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GRB 260610B

GCN Circular 45065

Subject
GRB 260610B: VLA detection
Date
2026-07-02T20:07:05Z (23 days ago)
From
Tanmoy Laskar at U of Utah <tanmoylaskar@gmail.com>
Via
Web form
T. Laskar (Utah), B. P. Gompertz (Birmingham), G. E. Anderson (Curtin), A. Levan (Radboud and Warwick), Daniele Malesani (DAWN/NBI and Radboud), Steve Schulze (Weizmann), Nial Tanvir (Leicester), A. J. van der Horst (GWU) report on behalf of a larger collaboration:

“We observed GRB 260610B (Fermi GBM team, GCN 44891; O'Neill et al., GCN 44903) with the Karl G. Jansky Very Large Array (VLA) at multiple frequencies beginning on 2026 June 13 at 07:23 UT (2.89 days after the burst).

In preliminary analysis, we detect the radio counterpart (Laskar et al., GCN 44941) at 9 GHz with a flux density of ~ 0.4mJy at the position:
RA(J2000) = 14:32:38.25 +/- 0.01”
Dec(J2000) = 27:00:17.64 +/- 0.01”

This is consistent with the optical position (O'Neill et al., GCN 44903) and radio position (Laskar et al., GCN 44941). Further observations are planned.

We thank the VLA staff for scheduling and executing these observations.”

GCN Circular 45062

Subject
GRB 260610B / AT2026owq: SAO RAS, CrAO and INASAN further optical observations and light curve
Date
2026-07-01T08:35:23Z (25 days ago)
From
Alexander Moskvitin at SAO RAS <mosk@sao.ru>
Via
email
A. Moskvitin (SAO RAS), N. Pankov (IKI), A. Pozanenko (IKI),
A. Volnova (IKI), V. Vlasyuk (SAO RAS), O. Spiridonova (SAO RAS),
V. Goranskij (SAI MSU, SAO RAS), N. Pit (CrAO), V. Rumyantsev (CrAO),
I. Nikolenko (INASAN) on behalf of IKI-GRB-FuN.

We continue R-band observations of the GRB 260610B (The Fermi GBM team,
GCN 44891; GCN 44901; Godwin and Meegan, GCN 44931; Yu et al.,
GCN 44944) on June 17--20, 22--25 and 27 nights with the 1-m
Zeiss-1000, the 0.5-m AS-500 (2) telescopes of SAO RAS,
the 1.25-m AZT-11 telescope of CrAO and the 1-m Zeiss-1000/Koshka
telescope of INASAN.

The OT (O'Neill et al., GCN 44903; Watson et al., GCN 44905;
Zhu et al., GCN 44909; Gillanders et al., GCN 44910; Akl et al.,
GCN 44911; O'Neill et al., GCN 44914; Moskvitin et al., GCN 44918;
Jackson-Horne et al., GCN 44919; Izzo, GCN 44920; Li et al.,
GCN 44921; Bochenek and Perley, GCN 44927; Moskvitin et al., GCN 44929;
Akl et al., GCN 44930; Bochenek and Perley, GCN 44932; Pankov et al.,
GCN 44946; Gillanders et al., GCN 44955; Angulo et al., GCN 44958;
Dimple et al., GCN 44959; Moskvitin et al., GCN 44963; Pankov et al.,
GCN 44977; Gassert et al., GCN 44981; Gupta et al., GCN 44993;
Corcoran et al., GCN 45004; Simon et al., GCN 45007) is clearly
detected in the stacked images derived from our observations
(except AZT-11 observations on 2026-06-22 and Zeiss-1000/Koshka
data obtained on 2026-06-24, 2026-06-25, 2026-06-27).

The observational log is provided below:

Date       UTstart  Exposure, s Filter Site    Telescope
---------- -------- ----------- ------ ------- ----------
2026-06-17 20:12:50  3600       R      SAO RAS AS-500
2026-06-17 19:28:53  2700       R      CrAO    AZT-11
2026-06-18 19:46:46  5400       R      SAO RAS AS-500
2026-06-18 20:01:52  3600       Rc     SAO RAS Zeiss-1000
2026-06-19 18:56:04  1800       R      CrAO    AZT-11
2026-06-20 21:36:05  5400       R      SAO RAS AS-500
2026-06-22 20:04:20  3600       R      CrAO    AZT-11
2026-06-22 22:24:19  2880       Rc     SAO RAS Zeiss-1000
2026-06-23 19:03:08  9660       R      SAO RAS AS-500
2026-06-24 19:37:50 14400       R      INASAN   Zeiss-1000/Koshka
2026-06-25 21:47:04  5400       R      SAO RAS AS-500
2026-06-25 19:16:02 14400       R      INASAN   Zeiss-1000/Koshka
2026-06-27 19:39:42  8100       R      INASAN   Zeiss-1000/Koshka

A preliminary light curve constructed from observations is available at:
https://heaiki.ru/lvc/r/GRBs/GRB260610B/GRB260610B_LC1.jpg
Besides of the relatively fast brightening started at about 5 days
after trigger (Gillanders et al., GCN 44955; Angulo et al., GCN 44958;
Dimple et al., GCN 44959; Moskvitin et al., GCN 44963; Gupta et al.,
GCN 44993; Corcoran et al., GCN 45004; Simon et al., GCN 45007)
we note slow fading till 15th day. The absolute magnitude
and time of maximum (assuming redshift of z = 0.473 the absolute
magnitude is about M_R = -20.8) do not contradict that brightening
could be a manifestation of a supernova.

GCN Circular 45007

Subject
GRB 260610B: Further GRANDMA Optical Observations and re-brightening detection
Date
2026-06-19T15:24:48Z (a month ago)
From
marion.pillas@ligo.org
Via
Web form
A. Simon (TShNU of Kyiv), M. Mašek (FZU), N. Kochiashvili (AbAO), M. Pillas (IAP), D. Akl (NYUAD), R. Hanna (IJCLAB), P. Putsathan (KKU), Y. Rajabov (UBAI), S. Antier (IJCLAB), P. Hello (IJCLAB), M. Coughlin (UNM), S. Karpov (FZU), C. Douzet (IJCLAB), D. Rusanova (TShNU of Kyiv), V. Shynkarova (TShNU of Kyiv), A. Korosko (TShNU of Kyiv), O. Burkhonov (UBAI), S. Ehgamberdiev (UBAI), Y. Tillayev (UBAI), P. Put (NARIT), K. Noysena (NARIT), M. Tanasan (CMU/NARIT), D. Turpin (CEA-Saclay/Irfu), M. Freeberg (KNC), C. Limonta (OCA), A. Dupin (OCA), Q. André (OCA), M. Molham (NRIAG), E. Elhosseiny (NRIAG), A. Takey (NRIAG), T. Kvernadze (AbAO) on behalf of on GRANDMA collaboration: 

We continued our observations of the field of the Fermi Long GRB 260610B (Fermi GBM team, GCN 44901). The additional observations started on June 2026, 16 up to now. 

We still detect the optical afterglow using TTT-200cm, T150 at Abastumani Observatory, UBAI-AZT-22 at Madainak Observatory, AZT-8 at Lisnyky Observatory, KAO-1.88m at Kottamia Astronomical Observatory. We detect a re-brightening, which is consistent with other measurements reported in GCN 44955, GCN 44958, GCN 44959, GCN 44963, GCN 44993, 44999, 45003, 45004. A subset of the photometry corrected for Galactic extinction is reported in the public ACME service Skyportal/ICARE https://skyportal-icare.ijclab.in2p3.fr/source/2026owq. To access, one simply needs to create an account using your ORCID id.

