Contribution to the 2025 Gravitation session of the 59th Rencontres de Moriond: Limits on the Ejecta Mass During the Search for Kilonovae Associated with Neutron Star-Black Hole Mergers

TL;DR

This paper assesses the observational strategies and constraints on ejecta mass in neutron star-black hole mergers by analyzing multi-messenger data and the timing of kilonova peaks, explaining the non-detection of expected signals.

Contribution

It provides an analysis of follow-up observation strategies and their effectiveness in detecting kilonovae from NSBH mergers, highlighting the importance of timing in observations.

Findings

Over 50% of simulated kilonovae peak within one day post-merger.

Observation timing critically impacts kilonova detection success.

Constraints on ejecta properties are derived from non-detections.

Abstract

This study evaluates ejecta properties from multi-messenger observations to understand the absence of detectable KN associated to the four NSBH candidates from May 2023 to July 2024: we use GW public information and joint observations taken from 05.2023 to 07.2024 (LVK, ATLAS, DECam, GECKO, GOTO, GRANDMA, SAGUARO, TESS, WINTER, ZTF) in the followup of S230518h, GW230529, S230627c and S240422ed. First, our analysis on follow-up observation strategies shows that, on average, more than 50\% of the simulated KNe associated with NSBH mergers reach their peak luminosity around one day after merger in the $g,r,i$- bands, which is not necessarily covered for each NSBH GW candidate. We also analyze the trade-off between observation efficiency and the intrinsic properties of the KN emission, to understand the impact on how these constraints affect our ability to detect the KN, and underlying ejecta properties for each GW candidate.