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GRB 260604C

GCN Circular 44887

Subject
Konus-Wind detection of GRB 260604C
Date
2026-06-09T16:52:29Z (2 days ago)
From
Anna Ridnaia at Ioffe Institute <ridnaia@mail.ioffe.ru>
Via
Web form
A. Ridnaia, D. Frederiks, A. Lysenko, D. Svinkin,
A. Tsvetkova,  M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:

The long-duration GRB 260604C
(Fermi-GBM detection: Fermi GBM team, GCN 44822,
Neights et al., GCN 44831;
SVOM-ECLAIRs detection: Gotz et al., GCN 44823,
Brunet et al., GCN 44883;
GECAM-B detection: Luo et al., GCN 44854;
AstroSat-CZTI detection: Arya et al., GCN 44885)
triggered Konus-Wind at T0=73128.568 s UT (20:18:48.568).

The burst light curve shows a single pulse
which starts at ~T0-3.2 s and has a duration of ~8.5 s.
The emission is seen up to ~4 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB260604_T73128/

As observed by Konus-Wind, the burst
had the total fluence of 3.60(-0.26,+0.26)x10^-5 erg/cm2,
and the 64-ms peak flux, measured from T0+3.056 s,
of 2.25(-0.18,+0.18)x10^-5 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+12.032 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.92(-0.05,+0.06),
the high energy photon index beta = -2.33(-0.19,+0.13),
the peak energy Ep = 421(-37,+40) keV
(chi2 = 82/89 dof).

The spectrum near the maximum count rate
(measured from T0+0.256 to T0+3.840 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.73(-0.06,+0.06),
the high energy photon index beta = -2.07(-0.07,+0.06),
the peak energy Ep = 374(-32,+35) keV
(chi2 = 60/71 dof).

All the quoted errors are at the 68% confidence level.
All the quoted values are preliminary.

GCN Circular 44885

Subject
GRB 260604C: AstroSat CZTI detection
Date
2026-06-09T13:26:28Z (2 days ago)
From
Anuraag Arya at IIT Bombay <aryaanuraag910@gmail.com>
Via
Web form
A. Arya (IITB), A. Goyal (IITB), Harsha K. H. (IUCAA), S. Salunke (IUCAA),  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 long GRB 260604C which was also detected by  Fermi/GBM (Fermi GBM team, GCN Circ. 44822) and SVOM/ECLAIRs (D. Gotz et.al., GCN Circ. 44823).

The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve peaks at 2026-06-04 20:18:49 UTC. The measured peak count rate associated with the burst is 1274 (+126, -136) counts/s above the background in the combined data of all quadrants, with a total of 1904 (+116, -105) counts. The local mean background count rate was 282 (+4, -7) counts/s. Using cumulative rates, we measure a T90 of 2.7 (+0.5, -0.2) s. 

The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2026-06-04 20:18:48 UTC. The measured peak count rate associated with the burst is 3712 (+112, -126) counts/s above the background in the combined data of all quadrants, with a total of 8150 (+207, -252) counts. The local mean background count rate was 1332 (+7, -6) counts/s. We measure a T90 of 3.3 (+0.2, -0.1) s from the cumulative Veto light curve.

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 data products like interactive and downloadable light curves for this GRB can be found at:
https://astrosat.iucaa.in/cift/cift_products/ET20260604T201844/ET20260604T201844_details.html

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

GCN Circular 44883

Subject
GRB 260604C: SVOM/ECLAIRs refined analysis
Date
2026-06-09T11:58:07Z (2 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
M. Brunet, O. Godet (IRAP), N. Dagoneau, D. Gotz, D. Adrien, S. Bisero (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 GRB 260604C (SVOM burst-id sb26060404 – GCN 44823, trigger time T0 = 2026-06-04T20:21:59 UTC), which was also detected by Fermi/GBM (GCN 44822, 44831) and GECAM-B (GCN 44854). 

The burst that triggered ECLAIRs onboard shows a multiple peak lightcurve. The burst duration is estimated to be around 255 s in the 4-20 keV energy band through imaging.

The refined burst localization is RA, Dec = 224.396, 28.742  degrees:

RA (J2000) = 14h57m35.0s
Dec (J2000) = 28d44m31.2s

with a 90% C.L. radius of 8.6 arcmin (including systematic error of 2 arcmin added in quadrature).

This localization is consistent with the one of the afterglow (MASTER OT detection, GCN 44827).

The time-averaged spectrum, before the slew (from T0-56.3 s to T0+41.9 s) in the 5-120 keV energy range is best fitted by a powerlaw model with alpha = -2.02 +0.17/-0.19. With this model, the 4-120 keV fluence is (4.5 +0.5/-1.3)e-7 erg/cm² and the 4-120 keV photon flux is 0.21 +0.01/-0.05 ph/cm²/s.

The time-averaged spectrum after the slew (from T0+100 s to T0+200 s) in the 5-120 keV energy range is best fitted by a powerlaw model with alpha = -2.09 +0.35/-0.48. With this model, the  4-120 keV fluence is (2.2 +0.2/-1.3 )e-7 erg/cm² and the 4-120 keV photon flux is 0.10 +0.01/-0.05 ph/cm²/s.

Fermi/GBM trigger occurred at T0-195 s at a time when ECLAIRs was not collecting data. So ECLAIRs missed the main part of the burst prompt emission. The spectral parameters we obtained may indicate that ECLAIRs observed the transition from the prompt to afterglow emission.

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: M. Brunet (IRAP) (marius.brunet at utoulouse.fr)

GCN Circular 44877

Subject
GRB 260604C: further SAO RAS optical observations
Date
2026-06-08T22:27:43Z (3 days ago)
From
Alexander Moskvitin at SAO RAS <mosk@sao.ru>
Via
Web form
A. Moskvitin (SAO RAS), O. Spiridonova (SAO RAS), A. Pozanenko (IKI),
A. Volnova (IKI), N. Pankov (IKI) report on behalf of GRB follow-up team

We observed the field of long GRB 260604C discovered by Fermi
(The Fermi GBM team, GCN 44822; Neights & Meegan, GCN 44831);
SVOM (Gotz et al., GCN 44823) and GECAM-B (Luo et al., GCN 44854)
with the Zeiss-1000 1-m telescope of SAO RAS equipped with the CCD
photometer. We obtained 12 x 300 sec. images in Rc band on June 08,
20:01:14--21:07:58 UT.