The data were 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. 

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 45004

Subject
GRB 260610B: VLT/FORS2 spectroscopic observation suggests the non-thermal nature of the optical rebrightening
Date
2026-06-19T14:42:59Z (a month ago)
From
Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>
Via
Web form
G. Corcoran (UCD), A. Martin-Carrillo (UCD), L. Izzo (INAF/OAC and DARK/NBI), J. An (NAOC), D. B. Malesani (DAWN/NBI and Radboud), B. P. Gompertz (Birmingham), A. J. Levan (Radboud and Warwick), A. de Ugarte Postigo (LAM), report on behalf of the Stargate collaboration:

We performed spectroscopic observations of GRB 260610B discovered by Fermi (Fermi GBM team, GCN 44901; Godwin & Meegan GCN 44931) and SVOM (Yu et al., GCN 44944), whose optical afterglow has recently experienced an optical rebrightening (O’Neill et al., GCNs 44903, 44914; Watson et al., GCN 44905; Zhu et al., GCN 44909; Gillanders et al., GCNs 44910, 44955; Akl et al., GCNs 44911, 44930; Moskvitin et al., GCNs 44918, 44929; Jackson-Horne et al., GCN 44919; Izzo et al., GCN 44920; Li et al., GCN 44921; Bochenek et al., GCNs 44927, 44932; Pankov et al., GCN 44946; Angulo et al., GCN 44958; Moskvitin et al., GCN 44963; Pankov et al., GCN 44977; Gassert et al., GCN 44981; Gupta et al., GCN 44993). 

We used the ESO Very Large Telescope (VLT) UT1 (Antu) equipped with the FORS2 spectrograph with the 300V grism, covering the wavelength range 4400-8650 AA. The observation consisted of 4 exposures of 600 s each. The observation started at 00:41:19 UT on June 17 2026 (6.04 days after the GRB). In our acquisition image, we measured an AB magnitude r ~ 20 (calibrated against Pan-STARRS, not corrected for Galactic extinction). Based on the reported light curve, this seems to indicate that our observation occurred during the peak of this late flaring episode.

The spectrum of the optical counterpart reveals a power-law continuum across the entire covered range suggesting that the observed recent activity is likely caused by late jet activity. Faint undulations are seen overimposed to the continuum, at a flux level consistent with the emergence of an early, still faint SN, but further observations will be required to conclude about its presence. Additionally, through the detection of the Ca II doublet in absorption and [O II] doublet in emission, we confirm the redshift reported by O’Neill et al. (GCN 44914), measuring z = 0.474.

Further spectroscopic observations of the source are planned to monitor the possible emergence of the supernova.

We acknowledge excellent support from the ESO observing staff in Paranal, in particular Martina Baratella and Leo Rivas, for rapidly executing these observations.

GCN Circular 44993

Subject
GRB 260610B/AT2026owq: 1.3m DFOT optical rebrightening detection
Date
2026-06-19T10:18:21Z (a month ago)
From
ANSHIKA GUPTA at ARIES <anshika05180@gmail.com>
Via
Web form
Anshika Gupta, Divyanshu Janghel, Debalina kar, Pankaj Pawar, Dhruv Jain and Kuntal Misra (ARIES) report:

We observed the field of GRB 260610B/AT 2026owq discovered by Fermi (The Fermi GBM team, GCN 44901; Godwin and Meegan; GCN 44931; Yu et al., GCN 44944)) with the 1.3m Devasthal Fast Optical Telescope (DFOT), located at the Devasthal Observatory of the Aryabhatta Research Institute of Observational Sciences (ARIES), India. The observations were started on 2026-06-15 at 16:53:40 UT, i.e., ~ 4.71 days 
after the Fermi trigger (GCN 44901). We have taken multiple frames with an exposure time of 300 s in the R and I filter. We stacked the images after the alignment. We detect a optical afterglow in our stacked image. We obtain the following preliminary magnitude in the stacked image:

Date Mid_UT T_start-T0 (days) Filter  Exp time (s)  Magnitude
=================================================================
2026-06-15 17:24:09.406   ~4.73   R     300s*12     20.88 +/-0.05


The optical detection of the burst is consistent with O'Neill et al., GCN 44903; Watson et al., GCN 44905; Zhu et al., GCN 44909; Gillanders et al., GCN 44910; Akl et al., GCN 44911; O'Neill et al., GCN 44914; Moskvitin et al., GCN 44918; Jackson-Horne et al., GCN 44919; Izzo, GCN 44920; Li et al., GCN 44921; Bochenek and Perley, GCN 44927; Moskvitin et al., GCN 44929; Akl et al., GCN 44930; Bochenek & Perley, GCN 44932; Pankov et al., GCN 44946; Gillanders et al., GCN 44955; Angulo et al., GCN 44958; Dimple et al., GCN 44959; Moskvitin et al., GCN 44963; Pankov et al., GCN 44977; Gassert et al., GCN 44981.

The magnitude is not corrected for the Galactic extinction in the direction of the burst. Photometric calibration is performed using the standard stars from the Gaia DR3 Synphot catalog.

GCN Circular 44981

Subject
GRB 260610B/AT 2026owq: FTW optical and NIR observations
Date
2026-06-18T19:32:02Z (a month ago)
From
Julius Gassert at Ludwig-Maximilians-Universität München <julius.gassert@campus.lmu.de>
Via
Web form
Julius Gassert (LMU/CMU), Malte Busmann (LMU), Brendan O'Connor (CMU), Daniel Gruen (LMU), Xander Hall (CMU), and Antonella Palmese (CMU) report:

We observed the afterglow of GRB260610B (AT 2026owq) (Fermi/GBM; GCN 44901; O’Neill et al., GCNs 44903, 44914; Watson et al., GCN 44905; Zhu et al., GCN 44909; Gillanders et al., GCN 44910; Akl et al., GCNs 44911, 44930; Moskvitin et al., GCNs 44918, 44929; Jackson-Horne et al., GCN 44919; Izzo et al., GCN 44920; Li et al., GCN 44921; Bochenek et al., GCNs 44927, 44932; Pankov et al., GCN 44946),  with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the g, r, i, z, and J bands for 8x 180s starting at 2026-06-17 22:48 UT. 

As previously reported (Gillanders et al., GCN 44955; Angulo et al., GCN44958; Dimple et al., GCN44959; Moksvitin et al. GCN44963; Pankov et al., GCN44977), we also detect the rebrightening/flattening in all bands, althought the i-band detection is marginal.

In the r band, we detect the counterpart at:

r = 21.31 +/- 0.10 AB mag

The magnitude is calibrated against the PS1 catalog and not corrected for Galactic extinction.

We thank Silona Wilke from the Wendelstein Observatory for obtaining these observations.

