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19 - The soft X-ray polarization in obscured AGN

from Part II - Polarized emission in X-ray sources

Published online by Cambridge University Press:  06 July 2010

S. Bianchi
Affiliation:
Università degli Studi Roma Tre, Italy
G. Matt
Affiliation:
Università degli Studi Roma Tre, Italy
F. Tamborra
Affiliation:
Università degli Studi Roma Tre, Italy
M. Chiaberge
Affiliation:
Space Telescope Science Institute, USA
M. Guainazzi
Affiliation:
XMM-Newton SOC, ESAC, ESA, Spain
A. Marinucci
Affiliation:
Università degli Studi Roma Tre, Italy
Ronaldo Bellazzini
Affiliation:
Istituto Nazionale di Fisica Nucleare (INFN), Rome
Enrico Costa
Affiliation:
Istituto Astrofisica Spaziale, Rome
Giorgio Matt
Affiliation:
Università degli Studi Roma Tre
Gianpiero Tagliaferri
Affiliation:
Osservatorio Astronomico di Brera
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Summary

The soft X-ray emission in obscured active galactic nuclei (AGN) is dominated by emission lines, produced in a gas photoionized by the nuclear continuum and probably spatially coincident with the optical narrow-line region (NLR). However, a fraction of the observed soft X-ray flux appears like a featureless power law continuum. If the continuum underlying the soft X-ray emission lines is due to Thomson scattering of the nuclear radiation, it should be very highly polarized. We calculated the expected amount of polarization assuming a simple conical geometry for the NLR, combining these results with the observed fraction of the reflected continuum in bright obscured AGN.

Introduction

The presence of a ‘soft excess’, i.e. soft X-ray emission above the extrapolation of the absorbed nuclear emission, is very common in low-resolution spectra of nearby X-ray obscured active galactic nuclei (AGN). It is generally very difficult to discriminate between thermal emission, as expected by gas heated by shocks induced by AGN outflows or episodes of intense star formation, and emission from a gas photoionized by the AGN primary emission. However, the high energy and spatial resolution of XMM-Newton and Chandra have allowed us to make important progress in the last few years.

The photoionization signatures

The high-resolution spectra of the brightest obscured AGN, made available by the gratings aboard Chandra and XMM-Newton, revealed that the ‘soft excess’ observed in CCD spectra was due to the blending of strong emission lines, mainly from He- and H-like transitions of light metals and L transitions of Fe (see Figure 19.1, e.g).

Type
Chapter
Information
X-ray Polarimetry
A New Window in Astrophysics
, pp. 130 - 135
Publisher: Cambridge University Press
Print publication year: 2010

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