information was received on the ground with low latency by the SVOM VHF Alert Network.
The burst was only detected by the Image Trigger (IMT), which produced a sequence of 2 alerts. IMT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 7.60 in the [8-120] keV energy band over a time window of 40.96 seconds starting at 2026-09-18T03:47:15.
The localization of the best alert is R.A., Dec. 354.8016, -8.9544 degrees:
R.A. (J2000) = 23h39m12.39s
Dec. (J2000) = -8d57m15.73s
with a 90% confidence level (C.L.) radius of 10.33 arcmin (including systematic error of 2 arcmin added in quadrature).
SVOM slewed to the burst.
The SVOM/MXT team is waiting for the full dataset to detect and localise the possible SVOM/MXT counterpart.
SVOM/VT began observing the field after the slew. The observations will be reported in circulars.
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 Yun Wang: wangyun@pmo.ac.cn.
Please contact the BA by email if you require additional information.