Dissecting the γ-Ray Emissions of the Nearby Galaxies NGC 1068 and NGC 253

Intrigued by recent high-energy study results for nearby galaxies with.-ray emission and in particular NGC 1068 that has been detected as a neutrino-emitting source by the IceCube Neutrino Observatory, we conduct a detailed analysis of the.-ray data of the galaxies NGC 1068 and NGC 253, obtained with the Large Area Telescope on board the Fermi.-ray Space Telescope. By checking their possible spectral features and then constructing light curves in the corresponding energy ranges, we identify spectral-change activity from NGC 1068 in the >= 2 GeV energy range and long-term, statistically significant changes for NGC 253 in the >= 5 GeV energy range. In the former, the emission appears harder in two half-year periods than in the otherwise "quiescent" state. In the latter, an similar to two-fold decrease in the detection significance after MJD = 57023 is clearly revealed by the test-statistic maps we obtain. Considering the previous studies carried out and the various models proposed for the gamma-ray emissions of the two sources, we discuss the implications of our findings. We suspect that a jet (or outflow) in NGC 1068 might contribute to the.-ray emission. The nature of the long-term statistically significant changes 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.