TITLE:   GCN CIRCULAR
NUMBER:  44955
SUBJECT: GRB 260610B / AT 2026owq: Possible re-brightening identified from Pan-STARRS observations
DATE:    26/06/16 15:30:51 GMT
FROM:    James Gillanders at University of Oxford <jhgillanders.astro@gmail.com>

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.