TITLE: GCN CIRCULAR NUMBER: 44910 SUBJECT: GRB 260610B / AT2026owq: Early ATLAS observations DATE: 26/06/11 11:00:56 GMT FROM: S. Srivastav at Oxford <shubhamsrivastav@gmail.com> 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. View this GCN Circular online at https://gcn.nasa.gov/circulars/44910. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...