Evidence for a second component in the high-energy core emission from Centaurus A?

TL;DR

This study analyzes four years of Fermi data from Centaurus A, revealing a potential second high-energy component in its core emission characterized by a spectral hardening above 4 GeV, with no significant variability detected.

Contribution

It provides the first evidence of a spectral break indicating a second emission component in Centaurus A's gamma-ray spectrum, expanding understanding of its high-energy processes.

Findings

Detection of gamma rays up to 50 GeV from Cen A's core.

Spectral hardening above 4 GeV suggests an additional emission component.

No significant short-term variability observed in the gamma-ray flux.

Abstract

We report on an analysis of \fermi data from four year of observations of the nearby radio galaxy Centaurus A (Cen A). The increased photon statistics results in a detection of high-energy ($>\:100$ MeV) $\gamma$-rays up to 50 GeV from the core of Cen A, with a detection significance of about 44$\sigma$. The average gamma-ray spectrum of the core reveals evidence for a possible deviation from a simple power-law. A likelihood analysis with a broken power-law model shows that the photon index becomes harder above $E_b \simeq 4$ GeV, changing from $\Gamma_1=2.74\pm0.03$ below to $\Gamma_2=2.09\pm0.20$ above. This hardening could be caused by the contribution of an additional high-energy component beyond the common synchrotron-self Compton jet emission. A variability analysis of the light curve with 15-, 30-, and 60-day bins does not provide evidence for variability for any of the components. Indications for a possible variability of the observed flux are found on 45-day time scale, but the statistics do not allow us to make a definite conclusion in this regards. We compare our results with the spectrum reported by H.E.S.S. in the TeV energy range and discuss possible origins for the hardening observed.