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GRB 260818B

GCN Circular 45459

Subject
GRB 260818B: Further EP-FXT observation
Date
2026-08-28T17:30:16Z (2 months ago)
From
Yu-Han Yang at University of Rome Tor Vergata <yyang@roma2.infn.it>
Via
Web form
Yu-Han Yang, Eleonora Troja (U Rome), Jun Yang (ZZU), Qinyu Wu (NAO, CAS) report:

We re-observed the field of the short GRB 260818B (SVOM/ECLAIRs; Cordier et al., GCN 45398) with EP-FXT through a ToO request. The observation started on 2026-08-27 at 16:48:47 UT (9.5 d post-trigger), with a total exposure of ~6 ks. 

Compared with the previous EP-FXT observation (Wu et al., GCN 45407), the X-ray source EPF_J025955.0+072535, located close to the radio source S10 (Schroeder et al., GCN 45418; Ricci et al. GCN 45452) and the candidate optical counterpart (Ducoin et al., GCN 45450; Yadav et al., GCN 45458), shows no significant variability.

No fading X-ray source is found within the 90% confidence region of the ECLAIRs localization (Cordier et al., GCN 45398). Expanding the search region to 5-sigma level, we identify a fading X-ray source EPF_J025906.7+072552 at R.A. = 44.7791 deg, Dec. = 7.4315 deg (J2000), with a localization uncertainty of 10 arcsec (radius, 90% C.L., statistical and systematic). The source lies 9.7 arcmin away from the center of the ECLAIRs localization.

We thank the EP team for scheduling and executing the ToO observation.

GCN Circular 45458

Subject
GRB 260818B: VLT optical and nIR observations
Date
2026-08-28T14:56:05Z (2 months ago)
From
muskan.yadav@students.uniroma2.eu
Via
Web form
Muskan Yadav, Massine El Kabir, Nicolás Monsalves, Hira Waseem, Yu-Han Yang and Eleonora Troja (U Rome) report on behalf of the ERC BHianca team:

We observed the field of short duration GRB 260818B detected by SVOM/ECLAIRs (Cordier et al., GCN 45398) with the FORS2 imager on the ESO VLT UT1 (Antu) and the HAWKI imager on the ESO VLT UT4 (Yepun). Both observations began 9.5 days after the trigger and were carried out at an average airmass ~1.2 and with an average seeing ~0.8 arcsec.

At the position of the candidate optical counterpart reported by COLIBRÍ (Ducoin et al., GCN 45450) we detect a faint (~24.7 AB) source in the z-band, but not in the J-band down to a 3-sigma limit >24.2 AB mag. Magnitudes are calibrated using nearby stars in the Pan-STARRS1 DR1 catalog (Chambers et al. 2016) and the 2MASS catalog (Skrutskie et al. 2006). Reported magnitudes are not corrected for Galactic extinction.

Our observations show that the optical source faded significantly between the two epochs, by at least ~2 mag between 6.5 d and 9.5 d after the burst. This would correspond to a power-law decay slope of ~-4.6, steeper than expected for standard GRB afterglows. 
The lack of a nIR detection suggests that its red color (r-z~1.8; Ducoin et al., GCN 45450) could be due to dust along the line of sight or, alternatively, that the late observations are dominated by the host galaxy light. 

Further observations are planned.

We thank the staff at the VLT for the rapid execution of these observations.

GCN Circular 45457

Subject
GRB 260818B: TNG NIR observations
Date
2026-08-28T10:09:39Z (2 months ago)
From
Riccardo Brivio at INAF-OAB <riccardo.brivio@inaf.it>
Via
Web form
R. Brivio (INAF-OAB), D. B. Malesani (DAWN/NBI and Radboud), M. Ferro (INAF-OAB), A. Rossi (INAF-OAS), A. Melandri (INAF-OAR), M. De Pasquale (U. of Messina), V. Lorenzi (INAF-TNG) report on behalf of the CIBO collaboration:

We observed the field of GRB 260818B detected by SVOM/ECLAIRs (Cordier et al., GCN 45398) with the Italian 3.6m TNG telescope equipped with the near-infrared camera NICS. A series of images were obtained with the J filter starting on 2026-08-28 at 02:19:43 UT (i.e. 9.26 days after the trigger). 

The transient candidate identified by the COLIBRI telescope (Ducoin et al., GCN 45450) is not detected in the co-added image. We measure a 3-sigma limiting magnitude of:

J > 21.9 (Vega; calibrated against the 2MASS catalogue),
 at a mid-time of 9.28 days after the trigger.

