Evidence for an intermediate-mass black hole from a gravitationally lensed gamma-ray burst
Abstract
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 GRB950830. The inferred lens mass depends on the unknown lens redshift zl, and is given by (1+zl)Ml = 5.5+1.7-0.9× 104 M (90% credibility), which we interpret as evidence for an intermediate-mass black hole. The most probable configuration, with a lens redshift zl 1 and a gamma-ray burst redshift zs 2, yields a present day number density of nimbh≈ 2.3+4.9-1.6×103 Mpc-3 (90% credibility) with a dimensionless energy density imbh ≈ 4.6+9.8-3.3×10-4. The false alarm probability for this detection is 0.6\% with trial factors. While it is possible that GRB950830 was lensed by a globular cluster, it is unlikely since we infer a cosmic density inconsistent with predictions for globular clusters gc ≈ 8 × 10-6 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.