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GCN Circular 34688

LIGO/Virgo/KAGRA S230904n: One ZTF candidate counterpart to coincident neutrino event
2023-09-13T20:35:14Z (10 months ago)
Jannis Necker at DESY <>
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Jannis Necker (DESY), Shreya Anand (Caltech), Simeon Reusch (DESY), Tomas Ahumada (Caltech), Gaurav Waratkar (IIT-B), Varun Bhalerao (IIT-B), Mansi Kasliwal (Caltech) report on behalf of the Zwicky Transient Facility (ZTF) and Global Relay of Observatories Watching Transients Happen (GROWTH) collaborations: 

We observed the combined sky localization of gravitational-wave candidate S230904n [1] and the coincident track-like neutrino event reported by IceCube [2] with the Palomar 48 inch telescope equipped with the 47 square degree Zwicky Transient Facility (ZTF) camera. This localization was generated by crossmatching the 90% confidence region of the neutrino error circle with the updated Bilby gravitational-wave localization [3].  We obtained a series of g- and r-band images covering 47.2 square degrees of the localization at least once, beginning at 2023-09-06T09:38:48.998 (2 days after the burst trigger time), corresponding to ~94% of the probability enclosed in the combined localization region.

We queried the ZTF alert stream using Kowalski (Duev et al. 2019) through Fritz (Coughlin et al. 2023), AMPEL (Nordin et al. 2019), and ZTFReST (Andreoni & Coughlin et al., 2021). We required at least 2 detections separated by at least 15 minutes to select against moving objects. Furthermore, we cross-matched our candidates with the Minor Planet Center to flag known asteroids, reject stellar sources (Tachibana and Miller 2018), and applied machine learning algorithms (Mahabal et al. 2019). We required that no spatially coincident ZTF alerts were issued before the detection time of the LVK trigger. Based on [4], we rejected candidates that were visibly offset from the host galaxy nucleus.

One candidate remains after human vetting:

| iau name   | alias        |       ra |     dec |
| AT2023rkw  | ZTF23abawyxp |  49.8224 | 37.7884 |

The details of the first detection of AT2023rkw are below:

|         mjd | mag±err (ab)   | filter   | instrument   |
| 60192.24512 | 20.32±0.06     | ztfg     | ZTF          |

AT2023rkw has since risen by ~0.7 mag in both g- and r-band. The WISE colors of its underlying host galaxy suggest that AGN contribution to this transient is subdominant.

Further follow-up of this localization region will continue as part of regular survey operations. Additional follow-up and monitoring of AT2023rkw is planned.

[1] The LIGO Scientific Collaboration, the Virgo Collaboration and the KAGRA collaboration, GCN 34612
[2] IceCube Collaboration, GCN 34616
[3] The LIGO Scientific Collaboration, the Virgo Collaboration and the KAGRA collaboration, GCN 34629
[4] Graham et al., 2023, ApJ, Volume 942, Issue 2

ZTF and GROWTH are worldwide collaborations comprising Caltech, USA; IPAC, USA, WIS, Israel; OKC, Sweden; JSI/UMd, USA; U Washington, USA; DESY, Germany; MOST, Taiwan; UW Milwaukee, USA; LANL USA; Tokyo Tech, Japan; IITB, India; IIA, India; LJMU, UK; TTU, USA; SDSU, USA and USyd, Australia. ZTF acknowledges the generous support of the NSF under AST MSIP Grant No 1440341. 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) and Kowalski (Duev et al. 2019). GROWTH India telescope is located at the Indian Astronomical Observatory (Hanle), operated by the Indian Institute of Astrophysics (IIA). GROWTH-India project is supported by SERB and administered by IUSSTF, under grant number IUSSTF/PIRE Program/GROWTH/2015-16 and IUCAA.
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