GCN Circular 45452

Subject
GRB 260818B: 6 GHz VLA observations
Date
2026-08-27T08:00:19Z (2 months ago)
From
Roberto Ricci at INAF-IRA <ricci@ira.inaf.it>
Via
Web form
R. Ricci, M. Yadav, E. Troja (U Rome) report:

We observed the field of short duration GRB 260818B, detected by SVOM (Cordier et al., GCN 45398) with the  Karl G. Jansky Very Large Array in C-band at the centre frequency of 6 GHz with a bandwidth of 4 GHz on August 25th 2026 at mid-time 13:37 UT(6.7 days after the trigger).

Data were calibrated and imaged using standard procedures in CASA (v6.6.1). 3C48 was used for flux density and bandpass calibration and J0321+1221 was used for complex gain calibration.

The full C-band field-of-view centred at the SVOM/ECLAIRs position (Cordier et al., GCN 45398) was imaged in CASA using the task tclean. We detect all the radio sources previously reported by Schroeder et al. (GCN 45418). 
S3 is detected at a flux of ~75 uJy and we cannot confirm it as a possible radio counterpart. Among the remaining sources, S5, S9 and S10 show a consistent fading trend across all epochs.  
S5 and S10 are coincident with catalogued sources and, given the relatively large beam size (11x9 arcsec), we cannot distinguish whether their variability is intrinsic or caused by the GRB afterglow.  However, we note that S10 lies close to the candidate optical afterglow (Ducoin et al., GCN 45450). 

Further observations are planned. We thank the VLA staff for executing the observations.

GCN Circular 45451

Subject
GRB 260818B: SVOM/VT optical upper limit and continuous observations
Date
2026-08-27T06:21:49Z (2 months ago)
Edited On
2026-08-28T11:45:11Z (2 months ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
H. L. Li, L. P. Xin, C. Wu, Y. L. Qiu, Y. N. Ma, Z. H. Yao, J. R. Xu, J. Wang, X. H. Han, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, J. S. Deng, J. Y. Wei (NAOC), J. T. Palmerio, Bertrand Cordier (CEA/Irfu) report on behalf of the SVOM/VT team.

SVOM/VT performed several ToO observations for GRB 260818B, which was triggered by SVOM/ECLAIRs (sb26081807, Cordier et al., GCN 45398; Yang et al., GCN 45405). This burst was also detected by SVOM/GRM (Ren et al., GCN 45411), HXMT (Wang et al., GCN 45408). The earliest observation started at 2026-08-18T23:33:18 UTC, about 3.4 hr post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously. Additional ToO observations were performed at 1.02 d and 7.5 d post trigger.

No credible candidate was found in the stacked images for each epoch, within the SVOM/ECLAIRs error box (Cordier et al., GCN 45398), the VLA radio sources (Schroeder et al., GCN 45418) and the Einstein Probe X-ray sources (Wu et al., GCN 45407). The typical 5-sigma upper limits are 23.0 mag in both channels. Our measurements are in AB magnitudes and are not corrected for Galactic extinction. This non-detection is consitent with the report from LCO (Shi et al., GCN 45399), SVOM/COLIBRÍ (Guelfand et al., GCN 45401), NOT (O'Neill et al., GCN 45403) and SVOM-CGFT (Guan et al., GCN 45417).

We noticed that there seemed to be an optical brightening trend for the VLA radio source S10 (Schroeder et al., GCN 45418). Its brightness in VT_R was 22.2+/-0.3 mag at 5.1 hr to 21.7+/-0.2 mag at day 7. The position of S10 is spatially coincident with EPF_J025955.0+072535 reported by EP-FXT (Wu et al., GCN 45407).

In the vicinity of S10, an optical candidate was proposed by SVOM/COLIBRÍ (FM-GFT) (Ducoin et al., GCN 45450). Some contributions from the COLIBRÍ candidate may be blended into our photometry, since the angular separation between the two sources is only 1.7 arcsec. Given the limited spatial resolution of VT and the faintness of the candidate (Ducoin et al., GCN 45450), we cannot confirm its detection. But the light curve of S10 shows some positive trend which needs to be further confirmed.

