Testing a Galactic Lensing Hypothesis with the Prompt Emission of GRB 221009A

Abstract

Abstract Even at modest amplification, the optical depth to gravitational lensing through the Galaxy is < 10−5. However, the large apparent isotropic-equivalent energy of GRB 221009A coupled with a path through low Galactic latitude suggests that the conditional probability that this particular GRB was lensed is greater than the very low a priori expectation. With the extreme brightness of the prompt emission, this Galactic lensing hypothesis can be constrained by autocorrelation analysis of Fermi photons on 0.1–1000 ms timescales. In relating lensing mass, magnification, and autocorrelation timescale, I show that a lensed-induced autocorrelation signature by stellar lenses falls below the minimal variability timescale (MVT) expected from a black hole central engine. However, lensing by Galactic dark matter MACHOs (M l  > 10–1000 M ⊙) could be confirmed with this approach. Regardless, at a peak γ-ray photon rate of > 30 ms−1, GRB 221009A represents a prime opportunity to measure the smallest MVTs of GRBs.