A strong radio brightening at the jet base of M87 during the elevated very-high-energy gamma-ray state in 2012

TL;DR

This study used high-resolution radio observations to link the 2012 elevated TeV gamma-ray activity in M87 to the jet base near the supermassive black hole, providing insights into gamma-ray origins.

Contribution

It presents the first direct radio evidence connecting TeV gamma-ray activity to the jet base in M87 during an elevated state.

Findings

Radio core flux increased during gamma-ray activity

HST-1 remained quiescent in radio during the event

Gamma-ray origin localized within 0.03 pc of the black hole

Abstract

The nearby radio galaxy M87 offers a unique opportunity for exploring the connection between gamma-ray production and jet formation at an unprecedented linear resolution. However, the origin and location of the gamma-rays in this source is still elusive. Based on previous radio/TeV correlation events, the unresolved jet base (radio core) and the peculiar knot HST-1 at >120 pc from the nucleus are proposed as candidate site(s) of gamma-ray production. Here we report our intensive, high-resolution radio monitoring observations of the M87 jet with the VLBI Exploration of Radio Astrometry (VERA) and the European VLBI Network (EVN) from February 2011 to October 2012. During this period, an elevated level of the M87 flux is reported at TeV with VERITAS. We detected a remarkable flux increase in the radio core with VERA at 22/43 GHz coincident with the VHE activity. Meanwhile, HST-1 remained quiescent in terms of its flux density and structure at radio. These results strongly suggest that the TeV gamma-ray activity in 2012 originates in the jet base within 0.03 pc (projected) from the central supermassive black hole.