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

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 ${\Gamma }_1=2.74\pm 0.03$ below to ${\Gamma }_2=2.09\pm 0.20$ 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.

submitted to ApJ Letters

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