TITLE: GCN CIRCULAR NUMBER: 39639 SUBJECT: GRB250309B: One candidate counterpart from the Zwicky Transient Facility DATE: 25/03/09 18:25:09 GMT FROM: Robert David Stein at JSI rdstein@umd.edu
Robert Stein (JSI), Tomas Ahumada (Caltech), Jannis Necker, Akshay Eranhalodi (DESY), and Anna Franckowiak (Ruhr University Bochum), Jesper Sollerman (Stockholm) report:
On behalf of the Zwicky Transient Facility (ZTF) and Global Relay of Observatories Watching Transients Happen (GROWTH) collaborations:
We observed the localisation of Gamma-Ray Burst GRB 250309B (McDermott et al., GCN 39635) with the Palomar 48-inch telescope equipped with the 47 square degree ZTF camera (Bellm et al. 2019, Graham et al. 2019). This GRB was reported to be in spatial/temporal coincidence with high-energy neutrino IC250309A (Zegarelli et. al, GCN 39631), and our observations included ToO observations (Stein et al., GCN 39638) conducted as part of the ZTF neutrino follow up program (Stein et al. 2023).
We started serendiptious observations of the GRB skymap in the g- and r-band beginning at 2025-03-09 07:51 UTC, approximately 0.3 hours after event time. We covered 80% of the reported localization region. This estimate accounts for chip gaps.
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). After applying standard candidate vetting procedures, we identified the following candidate optical afterglow:
±–––––––––––––––––––––––––––––––––––––––-+ | ZTF Name | IAU Name | RA (deg) | DEC (deg) | Filter | Mag | MagErr | ±–––––––––––––––––––––––––––––––––––––––-+ | ZTF25aaitvjt | AT2025dws | 210.801761 | -8.502842 | r | 18.48| 0.107 | ±–––––––––––––––––––––––––––––––––––––––-+
Using the smaller BALROG localisation (Greiner et al, GCN 39629), we covered 76.8% (1.9 sq deg) of the reported localization region. This estimate accounts for chip gaps. Repeating the same search, AT2025dws is again the only candidate found within the error contour of the GRB.
AT2025dws is red (g−r>0.4 mag) and is coincident with a faint Legacy Survey source that appears to be a possible host galaxy. From the ATLAS forced photometry service, we find that the transient had deeper upper limit (m>19.2) 1.67 hours before the GRB trigger. This suggests that AT2025dws is both fast-evolving, and temporally coincident with the GRB. Given this, we consider AT2025dws to be the likely afterglow counterpart.
We encourage spectroscopic observations to confirm the nature of AT2025dws as an afterglow. ToO observations have been requested with the Neil Gehrels Swift Observatory.
We note that none of ZTF25aaitvjt/AT2025dws is located 2.24 degrees from the center of the neutrino localisation. If this is indeed the afterglow, then the offset to the reported neutrino uncertainty region is >5 sigma, suggesting that the neutrino and GRB are not associated.
We will continue to observe this field as part of our standard ToO cadence for high-energy neutrinos (Stein et al. 2023).
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), Caltech/IPAC, and the University of Washington at Seattle, USA.
GROWTH acknowledges generous support of the NSF under PIRE Grant No 1545949. Alert distribution service provided by DIRAC@UW (Patterson et al. 2019). Alert database searches are done by AMPEL (Nordin et al. 2019). Alert filtering is performed with the nuztf (Stein et al. 2021, https://github.com/desy-multimessenger/nuztf ).
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