GCN Circular 36080

Vishwajeet Swain (IITB), Utkarsh Pathak (IITB), Viraj Karambelkar (Caltech), Theophile Jegou du Laz (Caltech), Tomas Ahumada (Caltech), Shreya Anand (Caltech), Igor Andreoni (UMD), Akash Anumarlapudi (UWM), Varun Bhalero (IITB), Eric Bellm (UW), Brad Cenko (NASA GSFC), Dave Cook (Caltech/IPAC), Michael Coughlin (UMN),  Matthew Graham (Caltech), Mansi Kasliwal (Caltech), Ravi Kumar (IITB), Antonella Palmese (CMU), Robert Stein (Caltech), Anirudh Salgundi (IITB), Aswin Suresh (IITB), Andrew Toivonen (UMN) Yogesh Wagh (IITB), and Gaurav Waratkar (IITB) report on behalf of the Zwicky Transient Facility (ZTF) and Global Relay of Observatories Watching Transients Happen (GROWTH) collaborations:

We observed the localization region of IGWN trigger S240413p (GCN #36075) with the Palomar 48-inch telescope, equipped with the 47 square degree ZTF camera (Bellm et al. 2019, Graham et al. 2019). We started observations in the g- and r-band beginning at 2024-04-13 03:14 UTC, approximately 0.9 hours after event time. We covered 77.9% (40.9 sq deg) of the reported localization region. This estimate accounts for chip gaps and IPAC pipeline processing completeness. Each exposure was 300s with a typical depth of ~22 mag. 
 
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). We queried the ZTF alert stream using Kowalski (Duev et al. 2019) through Fritz (Coughlin et al. 2023) and emgwcave (Karambelkar et al. in prep), AMPEL (Nordin et al. 2019) through nuztf (Stein et al. 2021), and ZTFReST (Andreoni & Coughlin et al., 2021). 

Since this GW event is classified as a binary black hole merger (GCN #36075), we checked for known AGN that were flaring and in the relevant GW volume.  WISEA J105644.96+105455.6 (ZTF18acvgziq) has a spectroscopic redshift of 0.067, which falls within the lower end of the 2 sigma distance distribution of S240413p. We find ZTF18acvgziq is possibly showing some flaring activity and needs further investigation.

Separately, we searched for new sources in the GW error circle and examined their photometric and host properties. 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 IGWN 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 IGWN trigger. 

We are left with the following high-significance transient candidates by our pipeline, all lying within the 95.0% localization of the skymap. We report the following eight sources to TNS. We note that the host galaxy of ZTF24aajmkte (AT2024gnd
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) has a spectroscopic redshift of 0.084 that is consistent within one-sigma of the GW distance, but the absolute magnitude of the detection is consistent with a supernova. We note that ZTF24aajmirr (AT2024gnb
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), ZTF24aajnbrw (AT2024gnh
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), ZTF24aajohse (AT2024gni
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), ZTF24aajmpex (At2024gne), ZTF24aajmjmz (AT2024gnc
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) have a median photometric redshift that is inconsistent with the GW skymap but the error bar on the redshift is large and they cannot be firmly rejected as too far. The sources ZTF24aajmzjy (AT2024gmz
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), ZTF24aajmrjl (AT2024gng
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) don't have a host photo-z but show no evidence of intra-night photometric evolution within the error bars (the baseline between the first and last observation was about four hours).

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), Kowalski (Duev et al. 2019), and SkyPortal (van der Walt et al. 2019; Coughlin et al. 2023). 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.