The OT (Lipunov, GCN 44827; Konno et al., GCN 44828; GCN 44836;
Zhu et al., GCN 44832; Becerra et al., GCN 44834; Hellot et al.,
GCN 44835; Gompertz et al., GCN 44837; Gompertz et al., GCN 44843;
Chen et al., GCN 44851; Moskvitin et al., GCN 44852; Sasaki et al.,
GCN 44857; Pankov et al., GCN 44858; Pankov et al., GCN 44862;
Burkhonov et al., GCN 44865; Lincetto et al., GCN 44873)
is detected in our stacked image with the following brightness:

Date       UTstart  t-T0    Exp.   Filter Mag +/- Err.    UL
                    (mid,d) (n*s)                         (3-sigma)  
2026.06.08 20:01:14 4.01102 12*300 Rc     23.08 +/- 0.18  23.8

The preliminary photometry is based on nearby SDSS stars (R-magnitudes
were obtained using Lupton 2005 transformation equations)
as in Moskvitin et al., GCN 44852, and was not corrected
for the Galactic extinction.

GCN Circular 44873

Subject
GRB 260604C: SVOM/COLIBRÍ (FM-GFT) colour evolution and confirmation of the late rebrightening
Date
2026-06-08T15:49:40Z (3 days ago)
From
Rosa Leticia Becerra Godínez at Instituto de Astronomía, UNAM <rbecerra@astro.unam.mx>
Via
Web form
Massimiliano Lincetto (CPPM), Leonardo García García (UNAM), Rosa L. Becerra (UNAM), Antonio de Ugarte Postigo (LAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Sarah Antier (IJCLAB), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), and Sofia Bisero (CEA Saclay) report:

We re-imaged the field of GRB 260604C (Fermi GBM Team, GCN Circ. 44822, Gotz et al., GCN Circ. 44823) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-06-08 06:37 to 09:24 UTC (from 3.43 to 3.54 days after the trigger) and obtained 64, 63, and 127 minutes of exposure in the g, r, and z filters respectively.

The data were reduced, coadded and analysed with the ASU COLIBRÍ 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 our first epoch reported by Becerra et al. (GCN Circ. 44827), we measured: 

g-r = 0.13 +/- 0.07 
g-z = 0.48 +/- 0.10 

In this epoch, we measured:

g-r = 0.47 +/- 0.06 
g-z = 0.86 +/- 0.09

Our measurements indicate a significant reddening of the source between the two epochs.

Moreover, we also observe the rebrightening first reported by the Mondy team (Pankov et al., GCN Circ. 44862). The rise began approximately 30 hours after the trigger and lasted for about 30 hours. 

Further observations and analysis 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 44872

Subject
GRB 260604C: SVOM/C-GFT upper limit
Date
2026-06-08T15:20:05Z (3 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form

C. Wu (NAOC), Z. Kang (CHO), L.P. Xin（NAOC), S.J. Yin (GXU), 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), N. Dagoneau, S. Bisero (CEA Saclay) report on behalf of the SVOM/C-GFT team:

We observed the field of GRB 260604C (SVOM burst-id sb26060404) detected by SVOM/ECLAIRs (GCN 44823

Loading...
 
 
) with LATIOS on SVOM/C-GFT. Observations started at 2026-06-05T15:54:29 UTC, ~19.54 h after the trigger.

We obtained 9X90s exposures in the SDSS g filter. After preliminary analysis，we do not detect the optical counterpart (Lipunov et al., GCN 44827

Loading...
 
 
; Konno et al., GCN 44828; Zhu et al., GCN 44832, Becerra et al., GCN 44834; Hellot et al., GCN 44835; Gompertz et al., GCN 44837; Gompertz et al., GCN 44843; Saccardi et al., GCN 44844; Chen et al., GCN 44851; Moskvitin et al., GCN 44852; Sasaki et al., GCN 44857 ; Pankov et al., GCN 44858 ; Pankov et al., GCN 44862 ; Burkhonov et al., GCN 44865 ) in the stacked image. The upper limit is measured as follows:

g > 21.1 AB (3-sigma, mid-time 19.68 h after the trigger).

The photometry was calibrated against Pan-STARRS1 DR1 catalog and no correction for Galactic dust extinction was applied.

We thank the observation assistants Hong-Xu Xue and Bo-Wen Li 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 44865

Subject
GRB 260604C: GRANDMA further observations
Date
2026-06-08T09:38:41Z (3 days ago)
Edited On
2026-06-08T10:03:18Z (3 days ago)
From
antier@ijclab.in2p3.fr
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of antier@ijclab.in2p3.fr
Via
Web form
O. Burkhonov (UBAI), M. Pillas (IAP), T. Leandro de Almeida (LNA), E. Elhosseiny (NRIAG), N. Kochiashvili (AbAO), L. Chen (TSU), A. Simon (TShNU of Kyiv), S. Antier (IJCLAB), M. Coughlin (UMN), S. Ehgamberdiev (UBAI), Y. Rajabov (UBAI), Y. Tillayev (UBAI), N. Sasaki (UEA,LNA), T. Kvernadze (AbAO), A. Takey (NRIAG), W. Corradi (LNA), I. Abdusamatjan (TNOT) on behalf of GRANDMA:

We conducted further follow-up observations of GRB 260604C (Fermi Team, GCN Circ. 44822, Gotz et al., GCN Circ. 44823) with GRANDMA up to now.

We observed a possible re-brightening in r, i, and R thanks to further observations from Two-meter Twin Telescope (TTT) sited at the Teide Observatory, UBAI-AZT-22 at Madainak Observatory, AbAO-T1.5m at Abastumani Observatory, KAO-1.88m at Kottamia Astronomical Observatory, and OPD-1.6m at Observatório do Pico dos Dias (Sasaki et al GCN 44857). This is shown in the public site group of the public ACME service Skyportal/ICARE https://skyportal-icare.ijclab.in2p3.fr/source/GCN-260604_202037. To access, one simply needs to create an account using your ORCID id.

Our measurements are aligned with Mondy's report (Pankov GCN 44862).

The observations are performed under the P7 campaign of GRANDMA to monitor SVOM alerts and LSST alerts from explosive fast transients filtered by BOOM/Babamul and FINK.

GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (kilonovacatcher.in2p3.fr).  Astrophysics Center for Multimessenger studies in Europe is supported Skyportal/ICARE (contact camille.douzet@ijclab.in2p3.fr, skyportal.io). The TTT Observing Time Rights (DTO) used were provided by Light Bridges, SL.

