A search for lensed gamma-ray bursts in 11 years of observations by Fermi GBM

TL;DR

This study conducted an 11-year search for gravitationally lensed gamma-ray bursts in Fermi GBM data, using spectral and temporal analysis methods, but found no convincing evidence of lensing events.

Contribution

First comprehensive search for macrolensed GRBs in Fermi GBM data employing multi-criteria analysis and cross-correlation techniques.

Findings

No convincing lens candidates identified.

Similarities within GRB populations can mimic lensing signals.

Observational uncertainties can obscure lensing signatures.

Abstract

Macrolensing of gamma-ray bursts (GRBs) is expected to manifest as a GRB recurring with the same light curve and spectrum as a previous one, but with a different flux and a slightly offset position. Identifying such lensed GRBs may give important information about the lenses, cosmology, as well as the GRBs themselves. Here we present a search for lensed GRBs among $\sim 2700$ GRBs observed by the Fermi Gamma-ray Burst Monitor (GBM) during 11 years of operations. To identify lensed GRBs, we perform initial cuts on position, time-averaged spectral properties and relative duration. We then use the cross-correlation function to assess the similarity of light curves, and finally analyze the time-resolved spectra of the most promising candidates. We find no convincing lens candidates. The most similar pairs are single-pulsed GRBs with relatively few time bins for the spectral analysis. This is best explained by similarities within the GRB population rather than lensing. However, the null result does not rule out the presence of macrolensed GRBs in the sample. In particular, we find that observational uncertainties and Poisson fluctuations can lead to significant differences within a pair of lensed GRBs.