TITLE: GCN CIRCULAR NUMBER: 45435 SUBJECT: GRB 260822A: the candidate optical afterglow is an unrelated type-Ia SN DATE: 26/08/24 14:07:25 GMT FROM: Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk> L. Izzo (INAF/OAC), A. Saccardi (CEA/Irfu), G. Corcoran (UCD), A. Martin-Carrillo (UCD), A. J. Levan (Radboud and Warwick), D. O’Neill (Birmingham), A. L. Thakur (INAF/IAPS), D. B. Malesani (DAWN/NBI and Radboud), N. Globus (UNAM), S. Basa (UAR Pytheas), on behalf of the Stargate collaboration, J. H. Gillanders (Oxford), K. W. Smith, S. J. Smartt (Oxford/QUB), M. Huber, K. C. Chambers (IfA, Univ. Hawaii), on behalf of the Pan-STARRS collaboration, report: We observed the candidate optical afterglow (Globus et al., GCN 45429; Li et al., GCN 45430) of the short GRB 260822A (Schanne et al., GCN 45422) using the ESO VLT UT4 (Yepun) equipped with the MUSE integral field unit spectrograph. Observations began on 2026 August 23 at 09:31:23 UT (0.69 days after the SVOM/ECLAIRs trigger), and consisted of four observations of about 12 min exposure time each. The optical transient lies close to a system of two galaxies, visible in both our data and in archival images. This could be plausibly the host system of the transient. We measure for them the following redshifts: RA = 00:09:45.94, Dec = +01:23:29.8: z = 0.290 from Halpha, [N II], [O III], Hbeta; RA = 00:09:45.91, Dec = +01:23:24.8: z = 0.296 from Halpha, [N II], [O III], Hbeta. At the position of the candidate optical afterglow (Globus et al., GCN 45429), a faint source is detected. Following preliminary extraction and flux calibration, the transient spectrum shows a continuum with a number of broad undulations, typical of supernovae. Template matching via SNID (Blondin & Tonry 2007, doi:10.1086/520494), leaving the redshift free, provides a match with a type-Ia SN post peak at redshift z = 0.291 +- 0.010. Good matches to post-peak type-Ia SNe are also obtained using SNID-SAGE (Stoppa et al. 2026, doi:10.1093/mnras/stag1066). Given the timing of our observation, the SN exploded well before the SVOM/ECLAIRs trigger. Examining recent data obtained during ordinary Pan-STARRS survey coverage (see Fulton et al. 2025, doi:10.1093/mnras/staf1165), we report the following archival detections of the transient: | MJD | Filter | Magnitude (AB) | | -------- | ------ | ------------------ | | 61020.23 | w | >22.8 (3-sigma UL) | | 61247.54 | i | 21.4 +/- 0.2 | | 61263.54 | w | 22.5 +/- 0.4 | These archival detections predate the GRB trigger (MJD 61274.7). Both the photometric and the spectroscopic data thus support the interpretation that this source is a pre-existing transient, unrelated to GRB 260822A. We acknowledge expert support from the observing staff at Paranal, in particular Zahed Wahhaj, Thomas Szeifert and Marcelo Lopez. The Pan-STARRS system comprises 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, and a broad w-filter, which is a composite of the gri-filters (Chambers et al., 2016, ArXiv e-prints, 1612.05560) Images were processed with the Pan-STARRS pipeline. After astrometric and photometric calibration, reference images were subtracted from the target images (Magnier et al., 2020a, ApJS, 251, 3; Magnier et al., 2020b, ApJS, 251, 6; Waters et al., 2020, ApJS, 251, 4). Discoveries from this program are a byproduct of the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) NEO survey observations. Operation of the Pan-STARRS1 and Pan-STARRS2 telescopes is primarily supported by the National Aeronautics and Space Administration under Grant No. NNX12AR65G and Grant No. NNX14AM74G issued through the SSO Near-Earth Object Observations Program. Data are processed at Queen's University Belfast enabled through the STFC grants ST/P000312/1 and ST/T000198/1. View this GCN Circular online at https://gcn.nasa.gov/circulars/45435. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...