GCN Circular 44862

Subject
GRB 260604C: Mondy optical observations: evidence of a possible brightening
Date
2026-06-08T09:03:44Z (3 days ago)
From
Nicolai Pankov at IKI <colinsergesen@gmail.com>
Via
Web form
N. Pankov (IKI), A. Pozanenko (IKI), E. Klunko (ISTP), A. Moskvitin (SAO RAS), A. Volnova (IKI) report on behalf of IKI-GRB-FuN:

We observed the field of GRB 260604C (The Fermi GBM team, GCN 44822; Neights & Meegan, GCN 44831; Gotz et al., GCN 44823) with the AZT-33IK 1.6-m telescope of the Sayan Solar Observatory (Mondy). The observations were conducted starting on 2026-06-07 16:34 UT, i.e. ~2.86 days since GRB. We aquired 30x120 sec exposures in the R-band using a CMOS-photometer.

The optical afterglow (Lipunov, GCN 44827; Konno et al., GCN 44828; GCN 44836; Zhu et al., GCN 44832; Becerra et al., GCN 44834; Hellot et al., GCN 44835; Konno et al., GCN 44836; Gompertz et al., GCNs 44837, 44843; Chen et al., GCN 44851; Moskvitin et al., GCN 44852; Sasaki et al., GCN 44857; Pankov et al., GCN 44858) is clearly detected in the co-add image and displays an evidence of a brightening in comparison with previous epochs of observations (Moskvitin et al., GCN 44852; Sasaki et al., GCN 44857; Pankov et al., GCN 44858). The observations details and preliminary photometry are summarized below:

Date       UTstart  t-T0    Exp.   Filter Mag    Err.    UL
                    (mid,d) (n*s)                      (3-sigma)
2026-06-07 16:34:49 2.86396 28x120 R      21.18  0.06   23.2

The photometry is based on nearby SDSS stars (R-magnitudes were obtained using Lupton 2005 transformation equations) as in Moskvitin et al., GCN 44852, and was not corrected for the Galactic extinction.

GCN Circular 44858

Subject
GRB 260604C: Simeiz Zeiss-1000 optical observations of the afterglow
Date
2026-06-07T19:31:45Z (4 days ago)
From
Nicolai Pankov at IKI <colinsergesen@gmail.com>
Via
Web form
N. Pankov (IKI), A. Pozanenko (IKI), A. Novichonok (KIAM), A. Moskvitin (SAO RAS), I. Nikolenko (INASAN), A. Volnova (IKI) report on behalf of IKI-GRB-FuN:

We observed the field of GRB 260604C (The Fermi GBM team, GCN 44822; Neights & Meegan, GCN 44831; Gotz et al., GCN 44823) with the Zeiss-1000 1-m telescope of the Simeiz Observatory (mnt. Koshka) of INASAN. The observations were conducted on 2026-06-05 and 2026-06-06 epochs using a CCD-photometer and the R-filter.

The optical afterglow (Lipunov, GCN 44827; Konno et al., GCN 44828; GCN 44836; Zhu et al., GCN 44832; Becerra et al., GCN 44834; Hellot et al., GCN 44835; Konno et al., GCN 44836; Gompertz et al., GCNs 44837, 44843; Chen et al., GCN 44851; Moskvitin et al., GCN 44852; Sasaki et al., GCN 44857) is clearly detected in the co-add images obtained on both epochs. The observations details and preliminary photometry are summarized below:

Date       UTstart  t-T0    Exp.   Filter Mag    Err.    UL
                    (mid,d) (n*s)                      (3-sigma)
2026-06-05 21:31:13 1.07291 26x150 R      21.35  0.14   22.4
2026-06-06 20:09:21 2.03341 46x150 R      21.53  0.19   22.3


The photometry is based on nearby SDSS stars (R-magnitudes were obtained using Lupton 2005 transformation equations) as in Moskvitin et al., GCN 44852, and was not corrected for the Galactic extinction.

GCN Circular 44857

Subject
GRB 260604C: OPD1.6m - GRANDMA observations - detection
Date
2026-06-07T13:35:13Z (4 days ago)
From
Nelio Sasaki at NEPA-UEA <nsasaki@uea.edu.br>
Via
Web form
Nélio Sasaki (UEA-Parintins), Wagner Corradi (LNA),  Leandro de Almeida (LNA), Felipe Navarete (LNA), S. Antier (OCA/IJCLAB), I. Tosta e Melo (IFN), A. Simon (TshNU of Kyiv), L. Chen (TSU), M. Coughlin (UMN) and P. Hello (IJCLAB) on behalf of Observatório do Pico dos Dias (OPD/LNA/Brazil) and the GRANDMA collaboration, report follow-up observations of GRB 260604C (Fermi Team, GCN Circ. 44822, Gotz et al., GCN Circ. 44823, Chen et al., GCN 44851; Moskvitin et al., GCN 44852) performing photometry of the GRB260604C source with SPARC4 in the Sloan bands (g,r,i,z), using exposures of 40 × 180 s in (g), 120 × 60 s in (r), 120 × 60 s in (i), and 120 × 60 s in (z). The observations began on 2026-06-07T00:18:36.036 UTC, corresponding to JD 2461198.512917. 

The recentered position of the source is:

RA = 14:57:49.54, Dec = +28:49:03.96

The measured magnitudes of the target are:


Filter | Mag (AB)
     g | 22.0938 ± 0.0008 (Magnitude limit: 24.6925)
     r | 21.7162 ± 0.0005 (Magnitude limit: 23.9245)
     i | 21.5773 ± 0.0008 (Magnitude limit: 25.1456)
     z | 21.4418 ± 0.0006 (Magnitude limit: 23.8440)

Photometric calibration was performed using Gaia DR3 field stars. For each comparison star, the Gaia magnitude (G) and color (G_BP-G_RP) were transformed to Sloan-type magnitudes (g,r,i,z). The photometric zero point in each filter was then obtained from the difference between the transformed magnitudes from the catalog and the instrumental magnitudes of the comparison stars, and applied to the instrumental magnitude of the target. The transformations in (g,r,i) follow the Gaia-SDSS color relationships of Evans et al. (2018), while the magnitude in (z) was estimated using the (G-g) versus (g-z) relationship of Jordi et al. (2010). The Gaia DR3 catalog is from Gaia Collaboration et al. / Vallenari et al. (2023).

The observations are performed under the P7 campaign of GRANDMA to monitor SVOM alerts and LSST alerts from explosive fast transients filtered by BOOM/Babamul and FINK. 

GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (kilonovacatcher.in2p3.fr).  Astrophysics Center for Multimessenger studies in Europe is supported by Skyportal/ICARE (contact camille.douzet@ijclab.in2p3.fr, skyportal.io)

GCN Circular 44854

Subject
GRB 260604C: GECAM-B observation
Date
2026-06-07T12:57:24Z (4 days ago)
From
Xinghao Luo at IHEP <2952704891@qq.com>
Via
Web form
Xing-Hao Luo, Chen-Wei Wang, Shao-Lin Xiong (IHEP) report on behalf of GECAM team:

GECAM-B was triggered in-flight by GRB 260604C, at 2026-06-04T20:18:44.600 UTC (denoted as T0), which is also detected by Fermi/GBM (Fermi GBM team, GCN #44822), and SVOM/ECLAIRs (D. Gotz et.al., GCN#44823).

