Pinpointing the TeV gamma-ray emission region in M87 using TeV and 43 GHz radio monitoring

TL;DR

This study combines TeV gamma-ray and 43 GHz radio observations of M87 to localize the gamma-ray emission region near the black hole, revealing a close connection between TeV flares and radio core activity.

Contribution

It provides the first direct evidence linking TeV gamma-ray flares to activity within a few tens of Schwarzschild radii of the black hole in M87.

Findings

TeV and radio flares are temporally coincident.

Gamma-ray emission originates within a few tens of Schwarzschild radii.

Radio core flux density increases during TeV flares.

Abstract

The TeV radio galaxy M87 is the first radio galaxy detected in the TeV regime. The structure of its jet, which is not pointing towards our line of sight, is spatially resolved in X-ray (by Chandra), optical and radio observations. In 2008, the three main Atmospheric Cherenkov Telescope observatories VERITAS, MAGIC and H.E.S.S. coordinated their observations in a joint campaign from January to May with a total observation time of approx. 120 hours. In February, strong and rapid day-scale TeV flares were detected. VLBA monitoring observations during the same period showed that the 43 GHz radio flux density of the unresolved core began to rise at the time of the TeV flares and eventually reached levels above any previously seen with VLBI. New jet components appeared during the flare. The localization accuracy of the TeV instruments of many arcseconds, even for strong sources, is inadequate to constrain the origin of the emission in the inner jets of AGNs. For M87, with a 6 billion solar mass black hole and a distance of 16.7 Mpc, the VLBA resolution instead corresponds to 30 by 60 Schwarzschild radii. This is starting to resolve the jet collimation region. The temporal coincidence of the TeV and radio flares indicates that they are related and provides the first direct evidence that the TeV radiation from this source is produced within a few tens of RS of the radio core, thought to be coincident to within the VLBA resolution with the black hole.