VERITAS Observations Contemporaneous with the LHAASO Detection of NGC 4278

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-$γ$ interactions and optically thin $γ$-ray emission. Within this framework, the implied neutrino signal is slightly below the current sensitivity of IceCube.

💡 Research Summary

The paper reports on VERITAS observations of the LINER/LLAGN galaxy NGC 4278 that were taken serendipitously while the array was pointed at three nearby TeV blazars (1ES 1218+304, 1ES 1215+303, and W Comae). This observing strategy allowed VERITAS to cover NGC 4278 during the same time window in which the Large High‑Altitude Air‑Shower Observatory (LHAASO) reported a significant detection of very‑high‑energy (VHE) gamma‑ray emission (1 TeV–20 TeV) from a point source (1LHAASO J1219+2915) spatially consistent with NGC 4278. LHAASO’s analysis identified an “active” period (MJD 59449–59589) during which the flux was about four times higher than during the surrounding “quasi‑quiet” intervals.

VERITAS accumulated a total of 6.93 hours of exposure between MJD 59280 and 59697 (2021 Mar 7 – 2022 Apr 28). Only 23.8 minutes overlapped with LHAASO’s active period, while the remaining 6.53 hours correspond to the quasi‑quiet state. Data were processed with the standard image‑template reconstruction, throughput corrections, and cuts optimized for a power‑law source with photon index Γ≈2.5 (scaled to 1 % of the Crab Nebula flux). The on‑source region was a 0.0707° radius circle centred on NGC 4278; background was estimated with reflected‑region and ring‑background methods, yielding on/off exposure ratios α of 0.02778 (active) and 0.02825 (quiet).

No statistically significant excess was found: +2.2σ for the active interval and –2.1σ for the quiet interval. Consequently, 95 % confidence upper limits on the differential flux were derived at the decorrelation energies where the uncertainty is minimal. For the active state the limit is 1.0 × 10⁻⁷ GeV cm⁻² s⁻¹ at 620 GeV (average energy threshold ≈240 GeV); for the quasi‑quiet state the limit is 8.3 × 10⁻⁹ GeV cm⁻² s⁻¹ at 680 GeV (threshold ≈290 GeV). These limits are more constraining than the contemporaneous Fermi‑LAT upper limits above 100 GeV.

When combined with the Fermi‑LAT data (100 MeV–1 TeV) and the LHAASO detection, the broadband spectral energy distribution (SED) of NGC 4278 appears to peak between 100 GeV and 2 TeV. This intermediate‑energy peak is atypical for a low‑luminosity AGN (LLAGN) and cannot be readily explained by standard synchrotron‑self‑Compton models that work for blazars.

The authors therefore propose a “corona” scenario: energetic protons are accelerated in the hot, magnetised corona surrounding the supermassive black hole. These protons interact with the intense X‑ray photon field via p‑γ collisions, producing neutral and charged pions. π⁰ decay yields VHE gamma‑rays that escape because the corona’s photon density in NGC 4278 is lower than in classic Seyfert galaxies, making the region optically thin at TeV energies. Charged pions decay into muons and subsequently electrons/positrons, which rapidly cool by synchrotron radiation, boosting the X‑ray flux and providing a feedback loop that enhances the p‑γ interaction rate. The model naturally reproduces the observed hard X‑ray component and the VHE gamma‑ray peak, while predicting a neutrino flux just below the current IceCube sensitivity (≈10⁻⁹ GeV cm⁻² s⁻¹).

Thus, the VERITAS non‑detection, rather than being a null result, places valuable constraints on the quasi‑quiet VHE emission and supports a transient, flare‑like behaviour during the LHAASO active window. It also highlights LLAGN coronae as plausible sites for high‑energy particle acceleration and multimessenger emission. The paper concludes that future observations with more sensitive VHE facilities (CTA, SWGO) and next‑generation neutrino detectors (IceCube‑Gen2) will be crucial to test the corona model and to establish LLAGN as a new class of VHE gamma‑ray and neutrino sources.