Tomas Ahumada (CIT), Vishwajeet Swain (IITB), Shreya Anand (CIT), Robert Stein (CIT), Viraj Karambelkar (CIT), Akash Anumarlapudi (UWM), Mansi Kasliwal (CIT), Anirudh Salgundi (IITB), Gaurav Waratkar (IITB), Avery Wold (IPAC), Theophile du Laz (CIT), Simeon Reusch (DESY), Igor Andreoni (SURA), Eric Bellm (UW), Varun Bhalerao (IITB), Brad Cenko (UMD), Michael Coughlin (UMN), Brian Healy (UMN), David Kaplan (UWM), Jannis Necker (DESY), D. Perley (LJMU) report on behalf of the ZTF and GROWTH collaborations:
We observed the localization region of the LVC trigger S230917af as part of routine Zwicky Transient Facility (ZTF; Graham et al., 2019; Bellm et al., 2019) survey operations. We obtained images in the r and g bands beginning at 2023-09-17T10:05:51 UT (8 seconds after the LVC trigger time), covering ~83% of the probability enclosed in the localization region. Our observations covered ~92% of the overlapping region between the MAXI low significance burst (GCN 34723) and the GW event.
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-match our candidates with the Minor Planet Center to flag known asteroids, reject stellar sources (Tachibana and Miller 2018), 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 LVC trigger. We also run forced photometry on ZTF images (Masci et al. 2019) and require no detections before the LVC trigger.
No candidates passed our filtering criteria. Furthermore, we run forced photometry over two years of ZTF data at the center of the Swift sources circulated in Evans et al. (GCN 34736) with a 1 arcsec radius, and we queried ZTF alerts within their 90% error region. Three of the six Swift sources have previous detections and can be associated to ZTF objects. These sources are unlikely to be associated with the GW trigger:
Swift name | ZTF name | JD discovery | magnitude at discovery
S230917af_X9 | ZTF18abvbsom | 2460152.82912 | g = 20.24
S230917af_X1 | ZTF18abvucic | 2460206.80333 | g = 18.93
S230917af_X10 | ZTF18abvzkzr | 2459817.80154 | g = 20.44
No predetections or alerts in the ZTF archive were found for the Swift sources S230917af_X3, S230917af_X4, and S230917af_X6. However, we note that (1) S230917af_X3 is 2.3 arcsec from a GALEX point-like UV source, at nuv = 20.03 mag, (2) S230917af_X4 is 2.5 arcsec from a Gaia source at g = 20.62 mag with a proper motion ~5.16 mas/year, and (3) S230917af_X6 is 9 arcsec from a WISE galaxy and its colors (W1-W2 = 0.9 mag, and W2-W3 = 2.6 mag) are indicative of a QSO/Seyfert galaxy.
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). 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.