VERITAS Observations Contemporaneous with the LHAASO Detection of NGC 4278

Technical Deep Dive: VERITAS Observations Contemporaneous with the LHAASO Detection of NGC 4278

Overview

The Large High Altitude Air Shower Observatory (LHAASO) recently detected very-high-energy (VHE) gamma-ray emission from the nearby galaxy NGC 4278. This is an unexpected observation, as VHE-emitting active galactic nuclei (AGN) are typically associated with blazar or radio-galaxy classes, both of which have highly relativistic jets. NGC 4278 is classified as a low-ionization nuclear emission-line region (LINER) galaxy, a low-luminosity variant of Seyfert galaxies.

This paper presents serendipitous observations of NGC 4278 by the VERITAS gamma-ray observatory, which were contemporaneous with the LHAASO detection. The VERITAS observations, along with data from the Fermi-LAT space telescope, provide new constraints on the broadband spectral energy distribution (SED) of NGC 4278 during both active and quasi-quiet states.

Methodology

• VERITAS observed the region around NGC 4278 for 6.93 hours between March 2021 and April 2022, with 23.8 minutes coinciding with the LHAASO active period.
• Events were reconstructed using the image template method, with cuts optimized for a power-law spectrum source with index Γ ≃ 2.5.
• Excess counts above background were calculated using the reflected-region method in an on-source region of radius 0.0707° centered on NGC 4278.
• The VERITAS data were split into active and quasi-quiet periods based on the LHAASO temporal analysis.
• Fermi-LAT data were analyzed for the same time period, using a binned-likelihood analysis with a point-source model centered on NGC 4278.

Results

• VERITAS did not detect significant emission from NGC 4278 during either the active or quasi-quiet periods.
• During the active period, VERITAS obtained a flux upper limit of 1.0 × 10^-7 TeV^-1 m^-2 s^-1 above 620 GeV.
• During the quasi-quiet period, VERITAS obtained a flux upper limit of 8.3 × 10^-9 TeV^-1 m^-2 s^-1 above 680 GeV.
• The VERITAS upper limits, combined with the LHAASO and Fermi-LAT data, constrain the VHE spectral peak to be between 100 GeV and 2 TeV.
• A log-parabolic fit to the active-period SED suggests a flux normalization of (7.6 ± 1.3) × 10^-10 photons TeV^-1 cm^-2 s^-1, a spectral index of 2.66 ± 0.21, and a curvature term of 0.125 ± 0.048.
• During the quasi-quiet period, the flux normalization is (1.32 ± 0.49) × 10^-10 photons TeV^-1 cm^-2 s^-1, a factor of 5.7 lower than the active period.

Interpretation

• The authors explore a hadronic, proton-gamma (p-γ) interaction model in the corona of the AGN as a viable explanation for the VHE emission.
• In this model, energetic protons in the corona interact with ambient X-ray photons to produce pions, which then decay into gamma rays, neutrinos, and secondary electrons.
• The rapid synchrotron cooling of the secondary electrons can account for the hard X-ray spectrum observed from NGC 4278, which is characteristic of an extreme high-peaked BL Lac object.
• The model predicts a neutrino flux slightly below the current sensitivity of IceCube, with about 0.2 neutrino events expected during the 140-day active period.
• The authors conclude that the corona of this LLAGN may be capable of producing transient low-level gamma-ray and neutrino emission, which could be further explored by future observations.

Limitations & Uncertainties

• The jet speed and Doppler factor for NGC 4278 are highly uncertain, with values implying both large and small Doppler boosts.
• The VERITAS observations were predominantly during the quasi-quiet period, while the LHAASO active period accounted for most of the detected flux, making a direct comparison difficult.
• The VERITAS upper limits only constrain the power-law index of the VHE spectrum during the quasi-quiet period, not the active period.
• The corona model presented is an example, not a definitive prediction, as the proton spectrum may be more complex than the assumed form.

What Comes Next

• Future, more sensitive observations of NGC 4278 and similar LLAGN by LHAASO, the Southern Wide-field Gamma-ray Observatory, and the Cherenkov Telescope Array will be important for understanding the nature of the VHE emission and testing the proposed corona model.
• Dedicated multiwavelength campaigns, including neutrino observations, could provide further insights into the particle acceleration and emission mechanisms in the coronae of LLAGN.