Y. Takeuchi, J. Kataoka, L. Stawarz, Y. Takahashi, K. Maeda, T. Nakamori, C. C. Cheung, A. Celotti, Y. Tanaka, T. Takahashi
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.
View original:
http://arxiv.org/abs/1202.0868
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