GRB 260529A

N. Habeeb, A. Kumar, D. O’Neill, R. Starling, B. Gompertz, G. Ramsay, S. Belkin, 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, 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 the Fermi/GBM detected GRB 260529A (fermi_801715802, Fermi GBM team, GCN 44733
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 ).  Observations covering the GBM localization began at  2026-05-29 03:00:27 UT (+0.18h post trigger) and continued through to 2026-05-29 04:55:50 UT(+2.10 h post trigger). Images consisted of 4x90s exposures in the GOTO L-band (400-700 nm).

Images were processed immediately after acquisition using the GOTO pipeline (Lyman et al. 2026) and 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.

A new optical transient, GOTO26fez (AT 2026nuz
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 ), is identified within the 90% localization region of the Fermi/GBM trigger. The source was detected in the GOTO L-band with magnitudes of 17.85 ± 0.06 and 17.90 ± 0.04 AB mag at +0.23 h and +1.3 h post-trigger, respectively. We would like to caution that, given the source location close to the Galactic plane, we cannot exclude the possibility that the transient is of Galactic origin, such as a stellar flare or a CV.

We find no evidence of this source prior to the GRB trigger time in previous GOTO observations, in the archival ATLAS forced photometry server (Shingles et al. 2021) to a magnitude limit of m_o > 18.06 AB mag ~26.3h prior to the trigger, nor in archival ZTF observations provided by the Lasair broker (Smith et al. 2019). We note, however, that the most recent pre-trigger observations from GOTO were obtained approximately one month prior to the burst, and therefore do not provide constraints on shorter-timescale pre-burst variability.

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).

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-OAFA robotic telescope  [1]  located in Argentina (OAFA observatory of San Juan National University) started inspect of the Fermi GRB 260529A ( Fermi GBM team, GCN 44733
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 ) errorbox  564 sec after notice time and 606 sec after trigger time at 2026-05-29 03:00:04 UT, with upper limit up to  17.5 mag. The observations began at zenith distance = 74 deg. The sun  altitude  is -66.9 deg. 

The galactic latitude b = -25 deg., longitude l = 28 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=3345808

We obtain a following upper limits.  

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

     666 | 2026-05-29 03:00:04 |         MASTER-OAFA | (20h 10m 08.98s , -15d 23m 33.3s) |   C |   120 | 17.3 |        
     807 | 2026-05-29 03:02:10 |         MASTER-OAFA | (20h 10m 09.05s , -15d 23m 24.1s) |   C |   150 | 17.5 |        
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

The Fermi GBM team reports the detection of a likely LONG GRB

At 02:49:57 UT on 29 May 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260529A (trigger 801715802.916756 / 260529118).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 302.8, Dec = -15.3 (J2000 degrees, equivalent to J2000 20h 11m, -15d 18'), with a statistical uncertainty of 4.2 degrees.

The angle from the Fermi LAT boresight is 90.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260529118/quicklook/glg_skymap_all_bn260529118.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/bn260529118/quicklook/glg_healpix_all_bn260529118.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260529118/quicklook/glg_lc_medres34_bn260529118.gif