According to the GECAM-B light curves in about 70-6000 keV, this burst mainly consists of a single pulse with a duration (T90) of 7 +/-0.5 s.

The GECAM-B light curve can be found here:
https://www.bursthub.cn//admin/static/gecambgrb260604C.png

In addition, the position of this burst, is determined by SVOM/ECLAIRs (RA=224.3461 , DEC=28.6558, with a 90% confidence level raiuds of 9.6 arcmins , D.Gotz et.al., GCN #43647). With this localization, the time-averaged spectrum from T0-0.5 to T0+7.5 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.41 +0.12/-0.09 and the cutoff energy, parameterized as Epeak, is 759 +342/-210 keV. The event fluence (10-1000keV) in this time interval is (2.88 +0.086/-0.11)E-05 erg/cm^2.

The 'Amati' relation diagram of GRB 260604C is shown at:
https://www.bursthub.cn//admin/static/gecambgrb260604C_amati.png

Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) mission originally consists of two micro-satellites (GECAM-A and GECAM-B) launched in Dec. 2020. As the third member of GECAM constellation, GECAM-C was launched onboard SATech-01 experimental satellite in July 2022. GECAM mission is funded by the Chinese Academy of Sciences (CAS).

GCN Circular 44853

Subject
GRB 260604C: 6 GHz VLA observations
Date
2026-06-07T10:22:39Z (4 days 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 GRB 260604C which triggered Fermi-GBM (Fermi-GBM team, GCN 44822) and SVOM (Gotz et al., GCN 44823) with Very Large Array in C-band at the centre frequency of 6 GHz with a bandwidth of 4 GHz on June 04th 2026 (38 hours after the trigger). 

Data were calibrated and imaged using standard procedures in CASA (v6.6.1). 1331+305 (3C286) was used for flux density and bandpass calibration and J1513+2338 was used for complex gain calibration.

No radio source was detected at the optical transient position (Lipunov et al., GCN 44827) down to a preliminary 3-sigma upper limit of ~24 uJy/beam.  

Further observations are planned.

We thank the VLA staff for executing the observations.

GCN Circular 44852

Subject
GRB 260604C: SAO RAS optical observations
Date
2026-06-06T20:42:04Z (5 days ago)
Edited On
2026-06-08T07:14:49Z (3 days ago)
From
Alexander Moskvitin at SAO RAS <mosk@sao.ru>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Alexander Moskvitin at SAO RAS <mosk@sao.ru>
Via
email
A. Moskvitin (SAO RAS), O. Spiridonova (SAO RAS), A. Pozanenko (IKI),
A. Volnova (IKI), N. Pankov (IKI) report on behalf of GRB follow-up team

We observed the field of long GRB 260604C discovered by Fermi
(The Fermi GBM team, GCN 44822; Neights & Meegan, GCN 44831)
and SVOM (Gotz et al., GCN 44823) with the Zeiss-1000 1-m telescope
of SAO RAS equipped with the CCD photometer. We obtained 12 x 300 sec.
images in Rc band on June 05, 22:25:58--23:33:55 UT.

The OT (Lipunov, GCN 44827; Konno et al., GCN 44828; GCN 44836;
Zhu et al., GCN 44832; Becerra et al., GCN 44834; Hellot et al.,
GCN 44835; Gompertz et al., GCN 44837; Gompertz et al., GCN 44843;
Chen et al.б GCN 44851) is clearly detected in our stacked image
with the following brightness:

Date       UTstart  t-T0    Exp.   Filter Mag +/- Err.    UL
                    (mid,d) (n*s)                         (3-sigma)
2026.06.05T22:25:58 1.11195 12*300 Rc     21.65 +/- 0.07  23.4

This preliminary photometry is based on nearby SDSS stars
(using Lupton 2005 transformation equations) and not corrected
for Galactic extinction.

GCN Circular 44851

Subject
GRB 260604C: GRANDMA observations
Date
2026-06-06T17:16:35Z (5 days ago)
From
antier@ijclab.in2p3.fr
Via
Web form
L. Chen (TSU), N. Sasaki (UEA,LNA), A. Simon (TShNU of Kyiv), S. Antier (IJCLAB), M. Coughlin (UMN) O. Burkhonov (UBAI), S. Ehgamberdiev (UBAI), Y. Rajabov (UBAI), Y. Tillayev (UBAI),  R. Inasaridze (AbAO), N. Kochiashvili (AbAO), V. Zhuzhunadze (AbAO), A. Klotz (IRAP), P. Put (NARIT), K. Noysena (NARIT), A. Manasanun (NARIT), M. Freeberg (KNC), E. Elhosseiny (NRIAG), A. Takey (NRIAG), M. Pillas (IAP), W. Corradi (LNA), T. Leandro de Almeida (LNA), I. Abdusamatjan (TNOT) on behalf of GRANDMA:

We conducted follow-up observations of GRB 260604C (Fermi Team, GCN Circ. 44822, Gotz et al., GCN Circ. 44823) with GRANDMA from 0.05 h post T0 (MJD  61195.84859954) up to now. To do this, we have collected data from KNC (Hellot et al., GCN 44835), TAROT/TCH, UBAI-NT60, UBAI-AZT22, AbAO-T70, KAO-1.8m, TRT-SRO, OPD-60, TTT and NOWT.

GRB 260604C optical afterglow (reported by Lipunov et al., GCN 44827) is detected at several epochs and a subset of the photometry corrected for Galactic extinction is reported in the public site group of the public ACME service Skyportal/ICARE https://skyportal-icare.ijclab.in2p3.fr/source/GCN-260604_202037. To access, one simply needs to create an account using your ORCID id. 

Our measurements are in agreement with Lipunov et al., GCN 44827; Konno et al., GCN 44828; Zhu et al., GCN 44832, Becerra et al., GCN 44834; GCN 44835; Gompertz et al., 44843,Saccardi et al. 44844.

The observations are performed under the P7 campaign of GRANDMA to monitor SVOM alerts and LSST alerts from explosive fast transients filtered by BOOM/Babamul and FINK. 

GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (kilonovacatcher.in2p3.fr).  Astrophysics Center for Multimessenger studies in Europe is supported Skyportal/ICARE (contact camille.douzet@ijclab.in2p3.fr, skyportal.io)

GCN Circular 44844

Subject
GRB 260604C: LCO optical upper limit
Date
2026-06-05T21:22:48Z (6 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
A. Saccardi (CEA/Irfu), D. B. Malesani (DAWN/NBI and Radboud), S. Bisero (CEA/Irfu) report on behalf of the SVOM mission team:

We observed the field of GRB 260604C (Fermi GBM Team, GCN 44822; Gotz et al., GCN 44823) with the LCO 1m telescope at South African Astronomical Observatory equipped with the Sinistro instrument.

Our observation started on 2026-06-05 at 20:08:32 UT (about 1 day after the trigger) and we obtained 3x200 s exposures in the SDSS r filter.

In the stacked image we do not detect the optical counterpart (Lipunov et al., GCN 44827; Konno et al., GCN 44828; Zhu et al., GCN 44832, Becerra et al., GCN 44834; Hellot et al., GCN 44835; Gompertz et al., GCN 44837; Gompertz et al., GCN 44843). We measure the following upper limit calibrated against the Pan-STARRS catalog, and not corrected for Galactic extinction:

r > 21.8 AB (3-sigma, mid-time 0.997 days after the trigger).

This project is funded by the SVOM collaboration.

GCN Circular 44843

Subject
GRB 260604C: Liverpool Telescope optical detection
Date
2026-06-05T14:01:06Z (6 days ago)
From
Dimple at University of Birmingham <dimplepanchal96@gmail.com>
Via
Web form
B. P. Gompertz (U. Birmingham), D. B. Malesani (DAWN/NBI and Radboud)  and Dimple (U. Birmingham) report on behalf of a larger collaboration:

We conducted follow-up observations of GRB 260604C (Fermi Team, GCN Circ. 44822, Gotz et al., GCN Circ. 44823) with the IO:O camera on the 2m robotic Liverpool Telescope (LT). Observations began at 00:14:01 UT on 2026-06-05, ~3.92 hr after the burst, and consisted of 5 x 120 s exposures in each of the SDSS r, g, i and z filters.

We detect the optical counterpart (Lipunov et al., GCN 44827; Konno et al., GCN 44828; Zhu et al., GCN 44832, Becerra et al., GCN 44834; Hellot et al., GCN 44835; Gompertz et al. 44837) in all the filters, and measure an AB magnitude of r = 18.60 ± 0.05.

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

GCN Circular 44837

Subject
GRB 260604C: GOTO detections of the optical afterglow
Date
2026-06-05T09:47:43Z (6 days ago)
From
Ben Gompertz at U of Birmingham <b.gompertz@bham.ac.uk>
Via
email
B. P. Gompertz,  S. Moran, D. O’Neill, A. Kumar, G. Ramsay, R. Starling, K. Ackley, M. Dyer, J. Lyman, K. Ulaczyk, D. Steeghs, D. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. Breton, J. Casares, L. Nuttall, B. Godson, T. Killestein and M. Pursiainen, on behalf of the GOTO collaboration:


We report on optical observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to GRB 260604C, discovered by Fermi/GBM and SVOM (Fermi GBM Team, GCN 44822; Gotz et al., GCN 44823).


Observations were taken in the GOTO-L filter (400 - 700nm) with 4x90s exposures. Images were processed immediately after acquisition using the GOTO pipeline. The optical counterpart (Lipunov et al., GCN 44827; Konno et al., GCN 44828; Zhu et al., GCN 44832, Becerra et al., GCN 44834; Hellot et al., GCN 44835) was detected in three epochs:


| mid-time(UT) | t-t0(hr) | ABmag |

| 2026-06-04 23:46:15 | 3.46 | 18.45 ± 0.04 |

| 2026-06-05 00:54:05 | 4.59 | 18.70 ± 0.08 |

| 2026-06-05 02:00:28 | 5.70 | 19.06 ± 0.12 |


Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and are not corrected for Galactic extinction.


GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester, the University of Birmingham and the Instituto de Astrofisica de Canarias (IAC).

GCN Circular 44836

Subject
GRB 260604C: Correction to GCN 44828
Date
2026-06-05T08:46:15Z (6 days ago)
From
Ruslan Konno at Weizmann Institute of Science <ruslankonno@gmail.com>
Via
Web form

R. Konno (WIS), S. Garrappa (WIS), E. A. Zimmerman (WIS), A. Horowicz (WIS), E. O. Ofek (WIS), S. Ben-Ami (WIS), D. Polishook (WIS), O. Yaron (WIS), S. Fainer (WIS), A. Krassilchtchikov (WIS), Y. M. Shani (WIS), E. Segre (WIS), A. Gal-Yam (WIS), and S. Spitzer (WIS) report on behalf of the LAST Collaboration.

This is a correction to the GCN Circular "GRB 260604C: LAST detection of optical counterpart candidate" (GCN 44828

Loading...
 
 
). The photometry within the circular was erroneously listed in the reverse order. The correct photometry is

Tmid-T0 (h)Mag (AB)
0.3316.42 +/- 0.02
0.4516.63 +/- 0.02
0.5716.85 +/- 0.02
0.7517.12 +/- 0.02

A correction of the archival circular has been requested, apologies for any confusion.

GCN Circular 44835

Subject
GRB 260604C: Kilonova-Catcher optical afterglow detection
Date
2026-06-05T08:27:06Z (6 days ago)
From
Damien Turpin at CEA-Saclay <dturpin-astro@hotmail.com>
Via
Web form

R. Hellot, D. Christen, M. Freeberg (KNC), D. Turpin (CEA-Saclay/Irfu), C. Andrade(UMN), S. Antier (IJCLab), M. Coughlin (UMN),S. Karpov (FZU), P. Hello (IJCLAB), M. Pillas (IAP) on behalf of the GRANDMA/Kilonova-Catcher collaboration:

We observed the field of GRB 260604C detected by Fermi/GBM (Fermi GBM team, GCN 44822

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) and SVOM (Götz et al., GCN 44823) with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with a CDK17 telescope located at AITP San Pedro Chile Observatory operated by R. Hellot and D. Cristen and the A1 CDK17 telescope located at the Alnitak Observatory operated by M. Freeberg. Our observations started at TGRB+1.0 hours and were taken with sdss-r and Rc filters.

In our stacked frames, subtracted from the PanSTARRS DR2 template image, we clearly detect the optical counterpart (Lipunov et al., GCN 44825

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; GCN 44826; GCN 44827; Konno et al., GCN 44828; Zhu et al., GCN 44832; Becerra et al., GCN 44834).