GCN Circular 44977

Subject
GRB 260610B: AbAO optical observations
Date
2026-06-18T10:47:10Z (a month ago)
From
Nicolai Pankov at IKI <colinsergesen@gmail.com>
Via
Web form
N. Pankov (IKI), R. Ya. Inasaridze (AbAO), A. Moskvitin (SAO RAS), V. Aivazyan (AbAO), A. Pozanenko (IKI), A. Volnova (IKI) report on behalf of IKI-GRB-FuN:

We observed the field of GRB 260610B (The Fermi GBM team, GCN 44891; GCN 44901) with the AS-32 70-cm telescope of the Abastumani Astrophysical Observatory (AbAO). We acquired a series of 98x60 sec images in the R-band using a CCD-photometer, starting on 2026-06-12 20:18 UT (i.e. 1.856 days since trigger). The OT (O'Neill et al., GCN 44903; Watson et al., GCN 44905; Zhu et al., GCN 44909; Gillanders et al., GCN 44910; Akl et al., GCN 44911; O'Neill et al., GCN 44914; Moskvitin et al., GCN 44918; Jackson-Horne et al., GCN 44919; Izzo, GCN 44920; Li et al., GCN 44921; Bochenek and Perley, GCN 44927; Moskvitin et al., GCN 44929; Akl et al., GCN 44930; Bochenek & Perley, GCN 44932; Pankov et al., GCN 44946; Gillanders et al., GCN 44955; Angulo et al., GCN 44958; Dimple et al., GCN 44959; Moskvitin et al., GCN 44963) is detected in the co-add image of 96x60 sec. The observational properties and preliminary photometry are provided further:

Date       UTstart    t-T0    Exp. Filter   Mag  Err   UL  
                     (mid,d) (n*s)                    (3-sigma)
2026-06-12 20:18:50 1.88931  96x60  R      20.59 0.10  22.0

The photometry is based on nearby SDSS stars (R-magnitudes obtained via Lupton 2005 transformation equations; Moskvitin et al., GCN 44918) and not corrected for the Galactic extinction.

GCN Circular 44963

Subject
GRB 260610B / AT2026owq: SAO RAS optical observations of possible re-brightening
Date
2026-06-17T09:33:10Z (a month ago)
From
Alexander Moskvitin at SAO RAS <mosk@sao.ru>
Via
Web form
A. Moskvitin (SAO RAS), O. Maslennikova (SAO RAS),
V. Goranskij (SAI MSU, SAO RAS), A. Pozanenko (IKI), A. Volnova (IKI),
N. Pankov (IKI) report on behalf of GRB follow-up team

We observed the field of GRB 260610B discovered by Fermi (The Fermi
GBM team, GCN 44891; GCN 44901; Godwin and Meegan; GCN 44931),
and also detected by SVOM (Yu et al., GCN 44944), with the Zeiss-1000 1-m
telescope of SAO RAS + CCD photometer in Rc band on June 14,
15 and 16.


The OT (O'Neill et al., GCN 44903; Watson et al., GCN 44905;
Zhu et al., GCN 44909; Gillanders et al., GCN 44910; Akl et al.,
GCN 44911; O'Neill et al., GCN 44914; Moskvitin et al., GCN 44918;
Jackson-Horne et al., GCN 44919; Izzo, GCN 44920; Li et al.,
GCN 44921; Bochenek and Perley, GCN 44927; Moskvitin et al., GCN 44929;
Akl et al., GCN 44930; Bochenek and Perley, GCN 44932; Pankov et al.,
GCN 44946; Gillanders et al., GCN 44955; Angulo et al., GCN 44958;
Dimple et al., GCN 44959) is detected in our stacked images
with the following brightness:

Date,      UTstart, t-T0,   Exp.,  Filter, Mag, Err., UL
                    (mid,d) (n*s)                    (3-sigma)
                   
2026-06-14 23:15:23 4.56847 15*180 Rc      21.14 0.12 22.3
2026-06-15 23:42:44 5.57617  8*180 Rc      20.98 0.17 21.8
2026-06-16 20:30:40 6.44388  3*600 Rc      21.06 0.18 21.9

This preliminary photometry is based on nearby SDSS stars
(using Lupton 2005 transformation equations) and not corrected
for Galactic extinction.
 
Although we cannot directly confirm that the afterglow brightness has increased, we can observe a plateau in the afterglow light curve.

GCN Circular 44959

Subject
GRB 260610B / AT 2026owq: Liverpool Telescope confirmation of continuing optical rebrightening
Date
2026-06-16T20:04:25Z (a month ago)
From
Dimple at University of Birmingham <dimplepanchal96@gmail.com>
Via
Web form
Dimple (U. Birmingham), B. P. Gompertz (U. Birmingham), A. Bochenek (LJMU), D. A. Perley (LJMU), D. O’Neill (U. Birmingham), D. B. Malesani (DAWN/NBI and Radboud) and A. Martin-Carrillo (UCD) report on behalf of a larger collaboration:

We have conducted nightly griz observations of the optical afterglow (O’Neill et al., GCNs 44903, 44914; Watson et al., GCN 44905; Zhu et al., GCN 44909; Gillanders et al., GCNs 44910, 44955; Akl et al., GCNs 44911, 44930; Moskvitin et al., GCNs 44918, 44929; Jackson-Horne et al., GCN 44919; Izzo et al., GCN 44920; Li et al., GCN 44921; Bochenek et al., GCNs 44927, 44932; Pankov et al., GCN 44946; Angulo et al., GCN 44958) of GRB 260610B (Fermi GBM Team, GCN 44901; Yu et al., GCN 44944) with the IO:O optical camera on the 2m robotic Liverpool Telescope.

In our two most recent epochs we detect the optical rebrightening first suggested by Gillanders et al. between their t-t0 = 3.3 and 4.3 day epochs (GCN 44955) in all filters. The measured g- and r-band AB magnitudes are:

|  t-t0(d)  |  filter  |  mag  |
|  3.99  |  g  |  21.71 ± 0.07  |
|  3.97  |  r  |  21.34 ± 0.07  |
|  5.08  |  g  |  21.49 ± 0.08  |
|  5.09  |  r  |  21.02 ± 0.07  |

These observations indicate that GRB 260610B continues experiencing a rebrightening episode beyond 5 days after trigger. We detect no change in g-r colour when comparing our observations to the earliest Liverpool Telescope epoch (Bochenek & Perley, GCN 44927), suggesting an achromatic rebrightening, inconsistent with expectations for an emerging supernova.

The above magnitudes are calibrated against nearby Pan-STARRS objects and are not corrected for Galactic extinction.

GCN Circular 44958

Subject
GRB 260610B / AT 2026owq: COLIBRÍ further observations and confirmation of the rebrigthening
Date
2026-06-16T19:58:21Z (a month ago)
From
Rosa Leticia Becerra Godínez at Instituto de Astronomía, UNAM <rbecerra@astro.unam.mx>
Via
Web form
Camila Angulo (UNAM), Marion Guelfand (CPPM), Rosa L. Becerra (UNAM), Nikos Mandarakas (LAM), Antonio de Ugarte Postigo (LAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Dalya Akl (NYUAD), Jean-Luc Atteia (IRAP),Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), 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 Alan M. Watson (UNAM) report:

We reimaged the field of AT 2026owq (O'Neill et al., GCN Circ. 44903, 44914; Watson et al., GCN Circ. 44905; Zhu et al., GCN Circ. 44909; Gillanders et al., GCN Circ. 44910; Akl et al., GCNs 44911, 44930; Moskvitin et al., GCNs 44918, 44929; Jackson-Horne et al., GCN Circ. 44919; Izzo et al., GCN Circ. 44920; Li et al., GCN Circ. 44921; Bochenek et al., GCNs 44927, 44932; Pankov et al., GCN Circ. 44946; Gillanders  et al., GCN Circ. 44995), which is the optical counterpart of GRB 260610B (Fermi GBM team, GCN Circ. 44901), using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. In our latest epoch, we observed from 2026-06-16 07:26 to 08:41 UTC (from 5.32 to 5.37 days after the GRB trigger) and obtained 6, 4, 5, and 14 minutes of exposure in the g, r, i, and z filters, respectively.

