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VERITAS Observations Contemporaneous with the LHAASO Detection of NGC 4278

The VERITAS Collaboration, A. Archer, P. Bangale, J. T. Bartkoske, W. Benbow, J. H. Buckley, Y. Chen, J. L. Christiansen, A. J. Chromey, A. Duerr, M. Errando, M. Escobar Godoy, S. Feldman, Q. Feng, S. Filbert, L. Fortson, A. Furniss, W. Hanlon, O. Hervet, C. E. Hinrichs, J. Holder, Z. Hughes, T. B. Humensky, M. Iskakova, W. Jin, M. N. Johnson, M. Kertzman, M. Kherlakian, D. Kieda, T. K. Kleiner, N. Korzoun, S. Kumar, S. Kundu, M. J. Lang, M. Lundy, G. Maier, C. McSorley, P. Moriarty, R. Mukherjee, W. Ning, S. O'Brien, M. Ohishi, M. A. Ong, A. Pandey, C. Poggemann, M. Pohl, E. Pueschel, J. Quinn, P. L. Rabinowitz, K. Ragan, P. T. Reynolds, D. Ribeiro, L. Rizk, E. Roache, I. Sadeh, L. Saha, H. Salzmann, M. Santander, G. H. Sembroski, R. Shang, M. Splettstoesser, D. Tak, A. K. Talluri, I. Thoreson, J. V. Tucci, J. Valverde, D. A. Williams, S. L. Wong, T. Yoshikoshi

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

VERITAS Observations Contemporaneous with the LHAASO Detection of NGC 4278

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 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- interactions and optically thin -ray emission. Within this framework, the implied neutrino signal is slightly below the current sensitivity of IceCube.
Paper Structure (10 sections, 10 equations, 2 figures, 2 tables)

This paper contains 10 sections, 10 equations, 2 figures, 2 tables.

Table of Contents

  1. Introduction
  2. LHAASO Observations
  3. VERITAS Observations
  4. Fermi-LAT Observations
  5. Spectral Energy Distribution
  6. Interpretation
  7. Hadronic interactions
  8. Secondary electrons
  9. Primary electrons
  10. Conclusions

Figures (2)

  • Figure 1: VERITAS sky map from observations of 1ES 1218+304, 1ES 1215+303, and W Comae contemporaneous with the LHAASO analysis. The location of each source, including NGC 4278 is marked on the map with a '+' symbol and the point spread function is shown in the bottom right. There is no evidence for the detection of NGC 4278 in the VERITAS sky map. A background fluctuation with a pre-trials significance of 3.1$\sigma$ can be seen 0.15$^\circ$ below the location of NGC 4278.
  • Figure 2: NGC 4278 SED during the active and quasi-quiet periods: LHAASO-WCDA fluxes and upper limits from LHAASO-KM2A (blue & purple), VERITAS (yellow & orange), and Fermi-LAT (green & light green). The BronziniFermi-LAT (gray) is from data spanning the 1LHAASO Catalog dates. The power-law fit to the LHAASO data is from LHAASO-Detection. A log-parabolic model was fit to the combined dataset for the active period (blue dotted line) to explore the curvature of the spectrum. During the quasi-quiet period (purple dotted line), the two LHAASO-WCDA flux points do not constrain the curvature, so only the normalization was fit assuming the same spectral shape as the active period. The example corona model presented in section 6.1 is also included.