VLBA22A-489

Resolving the Radio Afterglow of a Short Gamma-Ray Burst

Abstract

Gamma-ray bursts (GRB) with light curves showing two pulses have recently provided evidence for GRBs gravitationally lensed by intermediate-mass black holes (IMBH). There are only ~40 IMBH candidates today and lensed GRBs can provide a new way to discover IMBHs. Very long baseline interferometry resolving two lensed images would confirm the IMBH lensing scenario, but the GRB must be well-localized and have a bright enough radio afterglow. GRB 200716C is the only IMBH-lensed GRB candidate localized to a host galaxy. Motivated by a VLA detection of its radio emission on May 8th, 2022, we request 5 GHz continuum observation of the late-time radio afterglow of GRB 200716C to confirm the IMBH candidate, measure its mass, and reveal the nature of the binary neutron star merger remnant that powers the short GRB. If the remnant is a long-lived magnetar, it is predicted to accelerate the merger ejecta, which interacts with the circum-merger medium, producing radio synchrotron radiation years after the merger. The radio light curves constrain the kinetic energy and mass of the ejecta, providing a test for the magnetar remnant model. In response to the IMBH discovery in September 2021, DDT Exploratory Time is required to observe the fading afterglow.

Investigators

Name	Institution
John McKean	Netherlands Institute for Radio Astronomy; Kapteyn Astronomical Institute
Di Wen *	Kapteyn Astronomical Institute
GBT Operator	Green Bank Observatory

* indicates the PI