Evidence for Radiation Pressure Compression in the X-ray Narrow Line Region of Seyfert galaxies. (arXiv:1902.03076v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Bianchi_S/0/1/0/all/0/1">Stefano Bianchi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Guainazzi_M/0/1/0/all/0/1">Matteo Guainazzi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Laor_A/0/1/0/all/0/1">Ari Laor</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Stern_J/0/1/0/all/0/1">Jonathan Stern</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Behar_E/0/1/0/all/0/1">Ehud Behar</a>
The observed spatial and kinematic overlap between soft X-ray emission and
the Narrow Line Region (NLR) in obscured Active Galactic Nuclei (AGN) yields
compelling evidence that relatively low-density gas co-exists with higher
density gas on scales as large as 100s of pc. This is commonly interpreted as
evidence for a constant gas pressure multiphase medium, likely produced by
thermal instability. Alternatively, Radiation Pressure Compression (RPC) also
leads to a density distribution, since a gas pressure (and hence density)
gradient must arise within each cloud to counteract the incident ionising
radiation pressure. RPC leads to a well-defined ionization distribution, and a
Differential Emission Measure (DEM) distribution with a universal slope of
$sim-0.9$, weakly dependent on the gas properties and the illuminating
radiation field. In contrast, a multiphase medium does not predict the form of
the DEM. The observed DEMs of obscured AGN with XMM-Newton RGS spectra (the
CHRESOS sample) are in striking agreement with the predicted RPC DEM, providing
a clear signature that RPC is the dominant mechanism for the observed range of
densities in the X-ray NLR. In contrast with the constant gas pressure
multiphase medium, RPC further predicts an increasing gas pressure with
decreasing ionization, which can be tested with future X-ray missions using
density diagnostics.
The observed spatial and kinematic overlap between soft X-ray emission and
the Narrow Line Region (NLR) in obscured Active Galactic Nuclei (AGN) yields
compelling evidence that relatively low-density gas co-exists with higher
density gas on scales as large as 100s of pc. This is commonly interpreted as
evidence for a constant gas pressure multiphase medium, likely produced by
thermal instability. Alternatively, Radiation Pressure Compression (RPC) also
leads to a density distribution, since a gas pressure (and hence density)
gradient must arise within each cloud to counteract the incident ionising
radiation pressure. RPC leads to a well-defined ionization distribution, and a
Differential Emission Measure (DEM) distribution with a universal slope of
$sim-0.9$, weakly dependent on the gas properties and the illuminating
radiation field. In contrast, a multiphase medium does not predict the form of
the DEM. The observed DEMs of obscured AGN with XMM-Newton RGS spectra (the
CHRESOS sample) are in striking agreement with the predicted RPC DEM, providing
a clear signature that RPC is the dominant mechanism for the observed range of
densities in the X-ray NLR. In contrast with the constant gas pressure
multiphase medium, RPC further predicts an increasing gas pressure with
decreasing ionization, which can be tested with future X-ray missions using
density diagnostics.
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