TITLE:   GCN CIRCULAR
NUMBER:  36288
SUBJECT: LIGO/Virgo/KAGRA S240413p: DESGW DECam Follow-Up 
DATE:    24/04/24 19:39:16 GMT
FROM:    norafs@umich.edu

Nora Sherman (U. Mich.), Isaac McMahon (UZH), Elise Kesler (U. Mich.), Sean MacBride (U. Mich.), Andre Santos (CBPF), Simran Kaur(U. Mich.), Luidhy Santana-Silva (CBPF), Marcelle Soares-Santos (UZH), Clecio de Bom (CBPF), Ken Herner (FNAL), Kathy Vivas (NOIRLab)


On behalf of the DESGW team:


We report initial results of our DECam search for an optical counterpart of S240413p (LVK, GCN 36075) using DECam. Starting at 2024-04-16 00:33 UTC (~69.25hr post merger), we observed approximately the entirety of the 90% localization region provided in the updated sky map released on April 13, 2024 20:33:39 UTC.


Images were processed by our difference imaging pipeline (Herner et al. 2020) using DES and public DECam images as templates. We employ a machine learning code (autoscan, Goldstein et al. 2015) to reject subtraction artifacts. Candidates were initially selected by requiring at least two high signal to noise detections, which were separated in time in order to reject moving objects. We also require an autoscan score of at least 0.7 on at least one of those detections, and a host redshift consistent with the LVK reported distance within three s.d. We have identified three candidates coinciding with the nuclei of known AGNs, posted below. These are of interest for AGN-driven optical emission for BBH mergers (e.g. Graham et. al 2020).


AGN Name                  ||    RA     ||   DEC      ||  Band    ||    Mag    
================================================================================
WISEA J105831.80+115457.6    164.632471    11.915976     r           21.89
2MASS J10511539+05482488     162.814215    5.806858      i           18.99
WISEA J105144.54+061108.0    162.935722    6.185632      i           20.32



All magnitudes reported are observed magnitudes. We encourage spectroscopic followup of these candidates.

The DECam Search & Discovery Program for Optical Signatures of Gravitational Wave Events (DESGW) is carried out by the Dark Energy Survey (DES) collaboration in partnership with wide ranging groups in the community. DESGW uses data obtained with the Dark Energy Camera (DECam), which was constructed by the DES collaboration with support from the Department of Energy and member institutions, and utilizes data as distributed by the Science Data Archive at NOAO. NOAO is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation.