A Strong Radio Brightening at the Jet Base of M 87 During the Elevated Very High Energy Gamma-Ray State in 2012

Abstract

We report our intensive, high-angular-resolution radio monitoring observations of the jet in M 87 with the VLBI Exploration of Radio Astrometry (VERA) and the European VLBI Network (EVN) from February 2011 to October 2012, together with contemporaneous high-energy (HE; 100 MeV< E <100 GeV) γ-ray light curves obtained by the Fermi Large Area Telescope (LAT). During this period (specifically from February 2012 to March 2012), an elevated level of the M 87 flux is reported at very-high-energy (VHE; E > 100 GeV) γ-rays by VERITAS. We detected a remarkable (up to ∼70%) increase of the radio flux density from the unresolved jet base (radio core) with VERA at 22 and 43 GHz coincident with the VHE activity. Meanwhile, we confirmed with EVN at 5 GHz that the peculiar knot HST-1, which is an alternative favored γ-ray production site located at 120 pc from the nucleus, remained quiescent in terms of its flux density and structure. These results in the radio bands strongly suggest that the VHE γ-ray activity in 2012 originates in the jet base within 0.03 pc or 56 Schwarzschild radii (the VERA spatial resolution of 0.4 mas at 43 GHz) from the central supermassive black hole. We further conducted VERA astrometry for the M 87 core at six epochs during the flaring period, and detected core shifts between 22 and 43 GHz, a mean value of which is similar to that measured in the previous astrometric measurements. We also discovered a clear frequency-dependent evolution of the radio core flare at 43, 22 and 5 GHz; the radio flux density increased more rapidly at higher frequencies with a larger amplitude, and the light curves clearly showed a time-lag between the peaks at 22 and 43 GHz, the value of which is constrained to be within ∼ 35 − 124 days. This indicates that a new radio-emitting component was created near the black hole in the period of the VHE event, and then propagated outward with progressively decreasing synchrotron opacity. By combining the obtained core shift and time-lag, we estimated an apparent speed of the newborn component propagating through the opaque region between the cores at 22 and 43 GHz. We derived a sub-luminal speed (less than ∼0.2c) for this component. This value is significantly slower than the super-luminal (∼1.1c) features that appeared from the core during the prominent VHE flaring event in 2008, suggesting that the stronger VHE activity can be associated with the production of the higher Lorentz factor jet in M 87.

Radio Galaxies at VHE Energies

“…As the angular resolution of current VHE instruments is limited (to scales of ∼ 25 kpc for M87), coordinated VLBI radio observations, capable of probing down to scales of tens of gravitational radii, have been performed during the 2008, 2010 and 2012 high VHE states. These results indicate that the TeV emission is accompanied by (delayed) radio core flux enhancements, supporting proposals that the VHE emission originate at the jet base very near to the black hole [5,15,65,66]. The radio-VHE correlation along with the required compactness of the VHE zone have served as a strong motivation to explore plasma injection via gap-type magnetospheric processes close to the black hole [38,87,113].…”

The extragalactic gamma-ray sky in the Fermi era

“…M 87 is known to be a variable X-ray (see e.g., Harris et al 2003Harris et al , 2012 and TeV source (see e.g., Acciari et al 2010;Abramowski et al 2012). Fermi observations did not reveal any variation since its discovery (Abdo et al 2013a;Hada et al 2014). The lack of γ -ray variability in FR Is radio galaxies is consistent with the scenario in which the former class is the parent population of the latter simply viewed at larger angle along the line of sight.…”

The limb-brightened jet of M87 down to the 7 Schwarzschild radii scale

“…Thus, the inverse-Compton scenario disfavors the compact 86 GHz VLBI core region (∼ 11R sch ) as the dominant source of gamma-ray or TeV photons. Interestingly, the timing analysis of TeV and γray events associated with the M 87 nuclear region constrain the high-energy photon production site to be ∼ 20 − 50R sch (Acciari et al 2010;Abramowski et al 2012;Hada et al 2014), which is 2 − 5 times larger than the size of the 86 GHz VLBI core.…”