GCN Circular 36246

Tomás Ahumada (Caltech), Shreya Anand (CIT), Viraj Karambelkar (CIT), Eric Bellm (UW), Robert Stein (CIT), Mansi Kasliwal (CIT), Avery Wold (IPAC), Utkarsh Pathak (IITB),   Theophile du Laz (CIT), Simeon Reusch (DESY), Igor Andreoni (UMD),  Varun Bhalerao (IITB), Brad Cenko (UMD), Michael Coughlin (UMN), David Kaplan (UWM), Ravi Kumar (IITB), A. Ernsts (DESY), J. Nordin (DESY), Jannis Necker (DESY), D. Perley (LJMU), Anirudh Salgundi (IITB), Aswin Suresh (IITB), Vishwajeet Swain (IITB), Gaurav Waratkar (IITB), Jesper Sollerman (OKC) report on behalf of the ZTF and GROWTH collaborations:


We observed the localization region of the LVK trigger S240422ed (GCN 36236
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 ) with the Palomar 48-inch telescope equipped with the 47 square-degree Zwicky Transient Facility (ZTF) camera (Graham et al., 2019; Bellm et al., 2019). We obtained images in the g- and r-bands of the Bilby map (GCN 36240
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 ) We started observations in the g-, r- and i-band beginning at 2024-04-23 03:26 UTC, approximately 5.9 hours after event time, targeting 70% of the localization - detailed map of our coverage was reported to TreasureMap (Wyatt et al. 2020). Accounting for southern fields without reference images, chip gaps and processing success rate, we searched 48.8% (149.8 sq deg) of the reported localization region in real-time. Our observations were 300s, and reached a median depth of 19.8 in g-band, 20.6 in r-band and 19.8 in i-band.

We queried the ZTF alert stream using Kowalski (Duev et al. 2019) through Fritz (Coughlin et al. 2023), emgwcave (Karambelkar et al. in prep), and AMPEL (Nordin et al. 2019). We required at least 2 detections separated by at least 15 minutes to select against moving objects. Furthermore, we cross-match our candidates with the Minor Planet Center to flag known asteroids, reject stellar sources (Tachibana and Miller 2018 and using the GAIA catalog), and apply machine learning algorithms (Mahabal et al. 2019). We require that no spatially coincident ZTF alerts were issued before the detection time of the LVK trigger. We also ran forced photometry on ZTF images (Masci et al. 2019) and require no detections before the LVK trigger.

All candidates were rejected. Further follow-up of these fields will continue.

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 the 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). 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.