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

We detect the source AT2026owq with preliminary magnitudes of:

g = 21.52 +/- 0.13, 
r = 21.02 +/- 0.10,
i = 20.69 +/- 0.09, 
z = 20.43 +/- 0.10.

Compared with previous observations reported for this event, we also observe the rebrightening/flattening first reported by the Pan-STARRS team (Gillanders et al., GCN Circ. 44995), beginning approximately 80 hours after the trigger and visible in all of our filters.

We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams.

COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and México (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, México.

GCN Circular 44955

Subject
GRB 260610B / AT 2026owq: Possible re-brightening identified from Pan-STARRS observations
Date
2026-06-16T15:30:51Z (a month ago)
From
James Gillanders at University of Oxford <jhgillanders.astro@gmail.com>
Via
Web form
J. H. Gillanders (Oxford), M. Huber, K. C. Chambers (IfA, Univ. Hawaii), S. J. Smartt, K. W. Smith (Oxford/QUB), S. Srivastav (Oxford), M. Nicholl, D. Young (QUB), T.-W. Chen (NCU, Taiwan), A. S. B. Schultz, T. de Boer, J. Fairlamb, G. Paek, C. C. Lin, T. Lowe, E. Magnier, P. Minguez, I. A. Smith, R. J. Wainscoat (IfA, Univ. Hawaii).

We have been observing AT 2026owq (O’Neill et al., GCNs 44903, 44914; Watson et al., GCN 44905; Zhu et al., GCN 44909; Gillanders et al., GCN 44910; Akl et al., GCNs 44911, 44930; Moskvitin et al., GCNs 44918, 44929; Jackson-Horne et al., GCN 44919; Izzo et al., GCN 44920; Li et al., GCN 44921; Bochenek et al., GCNs 44927, 44932; Pankov et al., GCN 44946), the optical counterpart to GRB 260610B (detected by Fermi/GBM; GCN 44901) with a nightly cadence in grizy using the Pan-STARRS telescope system (Chambers et al., 2016) since MJD 61202.4. The Pan-STARRS system consists of two 1.8m telescope units located at the summit of Haleakala on the Hawaiian island of Maui, employing an SDSS-like filter system denoted as grizy.

Our observations consist of 200 second exposures, and have been processed with the standard Pan-STARRS pipeline. After astrometric and photometric calibration, reference images were subtracted from the target images (Magnier et al., 2020a; Magnier et al., 2020b; Waters et al., 2020).

Our most recent photometry indicates that the lightcurve evolution of AT 2026owq is no longer fading; instead, the transient appears to be flattening, or re-brightening, across all of griz. Full photometry is presented below. Note that these photometric measurements have not been corrected for Galactic extinction. 

MJD | Filter | Mag (AB)
61205.271 | g | 21.7+/-0.2
61205.273 | r | 21.2+/-0.1
61205.276 | i | 20.9+/-0.1
61205.278 | z | 20.7+/-0.2
61206.276 | g | 21.8+/-0.3
61206.270 | r | 21.3+/-0.2
61206.268 | i | 20.7+/-0.2
61206.265 | z | 20.6+/-0.2

At the redshift of AT 2026owq (z=0.473; O’Neill et al., GCN 44914), observer-frame i approximately corresponds to rest-frame g. Thus, the absolute magnitude of AT 2026owq on MJD 61206.3, in rest-frame g, is approximately -21.4. Given that, at MJD 61206.3, the transient is only ~2.9 rest-frame days post-explosion, this excess is too bright to be powered by the emergence of a typical GRB-supernova signal. Further follow-up is scheduled with Pan-STARRS to determine its nature.

GCN Circular 44946

Subject
GRB 260610B: Terskol optical observations
Date
2026-06-15T08:46:27Z (a month ago)
From
Nicolai Pankov at IKI <colinsergesen@gmail.com>
Via
Web form
N. Pankov (IKI), A. Salakhotdinova (INASAN), A. Shein (INASAN), E. Postnikova (INASAN), A. Pozanenko (IKI), A. Moskvitin (SAO RAS), A. Volnova (IKI) report on behalf of IKI-GRB-FuN:

We observed the field of GRB 260610B (The Fermi GBM team, GCN 44891; GCN 44901) with the Zeiss-2000 2-meter telescope of the Terskol Observatory on epochs 2026-06-11 and 2026-06-12. The R-band observations were carried out using the CCD-photometer. In the first epoch we acquired a series of 3x120+12x120 sec images, while in the latter epoch 9x120 sec images were obtained. The OT (O'Neill et al., GCN 44903; Watson et al., GCN 44905; Zhu et al., GCN 44909; Gillanders et al., GCN 44910; Akl et al., GCN 44911; O'Neill et al., GCN 44914; Moskvitin et al., GCN 44918; Jackson-Horne et al., GCN 44919; Izzo, GCN 44920; Li et al., GCN 44921; Bochenek and Perley, GCN 44927; Moskvitin et al., GCN 44929; Akl et al., GCN 44930; Bochenek & Perley, GCN 44932) is clearly detected in our co-add images. The observational properties and preliminary photometry are provided further:

Date       UTstart    t-T0    Exp.       Filter   Mag  Err    UL  
                     (mid,d) (n*s)                           (3-sigma)
2026-06-11 21:33:51 1.48481  3x120+12x90   R     19.81 0.06   22.4
2026-06-12 19:46:19 2.40802  9x120         R     20.70 0.13   22.3

The photometry is based on nearby SDSS stars (R-magnitudes obtained via Lupton 2005 transformation equations; Moskvitin et al., GCN 44918) and not corrected for the Galactic extinction.

GCN Circular 44944

Subject
GRB 260610B: SVOM/GRM observation
Date
2026-06-15T08:36:29Z (a month ago)
From
yzh807926@163.com
Via
Web form
SVOM/GRM team: Zheng-Hang Yu, Chen-Wei Wang, Yue Huang, Shi-Jie Zheng, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)

SVOM/ECLAIRs team: Marius Brunet (IRAP)

Report on behalf of the SVOM team:

SVOM/GRM was triggered on-ground by a burst GRB 260610B at 2026-06-10T23:46:14.100 UTC (T0), which is also detected by Fermi/GBM (Fermi GBM Team, GCN #44901).

With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of a main episode with two peaks a T90 of 65.0 +/-8.0 s in the 15-5000 keV band. 

The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb260610B.png

In addition, the position of this burst, as determined by Fermi/GBM (RA= 218.66 deg, DEC= 23.94 deg, ERR=2.77 deg, GCN#44901), is located at about 74 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view. 

With this localization, the time-averaged spectrum from T0-10 to T0+60 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.59 +0.18/-0.22 and the cutoff energy, parameterized as Epeak, is 34 +6/-8 keV. The event fluence (10-1000 keV) in this time interval is (9.48 +0.50/-0.47)E-06 erg/cm^2.

The 1s peak spectrum, measured from T0+1.9 to T0+2.9 s, if fitted by a power law function with an exponential high-energy cutoff, the power law index is -1.62 +0.16/-0.32 and the cutoff energy, parameterized as Epeak, is 25 +8/-7 keV. The flux (10-1000 keV) in this time interval is (3.98 +0.41/-0.37)E-07 erg/cm^2/s.

