High-energy particle acceleration at the radio-lobe shock of Centaurus A
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abstract
We present new results on the shock around the southwest radio lobe of
Centaurus A using data from the Chandra Very Large Programme observations. The
X-ray spectrum of the emission around the outer southwestern edge of the lobe
is well described by a single power-law model with Galactic absorption --
thermal models are strongly disfavoured, except in the region closest to the
nucleus. We conclude that a significant fraction of the X-ray emission around
the southwest part of the lobe is synchrotron, not thermal. We infer that in
the region where the shock is strongest and the ambient gas density lowest, the
inflation of the lobe is accelerating particles to X-ray synchrotron emitting
energies, similar to supernova remnants such as SN1006. This interpretation
resolves a problem of our earlier, purely thermal, interpretation for this
emission, namely that the density compression across the shock was required to
be much larger than the theoretically expected factor of 4. We estimate that
the lobe is expanding to the southwest with a velocity of ~2600 km/s, roughly
Mach 8 relative to the ambient medium. We discuss the spatial variation of
spectral index across the shock region, concluding that our observations
constrain gamma_max for the accelerated particles to be 10^8 at the strongest
part of the shock, consistent with expectations from diffusive shock
acceleration theory. Finally, we consider the implications of these results for
the production of ultra-high energy cosmic rays (UHECRs) and TeV emission from
Centaurus A, concluding that the shock front region is unlikely to be a
significant source of UHECRs, but that TeV emission from this region is
expected at levels comparable to current limits at TeV energies, for plausible
assumed magnetic field strengths.
