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GCN Circular 34089

LIGO/Virgo/KAGRA S230627c: Zwicky Transient Facility observations and candidates
2023-06-27T09:08:46Z (a year ago)
Akash Anumarlapudi (UWM), Tomas Ahumada (CIT), Mansi Kasliwal (CIT), Viraj Karambelkar (CIT), Harsh Kumar (IITB), Theophile du Laz (CIT), Simeon Reusch (DESY), Anirudh Salgundi (IITB), Robert Stein (CIT), Vishwajeet Swain (IITB), Gaurav Waratkar (IITB), Avery Wold (IPAC), Shreya Anand (CIT), Igor Andreoni (UMD),  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 LVK trigger S230627c (GCN 34086) 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 34087) beginning at ~2.2 hours after the LVK trigger time, covering 78.3% (105.3 sq deg) of the probability enclosed in the localization region.

We queried the ZTF alert stream using Kowalski (Duev et al. 2019) through Fritz (Coughlin et al. 2023), emgwcave (Karambelkar et al. in prep), 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 LVK trigger. We also run forced photometry on ZTF images (Masci et al. 2019) and ATLAS images (Tonry et al. 2018, Smith et al. 2020) and require no detections before the LVK trigger.

Four sources passed our criteria and are inside the 95% error region:

| ZTF Name     | IAU Name  | RA (deg)    | DEC (deg)   | Filter | Mag   | MagErr |
| ZTF23aaptsuy | AT2023lxu | 160.2019569 | +41.9681656 | r      | 20.20 | 0.08   |
| ZTF23aapttaw | AT2023lxt | 164.6898146 | +60.9545924 | r      | 21.11 | 0.20   |
| ZTF23aaptudb | AT2023lxs | 166.5566080 | +78.5596361 | r      | 20.86 | 0.16   |
| ZTF23aaptusa | AT2023lxx | 162.0445716 | +71.8414115 | g      | 20.89 | 0.19   |

ZTF23aaptsuy (AT2023lxu) is 2.7'' away from a WISE galaxy WISEA J104048.69+415805.3 with a spectroscopic redshift of z=0.092961 (luminosity distance of 440 Mpc assuming Planck+18 cosmology) which is at the edge of the 3-sigma boundary of the LVK line of sight distance estimate.

ZTF23aaptudb (AT2023lxs) is 0.07'' away from a galaxy that has a Legacy Survey DR8 (LS; Duncan, 2022) photometric redshift of photo-z = 0.118±0.07, which suggests that it might not be associated with the LVK trigger.

ZTF23aaptusa (AT2023lxx) is 0.75'' away from a galaxy that has an LS photometric redshift of photo-z = 0.175±0.044, suggesting that it might not be associated with the LVK trigger. We also caution that the centroid position shows a slight dispersion in the three detections.  

ZTF23aapttaw (AT2023lxt) is 0.236'' away from an LS source with a photo-z = 0.254±0.12, outside of the LVK volume.

Further follow-up of these candidates 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). 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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