We report part of our follow-up results in the table below :

Tmid-TGRB (hrs)Exp (s)FilterMagnitudeInstrument
1.4715 x 180sRc (Vega)17.43 +/- 0.03A1/CDK17
3.178 x 300sr (AB)18.41 +/- 0.05AITP/CDK17
4.0111 x 180sRc (Vega)18.43 +/- 0.06A1/CDK17

All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained with the sloan filters were calibrated using the PanSTARRS DR1 catalog. Images obtained with the Johnson-cousins filters were calibrated using the GAIA DR3 synphot catalog.

We use the SkyPortal application (skyportal.io) to monitor our observational campaign (Coughlin et al. 2023).

GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (http://kilonovacatcher.in2p3.fr/).

GCN Circular 44834

Subject
GRB 260604C: SVOM/COLIBRÍ (FM-GFT) optical observations
Date
2026-06-05T04:19:38Z (7 days ago)
From
Rosa Leticia Becerra Godínez at Instituto de Astronomía, UNAM <rbecerra@astro.unam.mx>
Via
Web form
Rosa L. Becerra (UNAM), Sarah Antier (IJCLAB), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (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), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), and Sofia Bisero (CEA Saclay) report:

We imaged the field of GRB 260604C (Fermi Team, GCN Circ. 44822, Gotz et al., GCN Circ. 44823) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-06-05 03:41 to 03:51 UTC (from 7.34 to 7.52 hours after the trigger) and obtained 3, 3, 3, 3 and 6 minutes of exposure in the g, r, i, z and y filters respectively.

The data were reduced, coadded and analysed with the ASU COLIBRÍ 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 detected the optical counterpart reported by Lipunov et al. (GCN Circ. 44827), at preliminary magnitudes of:

g = 19.69 +/- 0.04 
r = 19.56 +/- 0.03 
i = 19.38 +/- 0.04 
z = 19.21 +/- 0.06 
y = 19.26 +/- 0.08

These values are consistent with those reported by MASTER (Lipunov et al., GCN Circ. 44827), LAST (Gotz et al., GCN Circ. 44828), Jinshan (Zhu et al., GCN Circ. 44832). Comparing to the observations with LAST at about 0.75 hours, we estimate a temporal decay index of about -1.2.

Moreover, the detections in all filters and the values in the colours, suggest a low-redshift event. We also notice the presence of a red galaxy with g = 21.69 +/- 0.02, r = 20.15 +/- 0.01, z = 19.10 +/- 0.01 and a photo-z = 0.343 +/- 0.031 (DESI Legacy Survey catalog; Dey et al. 2019) at 18.9" from the optical counterpart position. This corresponds to a Pcc ≈ 0.16 and a projected offset of 95 kpc, which makes an association very unlikely.

We also comment that the light curve and analysis reported by the Fermi/GBM Team (GCN Circ. 44822, 44831) suggests hard emission with a duration that cannot completely rule out a Type I burst.

Further observations and analysis 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 44832

Subject
GRB 260604C: Jinshan optical observations
Date
2026-06-05T02:52:57Z (7 days ago)
Edited On
2026-06-05T14:31:12Z (6 days ago)
From
Zipei Zhu at NAOC <zpzhu@nao.cas.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Zipei Zhu at NAOC <zpzhu@nao.cas.cn>
Via
Web form
Z.P. Zhu, X. Liu, D. Xu, J. An,S.Q. Jiang, L.B. He (NAOC), S.Y. Fu, H.Z. Wu, A.D. Zhu, L. Lei (HUST), J.Z. Liu (XAO) report:

We observed the field of GRB260604C detected by Fermi-GBM (Fermi GBM team, GCN 44822) and SVOM (Gotz et al, GCN 44823), using the JinShan 100B telescope located at Altay, Xinjiang, China. Observation started at 2026-06-04T20:32:14 UT, i.e., about 13 min post-burst, and a series of r-band frames were obtained.

The previously reported optical counterpart (Lipunov et al, GCN 44827; Konno et al, GCN 44828) is detected in our first single frame with a brightness of R = 16.1 +/- 0.1, calibrated with Pan-STARRS DR2 stars in the field and not corrected for Galactic extinction.

We acknowledge the excellent support from T.Q. Chen, and J.F. Zhang for enabling these observations.

GCN Circular 44831

Subject
GRB 260604C: Fermi GBM Observation
Date
2026-06-05T02:49:32Z (7 days ago)
From
eliza.neights@gmail.com
Via
Web form
E. Neights (GWU, NASA GSFC) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 20:18:44.41 UT on 04 June 2026, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 260604C (trigger 802297129/260604846),
which was also detected by SVOM ECLAIRs (Gotz et al. 2026, GCN 44823), 
MASTER (Lipunov et al. 2026, GCN 44827), and LAST (Konno et al. 2026, GCN 44828).
The Fermi GBM on-ground location is consistent with the SVOM ECLAIRs position.

The angle from the Fermi LAT boresight is 102 degrees.

The GBM light curve consists of a single emission episode with a duration (T90)
of about 6.4 s (50-300 keV). The time-averaged spectrum
from T0-2.2 to T0+12.0 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -1.03 +/- 0.01 and the cutoff energy,
parameterized as Epeak, is 551 +/- 2 keV.

The event fluence (10-1000 keV) in this time interval is
(2.55 +/- 0.05)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+4.5 s in the 10-1000 keV band is 44.1 +/- 0.6 ph/s/cm^2.

A Band function fits the spectrum equally well
with Epeak= 394 +/- 3 keV, alpha = -0.95 +/- 0.01 and beta = -2 +/- 0.03.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog:
https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html

For Fermi GBM data and info, please visit the official Fermi GBM Support Page:
https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/"

GCN Circular 44828

Subject
GRB 260604C: LAST detection of optical counterpart candidate
Date
2026-06-04T23:52:38Z (7 days ago)
Edited On
2026-06-05T14:31:22Z (6 days ago)
From
Ruslan Konno at Weizmann Institute of Science <ruslankonno@gmail.com>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Ruslan Konno at Weizmann Institute of Science <ruslankonno@gmail.com>
Via
Web form

R. Konno (WIS), S. Garrappa (WIS), E. A. Zimmerman (WIS), A. Horowicz (WIS), E. O. Ofek (WIS), S. Ben-Ami (WIS), D. Polishook (WIS), O. Yaron (WIS), S. Fainer (WIS), A. Krassilchtchikov (WIS), Y. M. Shani (WIS), E. Segre (WIS), A. Gal-Yam (WIS), and S. Spitzer (WIS) report on behalf of the LAST Collaboration.

