Suzaku X-ray Imaging of the Extended Lobe in the Giant Radio Galaxy NGC6251 Associated with the Fermi-LAT Source 2FGLJ1629.4+8236

TL;DR

This study uses Suzaku X-ray imaging to detect diffuse X-ray emission from the north-west radio lobe of NGC6251, supporting the idea that extended radio lobes can produce gamma-rays via inverse-Compton scattering, indicating efficient particle acceleration.

Contribution

First detection of diffuse X-ray emission from NGC6251's radio lobe associated with Fermi-LAT gamma-ray source, linking lobe emission to high-energy particle acceleration.

Findings

Diffuse X-ray emission consistent with inverse-Compton scattering.

Gamma-ray flux can be explained by lobe emission.

Supports particle acceleration in radio galaxy lobes.

Abstract

We report the results of a Suzaku X-ray imaging study of NGC6251, a nearby giant radio galaxy with intermediate FR I/II radio properties. Our pointing direction was centered on the gamma -ray emission peak recently discovered with Fermi-LAT around the position of the north-west radio lobe 15 arcmin offset from the nucleus. After subtracting two "off-source" pointings adjacent to the radio lobe, and removing possible contaminants in the XIS field of view, we found significant residual X-ray emission most likely diffuse in nature. The spectrum of the excess X-ray emission is well fit by a power law with photon index \Gamma = 1.90 +- 0.15 and a 0.5 - 8 keV flux of 4 x 10^{-13} erg cm^{-2} s^{-1}. We interpret this diffuse X-ray emission component as being due to inverse-Compton up-scattering of the cosmic microwave background photons by ultrarelativistic electrons within the lobe, with only a minor contribution from the beamed emission of the large-scale jet. Utilizing archival radio data for the source, we demonstrate by means of broad-band spectral modeling that the -ray flux of the Fermi-LAT source 2FGL J1629.4+8236 may well be accounted for by the high-energy tail of the inverse-Compton continuum of the lobe. Thus, this claimed association of gamma-rays from the north-west lobe of NGC6251, together with the recent Fermi-LAT imaging of the extended lobes of Centaurus A, indicates that particles may be efficiently (re-)accelerated up to ultrarelativistic energies within extended radio lobes of nearby radio galaxies in general.