With the measured redshift z=0.473 (D. O'Neill et al., GCN#44914), we calculate the isotropic energy Eiso is about 8.8E51 erg. Thus GRB 260610B is well consistent with Type II GRBs in the 'Amati' relation diagram, as shown at:
https://www.bursthub.cn//admin/static/svgrb260610B_amati.png
The localization of GRB 260421B in the 'Yonetoku' relation diagram is shown at:
https://www.bursthub.cn//admin/static/svgrb260610B_yonetoku.png

The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. GRM is developed by the Institute of High Energy Physics (IHEP) of CAS.

The SVOM point of contact for this burst is: Zheng-Hang Yu(IHEP)(zhyu@ihep.ac.cn)

GCN Circular 44941

Subject
GRB 260610B: ALMA detection
Date
2026-06-14T21:57:00Z (a month ago)
From
Tanmoy Laskar at U of Utah <tanmoylaskar@gmail.com>
Via
Web form
T. Laskar (Utah), B. P. Gompertz (Birmingham), G. E. Anderson (Curtin), A. Levan (Radboud and Warwick), Daniele Malesani (DAWN/NBI and Radboud), Steve Schulze (Weizmann), Nial Tanvir (Leicester), A. J. van der Horst (GWU) report on behalf of a larger collaboration:

“We observed GRB 260610B (Fermi GBM team, GCN 44891; O'Neill et al., GCN 44903) with the Atacama Large Millimeter/Submillimeter Array (ALMA) at 43 GHz beginning on 2026 June 13 at 01:06 UT (2.62 days after the burst).

Preliminary analysis reveals a mm source with flux density of ~ 0.3 mJy at position:

RA (J2000) = 14:32:38.2
Dec (J2000) = +27:00:17.6

with an uncertainty of 0.2" in each coordinate. This is consistent with the optical position (O'Neill et al., GCN 44903). Further observations are planned.

We thank the P2G, AoD, JAO staff, and the entire ALMA team for their help with these observations."

GCN Circular 44932

Subject
GRB 260610B: further Liverpool Telescope optical observations
Date
2026-06-13T19:36:46Z (a month ago)
From
A. Bochenek at Liverpool John Moores University <a.m.bochenek@2023.ljmu.ac.uk>
Via
Web form
A. Bochenek, D. A. Perley (LJMU), report:

We observed the field of the optical counterpart  (O'Neil et al., GCN 44903) of GRB 260610b (Fu et al., GCN 44901) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained 5x120s exposures in SDSS griz filters, starting at 2026-06-13 01:22:29 UT, approximately 2.07 days after Fermi trigger.

Some exposures had to be discarded prior to stacking. We detect a source in the images in all filters at the position reported by O'Neil et al. (GCN 44903), observed also by Watson et al. (GCN 44905); Zhu et al. (GCN 44909); Gillanders et al. (GCN 44910); Akl et al. (GCN 44911); O'Neill et al. (GCN 44914); Moskvitin et al. (GCN 44918); Jackson-Horne et al. (GCN 44919); Izzo, (GCN 44920), Li et al. (GCN 44921), Bochenek et al. (GCN 44927), Moskvitin et al. (GCN 44929); and Akl et al. (GCN 44930) . The photometry is:

| MJD (mid)     | T_mid-T_0   | Filter    | Mag. (AB)     |
|---------------|-------------|-----------|---------------|
| 61204.05962   | 49.66 h     | r         | 21.05 ± 0.07  |
| 61204.07237   | 49.97 h     | i         | 20.61 ± 0.09  |
| 61204.07751   | 50.08 h     | z         | 20.28 ± 0.12  |
| 61204.08753   | 50.33 h     | g         | 21.40 ± 0.14  |

The photometry was calibrated using nearby PanSTARRS secondary standards and was not corrected for extinction.

GCN Circular 44931

Subject
GRB 260610B: Fermi GBM Observation
Date
2026-06-13T18:21:44Z (a month ago)
From
Matt Godwin <msg0028@uah.edu>
Via
Web form
Matt Godwin (UAH) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team:

"At 23:46:14.25 UT on 10 June 2026, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 260610B (trigger 802827979/260610990).
An optical counterpart is identified by GOTO (O'Neill, et al. 2026, GCN 44903). The redshift of the source is measured to be z = 0.473, ALFOSC/NOT (ONeill et al. 2026, GCN 44914).
The Fermi GBM on-ground location is consistent with the GOTO position.

The angle from the Fermi LAT boresight is 97 degrees.

The GBM light curve consists of three peaks with a duration (T90)
of about 62 s (50-300 keV). The time-averaged spectrum
from T0-2.4 to T0+55.4 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -1.50 +/- 0.01 and the cutoff energy,
parameterized as Epeak, is 46.8 +/- 0.4 keV.

The event fluence (10-1000 keV) in this time interval is
(1.30 +/- 0.02)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+3.8 s in the 10-1000 keV band is 7.7 +/- 0.3 ph/s/cm^2.

A Band function fits the spectrum equally well
with Epeak= 45.9 +/- 0.5 keV, alpha = -1.4800 +/- 0.0009 and beta = -3.17 +/- 0.01.

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 44930

Subject
GRB 260610B / AT2026owq: GRANDMA optical observations
Date
2026-06-13T13:50:34Z (a month ago)
Edited On
2026-06-13T14:41:33Z (a month ago)
From
Dalya Akl at New York University Abu Dhabi <dka2010@nyu.edu>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Dalya Akl at New York University Abu Dhabi <dka2010@nyu.edu>
Via
Web form
D. Akl (NYUAD), M. Mašek (FZU), R. Hanna (IJCLAB), P. Putsathan (KKU), Y. Rajabov (UBAI), S. Antier (IJCLAB), P. Hello (IJCLAB), M. Coughlin (UNM), S. Karpov (FZU), M. Pillas (IAP), C. Douzet (IJCLAB),  A. Simon (TShNU of Kyiv), D. Rusanova (TShNU of Kyiv), V. Vasylenko (NC JASU), O. Burkhonov (UBAI), S. Ehgamberdiev (UBAI), Y. Tillayev (UBAI), P. Put (NARIT), K. Noysena (NARIT), M. Tanasan (CMU/NARIT), D. Turpin (CEA-Saclay/Irfu), M. Freeberg (KNC), A. Klotz (IRAP), C. Limonta (OCA), A. Dupin (OCA), Q. André (OCA), M. Molham (NRIAG), E. Elhosseiny (NRIAG), A. Takey (NRIAG), N. Kochiashvili (AbAO), T. Kvernadze (AbAO) on behalf of the GRANDMA collaboration: 
 
We observed the field of the Fermi Long GRB 260610B (Fermi GBM team, GCN 44901). The observations started at 16:29:50 UTC on the 11 of June 2026, 0.7 days post the Fermi Trigger.

We detected the optical afterglow candidate, reported by GOTO (O'Neill et al, GCN 44903), in several epochs, using TRT-SRO in California state, UBAI-NT60, UBAI-AZT-22 at Madainak Observatory, AZT-8 at Lisnyky Observatory, KAO-1.88m at Kottamia Astronomical Observatory, TAROT-TCA/TCH at Calern and La Silla, and Kilonova Catcher. A subset of the photometry corrected for Galactic extinction is reported in the public ACME service Skyportal/ICARE https://skyportal-icare.ijclab.in2p3.fr/source/2026owq. To access, one simply needs to create an account using your ORCID ID. 