We report observations of GRB 260604C, detected by Fermi-GBM (Fermi GBM team, GCN 44822

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) and SVOM (Gotz et al, GCN 44823). Observations were conducted with the Large Array Survey Telescope (LAST; Ofek et al. 2023, PASP 135, 5001; Ben-Ami et al. 2023, PASP 135, 5002).

Observations of GRB 260604C were taken over 4 sequential epochs in clear band (similar to the Gaia Bp band). The first epoch began at 2026-06-04 20:35:04 UTC (T-T0 = 0.275 h). Each epoch consists of 20x20s exposures per telescope.

We detect one credible optical transient within the field reported by SVOM (Gotz et al, GCN 44823

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) consistent with the position of AT 2026ofi (TNS Report No. 307026) reported by MASTER (Lipunov et al, GCN 44827). The transient is well detected within each epoch and shows a rapid decay, consistent with afterglow emission.

Tmid-T0 (h)Mag (AB)
0.3316.42 +/- 0.02
0.4516.63 +/- 0.02
0.5716.85 +/- 0.02
0.7517.12 +/- 0.02

Previous observations of the field yield no detections on the source positions up to 2026-06-01 20:46:47 UTC (T0-T = 3d) with a 5-sigma limiting magnitude of 20.15 (AB).

We encourage further follow-up observations.

LAST is a survey telescope array of the Weizmann Astrophysical Observatory (https://www.weizmann.ac.il/wao/).

GCN Circular 44827

Subject
GRB 260604C: MASTER OT J145749.59+284903.0 optical counterpart discovery
Date
2026-06-04T22:45:21Z (7 days ago)
From
Vladimir Lipunov at Lomonosov Moscow State University <lipunov@sai.msu.ru>
Via
Web form
MASTER OT J145749.59+284903.0 discovery
 

MASTER-Tavrida auto-detection system ( Lipunov et al., "MASTER Global Robotic Net", Advances in Astronomy, 2010, 30L )
 discovered OT source at (RA, Dec) = 14h 57m 49.59s +28d 49m 03.0s on 2026-06-04.86020 UT.

The OT magnitude (unfiltered) is 17.5m at04.86020UT (mlim=19.4).

The OT is seen in 14 images. There is no minor planet at this place.

We have reference image on 2020-09-17.71399 UT with  unfiltered mlim=  18.1m.

Spectral observations are required.

GCN Circular 44826

Subject
Fermi GRB 260604C: Global MASTER-Net observations report
Date
2026-06-04T21:20:24Z (7 days ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
email
V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, 
G.Antipov,  A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),
O.Gress, N.Budnev (ISU),
M. Segura,  F. Podesta, R.Podesta, E. Gonzalez  (Observatorio Astronomico Felix Aguilar (OAFA),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A.Sosnovskij (CrAO),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity),
D.Buckley (SAAO),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) 

MASTER-Tavrida robotic telescope  [1]  located in Russia (Lomonosov MSU, SAI Crimea astronomical station) started inspect of the Fermi GRB 260604C ( Fermi GBM team, GCN 44822) errorbox  68 sec after notice time and 121 sec after trigger time at 2026-06-04 20:20:45 UT, with upper limit up to  19.9 mag. The observations began at zenith distance = 13 deg. The sun  altitude  is -20.4 deg. 

MASTER-Kislovodsk robotic telescope  located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) started inspect of the Fermi GRB 260604C errorbox  2607 sec after notice time and 2660 sec after trigger time at 2026-06-04 21:03:04 UT, with upper limit up to  18.5 mag. The observations began at zenith distance = 28 deg. The sun  altitude  is -23.8 deg. 

The galactic latitude b = 65 deg., longitude l = 54 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=3361051

We obtain a following upper limits.  

Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

     132 | 2026-06-04 20:20:45 |      MASTER-Tavrida | (14h 45m 39.13s , +34d 23m 10.1s) |   C |    20 | 18.2 |        
     159 | 2026-06-04 20:21:08 |      MASTER-Tavrida | (14h 45m 39.22s , +34d 23m 08.2s) |   C |    30 | 18.5 |        
     201 | 2026-06-04 20:21:45 |      MASTER-Tavrida | (14h 45m 39.31s , +34d 23m 05.7s) |   C |    40 | 18.7 |        
     244 | 2026-06-04 20:22:28 |      MASTER-Tavrida | (14h 45m 39.40s , +34d 23m 03.5s) |   C |    40 | 18.7 |        
     292 | 2026-06-04 20:23:11 |      MASTER-Tavrida | (14h 45m 39.48s , +34d 23m 00.9s) |   C |    50 | 18.9 |        
     351 | 2026-06-04 20:24:05 |      MASTER-Tavrida | (14h 45m 39.57s , +34d 22m 58.3s) |   C |    60 | 19.0 |        
     424 | 2026-06-04 20:25:08 |      MASTER-Tavrida | (14h 45m 39.70s , +34d 22m 55.4s) |   C |    80 | 19.9 |        
     512 | 2026-06-04 20:26:31 |      MASTER-Tavrida | (14h 45m 39.83s , +34d 22m 52.2s) |   C |    90 | 18.1 |        
     616 | 2026-06-04 20:28:05 |      MASTER-Tavrida | (14h 45m 39.93s , +34d 22m 49.5s) |   C |   110 | 18.8 |        
     753 | 2026-06-04 20:30:17 |      MASTER-Tavrida | (14h 41m 46.90s , +33d 53m 19.6s) |   C |   120 | 19.3 |        
    2690 | 2026-06-04 21:03:04 |   MASTER-Kislovodsk | (14h 31m 34.56s , +34d 00m 31.5s) |   C |    60 | 18.4 |        
    2780 | 2026-06-04 21:04:34 |   MASTER-Kislovodsk | (14h 41m 05.21s , +33d 59m 44.5s) |   C |    60 | 18.5 |        
    2868 | 2026-06-04 21:06:02 |   MASTER-Kislovodsk | (14h 36m 52.22s , +35d 55m 19.9s) |   C |    60 | 18.3 |        
    2957 | 2026-06-04 21:07:31 |   MASTER-Kislovodsk | (14h 46m 41.72s , +35d 54m 08.3s) |   C |    60 | 18.4 |        
    3046 | 2026-06-04 21:09:00 |   MASTER-Kislovodsk | (14h 45m 53.08s , +34d 00m 34.0s) |   C |    60 | 18.5 |        
Filter C is a clear (unfiltred) band. 


The observation and reduction will continue. 
The message may be cited.

