X-ray Spectral Analysis of the Jet Termination Shock in Pictor A on Sub-Arcsecond Scales with Chandra. (arXiv:2208.10648v2 [astro-ph.HE] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Thimmappa_R/0/1/0/all/0/1">R. Thimmappa</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Stawarz_L/0/1/0/all/0/1">L. Stawarz</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Neilsen_J/0/1/0/all/0/1">J. Neilsen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ostrowski_M/0/1/0/all/0/1">M. Ostrowski</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Reville_B/0/1/0/all/0/1">B. Reville</a>
Hotspots observed at the edges of extended radio lobes in high-power radio
galaxies and quasars mark the position of mildly-relativistic termination
shock, where the jet bulk kinetic energy is converted to the internal energy of
the jet particles. These are the only astrophysical systems where
mildly-relativistic shocks can be directly resolved at various wavelengths of
the electromagnetic spectrum. The western hotspot in the radio galaxy Pictor,A
is an exceptionally good target in this respect, due to the combination of its
angular size and high surface brightness. In our previous work, after a careful
{it Chandra} image deconvolution, we resolved this hotspot into a disk-like
feature perpendicular to the jet axis, and identified this as the front of the
jet termination shock. We argued for a synchrotron origin of the observed X-ray
photons, which implied maximum electron energies of the order of 10–100,TeV.
Here we present a follow-up on that analysis, proposing in particular a novel
method for constraining the shape of the X-ray continuum emission with
sub-arcsec resolution. The method is based on a {it Chandra} hardness map
analysis, using separately de-convolved maps in the soft and hard X-ray bands.
In this way, we have found there is a systematic, yet statistically significant
gradient in the hardness ratio across the shock, such that the implied electron
energy index ranges from $sleq 2.2$ at the shock front to $s> 2.7$ in the near
downstream. We discuss the implications of the obtained results for a general
understanding of particle acceleration at mildly-relativistic shocks.
Hotspots observed at the edges of extended radio lobes in high-power radio
galaxies and quasars mark the position of mildly-relativistic termination
shock, where the jet bulk kinetic energy is converted to the internal energy of
the jet particles. These are the only astrophysical systems where
mildly-relativistic shocks can be directly resolved at various wavelengths of
the electromagnetic spectrum. The western hotspot in the radio galaxy Pictor,A
is an exceptionally good target in this respect, due to the combination of its
angular size and high surface brightness. In our previous work, after a careful
{it Chandra} image deconvolution, we resolved this hotspot into a disk-like
feature perpendicular to the jet axis, and identified this as the front of the
jet termination shock. We argued for a synchrotron origin of the observed X-ray
photons, which implied maximum electron energies of the order of 10–100,TeV.
Here we present a follow-up on that analysis, proposing in particular a novel
method for constraining the shape of the X-ray continuum emission with
sub-arcsec resolution. The method is based on a {it Chandra} hardness map
analysis, using separately de-convolved maps in the soft and hard X-ray bands.
In this way, we have found there is a systematic, yet statistically significant
gradient in the hardness ratio across the shock, such that the implied electron
energy index ranges from $sleq 2.2$ at the shock front to $s> 2.7$ in the near
downstream. We discuss the implications of the obtained results for a general
understanding of particle acceleration at mildly-relativistic shocks.
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