GCN Circular 42677

S. Anand (Stanford), R. Stein (UMD), X. J. Hall (CMU), V. Swain (IIT-B), A. Saikia (IIT-B), T. Mohan (IIT-B), V. Bhalerao (IIT-B), D. Cook (Caltech/IPAC), A. Singh (SU), M. M. Kasliwal (Caltech), E. C. Bellm (UW), M. Coughlin (UMN), T. du Laz (Caltech), V. Karambelkar (Columbia), G. Waratkar (Caltech), I. Andreoni (UNC), A. 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 sub-solar mass candidate S251112cm (GCN 42650
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 ) 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 2025-11-12 09:21 UTC, approximately 18 hours after merger, covering 38.3% of the probability within the 90% credible region of the LVK skymap in g- and r-bands. This estimate accounts for chip gaps. Each exposure was 300s with a median r-band depth of 21.14 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. 

Because of the nature of the GW trigger as a sub-solar mass merger candidate, we do not exclude any candidates that may be consistent with being young stripped envelope supernovae (SESNe). Disk fragmentation or core fissioning in SESNe may lead to the formation of subsolar mass neutron stars (Metzger et al. 2024, Kasliwal et al. 2025).

After filtering, 8 transient candidates remain, all within the 95.0% localization of the skymap. Candidates marked as FAR have redshifts from Legacy Survey or from SDSS (shown below). Redshift lower limits come from Legacy Survey (l95; a conservative lower limit on the photometric redshift). We list the 8 candidates found in the table below: 

| ZTF Name      | RA (deg)   | DEC (deg) | MJD      | Filt | Mag   | MagErr | Redshift                | Note                                     |
| ------------- | ---------- | --------- | -------- | ---- | ----- | ------ | --------------------------------------- | ---------------------------------------- |
| ZTF25aceekrn* | 180.101460 | 49.047764 | 60992.43 | g    | 21.04 | 0.22   | photo-z=0.14 (LS); photo-z=0.189 (SDSS) | FAR |
| ZTF25aceekwp  | 181.804049 | 55.653636 | 60992.44 | g    | 21.23 | 0.20   | photo-z>0.24 (LS)                       | FAR |
| ZTF25aceelgz  | 190.811137 | 57.328886 | 60992.47 | g    | 21.15 | 0.13   | spec-z=0.476 (SDSS)                     | AGN-like; w1-w2=0.97 |
| ZTF25aceelbm  | 169.544486 | 41.094776 | 60992.41 | g    | 21.20 | 0.23   | photo-z=0.21 (LS); photo-z=0.24 (SDSS)  | FAR |
| ZTF25aceemla  | 185.395941 | 56.586289 | 60992.46 | r    | 21.25 | 0.23   | photo-z>0.28 (LS)                       | FAR |
| ZTF25aceekzz  | 169.076786 | 39.863435 | 60992.41 | g    | 21.32 | 0.26   | photo-z=0.528 (LS)                      | AGN-like; ZTF pre-detections; w1-w2=1.13 |
| ZTF25aceelmh  | 184.916369 | 57.167778 | 60992.47 | g    | 21.61 | 0.22   | photo-z>0.41 (LS); photo-z=0.42 (SDSS)  | FAR |
| ZTF25aceempu  | 169.569004 | 35.985827 | 60992.49 | r    | 21.60 | 0.21   | photo-z>0.23 (LS)                       | FAR |

*ZTF25aceekrn (AT 2025adht
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 ) was previously reported by the TROVE team (GCN 42675
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 ) with a photo-z=0.11 +/- 0.07 (nominally consistent with the LVK distance posterior). A spectrum can help resolve this discrepancy.

None of the other candidates reported here warrant additional follow-up.

All candidates have been reported to the Transient Name Server, and some are pending assignment of a TNS name. 


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.