[1] - V.M. Lipunov, V.G. Kornilov, E.S. Gorbovskoy, N.A. Tiurina & A.S.Kuznetsov, 2023,  Astronomical Robotic Networks and Operative Multichanel Astrophysics, Lomonosov MSU PRESS, 591pp.
http : // www.pereplet.ru/lipunov/625.html

GCN Circular 44825

Subject
Fermi GRB 260604C: Global MASTER-Net observations report
Date
2026-06-04T21:18:02Z (7 days ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
email
V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, 
G.Antipov,  A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),
O.Gress, N.Budnev (ISU),
M. Segura,  F. Podesta, R.Podesta, E. Gonzalez  (Observatorio Astronomico Felix Aguilar (OAFA),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A.Sosnovskij (CrAO),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity),
D.Buckley (SAAO),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) 

MASTER-Tavrida robotic telescope  [1]  located in Russia (Lomonosov MSU, SAI Crimea astronomical station) started inspect of the Fermi GRB 260604C ( Fermi GBM team, GCN 44822) errorbox  68 sec after notice time and 121 sec after trigger time at 2026-06-04 20:20:45 UT, with upper limit up to  19.9 mag. The observations began at zenith distance = 13 deg. The sun  altitude  is -20.4 deg. 

MASTER-Kislovodsk robotic telescope  located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) started inspect of the Fermi GRB 260604C errorbox  2607 sec after notice time and 2660 sec after trigger time at 2026-06-04 21:03:04 UT, with upper limit up to  18.5 mag. The observations began at zenith distance = 28 deg. The sun  altitude  is -23.8 deg. 

The galactic latitude b = 65 deg., longitude l = 54 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=3361051

We obtain a following upper limits.  

Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

     132 | 2026-06-04 20:20:45 |      MASTER-Tavrida | (14h 45m 39.13s , +34d 23m 10.1s) |   C |    20 | 18.2 |        
     159 | 2026-06-04 20:21:08 |      MASTER-Tavrida | (14h 45m 39.22s , +34d 23m 08.2s) |   C |    30 | 18.5 |        
     201 | 2026-06-04 20:21:45 |      MASTER-Tavrida | (14h 45m 39.31s , +34d 23m 05.7s) |   C |    40 | 18.7 |        
     244 | 2026-06-04 20:22:28 |      MASTER-Tavrida | (14h 45m 39.40s , +34d 23m 03.5s) |   C |    40 | 18.7 |        
     292 | 2026-06-04 20:23:11 |      MASTER-Tavrida | (14h 45m 39.48s , +34d 23m 00.9s) |   C |    50 | 18.9 |        
     351 | 2026-06-04 20:24:05 |      MASTER-Tavrida | (14h 45m 39.57s , +34d 22m 58.3s) |   C |    60 | 19.0 |        
     424 | 2026-06-04 20:25:08 |      MASTER-Tavrida | (14h 45m 39.70s , +34d 22m 55.4s) |   C |    80 | 19.9 |        
     512 | 2026-06-04 20:26:31 |      MASTER-Tavrida | (14h 45m 39.83s , +34d 22m 52.2s) |   C |    90 | 18.1 |        
     616 | 2026-06-04 20:28:05 |      MASTER-Tavrida | (14h 45m 39.93s , +34d 22m 49.5s) |   C |   110 | 18.8 |        
     753 | 2026-06-04 20:30:17 |      MASTER-Tavrida | (14h 41m 46.90s , +33d 53m 19.6s) |   C |   120 | 19.3 |        
    2690 | 2026-06-04 21:03:04 |   MASTER-Kislovodsk | (14h 31m 34.56s , +34d 00m 31.5s) |   C |    60 | 18.4 |        
    2780 | 2026-06-04 21:04:34 |   MASTER-Kislovodsk | (14h 41m 05.21s , +33d 59m 44.5s) |   C |    60 | 18.5 |        
    2868 | 2026-06-04 21:06:02 |   MASTER-Kislovodsk | (14h 36m 52.22s , +35d 55m 19.9s) |   C |    60 | 18.3 |        
    2957 | 2026-06-04 21:07:31 |   MASTER-Kislovodsk | (14h 46m 41.72s , +35d 54m 08.3s) |   C |    60 | 18.4 |        
    3046 | 2026-06-04 21:09:00 |   MASTER-Kislovodsk | (14h 45m 53.08s , +34d 00m 34.0s) |   C |    60 | 18.5 |        
Filter C is a clear (unfiltred) band. 


The observation and reduction will continue. 
The message may be cited.

[1] - V.M. Lipunov, V.G. Kornilov, E.S. Gorbovskoy, N.A. Tiurina & A.S.Kuznetsov, 2023,  Astronomical Robotic Networks and Operative Multichanel Astrophysics, Lomonosov MSU PRESS, 591pp.
http : // www.pereplet.ru/lipunov/625.html

GCN Circular 44823

Subject
GRB 260604C: SVOM detection of a long burst
Date
2026-06-04T20:43:09Z (7 days ago)
From
Diego Gotz at CEA <diego.gotz@cea.fr>
Via
Web form
D. Gotz, D. Adrien, N. Dagoneau, S. Bisero (CEA Saclay) report on behalf of the SVOM mission team:
At 2026-06-04T20:21:59 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 260604C (SVOM burst-id sb26060404).

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 4 alerts. IMT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 8.23 in the [5-20] keV energy band over a time window of 163.84 seconds starting at 2026-06-04T20:19:56 UTC.

The localization of the best alert is R.A., Dec. 224.3461, 28.6558 degrees:
R.A. (J2000) = 14h57m23s
Dec. (J2000) = 28d39m20s
 with a 90% confidence level (C.L.) radius of 9.6 arcmin (including systematic error of 2 arcmin added in quadrature).

This position is compatible with the GRB detected by Fermi/GBM GCN 44822 .

SVOM slewed to the burst position.

No X-ray observation could be performed by SVOM/MXT for the time being.

Optical observations by VT will be published in a further circular.


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 Sofia Bisero: sofia.bisero@cea.fr.
Please contact the BA by email if you require additional information.

GCN Circular 44822

Subject
GRB 260604C: Fermi GBM Final Real-time Localization
Date
2026-06-04T20:31:01Z (7 days ago)
From
Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply@GIOC.nsstc.nasa.gov>
Via
email
The Fermi GBM team reports the detection of a likely LONG GRB

At 20:18:44 UT on 4 Jun 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260604C (trigger 802297129.411688 / 260604846).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 220.5, Dec = 33.4 (J2000 degrees, equivalent to J2000 14h 41m, 33d 23'), with a statistical uncertainty of 1.0 degrees.

The angle from the Fermi LAT boresight is 108.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260604846/quicklook/glg_skymap_all_bn260604846.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/bn260604846/quicklook/glg_healpix_all_bn260604846.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260604846/quicklook/glg_lc_medres34_bn260604846.gif
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