GRB 260818A
GCN Circular 45560
Subject
GRB 260818A: Continued ATCA observations
Date
2026-09-14T03:26:21Z (11 days ago)
From
Tao An at USTC <antao2008@ustc.edu.cn>
Via
Web form
Tao An (USTC), Ailing Wang (YNAO, CAS) report on behalf of a large collaboration:
We report a second-epoch Australia Telescope Compact Array (ATCA) observation of the candidate radio counterpart to GRB 260818A (Yin et al., GCN 45391; EP-FXT X-ray afterglow: Wang et al., GCN 45400; Plasse et al., GCN 45434; first ATCA detection: An & Wang, GCN 45494).
The new observation started at 2026-09-12 14:01 UTC (~25.5 days after the SVOM/ECLAIRs trigger). Data were obtained simultaneously at central frequencies of 6.53 GHz and 10.37 GHz. For a homogeneous comparison, we also re-imaged the first-epoch data (starting 2026-08-30 10:49 UTC; T0+12.4 d) with the same spectral-window selection.
A radio source is detected at both epochs near R.A. (J2000) = 23:20:25.422, Dec. (J2000) = −32:05:49.60, consistent with the fading X-ray afterglow EPF_J232025.8-320549. Preliminary peak brightnesses and image rms values are:
Epoch T0+ (d) Peak_6GHz rms S/N Peak_10GHz rms S/N
1 12.41 71.9 8.2 8.8 50.2 7.5 6.7
2 25.54 61.7 7.7 8.0 32.3 7.0 4.6
The peak flux density and rms are in unit of μJy/beam.
Between the two epochs, the 6.53 GHz peak declined by ~14% and the 10.37 GHz peak by ~36%. Approximating the unresolved peak brightness as the flux density, the two-point spectral index (S_ν ∝ ν^α) evolved from α = −0.78 ± 0.41 to α = −1.40 ± 0.54. The negative spectral indices are consistent with optically thin synchrotron emission. The apparent fading and spectral steepening are in the direction expected for a GRB radio afterglow. Absolute flux-calibration systematics are not included in the quoted uncertainties.
We thank the CSIRO Space and Astronomy staff, in particular Jamie Stevens, for supporting these observations. The Australia Telescope Compact Array is part of the Australia Telescope National Facility which is funded by the Australian Government for operation as a National Facility managed by CSIRO.
GCN Circular 45494
Subject
GRB 260818A: ATCA detection of a candidate radio counterpart
Date
2026-09-02T03:38:38Z (23 days ago)
From
Tao An at USTC <antao@shao.ac.cn>
Via
Web form
Tao An (USTC), Ailing Wang (YNAO, CAS) report on behalf of a large collaboration:
We observed the field of GRB 260818A (Yin et al., GCN 45391; EP-FXT X-ray afterglow: GCN 45400, 45434) with the Australia Telescope Compact Array (ATCA), starting at 2026-08-30 10:48:49 UTC (12.4 days after the SVOM/ECLAIRs trigger). Data were obtained simultaneously at central frequencies of 6.53 GHz and 10.36 GHz.
The 1-sigma image rms noise levels are 9.5 uJy/beam at 6.53 GHz and 7.5 uJy/beam at 10.36 GHz.
A radio source is detected both frequencies at a significance >~ 7 sigma at R.A. (J2000) = 23:20:25.422, Dec. (J2000) = −32:05:49.60, consistent with the fading X-ray afterglow EPF_J232025.8-320549 (Plasse et al., GCN 45434). A detailed analysis is underway.
Further ATCA observations are being considered to test for radio variability and to confirm the association with the high-energy transient.
We thank the CSIRO Space and Astronomy staff, in particular Jamie Stevens, for supporting these observations. The Australia Telescope Compact Array is part of the Australia Telescope National Facility which is funded by the Australian Government for operation as a National Facility managed by CSIRO.
GCN Circular 45434
Subject
GRB 260818A: further EP-FXT observations and X-ray afterglow confirmation
Date
2026-08-24T06:29:10Z (a month ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
C. Plasse (HKU), Z.M. Wang (BNU) and H. Goto (RIKEN, CEA) report on behalf of the SVOM and Einstein Probe teams:
We performed two additional observations at the location of the SVOM/ECLAIRs GRB 260818A (Yin et al., GCN 45391) with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) mission. The observations started on 2026-08-18 at 18:10:40 UT (0.76 days after the trigger) and on 2026-08-20 at 18:05:05 UTC (2.75 days after the trigger), both with a total exposure time of ~6000 s.
