TITLE: GCN CIRCULAR NUMBER: 36285 SUBJECT: LIGO/Virgo/KAGRA S240422ed: Vetting of DECam Candidates Using Real-Time Resources and SAGUARO Infrastructure DATE: 24/04/24 16:01:50 GMT FROM: Jillian Rastinejad at Northwestern Univ. jillianrastinejad2024@u.northwestern.edu

Jillian C. Rastinejad (NU), David J. Sand (UA), Griffin Hosseinzadeh (UA), Charles D. Kilpatrick (NU), Wen-fai Fong (NU), Manisha Shrestha (UA), Jennifer E. Andrews (Gemini), K. Azalee Bostroem (UA), Philip N. Daly (UA), Michael J. Lundquist (Keck), Kerry Paterson (MPIA), Jeniveve Pearson (UA), Samuel D. Wyatt (GSFC) report:

We performed a vetting analysis of the candidates reported from the DECam GW-MMADS follow-up (Hu et al. 2024, GCN 36273). Following the methods outlined in Rastinejad et al. (2022) and Hosseinzadeh et al. (2024), we crossmatch each candidate with the TNS (Gal-Yam 2021) and point source catalogs (Tachibana and Miller 2018; Jayasinghe et al. 2019; Flesch et al. 2021; Gaia Collaboration 2023), search public ZTF photometry (Bellm et al. 2019), and run ATLAS forced photometry (Shingles et al. 2021) at the position of the candidate to rule out transients unrelated to the GW event. We determine a most likely host galaxy using the probability of chance coincidence method (Pcc; Bloom et al. 2002) and search for an associated spectroscopic or photometric redshift in public galaxy catalogs (White et al. 2011; Alam et al. 2015; Beck et al. 2016, 2021; Dalya et al. 2021; DESI Collaboration et al. 2023).

Candidate | Vetting result —————————————————————————————————— AT2024hdk | Three marginal (~3-sigma) ATLAS forced photometry | detections ~3-5 days prior to GW event. AT2024hdl | Marginal (~4-sigma) ATLAS forced photometry detection ~4 | days prior to GW event. AT2024hdm | Marginal (<5-sigma) ATLAS forced photometry detections ~2 | months to 4 days prior to GW event. AT2024hdn | Five ATLAS forced photometry detections ~4-24 days prior to | GW event. AT2024hdo | Associated (Pcc = 0.002) with host galaxy | WISEJ080327.75-260039.2 from GLADE at z = 0.09 +/- 0.02 | (photometric redshift; D = 404 Mpc). AT2024hdp | Associated (Pcc < 0.001) with host galaxy | WISEJ080210.31-271529.7 from GLADE at z = 0.09 +/- 0.02 | (photometric redshift; D = 411 Mpc). ATLAS forced | photometry detections ~3-18 days prior to GW event. AT2024hdq | ZTF detections indicate periodic behavior since 2022. AT2024hdr | ZTF detections in 2021-2022. ATLAS forced photometry | detections extending >200 days prior to GW event.

Overall, we determine that AT2024hdq and AT2024hdr are unrelated to S240422ed. We find candidates AT2024hdn, AT2024hdm, AT2024hdk, AT2024hdl and AT2024hdp to be likely unrelated to the GW event based on detections prior to the GW event, though we note the significance of the detections varies. In particular, combined with previously reported color and spectroscopic information, we strongly disfavor AT2024hdn and AT2024hdm as the counterparts of S240422ed (Tinyanont et al. 2024, GCN 36281; Karambelkar et al. 2024, GCN 36279). Finally, for AT2024hdo, while the most probable host galaxy photometric redshift is inconsistent with the GW-inferred distance, we cannot rule it out as a possible counterpart given large uncertainties in photometric redshift estimates in the Local Universe.

SAGUARO stands for Searches After Gravitational-waves Using ARizona's Observatories (Hosseinzadeh et al. 2024). It is a partnership between the University of Arizona and Northwestern University.

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