Fermi-GBM Observations of GRB 210812A: Signatures of a Million Solar Mass Gravitational Lens

Abstract

Observing gravitationally lensed objects in the time domain is difficult, and well-observed time-varying sources are rare. Lensed gamma-ray bursts (GRBs) offer improved timing precision for this class of objects, complementing observations of quasars and supernovae. The rate of lensed GRBs is highly uncertain, approximately one in 1000. The Gamma-ray Burst Monitor on board the Fermi Gamma-ray Space Telescope has observed more than 3000 GRBs, making it an ideal instrument to uncover lensed bursts. Here we present observations of GRB 210812A showing two emission episodes, separated by 33.3 s and with a flux ratio of about 4.5. An exhaustive temporal and spectral analysis shows that the two emission episodes have the same pulse and spectral shape, which poses challenges to GRB models. We report multiple lines of evidence for a gravitational lens origin. In particular, modeling the lightcurve using nested sampling, we uncover strong evidence in favor of the lensing scenario. Assuming a point-mass lens, the mass of the lensing object is about 1 million solar masses. High-resolution radio imaging is needed for future lens candidates to derive tighter constraints.

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

“…170 candidates, whose a(∆t) − ∆t curve contains at least one effective peak (peak at ∆t = 0 doesn't count), are selected as the preliminary candidates. ∆t of the highest effective peak could be taken as the possible millilensing time delay, whose uncertainty could be estimated by adding Poisson noise to the original lightcurve (Ukwatta et al 2010;Hakkila et al 2018;Ji et al 2018;Veres et al 2021, for a similar approach).…”

“…Hardness test has been widely used to justify the lensing effect (Paynter et al 2021;Kalantari et al 2021;Wang et al 2021;Veres et al 2021), based on the hypothesis that the flux ratio between gravitationally lensed pulses should not depend on energy (Paczynski 1987). In this work, we adopt three energy channels to carry out the hardness test: Low energy channel (8-50 keV), Medium energy channel (50-110 keV) and High energy channel (110-323 keV).…”

“…Paynter et al (2021) claims the first convincing evidence for millilensing events in the light curve of BATSE GRB 950830 and thus claims the existence of intermediatemass black holes 2 . Later on, an increase of millilensing GRB candidates from the data of Fermi/GBM have been claimed (Kalantari et al 2021;Wang et al 2021;Yang et al 2021;Veres et al 2021). For instance, Wang et al (2021) and Yang et al (2021) made independent analysis for GRB 200716C and argued it showing millilensing signatures.…”

“…For instance, Wang et al (2021) and Yang et al (2021) made independent analysis for GRB 200716C and argued it showing millilensing signatures. Veres et al (2021) made an exhaustive temporal and spectral analysis to claim that GRB 210812A shows strong evidence in favor of the millilensing effects.…”

“…In the study of GRB 210812A, Veres et al (2021) shows that time-resolved spectrum is a more powerful tool than hardness to help justify the lensing effect. Taking advantage of its two main instruments (GBM and LAT), it is possible to study the γ-ray spectra of Fermi GRBs in unprecedented detail.…”