Evidence for an intermediate-mass black hole from a gravitationally lensed gamma-ray burst

James Paynter; Rachel Webster; Eric Thrane

doi:10.1038/s41550-021-01307-1

paper

Evidence for an intermediate-mass black hole from a gravitationally lensed gamma-ray burst

arXiv:2103.15414 · doi:10.1038/s41550-021-01307-1

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 $\sim 1 0^{4}$ 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 $z_{l}$ , and is given by $(1 + z_{l}) M_{l} = 5. 5_{- 0.9}^{+ 1.7} \times 1 0^{4}$ M $_{⊙}$ (90% credibility), which we interpret as evidence for an intermediate-mass black hole. The most probable configuration, with a lens redshift $z_{l} \sim 1$ and a gamma-ray burst redshift $z_{s} \sim 2$ , yields a present day number density of $n_{imbh} \approx 2. 3_{- 1.6}^{+ 4.9} \times 1 0^{3} Mpc^{- 3}$ (90% credibility) with a dimensionless energy density $Ω_{imbh} \approx 4. 6_{- 3.3}^{+ 9.8} \times 1 0^{- 4}$ . The false alarm probability for this detection is $\sim 0.6%$ 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 $Ω_{gc} \approx 8 \times 1 0^{- 6}$ 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.

22 pages, 12 figures

Evidence for an intermediate-mass black hole from a gravitationally lensed gamma-ray burst

Abstract

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