Discovery of Very-high-energy Gamma-ray Emissions from the Low Luminosity AGN NGC 4278 by LHAASO

TL;DR

This paper reports the first detection of very-high-energy gamma-ray emissions from the low-luminosity AGN NGC 4278, demonstrating its jet's capability to accelerate particles to TeV energies.

Contribution

It provides the first evidence of TeV gamma-ray emissions from NGC 4278, a low-luminosity AGN, expanding understanding of particle acceleration in such sources.

Findings

Detection of TeV gamma-ray emissions from NGC 4278 with 8.8 sigma significance.

Best-fit photon spectral index of 2.56±0.14 and flux about 5% of the Crab Nebula.

Indication of variability at a few months timescale in the TeV band.

Abstract

The first source catalog of Large High Altitude Air Shower Observatory reported the detection of a very-high-energy gamma ray source, 1LHAASO J1219+2915. In this paper a further detailed study of the spectral and temporal behavior of this point-like source have been carried. The best-fit position of the TeV source ($\rm{RA}=185.05^{\circ}\pm0.04^{\circ}$, $\rm{Dec}=29.25^{\circ}\pm0.03^{\circ}$) is compatible with NGC 4278 within $\sim0.03$ degree. Variation analysis shows an indication of the variability at a few months level in the TeV band, which is consistent with low frequency observations. Based on these observations, we report the detection of TeV $\gamma$-ray emissions from this low-luminosity AGN NGC 4278. The observations by LHAASO-WCDA during active period has a significance level of 8.8\,$\sigma$ with best-fit photon spectral index $\varGamma=2.56\pm0.14$ and a flux $f_{1-10\,\rm{TeV}}=(7.0\pm1.1_{\rm{sta}}\pm0.35_{\rm{syst}})\times10^{-13}\,\rm{photons\,cm^{-2}\,s^{-1}}$, or approximately $5\%$ of the Crab Nebula. The discovery of VHE from NGC 4278 indicates that the compact, weak radio jet can efficiently accelerate particles and emit TeV photons.