Dissecting the gamma-ray emissions of the nearby galaxies NGC 1068 and NGC 253

TL;DR

This study analyzes gamma-ray data from NGC 1068 and NGC 253, revealing spectral changes and long-term detection variability, with implications for understanding their emission mechanisms and potential links to neutrino signals.

Contribution

It provides a detailed spectral and temporal analysis of gamma-ray emissions from NGC 1068 and NGC 253, highlighting activity changes and discussing their possible origins.

Findings

Spectral-hardening activity in NGC 1068 during specific periods.

Detection significance decrease in NGC 253 after a certain date.

Potential jet contribution to NGC 1068's gamma-ray emission.

Abstract

Intrigued by recent high-energy study results for nearby galaxies with gamma-ray emission and in particular NGC~1068 that has been detected as a neutrino-emitting source by the IceCube Neutrino Observatory, we conduct detailed analysis of the $\gamma$-ray data for the galaxies NGC~1068 and NGC~253, obtained with the Large Area Telescope onboard {\it the Fermi Gamma-ray Space Telescope}. By checking for their possible spectral features and then constructing light curves in corresponding energy ranges, we identify spectral-change activity from NGC ~1068 in $\geq$2\,GeV energy range and long-term detection significance changes for NGC~253 in $\geq$5\,GeV energy range. In the former, the emission appears harder in two half-year time periods than that in the otherwise `quiescent' state. In the latter, a $\sim$2-times detection significance decrease after MJD~57023 is clearly revealed by the test-statistic maps we obtain. Considering studies carried out and models proposed for the $\gamma$-ray emissions of the two sources, we discuss the implications of our findings. We suspect that the jet (or outflow) in NGC~1068 might contribute to the \gr\ emission. The nature of the long-term detection significance change for NGC~253 is not clear, but since the part of the GeV emission may be connected to the very-high-energy (VHE) emission from the center of the galaxy, it could be further probed with VHE observations.