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GRB 260604C

GCN Circular 44844

Subject
GRB 260604C: LCO optical upper limit
Date
2026-06-05T21:22:48Z (6 hours 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 (14 hours 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 (18 hours 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 (19 hours 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

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). 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 (19 hours 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 (a day 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 (a day ago)
Edited On
2026-06-05T14:31:12Z (13 hours 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 (a day 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 (a day ago)
Edited On
2026-06-05T14:31:22Z (13 hours 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 (a day 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 (a day 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 (a day 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 (a day 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 (a day 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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