TeV Emission in NGC 4278: Shock Energetics Insufficient, Compact Origin Favored

Abstract

The Large High Altitude Air Shower Observatory recently detected TeV emission from the low-luminosity active galactic nucleus (LLAGN) NGC 4278. Integral-field spectroscopy with MEGARA revealed ionized outflows on hundred-parsec scales with kinetic power ${\dot{E}}_{\mathrm{shock}}\sim 10^{38}$ erg s${}^{-1}$, well below the TeV luminosity $L_{\gamma }\gtrsim 10^{41}$ erg s${}^{-1}$. The implied efficiencies, $\eta \gtrsim 1000$, are far above unity, showing that shocks cannot account for the TeV emission. Contemporaneous Fermi-Large Area Telescope and Swift data, consistent with a compact synchrotron self-Compton jet component, favor a nuclear origin. NGC 4278 is the first low-ionization nuclear emission-line region (LINER)/LLAGN of its class detected at TeV energies, a benchmark for LINERs with outflows and a prime target for the Cherenkov Telescope Array Observatory. I conclude that the TeV photons arise from a compact nuclear accelerator, while optical outflows trace larger-scale jet--ISM interactions that coexist but do not power the TeV radiation.

Published in Research Notes of the American Astronomical Society