TITLE:   GCN CIRCULAR
NUMBER:  45062
SUBJECT: GRB 260610B / AT2026owq: SAO RAS, CrAO and INASAN further optical observations and light curve
DATE:    26/07/01 08:35:23 GMT
FROM:    Alexander Moskvitin at SAO RAS <mosk@sao.ru>

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.