GCN Circular 42594

Shreya Anand (Stanford), Anirudh Salgundi (UNC), Robert Stein (UMD), Vishwajeet Swain (IIT-B), Yogesh Wagh (IIT-B), Xander J. Hall (CMU), Mansi Kasliwal (Caltech), Eric Bellm (UW), Michael Coughlin (UMN), Theophile Jegou du Laz (Caltech), and Antonella Palmese (CMU) report,

On behalf of the Zwicky Transient Facility (ZTF) and Global Relay of Observatories Watching Transients Happen (GROWTH) collaborations: 

We observed the localization region of the LVK low-significance burst trigger S251031dw with the Palomar 48-inch telescope, equipped with the 47 square degree ZTF camera (Bellm et al. 2019, Graham et al. 2019). We started observations in the g- and r-band beginning at 2025-11-01 12:22 UTC, approximately 15.2 hours after merger. We covered 23.1% of the reported localization region on 2025-10-31 and 60.9% of the localization on 2025-11-01. This estimate accounts for chip gaps. ~30% of the tiled localization was at a low galactic latitude. Each exposure was 200s with a median r-band depth of 21.1 mag. 
 
The images were processed in real-time through the ZTF reduction and image subtraction pipelines at IPAC to search for potential counterparts (Masci et al. 2019). AMPEL (Nordin et al. 2019, Stein et al. 2021) was used to search the alerts database for candidates. We reject stellar sources (Tachibana and Miller 2018) and moving objects, and apply machine learning algorithms (Mahabal et al. 2019), removing candidates with history of variability prior to the merger time. 

After filtering, 58 transient candidates remain, all within the 95.0% localization of the skymap. None of these candidates have fast evolution consistent with kilonovae. We perform additional vetting of these candidates, and highlight 9 candidates that appear to be young and SN-like, or AGN-like, as candidates of interest in the burst search, in the table below: 


| ZTF Name     | IAU Name   | RA (deg)   | DEC (deg)  | MJD      | Filt | Mag   | MagErr | Note     |
| ------------ | ---------- | ---------- | ---------- | -------- | ---- | ----- | ------ | -------- |
| ZTF25acchitl | --         | 127.996770 | -27.778576 | 60980.52 | r    | 20.63 | 0.11   | AGN-like |
| ZTF25accnabz | --         | 128.305827 | -23.858020 | 60980.52 | r    | 21.60 | 0.19   | AGN-like |
| ZTF25acchdbq | --         | 121.061164 | -19.087422 | 60980.50 | r    | 20.35 | 0.21   | AGN-like |
| ZTF25acchjjz | --         | 127.829510 | -20.792424 | 60980.52 | r    | 20.88 | 0.12   | AGN-like |
| ZTF25acchljm | --         | 130.363870 | -23.822309 | 60980.52 | r    | 20.87 | 0.13   | AGN-like |
| ZTF25accniqx | --         | 131.262056 | -20.691176 | 60980.54 | r    | 20.16 | 0.15   | AGN-like |
| ZTF25acchigq | --         | 128.491126 | -24.746190 | 60980.52 | r    | 21.26 | 0.15   | AGN-like |
| ZTF25acchszc | AT2025acno
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  | 137.799671 | -22.017226 | 60980.54 | r    | 20.93 | 0.13   | SN-like  |
| ZTF25acchppz | AT2025acep
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  | 135.228502 | -23.983061 | 60980.54 | r    | 19.48 | 0.05   | SN-like  |

ZTF25acchszc appears to be a rising young, blue supernova in a host galaxy (no redshift available). ZTF25acchppz is offset 9" from a potential host galaxy at z=0.082, and is therefore unlikely to be associated with the burst source. Further follow-up is encouraged.

Based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Award #2407588 and a partnership including Caltech, USA; Caltech/IPAC, USA; University of Maryland, USA; University of California, Berkeley, USA; University of Wisconsin at Milwaukee, USA; Cornell University, USA; Drexel University, USA; University of North Carolina at Chapel Hill, USA; Institute of Science and Technology, Austria; National Central University, Taiwan; Operations are conducted by Caltech's Optical Observatory (COO) and Caltech/IPAC. GROWTH acknowledges the generous support of the NSF under PIRE Grant No 1545949. Alert database searches are done by AMPEL (Nordin et al. 2019) and Kowalski (Duev et al. 2019). The GROWTH India Telescope (GIT, Kumar et al., 2022) is set up by the Indian Institute of Astrophysics (IIA) and the Indian Institute of Technology Bombay (IITB) with funding from DST-SERB and IUSSTF. Its operations are partially supported by funding from the IIT Bombay alumni batch of 1994. The Fritz and SkyPortal projects acknowledge the generous support of The Gordon and Betty Moore Foundation.