The data were 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. 

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. 

The results are consistent with other measurements reported (Zhu et al., GCN 44909; Akl et al., GCN 44911; O'Neill et al., GCN 44914; Moskvitin et al., GCN 44918; Jackson-Horne et al., GCN 44919; Li et al., GCN 44921; Bochenek and Perley, GCN 44927; Moskvitin et al., GCN 44929)

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 44929

Subject
GRB 260610B / AT2026owq: further SAO RAS optical observations
Date
2026-06-13T06:35:11Z (a month 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 GRB 260610B discovered by Fermi (The Fermi
GBM team, GCN 44891; GCN 44901) with the Zeiss-1000 1-m telescope
of SAO RAS + CCD photometer in BVRcIc filters on June 12.

The OT (O'Neill et al., GCN 44903; Watson et al., GCN 44905;
Zhu et al., GCN 44909; Gillanders et al., GCN 44910; Akl et al.,
GCN 44911; O'Neill et al., GCN 44914; Moskvitin et al., GCN 44918;
Jackson-Horne et al., GCN 44919; Izzo, GCN 44920; Li et al.,
GCN 44921; Bochenek and Perley, GCN 44927) is clearly detected in
our frames with the following brightness:

Date,      UTstart, t-T0,   Exp., Filter, Mag, Err., UL
                    (mid,d) (n*s)                    (3-sigma)
2026-06-12 19:21:03 2.41821 5*300 Rc      20.61 0.03 23.5

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

GCN Circular 44927

Subject
GRB 260610b: Liverpool Telescope optical observations
Date
2026-06-12T18:02:21Z (a month ago)
From
A. Bochenek at Liverpool John Moores University <a.m.bochenek@2023.ljmu.ac.uk>
Via
Web form

A. Bochenek, D. A. Perley (LJMU), report:

We observed the field of the optical counterpart (O'Neil et al., GCN 44903

Loading...
 
 
) of GRB 260610b (Fu et al., GCN 44901) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained 5x120s exposures in SDSS griz filters, starting at 2026-06-10 23:39:02 UT, approximately 23.9 hours after trigger.

We detect a source in the stacked images in all filters at the position reported by O'Neil et al. (GCN 44903

Loading...
 
 
), observed also by Watson et al. (GCN 44905); Zhu et al. (GCN 44909); Gillanders et al. (GCN 44910); Akl et al. (GCN 44911); O'Neill et al. (GCN 44914); Moskvitin et al. (GCN 44918); Jackson-Horne et al. (GCN 44919); Izzo, (GCN 44920), and Li et al. (GCN 44921). The photometry is:

MJD (mid)T_mid-T_0FilterMag. (AB)
61202.9894723.98 hr20.22 ± 0.02
61202.9979524.18 hi19.89 ± 0.02
61203.0064624.38 hz19.78 ± 0.05
61203.0149324.59 hg20.66 ± 0.03

The photometry is in the AB magnitude system, was calibrated using nearby PanSTARRS secondary standards and was not corrected for extinction.

GCN Circular 44921

Subject
GRB 260610B / AT2026owq: SVOM/VT optical observations
Date
2026-06-12T08:27:15Z (a month ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
H. L. Li, L. P. Xin, Y. L. Qiu, C. Wu, J. R. Xu, Y. N. Ma, Z. H. Yao, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, J. S. Deng, J. Y. Wei (NAOC) and J. Palmerio (CEA/Irfu) report on behalf of the SVOM/VT team.

SVOM/VT performed a ToO observation of GOTO26fua/AT2026owq (O'Neil et al., GCN 44903; Watson et al., GCN 44905; Zhu et al.,  GCN 44909; Gillanders et al., GCN 44910; Akl et al., GCN 44911; O'Neill et al., GCN 44914; Moskvitin et al., GCN 44918; Jackson-Horne et al., GCN 44919; Izzo, GCN 44920), a possible counterpart of GRB 260610B (Fermi GBM Team, GCN 44901). The observation started at 2026-06-11T13:05:40 UTC, i.e., 13.32 hours post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously.

The OT was clearly detected in both channels. The following measurements are in the AB magnitude and are not corrected for Galactic extinction:

Mid time | Band | Exposure Time | Brightness
14.50 h    VT_B    30*70 s       20.10 +/-0.06 mag
16.12 h    VT_B    29*70 s       20.16 +/-0.06 mag
14.52 h    VT_R    28*70 s       19.38 +/-0.06 mag
16.15 h    VT_R    26*70 s       19.49 +/-0.06 mag

The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS.

GCN Circular 44920

Subject
GRB 260610B / AT2026owq: OASDG optical observations
Date
2026-06-12T07:54:38Z (a month ago)
Edited On
2026-06-13T14:41:45Z (a month ago)
From
luca.izzo@inaf.it
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of luca.izzo@inaf.it
Via
Web form
L. Izzo (INAF-OACn, DARK/NBI, OASDG) reports:

We observed the counterpart of GRB 260610B (Fermi GBM team, GCN #44901) using the 0.5-m T1 telescope of the Osservatorio Astronomico S. Di Giacomo (OASDG), located in Agerola, Italy. Our observations started on 2026 June 11 at 21:44 UT, 0.917 days after the GRB trigger. We acquired a series of 8 × 300 s images in the Rc and Ic filters.

The optical counterpart, also reported as AT2026owq (O'Neill et al., GCN #44903; Watson et al., GCN #44905; Zhu et al., GCN #44909; Gillanders et al., GCN #44910; Akl et al., GCN #44911; O'Neill et al., GCN #44914; Moskvitin et al., GCN #44918; Perley et al., GCN #44919), is clearly detected in both stacked images.

We measure the following preliminary magnitudes (AB), which have been calibrated against nearby stars from the Pan-STARRS catalog and appropriately transformed to Rc and Ic magnitudes. 

Rc = 19.44 +/- 0.16 mag
Ic = 19.28 +/- 0.16 mag

GCN Circular 44919

Subject
GRB 260610B: IAC-80 Optical observations
Date
2026-06-12T07:06:28Z (a month ago)
Edited On
2026-06-14T21:03:45Z (a month ago)
From
Daniel Perley at Liverpool JMU <d.a.perley@ljmu.ac.uk>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Daniel Perley at Liverpool JMU <d.a.perley@ljmu.ac.uk>
Via
Web form

Dominic Jackson-Horne (UOL/LJMU), Danny McDonnell (UOL/LJMU), Esme Williams (UOL/LJMU), Dafydd Thomas (UOL/LJMU), Ruby Keeling Roberston (UOL/LMJU), Farhan Qamar Salmo (UOL/LJMU), Oliver King (UOL/LJMU), Daniel Perley (LJMU), H.-M. S. Grabham (LJMU), and Daniel Harman (LJMU) report:

We observed the optical afterglow (D. O'Neill, et al., GCN 44903

Loading...
 
 
) of GRB 260610B (The Fermi GBM team, GCN 44901) using the 0.82m IAC-80 telescope of Teide Observatory, equipped with the CAMELOT2 optical imager. Our first set of observations commenced at 21:41 UT and concluded at 22:20 UT on June 11; a second set of observations was then taken between 01:20 UT and 02:05 UT on June 12.

