A Search for Millilensing Gamma-Ray Bursts in the Observations of Fermi GBM

TL;DR

This study systematically searched for millilensing signatures in 3000 Fermi GBM gamma-ray bursts, identifying four candidates and constraining the density of potential lensing objects with masses around 10^6 solar masses.

Contribution

It presents the first large-scale systematic search for millilensing in gamma-ray bursts and provides constraints on the abundance of compact dark matter objects.

Findings

Four candidate millilensed GRBs identified.

Constraints on dark matter lens density parameter for 10^6 M_sun objects.

Two strong candidate GRBs with significant temporal and spectral features.

Abstract

Millilensing of Gamma-Ray Bursts (GRBs) is expected to manifest as multiple emission episodes in a single triggered GRB with similar light-curve patterns and similar spectrum properties. Identifying such lensed GRBs could help improve constraints on the abundance of compact dark matter. Here we present a systemic search for millilensing among 3000 GRBs observed by the \textit{Fermi} GBM up to 2021 April. Eventually we find 4 interesting candidates by performing auto-correlation test, hardness test, and time-integrated/resolved spectrum test to the whole sample. GRB 081126A and GRB 090717A are ranked as the first class candidate based on their excellent performance both in temporal and spectrum analysis. GRB 081122A and GRB 110517B are ranked as the second class candidates (suspected candidates), mainly because their two emission episodes show clear deviations in part of the time-resolved spectrum or in the time-integrated spectrum. Considering a point mass model for the gravitational lens, our results suggest that the density parameter of lens objects with mass $M_{\rm L}\sim10^{6} M_{\odot}$ is larger than $1.5\times10^{-3}$.