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 45450

Subject
GRB 260818B: SVOM/COLIBRÍ (FM-GFT) optical counterpart candidate
Date
2026-08-26T16:27:03Z (2 months ago)
From
J.-G. Ducoin at CPPM <ducoin@cppm.in2p3.fr>
Via
Web form
Jean-Grégoire Ducoin (CPPM), Marion Guelfand (CPPM), Diego López-Cámara (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), Francis Fortin (IRAP),  Leonardo García-García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), Alan M. Watson (UNAM) and Clare Plasse (HKU) report:

We continued imaging the field of the SVOM GRB 260818B (Cordier et al., GCN Circ. 45398) using the DDRAGO two-channel wide-field imager on the SVOM/COLIBRÍ (FM-GFT) telescope. We observed from 2026-08-25T07:59:01 to 11:12:53.575 UTC (from 155.80 to 158.95 hours after the trigger) and obtained 144 minutes of simultaneous exposure in the r and z filters.
The data were reduced and coadded with the COLIBRÍ ASU pipeline and analysed with STDWeb/STDPipe (Karpov 2025). 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 detect a possible optical transient (revealed by image subtraction using Legacy Survey as template) consistent with the ECLAIRs 4.6 arcmin error circle (Cordier et al., GCN Circ. 45398) at: 

  RA(J2000) = 02:59:54.374 = 44.97656 degrees
  Dec(J2000) = 07:25:28.24 = 7.424511 degrees

with an uncertainty of 0.5 arcsec.

The preliminary photometry of the transient candidate, measured on the unsubtracted images, is:

First epoch, 2026-08-19, 13.20 hr after the trigger:
  r = 24.24 +/- 0.15 mag
  z > 23.81 mag (3-sigma upper limit)

Second epoch, 2026-08-25, 157.38 h after the trigger:
  r = 24.60 +/- 0.29 mag
  z = 22.84 +/- 0.15 mag
  
At the first epoch, the transient candidate is brighter than the reference Legacy Survey source in the r band. At the second epoch, it is also brighter in the z band, showing a clear brightening relative to the first epoch.

The source is located 11.1 arcseconds from the X-ray source EPF_J025955.0+072535 reported by Wu et al. (GCN Circ. 45407), and 1.9 arcseconds from the radio source S10 reported by Schroeder et al. (GCN Circ. 45418).

We also retrieved an r-band template image from the Hyper Suprime-Cam Legacy Archive. A source is visible at the position of the optical transient candidate and may correspond to its host galaxy.

At this stage, we cannot draw firm conclusions regarding the nature of the transient or a possible association with GRB 260818B. 

Further observations are ongoing. 

We encourage additional follow-up, particularly at redder wavelengths, to better constrain the nature of this source and assess its possible association with GRB 260818B.


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 45418

Subject
GRB 260818B: VLA radio observations
Date
2026-08-21T22:41:23Z (2 months ago)
From
Genevieve Schroeder at Cornell University <genevieveschroeder@u.northwestern.edu>
Via
Web form

G. Schroeder (U. Toronto), W. Fong (Northwestern), J. Rastinejad (UMD) report:

We observed the location of the short-duration GRB 260818B (Cordier et al., GCN 45398

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; Wang et al., GCN 45408; Ren et al, GCN 45411) with the Karl G. Jansky Very Large Array (VLA) in D configuration under program 26A-062 (PI Schroeder) for two 0.75 hour epochs at a mean frequency of 6 GHz and mid times of 2026 Aug 20 at 07:58 UT (1.5 days post-burst) and 2026 Aug 21 at 11:23 UT (2.6 days post-burst). The images reached typical rms values of 7-10 uJy.

In preliminary analysis, we detect 13 >= 5-sigma radio sources within the ECLAIRs error region across both epochs. Of them, 9 appear to vary on the 1 sigma level across the two epochs, only 1 of which appears to have faded by 3 sigma across the two epochs (S3). The details of the variable sources are as follows:

SourceR.A.Dec.Error (arcsec)Fnu e1 (uJy)Fnu e2 (uJy)Notes
S202:59:56.59+07:29:28.52.383107Close to apparent galaxy
S302:59:36.59+07:28:16.03.09634Faded by >= 3sigma
S402:59:51.45+07:27:41.85.26390Close to apparent galaxy
S503:00:00.19+07:27:23.40.412661200In VLASS/RACS
S702:59:38.44+07:26:20.52.711491
S802:59:49.96+07:25:46.93.5168110
S902:59:50.37+07:25:41.04.016991
S1002:59:54.25+07:25:28.70.312321139Coincident with EPF_J025955.0+072535, in VLASS/RACS
S1102:59:44.77+07:25:25.01.1200178Close to apparent galaxy

Additionally, we note that EPF_J025955.0+072535 and EPF_J025936.6+072418 (Wu et al., GCN 45407

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) are coincident with long-lived radio sources present in VLASS and RACS (which are present in our observations as well). There is a faint radio source coincident with EPF_J025951.6+072917, however the flux density doesn’t vary significantly over the two epochs (~50 uJy).

