2021NatAs...5..560P

If gamma-ray bursts are at cosmological distances, they must be gravitationally lensed occasionally. The detection of lensed images with millisecond-to-second time delays provides evidence for intermediate-mass black holes, a population that has been difficult to observe. Several studies have searched for these delays in gamma-ray burst light curves, which would indicate an intervening gravitational lens. Among the ∼10⁴ gamma-ray bursts observed, there have been a handful of claimed lensing detections, but none have been statistically robust. Here we present a Bayesian analysis identifying gravitational lensing in the light curve of GRB 950830. The inferred lens mass Ml depends on the unknown lens redshift zl, and is given by (1+z_l) M_l=5.5_0.9+^1.7x10⁴M_☉ (90% credibility), which we interpret as evidence for an intermediate-mass black hole. The most probable configuration, with a lens redshift z_l ≈ 1 and a gamma-ray burst redshift z_s ≈ 2, yields a present-day number density of about 2.3_1.6+^4.9 x 10³Mpc^–3 (90% credibility) with a dimensionless energy density Ω_IMBH≈ 4.6_3.3^+9.8x10^–4. The false alarm probability for this detection is ∼0.6% with trial factors. While it is possible that GRB 950830 was lensed by a globular cluster, it is unlikely as we infer a cosmic density inconsistent with predictions for globular clusters Ω_GC ≈ 8 x 10⁶ at 99.8% credibility. If a significant intermediate-mass black hole population exists, it could provide the seeds for the growth of supermassive black holes in the early Universe.