The source EPF_J232025.8-320549, detected by EP/FXT about 1.67 hr after the SVOM trigger, reported in GCN 45400, fades by a factor 2 in the second observation, from a flux of (13.1 +/- 2.6) x 10^-14 erg/s/cm^-2 down to a flux of (6.7 +/- 1.9) x 10^-14 erg/s/cm^-2 (0.3-10 keV; 1-sigma uncertainty). At this source position of RA, Dec: 350.1064 -32.0978 (with an error radius of 10 arcsec ), 5.9 arcmin from ECLAIRs position, no X-ray source is detected in the third observation.
The fading behaviour of this X-ray source confirms it as the afterglow of GRB 260818A.
We encourage further multi-wavelength follow-up of this source.
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 45406
Subject
GRB 260818A: J-band observations with WINTER
Date
2026-08-20T03:58:10Z (a month ago)
From
tanishk.mohan@iitb.ac.in
Via
Web form
Tanishk Mohan (IITB), Geoffrey Mo (Caltech/Carnegie), Viraj Karambelkar (Columbia), Benjamin Schneider (LAM), Tomas Ahumada (NOIRLab), Robert Stein (UMD), Danielle Frostig (CfA), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:
We observed the field of GRB 260818A (Yin et al., GCN 45391) in the near-infrared J band with the Palomar 1-m telescope, equipped with the 1.2-square degree WINTER camera (Lourie et al. 2020, Frostig et al. 2024). Observations began at 2026-08-18T07:40:58.776 UTC in the J band (6.61 hr after the SVOM/ECLAIRs trigger), consisting of 15*120 sec exposures. The images were processed using the WINTER data reduction pipeline implemented with mirar (https://github.com/winter-telescope/mirar, https://doi.org/10.5281/zenodo.13352565).
Of the five unique uncatalogued X-ray sources (EPF_J231953.3-320838 is repeated in the FXT circular) reported by FXT (Wang et al., GCN 45400), four lie within our WINTER image, with EPF_J231912.9-320034 falling outside the field of view. Of these four sources, two are coincident with catalogued sources: EPF_J231936.3-320537 is classified as a star, while EPF_J231953.3-320838 is classified as a quasar in the Gaia catalog. We also note that EPF_J231953.3-320838 has a measured redshift of z=0.1898 in the GAMA survey (Yao et al. 2020; https://ui.adsabs.harvard.edu/abs/2020ApJ...903...91Y/abstract). We do not detect counterparts at the positions of the remaining two FXT sources (Ducoin et al., GCN 45393; Shi et al., GCN 45395). We obtain a 5-sigma upper limit of J=19.14 mag (AB).
WINTER (Wide-field INfrared Transient ExploreR) is a partnership between MIT and Caltech, housed at Palomar Observatory, and funded by NSF MRI, NSF AAG, the David and Lucile Packard Foundation, and the MIT Kavli Institute for Astrophysics and Space Research.
GCN Circular 45404
Subject
GRB 260818A: SVOM/ECLAIRs refined analysis
Date
2026-08-19T20:26:12Z (a month ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
H. Yang, O. Godet (IRAP), W.-J. Xie (NAOC), Clara Plasse (HKU)
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 possible faint burst GRB 260818A (SVOM burst-id sb26081801, GCN 45391, trigger time T0 = 2026-08-18T01:04:36 UTC).
The burst that triggered ECLAIRs shows a featureless light curve, and is detected from T0-82 s to T0 in the 8-120 keV energy band.
The time-averaged spectrum from T0-82 s to T0 in the 5-120 keV is well fitted by a blackbody model with a temperature of (7 +2/-1) keV, a 4-120 keV photon flux of (0.26 +0.06/-0.05) photons/cm^2/s, and a corresponding 4-120 keV fluence of (7 +/-2)e-7 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 45400
Subject
GRB260818A: EP-FXT follow-up observation
Date
2026-08-19T03:49:35Z (a month ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Z.M. Wang(BNU), C.Y. Dai(NJU), D. F. Hu(PMO,CAS) and H.W. Pan(NAOC) on behalf of the Einstein Probe (EP) team:
EP-FXT performed a follow-up observation of GRB 260818A detected by SVOM/ECLAIRs (Yin et al., GCN 45391). The observation started at 2026-08-18 02:43:32(UTC), about 1.67 hr after the SVOM trigger, with a total exposure time of around 4.6 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_J232025.8-320549 | 350.1077 | -32.0971 | 6.101382 | 1.3205e-13
EPF_J231936.3-320537 | 349.9012 | -32.0935 | 6.582593 | 5.9746e-14
EPF_J231953.3-320838 | 349.9729 | -32.1449 | 6.812141 | 2.0801e-13
EPF_J231912.9-320034 | 349.8036 | -32.0095 | 10.701601 | 1.0260e-12
EPF_J231953.3-320838 | 349.9722 | -32.1440 | 6.770902 | 2.0112e-13
EPF_J232037.7-320633 | 350.1572 | -32.1093 | 8.604395 | 4.5878e-14
with uncertainties of 10 arcsec (radius, 90% C.L. statistical and systematic).