We thank the VLA staff for quickly approving and executing these observations.

GCN Circular 45417

Subject
GRB 260818B: SVOM/C-GFT optical upper limit
Date
2026-08-21T16:21:24Z (2 months ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
Y.X Guan(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), Bertrand Cordier (CEA), Hatsune Goto (RIKEN, CEA) , report on behalf of the SVOM/C-GFT team:
 
We observed the field of GRB 260818B detected by SVOM/ECLAIRs (Bertrand Cordier et al., GCN 45398)， HXMT (Wang et al., GCN 45408), and SVOM/GRM (Ren et al., GCN 45411) with LATIOS on SVOM/C-GFT. Observations started at 2026-08-20T17:09:40.28 UTC, ~1.87 days after the trigger. 
 
We obtained i-band follow-up imaging. After preliminary analysis, no credible candidate was detected within the error box provided by EP-FXT (Wu et al., GCN 45407) in our images. We measure the following upper limit calibrated against the Pan-STARRS DR1 catalog, and not corrected for Galactic extinction:

i > 19.9 AB (3-sigma, mid-time 1.88 days after the trigger).

This is consistent with the non-detection reported by Shi et al.(GCN 45399) and Neill et al.(GCN 45403)

We thank the observation assistants Y.-H. Hao and C.-L. Guo 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 45411

Subject
GRB 260818B: SVOM/GRM observation
Date
2026-08-20T09:16:25Z (2 months ago)
From
renyz16607@163.com
Via
Web form
SVOM/GRM team: Yang-Zhao Ren, Chen-Wei Wang, Shi-Jie Zheng, Yue Huang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)

SVOM/ECLAIRs team: Sebastien Guillot (IRAP)
Report on behalf of the SVOM team:

SVOM/GRM was triggered in-flight by GRB 260818B at 2026-08-18T20:11:12.700 UTC (T0), which is also detected by SVOM/ECLAIRs (Cordier et. al., GCN #45398) and HXMT (Wang et. al., GCN #45408).

With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of a pulse with a T90 of 0.16 +0.12/-0.04 s in the 15-5000 keV band.

The SVOM/GRM light curve can be found here:
https://www.bursthub.cn/gcn/2026/svgrb260818B.png

In addition, the position of this burst, as determined by SVOM/ECLAIRs (RA= 44.9407, Dec=7.4547, ERR= 4.60 arcmin, GCN #45398), is located at about 29 degrees from the SVOM optical axis. 

With this localization, the time-averaged spectrum from T0-0.08 to T0+0.12 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.00 +0.22/-0.23 and the cutoff energy, parameterized as Epeak, is 70 +7/-6 keV. The event fluence (10-1000 keV) in this time interval is (2.67 +0.18/-0.16)E-07 erg/cm^2.

The localization of GRB 260818B in the 'Amati' relation diagram is shown at: 
https://www.bursthub.cn/gcn/2026/svgrb260818B_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: Yang-Zhao Ren (IHEP)(renyz@ihep.ac.cn)

GCN Circular 45408

Subject
GRB 260818B: Insight-HXMT detection
Date
2026-08-20T05:32:14Z (2 months ago)
From
Chenwei Wang at IHEP <cwwang@ihep.ac.cn>
Via
Web form
Chen-Wei Wang, Yang-Zhao Ren, Shao-Lin Xiong, Cheng-Kui Li, and Chao Zheng report on behalf of the Insight-HXMT team:

At 2026-08-18T20:11:12.600 (T0), Insight-HXMT/HE detected the short burst GRB 260818B, which is also detected by SVOM/ECLAIRs (Bertrand Cordier et al., GCN#45398) and SVOM/GRM.
The Insight-HXMT/HE light curve mainly consists of a narrow pulse with a T90 of 0.15 +0.15/-0.10 s. 

The HXMT/HE light curve can be found here:
https://www.bursthub.cn/gcn/2026/hxmtgrb260818B.png

All measurements above are made with the CsI detectors operating in the regular mode with the energy range of about 60-900 keV (deposited energy). Only gamma-rays with energy greater than about 200 keV can penetrate the spacecraft and leave signals in the CsI detectors installed inside of the telescope. 

