Constraining possible $γ$-ray burst emission from GW230529 using Swift-BAT and Fermi-GBM

Kavli Affiliate: Lijing Shao

| First 5 Authors: Samuele Ronchini, Suman Bala, Joshua Wood, James Delaunay, Simone Dichiara

| Summary:

GW230529 is the first compact binary coalescence detected by the
LIGO-Virgo-KAGRA collaboration with at least one component mass confidently in
the lower mass-gap, corresponding to the range 3-5$M_{odot}$. If interpreted
as a neutron star-black hole merger, this event has the most symmetric mass
ratio detected so far and therefore has a relatively high probability of
producing electromagnetic (EM) emission. However, no EM counterpart has been
reported. At the merger time $t_0$, Swift-BAT and Fermi-GBM together covered
100$%$ of the sky. Performing a targeted search in a time window $[t_0-20
text{s},t_0+20 text{s}]$, we report no detection by the Swift-BAT and the
Fermi-GBM instruments. Combining the position-dependent $gamma-$ray flux upper
limits and the gravitational-wave posterior distribution of luminosity
distance, sky localization and inclination angle of the binary, we derive
constraints on the characteristic luminosity and structure of the jet possibly
launched during the merger. Assuming a top-hat jet structure, we exclude at
90$%$ credibility the presence of a jet which has at the same time an on-axis
isotropic luminosity $gtrsim 10^{48}$ erg s$^{-1}$, in the bolometric band 1
keV-10 MeV, and a jet opening angle $gtrsim 15$ deg. Similar constraints are
derived testing other assumptions about the jet structure profile. Excluding
GRB 170817A, the luminosity upper limits derived here are below the luminosity
of any GRB observed so far.

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