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 45395
Subject
GRB 260818A: LCO optical upper limits
Date
2026-08-18T15:08:30Z (a month ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
Y.Y. Shi (GXU), C. Wu, L.P. Xin (NAOC), A. Saccardi, D. Turpin (CEA/Irfu), Clara Plasse (HKU), Donghua Zhao (NAOC), report on behalf of the SVOM mission team:
We observed the field of GRB 260818A (SVOM burst-id sb26081801) detected by SVOM (Yin et al., GCN 45391) with the LCO 1m telescope at Teide Observatory and South African Astronomical Observatory equipped with the Sinistro instrument.
Our observation started on 2026-08-18 at 01:46:28.00 UT (about 41.87 min after the trigger) and we obtained 6x200 s exposures in the SDSS r and 3x200 s exposures in the PS1 zs filter.
No credible uncatalogued source was detected in our stacked frames in the ECLAIRs localization region as also reported by SVOM/COLIBRÍ (Ducoin et al., GCN 45393).
We measure the following upper limits calibrated against the DESI Legacy Imaging Surveys DR10 catalog, and not corrected for Galactic extinction:
r > 21.8 AB (5-sigma, mid-time 1.01 hr after the trigger),
z > 20.5 AB (5-sigma, mid-time 1.14 hr after the trigger).
This project is funded by the SVOM collaboration.
GCN Circular 45393
Subject
GRB 260818A: SVOM/COLIBRÍ (FM-GFT) optical upper limit
Date
2026-08-18T09:54:29Z (a month ago)
From
J.-G. Ducoin at CPPM <ducoin@cppm.in2p3.fr>
Via
Web form
Jean-Grégoire Ducoin (CPPM), Camila Angulo (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), 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), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), 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 Clara Plasse report:
We imaged the field of the SVOM GRB 260818A (Yin et al., GCN Circ. 45391) using the DDRAGO two-channel wide-field imager on the SVOM/COLIBRÍ (FM-GFT) telescope. We observed from 2026-08-18T07:52:19 to 2026-08-18T08:35:21 UTC (from 6.82 to 7.54 hours after the trigger) and obtained 32 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 SkyMapper DR4 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 (Yin et al., GCN Circ. 45391) down to the following 10-sigma limit:
r > 22.3
z > 20.4
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 45391
Subject
GRB 260818A: SVOM possible detection of a long and faint GRB
Date
2026-08-18T01:59:59Z (a month ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
Y.-H. I. Yin (HKU), Hatsune Goto (RIKEN, CEA), Clara Plasse (HKU), Donghua Zhao (NAOC), Wenjin Xie (NAOC) report on behalf of the SVOM mission team:
At 2026-08-18T01:04:36 UTC (T0), SVOM/ECLAIRs triggered and located a possible GRB (SVOM burst-id sb26081801). 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 1 alert. IMT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 6.86 in the [8-120] keV energy band over a time window of 81.92 seconds starting at 2026-08-18T01:03:14.
The localization of the best alert is R.A., Dec. 350.0116, -32.0362 degrees:
R.A. (J2000) = 23h20m02.79s
Dec. (J2000) = -32d02m10.23s
with a 90% confidence level (C.L.) radius of 11.41 arcmin (including systematic error of 2 arcmin added in quadrature).
We notice the presence of several quasars in this error box, among which the closest is [VV2006] J232014.7-320423 located at 3.3 arcmin.
Due to the detection significance being below the slew threshold, no immediate slew was performed on this burst. ToOs of SVOM/MXT and SVOM/VT have been scheduled 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 Clara Plasse: clara.plasse@outlook.fr.
Please contact the BA by email if you require additional information.