Insight-HXMT is the first Chinese space X-ray telescope, which was funded jointly by the China National Space Administration (CNSA) and the Chinese Academy of Sciences (CAS). More information about it could be found at: http://www.hxmt.org.

GCN Circular 45407

Subject
GRB 260818B: EP-FXT follow-up observation
Date
2026-08-20T05:17:32Z (2 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Q. Y. Wu (NAO, CAS), H. Zhou, Y. L. Hua (PMO, CAS), W. D. Zhang (NAO, CAS) on behalf of the Einstein Probe (EP) team:

EP-FXT performed a follow-up observation of GRB 260818B detected by SVOM/ECLAIRs (Cordier et al., GCN 45398). The observation started at 2026-08-18 22:01:25(UTC), about 1.84 hr after the SVOM trigger, with a total exposure time of around 6.0 ks.

On-ground analysis of the FXT data found several uncataloged sources inside the ECLAIRs error region
Name | RA | DEC | Sep_obj (arcmin) | Flux (erg/cm^2/s)
EPF_J025951.6+072917 | 44.9651 | 7.4881 |  2.47 | 4.4e-14
EPF_J025955.0+072535 | 44.9791 | 7.4263 |  2.85 | 4.5e-14
EPF_J025936.6+072418 | 44.9024 | 7.4049 |  3.76 | 1.5e-12
EPF_J025950.9+073127 | 44.9619 | 7.5242 |  4.36 | 9.8e-14
with uncertainties of 10 arcsec (radius, 90% C.L. statistical and systematic). None of these sources showed significant variability during this observation.

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 45405

Subject
GRB 260818B: SVOM/ECLAIRs refined analysis
Date
2026-08-19T20:28:53Z (2 months ago)
Edited On
2026-08-20T15:43:01Z (2 months ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
H. Yang, O. Godet, S. Guillot (IRAP),  Bertrand Cordier, S. Schanne (CEA), Hatsune Goto (RIKEN, CEA) 
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 the short burst GRB 260818B (SVOM burst-id sb26081807, GCN 45398, trigger time T0 = 2026-08-18T20:11:12.618 UTC). 

The burst that triggered ECLAIRs shows a typical fast-rise, exponential-decay (FRED) light curve in the 4-120 keV band, with T90(4-120 keV) = (0.22 +0.07/-0.02) s. The light curve in the 4-20 keV band shows possible extended emission following the main pulse, with a corresponding T90(4-20 keV) = (0.71 +0.40/-0.39) s. 

The time-averaged spectrum of the main pulse, extracted from T0 to T0 + 0.22 s in the 5-120 keV range, is well fitted by a cutoff power-law model with a photon index of -0.61 +/-0.14, a peak energy of (56 +16/-12) keV, a 4-120 keV photon flux of (32.3 +0.1/-4.8) photons/cm^2/s, and a corresponding 4-120 keV fluence of (2.9 +/-0.2)e-7 erg/cm^2. 

The time-averaged spectrum of the possible extended emission, extracted from T0 + 0.22 s to T0 + 0.71 s in the 5-20 keV, can be well fitted by a blackbody model with a temperature of (1.6 +0.4/-0.3) keV, a 4-120 keV photon flux of (2.1 +0.3/-0.7) photons/cm^2/s, and a corresponding 4-120 keV fluence of (1.1+/-0.3) e-8 erg/cm^2.

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: Hui Yang (IRAP) (hui.yang@cnrs.fr).

GCN Circular 45403

Subject
GRB 260818B: NOT optical upper limits
Date
2026-08-19T16:13:31Z (2 months ago)
Edited On
2026-08-19T18:11:29Z (2 months ago)
From
d.s.oneill@bham.ac.uk
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of d.s.oneill@bham.ac.uk
Via
Web form
D O’Neill (U. Birmingham), B. Schneider (LAM), G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud), D. Xu (NAOC), A. L. Bouquin (NOT), A. Schenone Zanuzzi (NOT), report on behalf of a larger collaboration:

We observed the field of GRB 260818B detected by SVOM/ECLAIRs (Cordier et al., GCN  45398), using the ALFOSC camera mounted on the Nordic Optical Telescope (NOT). We obtained 4x200 s exposures in the SDSS r-band starting on 2026-08-19 at 03:43:55 UT (7.55 hr post trigger) and 5x200 s in the SDSS z-band starting on 2026-08-19 at 03:59:24 UT (7.80 hr post trigger). 

