ON THE GAMMA-RAY EMISSION FROM THE CORE AND RADIO LOBES OF THE RADIO GALAXY CENTAURUS A

Narek Sahakyan,Emma De Ona-Wilhelmi,Felix Aharonian,Ruizhi Yang,Frank M Rieger

doi:10.1142/s2010194514601823

Abstract

We summarize recent results based on an analysis of Fermi-LAT data for the lobes and the core of the nearby radio galaxy Centaurus A (Cen A). In the case of the core, high-energy (HE; > 100 MeV) γ-rays up to 50 GeV have been detected with a detection significance of about 44σ. The average gamma-ray spectrum of the core reveals interesting 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 substantially harder above Eb ≃ 4 GeV, changing from Γ1 = 2.74 ± 0.03 below to Γ2 = 2.09 ± 0.20 above. It seems possible that this hardening marks the contribution of an additional high-energy component beyond the common synchrotron-self Compton jet emission. In the case of the lobes, the high-energy gamma-ray emission extends up to 6 GeV, with a significance of more than 10 and 20 σ for the north and the south lobe, respectively. Based on a detailed spatial analysis and comparison with the associated radio lobes, a substantial extension of the HE γ-ray emission beyond the WMAP radio image for the northern lobe of Cen A is found. We provide a short discussion of the lobe's spectral energy distribution (SED) in the context of hadronic and time-dependent leptonic scenarios.

Highlights

The prominent radio galaxy Centaurus A (NGC 5128), at a distance of 3.8 Mpc,[1] is our closest active galaxy
We summarize recent results based on an analysis of Fermi-LAT data for the lobes and the core of the nearby radio galaxy Centaurus A (Cen A)
The Fermi-LAT collaboration has early on reported the detection of HE γ-rays from both the core and the giant radio lobes of Cen A:6,7 An analysis of the available ten-month LAT data set revealed a point-like HE emission region coincident with the position of the radio core of Cen A, and two large extended emission regions detected with a significance of 5 and 8σ for the northern and the southern lobe, respectively

Summary

Introduction

The prominent radio galaxy Centaurus A (NGC 5128), at a distance of 3.8 Mpc,[1] is our closest active galaxy. The Fermi-LAT collaboration has early on reported the detection of HE γ-rays from both the core (i.e., within ∼ 0.1◦) and the giant radio lobes of Cen A:6,7 An analysis of the available ten-month LAT data set revealed a point-like HE emission region coincident with the position of the radio core of Cen A, and two large extended emission regions detected with a significance of 5 and 8σ for the northern and the southern lobe, respectively. The HE emission from the extended regions seemed to be morphologically correlated with the giant radio lobes, contributing more than 50 % to the total HE source emission These regions appeared spectrally well described by a power-law function extending up to 2 or 3 GeV with photon indices of Γ ∼ 2.6. We report on our results obtained using a much larger Fermi-LAT data set, allowing for a detailed investigation of the spectrum and morphology of the ‘lobes’ and for an improved analysis of the spectral and temporal characteristics of HE core emission

The core of Cen A

The lobes of Cen A

Discussion

Findings

Conclusions