Fermi-LAT and Suzaku Observations of the Radio Galaxy Centaurus B

TL;DR

This study confirms gamma-ray emission from Centaurus B, analyzes its X-ray and gamma-ray properties, and explores whether the lobes or core of the galaxy produce the observed gamma rays, using multi-wavelength data and modeling.

Contribution

It provides the first detailed analysis combining Fermi-LAT and Suzaku data for Centaurus B, exploring emission scenarios for the lobes and core.

Findings

Centaurus B is a steady gamma-ray emitter detected by Fermi-LAT.

No diffuse X-ray emission detected from the radio lobes, with upper limits close to previous claims.

Gamma-ray emission could originate from either the lobes or the core, depending on the X-ray emission scenario.

Abstract

Centaurus B is a nearby radio galaxy positioned in the Southern hemisphere close to the Galactic plane. Here we present a detailed analysis of about 43 months of accumulated Fermi-LAT data of the gamma-ray counterpart of the source initially reported in the 2nd Fermi-LAT catalog, and of newly acquired Suzaku X-ray data. We confirm its detection at GeV photon energies, and analyze the extension and variability of the gamma-ray source in the LAT dataset, in which it appears as a steady gamma-ray emitter. The X-ray core of Centaurus B is detected as a bright source of a continuum radiation. We do not detect however any diffuse X-ray emission from the known radio lobes, with the provided upper limit only marginally consistent with the previously claimed ASCA flux. Two scenarios that connect the X-ray and gamma-ray properties are considered. In the first one, we assume that the diffuse non-thermal X-ray emission component is not significantly below the derived Suzaku upper limit. In this case, modeling the inverse-Compton emission shows that the observed gamma-ray flux of the source may in principle be produced within the lobes. This association would imply that efficient in-situ acceleration of the radiating electrons is occurring and that the lobes are dominated by the pressure from the relativistic particles. In the second scenario, with the diffuse X-ray emission well below the Suzaku upper limits, the lobes in the system are instead dominated by the magnetic pressure. In this case, the observed gamma-ray flux is not likely to be produced within the lobes, but instead within the nuclear parts of the jet. By means of synchrotron self-Compton modeling we show that this possibility could be consistent with the broad-band data collected for the unresolved core of Centaurus B, including the newly derived Suzaku spectrum.