Coronal properties of AGN

The primary hard X-ray continuum in Seyfert galaxies arises from repeated Compton up-scattering of UV/soft X-ray accretion disk photons in a hot, trans-relativistic plasma. This process results in a power-law spectrum with a high-energy cutoff (Ecut), determined by the electron temperature in the hot corona. The power-law index  is a function of the plasma temperature and optical depth. Only recently, the high S/N of the NuSTAR spectra (in the 3-79 keV range) allowed precise constraints on Ecut in some of the brightest local AGN.
If the corona is compact, then any increase in the source luminosity for a fixed size will generate electron-positron pairs rather than heating the coronal plasma. The pair production mechanism acts like a thermostat, regulating the source temperature as a function of its compactness. Current measurements, preformed at low redshift, show that AGN coronae are in agreement with this scenario.

If the whole 0.3-80 keV band is observed, a measurement similar to those obtained for much brighter (but less luminous and less massive) local AGN is possible. The lower flux (compared to nearby objects) is compensated for by the shift of Ecut to lower energies.

QSO B2202-209

The redshift of the source was estimated by Reboul et al. (1987) to be z = 1.77, implying a luminosity of 3E+46 erg/s, which would make this source erroneously the most luminous RQQ within its redshift. Reboul et al. (1987) determined the redshift of the source by identifying a broad emission line observed at ∼4290Å with a CIV λ1549 line. However, instead identifying the line with MgIIλ2800 implies a redshift of 0.532, compatible with the redshift of the X-ray absorber. This discrepancy motivated us to obtain a higher quality optical spectrum using the Hale Telescope at Palomar Observatory. The Palomar data show that the source is actually situated at a redshift of 0.532.

The joint XMM–Newton and NuSTAR observations of B2202 resulted in a high-quality 0.5–30 keV spectrum that could be fitted with an absorbed (a neutral absorber and a partially ionized absorber at the redshift of the source) power law with an exponential high-energy cutoff (Ecut ∼ 153 keV, at the rest frame of the source).

Read more here: Kammoun et al. (2017a)

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