Because of the size of the ALFOSC field of view, only 58% of the ECLAIRs error circle could is covered in our stacked images.

In the stacked images we identify no new source and obtain the following upper limits:

r > 23.7 AB mag
z > 22.6 AB mag

Magnitudes are calibrated against nearby stars from the Pan-STARRS catalog, and not corrected for Galactic extinction.

GCN Circular 45401

Subject
GRB 260818B: SVOM/COLIBRÍ (FM-GFT) optical upper limit
Date
2026-08-19T11:53:08Z (2 months ago)
From
Marion G <marion.guelfand@gmail.com>
Via
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Marion Guelfand (CPPM), Diego López-Cámara (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), Noémie Globus (UNAM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), Alan M. Watson (UNAM) and Clare Plasse (HKU) report:

We imaged the field of the SVOM GRB 260818B (Cordier et al., GCN Circ. 45398) using the DDRAGO two-channel wide-field imager on the SVOM/COLIBRÍ (FM-GFT) telescope. We observed from 2026-08-19 08:03:24 to 10:40:40 UTC (from 11.87 to 14.51 hours after the trigger) and obtained 118 minutes of simultaneous exposure in the r and z filters.

The data were reduced and coadded and analysed with the COLIBRÍ pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR2 catalog, is in the AB system, and is not corrected for Galactic extinction.

In the stacked image, we do not detect any new source at the ECLAIRs source position (Cordier et al., GCN Circ. 45398) down to the following 10-sigma limit:

r > 23.8
z > 22.4

This is consistent with the non-detection reported by the LCO 1m telescope (Shi et al., GCN 45399).

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 45399

Subject
GRB 260818B: LCO optical upper limits
Date
2026-08-19T03:42:13Z (2 months ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
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Y.Y. Shi (GXU), C. Wu, L.P. Xin (NAOC), A. Saccardi, D. Turpin (CEA/Irfu), Bertrand Cordier (CEA), Hatsune Goto (RIKEN, CEA), report on behalf of the SVOM mission team:


We observed the field of GRB 260818B (SVOM burst-id sb26081807) detected by SVOM (Cordier et al., GCN 45398) with the LCO 1m telescope at South African Astronomical Observatory equipped with the Sinistro instrument.

Our observation started on 2026-08-18 at 23:44:55 UT (about 3.56 hr after the trigger) and we obtained 3x200 s exposures in the SDSS r and PS1 zs filter.

In the stacked image, we do not detect any new source within the SVOM/ECLAIRs error box. We measure the following upper limits calibrated against the Pan-STARRS DR1 catalog, and not corrected for Galactic extinction:

r > 21.9  AB (5-sigma, mid-time 3.65 hr after the trigger),
z > 19.7  AB (5-sigma, mid-time 3.65 hr after the trigger).

This project is funded by the SVOM collaboration.

GCN Circular 45398

Subject
GRB 260818B: SVOM detection of a short burst
Date
2026-08-18T21:18:05Z (2 months ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
Bertrand Cordier (CEA), Hatsune Goto (RIKEN, CEA) report on behalf of the SVOM mission team:

At 2026-08-18T20:11:12 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 260818B (SVOM burst-id sb26081807).

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 Count-Rate Trigger (CRT), which produced a sequence of 2 alerts. CRT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 18.60 in the [8-120] keV energy band over a time window of 0.08 seconds starting at 2026-08-18T20:11:12.

The localization of the best alert is R.A., Dec. 44.9407, 7.4547 degrees:
R.A. (J2000) = 2h59m45.76s
Dec. (J2000) = 7d27m16.93s
 with a 90% confidence level (C.L.) radius of 4.60 arcmin (including systematic error of 2 arcmin added in quadrature).

The SVOM/ECLAIRs light curve showed a single peak structure with a duration of about 1 second.

This burst also triggered SVOM/GRM at 2026-08-18T20:11:12 on a timescale of 0.10 seconds with an SNR of 31.10.

Due to a platform constraint, no immediate slew was performed on this burst.
No X-ray observation could be performed by SVOM/MXT for the time being. No optical observation could be performed by SVOM/VT for the time being.

We will schedule a SVOM ToO observation for this burst.

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 Bertrand Cordier: bcordier@cea.fr.
Please contact the BA by email if you require additional information.
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