Constraints on the electromagnetic counterpart of the Neutron Star Black Hole merger GW200115

TL;DR

This study reports optical follow-up observations of the neutron star-black hole merger GW200115, constraining possible electromagnetic counterparts and providing insights into the merger's ejecta and jet properties.

Contribution

First comprehensive optical follow-up of GW200115, setting limits on ejecta mass and jet emission, and highlighting the importance of prompt localization for future observations.

Findings

Disfavors high-mass ejecta (>0.1 solar masses)

Rules out standard short gamma-ray burst for viewing angles less than 15 degrees

No confirmed electromagnetic counterpart detected

Abstract

We report the results of our follow-up campaign for the neutron star - black hole (NSBH) merger GW200115 detected during the O3 run of the Advanced LIGO and Advanced Virgo detectors. We obtained wide-field observations with the Deca-Degree Optical Transient Imager (DDOTI) covering ~20% of the total probability area down to a limiting magnitude of $w$=20.5 AB at ~23 h after the merger. Our search for counterparts returns a single candidate (AT2020aeo), likely not associate to the merger. In total, only 25 sources of interest were identified by the community and later discarded as unrelated to the GW event. We compare our upper limits with the emission predicted by state-of-the-art kilonova simulations and disfavor high mass ejecta (>0.1$M_{\odot}$), indicating that the spin of the system is not particularly high. By combining our optical limits with gamma-ray constraints from $Swift$ and $Fermi$, we disfavor the presence of a standard short duration burst for viewing angles $\lesssim$15 deg from the jet axis. Our conclusions are however limited by the large localization region of this GW event, and accurate prompt positions remain crucial to improving the efficiency of follow-up efforts.