We report the following photometry:

DateUTstartt-T0 (d)Exp (n*s)FilterMagErr
2026-06-1121:41:520.9133*180SDSS-r20.040.06
2026-06-1122:03:580.9283*180Rc19.850.06
2026-06-1122:17:050.9383*180Ic19.230.05
2026-06-1122:25:330.9433*180B20.940.13
2026-06-1201:20:481.0653*120B21.470.26
2026-06-1201:27:401.0703*120V20.350.09
2026-06-1201:34:331.0753*120Ic19.400.08
2026-06-1201:41:261.0803*120SDSS-r20.260.09

This preliminary photometry is calibrated relative to the SDSS star J143237.88+270028.3 (using the Lupton 2005 transformation equations) and not corrected for Galactic extinction.

GCN Circular 44918

Subject
GRB 260610B / AT2026owq: SAO RAS optical observations
Date
2026-06-11T22:05:25Z (a month 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 GRB 260610B discovered by Fermi (The Fermi
GBM team, GCN 44891; GCN 44901) with the Zeiss-1000 1-m telescope
of SAO RAS equipped with the CCD photometer. The observations
carried out in BVRcIc filters starting June 11, 19:43:01 UT
(since ~ 1.4 days since trigger).

The OT (O'Neill et al., GCN 44903; Watson et al., GCN 44905;
Zhu et al., GCN 44909; Gillanders et al., GCN 44910; Akl et al.,
GCN 44911; O'Neill et al., GCN 44914) is clearly detected in
our frames with the following brightness:

Date,      UTstart, t-T0,   Exp., Filter, Mag, Err., UL
                    (mid,d) (n*s)                    (3-sigma)
2026-06-11 19:43:01 1.40315 2*300 Rc      19.73 0.03 22.6

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

GCN Circular 44914

Subject
GRB 260610B: optical counterpart shows spectroscopic redshift of z=0.473
Date
2026-06-11T13:44:50Z (a month ago)
From
Ben Gompertz at U of Birmingham <b.gompertz@bham.ac.uk>
Via
email
D. O'Neill, J. Mills, M. Pursiainen, T. Killestein, D. B. Malesani, B. P. Gompertz, J. Lyman, R. Starling, K. Ackley, M. Dyer, K. Ulaczyk, D. Steeghs, D. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. Breton, J. Casares, L. Nuttall, B. Godson and A. Kumar report on behalf of the GOTO collaboration.


We observed GOTO26fua/AT2026owq (O'Neill et al., GCN Circ. 44903, Watson et al., GCN Circ. 44905, Zhu et al. GCN Circ 44909, Gillanders et al. GCN Circ 44910, Akl et al. GCN Circ 44911), the optical counterpart of GRB 260610B (Fermi GBM team, GCN Circ. 44891), with the Alhambra Faint Object Spectrograph (ALFOSC) mounted on the Nordic Optical Telescope (NOT) as part of the GOTO Fast Analysis and Spectroscopy of Transients program (GOTO-FAST; Godson et al., AstroNote 2023-224).


We obtained spectroscopy with ALFOSC, 2x900s total with grism #4, starting at 2026-06-10 02:10:00 (+2.5h post-trigger). We identify Mg II and Ca II absorption features at a common redshift of z=0.473, which we suggest to be the redshift of GRB 260610B.


We also obtained multi-colour photometry starting at 2026-06-11 02:35 (+2.8h post-trigger)


| MJD            | T - T0 (hours)     | Filter     | AB mag     |

|------------    |----------------    |----------    |--------    |

| 61202.1079     | 2.81               | g            | 18.85      |

| 61202.1090     | 2.84               | r            | 18.40      |

| 61202.1100     | 2.87               | i            | 18.14      |

| 61202.1111     | 2.90               | z            | 17.92      |

| 61202.1264     | 3.27              | g            | 19.00      |

| 61202.1275     | 3.29               | r            | 18.49      |

| 61202.1285     | 3.31               | i            | 18.25      |

| 61202.1296     | 3.34               | z            | 18.04      |


Magnitudes are reported in the AB system, calibrated against SDSS DR16 photometry, and are not corrected for foreground 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 44911

Subject
GRB 260610B / AT 2026owq: COLIBRÍ optical observations
Date
2026-06-11T11:14:07Z (a month ago)
From
Dalya Akl at New York University Abu Dhabi <dka2010@nyu.edu>
Via
Web form
GRB 260610B / AT 2026owq: COLIBRÍ optical observations

Dalya Akl (NYUAD), Enrique Moreno Méndez (UNAM), Jean-Grégoire Ducoin (CPPM), Rosa L. Becerra (UNAM), Noémie Globus (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM) report:

We imaged the field of GOTO26fua/AT2026owq (O'Neill et al., GCN Circ. 44904), a possible counterpart of the Fermi GBM GRB 260610B (Fermi GBM team, GCN Circ. 44901), using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-06-11 06:51 to 08:13 UTC (from 7.08 to 8.45 hours after the GRB trigger) and obtained 15, 16, 15, 16, and 30 minutes, respectively, of exposure in the g, r, i, z, and y filters.

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

We detect the source AT2026owq with preliminary magnitudes of:

g = 19.48 +/- 0.10
r = 19.02 +/- 0.03
i = 18.61 +/- 0.11
z = 18.54 +/- 0.08
y = 18.25 +/- 0.28

The absence of a strong break within our five filter measurements suggests a low-redshift event. Given the brightness, we encourage spectroscopic observations.

Our values are consistent with those reported by GOTO (O'Neill et al., GCN Circ. 44903) and DDOTI (Watson et al., GCN Circ. 44905), given that the source continues to exhibit the same temporal decay reported by Watson et al. (GCN Circ. 44905), with a decay index of alpha ≈ -0.6.

We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams.

COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and México (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, México.

GCN Circular 44910

Subject
GRB 260610B / AT2026owq: Early ATLAS observations
Date
2026-06-11T11:00:56Z (a month ago)
From
S. Srivastav at Oxford <shubhamsrivastav@gmail.com>
Via
Web form
J. H. Gillanders (Oxford), S. J. Smartt, K. W. Smith (Oxford, QUB), S. Srivastav, (Oxford), M. Nicholl, D. Young (QUB), T.-W. Chen (NCU, Taiwan), J. Tonry, L. Denneau, H. Weiland (IfA, University of Hawaii), N. Erasmus, W. Koorts (South African Astronomical Observatory), A. Jordan, V. Suc (UAI, Obstech), M. R. Alarcón, J. Licandro, P. Nichita (IAC), D. R. Young, M. Nicholl, T. Moore, J. Weston, X. Sheng, C. R. Angus, A. Wilson, A. Aamer, D. Magill, P. J. Broda, A. J. Smith (QUB), P. Ramsden (Birmingham/QUB), L. Shingles (GSI/QUB), H. Stevance, A. J. Cooper, F. Stoppa, J. Tweddle, L. Eastman (Oxford), L. Rhodes (TSI/McGill), A. Rest (STScI), C. Stubbs (Harvard), J. S. Sommer (LMU), B. P. Schmidt (ANU)

The ATLAS survey comprises 5 optical telescopes situated around the globe (Hawaii x2, Chile, South Africa, Tenerife) that are continuously observing the night sky with a cadence of ~12-36 hours (Tonry et al., 2018; Licandro et al., 2023, 2025). Each point of the sky is imaged 4 times across a ~1 hour period so as to identify nearby moving objects. This high-cadence set of observations can also provide tight explosion constraints on extragalactic phenomena, provided an ATLAS unit is serendipitously pointing at the explosion site. Here we report ATLAS-Teide (the Tenerife unit) observations of the optical counterpart (AT2026owq; discovered by GOTO; O’Neill et al., GCN 44903; see also Watson et al., GCN 44905 and Zhu et al., GCN 44909) to GRB 260610B (detected by Fermi/GBM; GCN 44901).

ATLAS-Teide observed the sky location of AT2026owq four times on MJDs 61201.96762, 61201.98177, 61201.99593 and 61202.01008 as part of regular survey operations. Each exposure lasted 30s and was acquired with the wide w-band filter (described by Tonry et al., 2025). The first and second exposures returned 5-sigma upper limits of >20.20 and >20.35 AB mag, respectively. The third and fourth exposures detected the transient, with magnitudes of w = 16.26+/-0.02 and w = 16.29+/-0.02 AB mag, respectively.

Fermi/GBM reported the detection time of GRB 260610B (GCN 44901) to be MJD 61201.99044. This means that our third exposure, in which we first detected AT2026owq, was obtained just 7.9 minutes post-explosion. Similarly, our second exposure provides an upper limit corresponding to 12.5 minutes pre-explosion.

We further note that although a host association is not obvious, the location of GRB 260610B / AT2026owq is in the vicinity of a number of nearby (z ~ 0.1, or D ~ 400-500 Mpc) galaxies; as such, further follow-up is scheduled to quantify the evolution of this (possibly low-z) GRB.

The Asteroid Terrestrial-impact Last Alert System (ATLAS) project is primarily funded to search for Near-Earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogs from the survey area. This work was partially funded by Kepler/K2 grant J1944/80NSSC19K0112 and HST GO-15889, and STFC grants ST/T000198/1 and ST/S006109/1. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy, University of Oxford, the Queen's University Belfast, the Space Telescope Science Institute, the South African Astronomical Observatory, and The Millennium Institute of Astrophysics (MAS), Chile.

GCN Circular 44909

Subject
GRB 260610B: TRT optical observation
Date
2026-06-11T09:13:48Z (a month ago)
From
Zipei Zhu at NAOC <zpzhu@nao.cas.cn>
Via
Web form
Z.-P. Zhu (NAOC), K. Noysena, K. Chanchaiworawit, S. Tinyanont (NARIT), J. An (NAOC), X. Liu, S.-Q. Jiang, L.-B. He, D. Xu (NAOC), S.-Y. Fu (HUST) report:

We observed the field of GRB260610B detected by Fermi (Fermi GBM team, GCN 44901) , using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at Fresno, California, U.S.A (SRO). Observation started at 2026-06-11 08:22:08 UT, i.e., ~ 0.36 days post-burst, and a series of R-band frames were obtained.

The previously reported optical counterpart (O'Neill et al., GCN 44903; Watson et al., GCN 44905) is clearly detected in our stacked images with a brightness of R = 18.92 +/- 0.05, calibrated with Pan-STARRS DR2 stars in the field and not corrected for Galactic extinction.

GCN Circular 44905

Subject
GRB 260610B: DDOTI optical observations
Date
2026-06-11T06:31:15Z (a month ago)
From
Alan Watson at UNAM <alan@astro.unam.mx>
Via
Web form
Alan M. Watson (UNAM), Camila Angulo Valdez (UNAM), Rosa L. Becerra (UNAM), Nat Butler (ASU), Tsvetelina Dimitrova (ASU), William H. Lee (UNAM), and Margarita Pereyra (UNAM) report:

We observed the field of GRB 260610B (Fermi Team, GCN Circ. 44901) with the DDOTI wide-field imager at the Observatorio Astronómico Nacional on Sierra San Pedro Mártir from 2026-06-11 04:53 to 05:44 UTC (5.1 to 5.9 hours after the trigger) obtaining total exposure of 27 minutes.

We detect the candidate afterglow GOTO26fua/AT2026owq reported by O'Neill et al. (GCN Circ. 44903) with a preliminary AB magnitude of:

w = 18.82 +/- 0.04

Compared to the observations reported by O'Neill et al. at 0.21 hours, the candidate continues to fade and has an estimated temporal decay index of approximately −0.6, in agreement with that measured by O'Neill et al. between 0.21 and 1.4 hours.

We thank the staff of the Observatorio Astronómico Nacional on the Sierra of San Pedro Mártir.

GCN Circular 44903

Subject
GRB 260610B: GOTO optical counterpart candidate
Date
2026-06-11T02:49:52Z (a month ago)
Edited On
2026-06-11T17:29:11Z (a month ago)
From
d.s.oneill@bham.ac.uk
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of d.s.oneill@bham.ac.uk
Via
Web form
D. O'Neill, J. Mills, M. Pursiainen, J. Lyman, K. Ackley, M. Dyer, K. Ulaczyk, D. Steeghs, D. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. Breton, J. Casares, L. Nuttall, R. Starling, B. Gompertz, B. Godson, T. Killestein, A. Kumar, on behalf of 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 the Fermi/GBM alert GRB 260610B (Fermi GBM Team, GCN #44901).

Observations covering the localisation area began at 2026-06-10 23:58:04 UT, (+0.2h post trigger) and continued through to 2026-06-11 00:31:39 UT (+0.76h post trigger). 63 images were taken, across 10 unique pointings, covering 241.5 sq. deg within the 90% localisation contour. ~89.6% of the total 2D localisation probability was covered, with an average 5-sigma depth of 20.6 mag.

Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations. Source candidates were initially filtered using a classifier (Killestein et al. 2021) and cross-matched against a variety of contextual and minor planet catalogs. Human vetting was carried out in real time on any candidates that passed the above checks (Lyman et al. 2026).

A new optical source GOTO26fua/AT2026owq is identified within the GBM 90% localisation region (25% contour) with co-ordinates:

    RA,DEC (J2000) = 218.159414, 27.004935,
                    14:32:38.26, +27:00:17.76

The source was detected with L = 16.66 ± 0.01 AB mag (+0.21h) before fading to L = 17.83 ± 0.03 AB mag (+1.4h) with the light curve consistent with a decay index t^-0.56. We find no evidence of the source prior to the GRB trigger time in previous GOTO observations, the ZTF observations provided by the Lasair broker (Smith et al. 2019), or the ATLAS forced photometry server (Shingles et al. 2021) at m_o > 19.45 mag (-38.99h). The temporal and spatial coincidence along with the power-law like decay strongly suggest this is the optical counterpart of GRB 260610B.

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 44901

Subject
GRB 260610B: Fermi GBM Final Localization
Date
2026-06-11T01:29:13Z (a month ago)
From
Matt Godwin <msg0028@uah.edu>
Via
Web form
The Fermi GBM team reports the detection of a likely LONG GRB

"At 23:46:14.25 UT on 10 June 2026, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 260610B (trigger 802827979/260610990).

The on-ground calculated location, using the Fermi GBM trigger data,
is RA = 218.66, Dec = 23.94 (J2000 degrees, equivalent to J2000 14h 34m, +23d 56'),
with a statistical uncertainty of 2.77 degrees.

The angle from the Fermi LAT boresight is 84 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260610990/quicklook/glg_skymap_all_bn260610990.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/bn260610990/quicklook/glg_healpix_all_bn260610990.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260610990/quicklook/glg_lc_medres34_bn260610990.gif"
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