A Tentative Double Excess in the Gamma-Ray Spectrum of the Fermi Blazar 4FGL J0604.9-0000

Abstract

Based on 17.75 years of Fermi-LAT observations, we report a tentative double excess in the gamma-ray spectrum of the blazar 4FGL J0604.9-0000, with peak energies at approximately 1.5 GeV and 11 GeV. The two excesses are modeled as double Gaussian components. For the lower-energy excess, the best-fit centroid is $E_1 = 1.59\pm0.07$ GeV with a width fixed to the instrumental resolution ($\sigma_1 = 0.145$ GeV). For the higher-energy excess, the centroid is $E_2 = 11.15\pm0.61$ GeV and the width is constrained to $\sigma_2 = 1.185$ GeV (about 10\% of the peak energy), which is comparable to the Fermi-LAT energy resolution at that energy. The two features have local significances of $2.6\sigma$ and $3.7\sigma$ (3 dof), respectively. A joint likelihood analysis yields a combined test statistic of TS $\simeq 27$, corresponding to a local significance of approximately $4.3\sigma$ (4 dof, without correction for trials and the look-elsewhere effect) or about $4.8\sigma$ (2 dof). To our knowledge, no other active galactic nucleus has been reported to show a similar double-excess candidate. The observed energy ratio of $\sim$1:7 is difficult to explain with standard astrophysical emission processes. However, the energies and their ratio are consistent with dark matter annihilation (e.g., $\chi\chi\to\gamma\gamma$ and $\chi\chi\to\gamma\gamma'$) for a particle mass near 11 GeV, making this source a promising target for follow-up observations with next-generation gamma-ray telescopes.