TITLE: GCN CIRCULAR NUMBER: 41493 SUBJECT: LIGO/Virgo/KAGRA S250818k: Pan-STARRS coverage of the skymap and candidate counterparts DATE: 25/08/22 14:09:35 GMT FROM: James Gillanders at University of Oxford <jhgillanders.astro@gmail.com> S. J. Smartt, J. H. Gillanders (Oxford), M. E. Huber, K. C. Chambers (IfA, Univ. Hawaii), K. W. Smith (Oxford/QUB), S. Srivastav, F. Stoppa, H. Stevance, J. Tweddle (Oxford), M. Nicholl, D. R. Young, A. Aamer, C. R. Angus, M. D. Fulton, D. Magill, M. McCollum, T. Moore, S. Sim, J. Weston, X. Sheng (QUB), T.-W. Chen (NCU, Taiwan), L. Shingles (GSI/QUB), P. Ramsden (Birmingham/QUB), A. S. B. Schultz, T. de Boer, J. Fairlamb, C. C. Lin, T. Lowe, E. Magnier, P. Minguez, G. Paek, I. A. Smith, R. J. Wainscoat (IfA, Univ. Hawaii), A. Rest (STScI), C. Stubbs (Harvard). We surveyed the skymap of the gravitational wave event S250818k (LVK Collaboration, GCNs 41437, 41440) using the Pan-STARRS twin telescope system (Chambers et al., 2016, arXiv e-prints, 1612.05560). Following on from the reported optical candidate AT2025ulz (Stein et al., GCN 41414; Busmann et al., GCN 41421; Hall et al., GCN 41433; Karambelkar et al., GCN 41436; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454; Liu et al., GCN 41461; Perley et al., GCN 41480, D’Avanzo et al. GCN 41489), we have been taking targeted grizy imaging of this transient (Gillanders et al., GCN 41454). At this specific region we have proprietary data to make significantly deeper reference stacks than those that are available through the STScI public website, and monitoring with Pan-STARRS is ongoing. For the rest of the northern portion of the GW skymap, we surveyed the region in the i-band over three nights: 2025 Aug 20, 21, 22. We covered ~288 square degrees and ~33% of the bilby.fits skymap 90% area each night, with multiple 45s (Aug 20) and 120s (Aug 21, 22) exposures. The images were processed with the Pan-STARRS pipeline. After astrometric and photometric calibration, reference images were subtracted from the target stacked images (Magnier et al., 2020a, ApJS, 251, 3; Magnier et al., 2020b, ApJS, 251, 6; Waters et al., 2020, ApJS, 251, 4). Transient candidates were identified and run through our standard filtering procedures, including rejection of artefacts with machine learning tools and cross-matching with galaxy, stellar and solar-system catalogs (e.g., Smith et al., 2020, PASP, 132, 1; Smartt et al., 2024, MNRAS 528, 2299). The depth of our images are typically i ~ 21 - 21.5 per exposure, with the first night having better seeing and transparency conditions. We detected 68 candidate extragalactic transients, of which 30 were already registered on the IAU Transient Name Server (TNS) as having been discovered and detected before S250818k merger time. We have registered the other 38 sources on the TNS. To down-select for optical counterpart candidates of S250818k, we applied the following filters: 1. Spatially associated with a host galaxy that has a photometric or spectroscopic redshift within the 2-sigma limit from LVK of 0.025 < z < 0.08, 2. Has some indication of lightcurve evolution over the three nights of observation, 3. We rejected all faint transients spatially coincident with the nucleus of galaxies as being due to either AGN variability or possible subtraction artefacts. We found no plausible candidate for the optical counterpart of S250818k other than AT2025ulz. Six sources with plausible kilonova-like absolute magnitudes and in hosts with consistent redshifts (within errors) with S250818k were found (AT2025uuw, AT2025uso, AT2025uuf, AT2025utr, AT205usy, AT2025uxs), but their lightcurves are either rising or are flat and show no evolution over 48hrs, and they do not appear to be viable candidates at this point. Discovery details and magnitudes of the ATs can be found on the TNS. Operation of the Pan-STARRS1 and Pan-STARRS2 telescopes is primarily supported by the National Aeronautics and Space Administration under Grant No. NNX12AR65G and NNX14AM74G, issued through the SSO Near-Earth Object Observations Program. Data processing is enabled by Queen's University Belfast and the University of Oxford, enabled through STFC grants ST/Y001605/1, ST/T000198/1 and ST/X001253/1, the Royal Society, and the Hintze Centre for Astrophysical Surveys. View this GCN Circular online at https://gcn.nasa.gov/circulars/41493. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...