VERITAS Observations Contemporaneous with the LHAASO Detection of NGC 4278

TL;DR

This paper reports VERITAS observations of NGC 4278 during a gamma-ray active period, providing upper limits that help constrain its spectral energy distribution and suggesting proton acceleration in the AGN corona.

Contribution

First simultaneous VERITAS observations of NGC 4278 during LHAASO activity, setting upper limits that refine the SED and proposing a proton acceleration model.

Findings

VERITAS did not detect gamma-ray emission from NGC 4278.

The flux upper limits suggest the SED peaks between 100 GeV and 2 TeV.

A proton acceleration model explains the broadband emission and neutrino predictions.

Abstract

Significant gamma-ray emission between 1 TeV and 20 TeV from a point source, 1LHAASO J1219+2915, consistent with the location of the LINER/LLAGN galaxy NGC 4278 was recently reported by the LHAASO collaboration. These data were later split into active and quasi-quiet states, with most of the LHAASO significance coming from the active state (MJD 59449-59589). Subsequent analysis of Fermi-LAT and Swift-XRT observations have been used to explore the double-peaked broad-band emission. Models of the spectral energy distribution (SED) are currently unconstrained due to the lack of contemporaneous multi-wavelength data at either peak. Here we report serendipitous observations of NGC 4278 with VERITAS, made possible by the contemporaneous observations of the nearby blazars 1ES 1218+304, 1ES 1215+303, and W Comae, each of which are located within $2^\circ$ of NGC 4278. VERITAS did not detect any gamma-ray emission and a flux upper limit was calculated. The flux upper limits constrain the photon spectrum of the quasi-quiet period, and together with Fermi-LAT, indicate a peak in the SED between 100 GeV and 2 TeV. We present an interpretation of the broadband SED that is based on acceleration of protons in the corona of the AGN, followed by p-$\gamma$ interactions and optically thin $\gamma$-ray emission. Within this framework, the implied neutrino signal is slightly below the current sensitivity of IceCube.