TITLE: GCN CIRCULAR NUMBER: 41540 SUBJECT: LIGO/Virgo/KAGRA S250818k: Pan-STARRS imaging confirms re-brightening of SN2025ulz DATE: 25/08/25 13:07:11 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 have been observing the optical transient SN2025ulz (Banerjee et al., GCN 41532), formerly AT2025ulz (Stein et al., GCN 41414), the candidate optical counterpart 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); see also Gillanders et al. (GCN 41454) and Smartt et al. (GCN 41493). Our observations span grizy-bands, but here we report on the optical re-brightening evident in our i-band observations. On each of MJDs 60909.26, 60910.26, 60911.26 and 60912.26, we observed SN2025ulz for a total exposure time of 1800s. The images were processed with the Pan-STARRS pipeline, where, 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). For this process, we utilise proprietary data to compile significantly deeper reference stacks (~3400s) than those that are publicly available through the STScI website (~300s) and our stacked target images (1800s). From these difference images, we measure the following preliminary AB magnitudes: MJD t-t0 (days) Filter AB mag 60909.26 4.2 i 22.4 +/- 0.3 60910.26 5.2 i 22.5 +/- 0.3 60911.26 6.2 i 22.1 +/- 0.2 60912.26 7.2 i 21.6 +/- 0.1 Here, t0 corresponds to the GW trigger time (LVK Collaboration, GCN 41437). SN2025ulz has risen in our i-band observations by ~0.5 mag (~0.9 mag) in the last one day (two days). While there are barriers to accurately measuring the photometry of this source (e.g., it is relatively faint; it lies close to the bright nucleus and body of the host), our photometry values, and the inferred re-brightening, are robust, given our access to deep reference stacks. We note that these are vital to accurately infer the true brightness of the transient, as performing difference imaging with a shallower reference image can lead to systematic errors (as much as ~0.7 AB mag, in the specific case of SN2025ulz utilising the public reference stack available through the STScI website). Re-brightening of SN2025ulz has been previously noted by Liu et al. (GCN 41461), Freeburn et al. (GCN 41507) and Angulo et al. (GCN 41518). Our observed re-brightening supports the interpretation of SN2025ulz being a young SN, based on the VLT spectral observations reported by ENGRAVE (Banerjee et al., GCN 41532). Our consistent i-band rise is likely to suggest it is now on a radioactively powered rise following an initial shock-cooling phase, and thus indicates that SN2025ulz is likely not the optical counterpart to S250818k. Further Pan-STARRS observations are continuing to determine if the colour and lightcurve shape remains consistent with a type II or IIb supernova, to compare with the conclusions of Kasliwal et al. (GCN 41538). 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/41540. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...