TITLE: GCN CIRCULAR NUMBER: 41454 SUBJECT: LIGO/Virgo/KAGRA S250818k: Pan-STARRS grizy-band imaging and photometry of AT2025ulz DATE: 25/08/20 18:11:46 GMT FROM: James Gillanders at University of Oxford <jhgillanders.astro@gmail.com> J. H. Gillanders (Oxford), M. E. Huber, K. C. Chambers (IfA, Univ. Hawaii), S. J. Smartt, 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 observed the optical transient AT2025ulz (Stein et al., GCN 41414), the transient plausibly associated with the (sub-threshold) 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). Our observation consisted of 6x150s exposures in each of the grizy filters in both telescopes. 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). From these difference images, we measure the following preliminary AB magnitudes, from averages of all forced photometry from the two GPC1 and GPC2 cameras: MJD t-t0 (days) Filter AB mag 60907.30 2.25 g 22.9 +/- 0.4 60907.31 2.26 r 22.2 +/- 0.2 60907.33 2.27 i 21.8 +/- 0.2 60907.34 2.28 z 21.4 +/- 0.2 Here, t0 corresponds to the GW trigger time (LVK Collaboration, GCN 41437). Our difference imaging and photometric measurements indicate a consistent flux excess at the position of AT2025ulz (Stein et al., GCN 41414), although we note that these current magnitude estimates are derived from difference images generated with reference images shallower than the target images (900s vs. ~300s, respectively). We caution these are preliminary, given the differencing issues and the closeness of the source to the bright nucleus and body of the host. Comparing our current Pan-STARRS gr-band data to that previously reported by Busmann et al. (GCN 41421) and Hall et al. (GCN 41433), we find that AT2025ulz appears to have faded further, if both sets of photometry are accurate. We can certainly conclude that no re-brightening (which may be expected in SNe IIb or shock cooling) is yet taking place. Work is ongoing to generate a deeper reference image, and thus improved magnitude estimates. Further multi-band observations are scheduled. View this GCN Circular online at https://gcn.nasa.gov/circulars/41454. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...