The X-Ray Polarization of the Accretion Disk Coronae of Active Galactic Nuclei

Abstract

Hard X-rays observed in Active Galactic Nuclei (AGNs) are thought to originate from the Comptonization of the optical/UV accretion disk photons in a hot corona. Polarization studies of these photons can help to constrain the corona geometry and the plasma properties. We have developed a ray-tracing code that simulates the Comptonization of accretion disk photons in coronae of arbitrary shape, and use it here to study the polarization of the X-ray emission from wedge and spherical coronae. We study the predicted polarization signatures for the fully relativistic and various approximate treatments of the elemental Compton scattering processes. We furthermore use the code to evaluate the impact of nonthermal electrons and cyclo-synchrotron photons on the polarization properties. Finally, we model the NuSTAR observations of the Seyfert I galaxy Mrk 335 and predict the associated polarization signal. Our studies show that X-ray polarimetry missions such as NASA's Imaging X-ray Polarimetry Explorer (IXPE) and the X-ray Imaging Polarimetry Explorer (XIPE) proposed to ESA will provide valuable new information about the physical properties of the plasma close to the event horizon of AGN black holes.

MoCA: A Monte Carlo code for Comptonisation in Astrophysics

“…In this case we expect to see a (probably still small) difference in the polarisation signal in the IXPE band for the two regimes but we defer this study to future papers focussed on the exploration of the coronal parameters space in AGN. Beheshtipour et al (2017) found an interesting difference in the polarisation signal when comparing the proper K-N treatment VS the Thomson one but they compared models which were giving the same spectral index in the 2-10 keV energy band leading to an optical depth per proper length which is more than two times larger for the Thomson case with respect to the K-N one (Fig.6 in their paper).…”

“…For τ = 2 the differences are relevant even below 100 keV which can be important to consider when using, for example, NuSTAR data up to 80 keV in order to measure the high-energy cut-off and therefore the thermal energy of the corona. Beheshtipour et al (2017) performed a similar test but for a very compact and thick spherical corona lying within 15 r g from the central object and their spectra are in the form of histograms. However, they found a qualitatively similar result with Thomson spectra being steeper than K-N ones (model 1 VS model 3 in Fig.4 of their paper).…”

Exploiting High-Energy Polarization in Blazars

“…For example, the photon packet approach adopted by Schittman and Krolik [27] can mostly be used in the Thomson regime, and the energy exchange can be taken into account only by the boost from the emission frame to the reference frame of the electron, while the scattering is always elastic. Beheshtipour et al [28] expanded on the work of Krawczynski [29] and manage to include the proper Klein-Nishina treatment for Comptonization, as well as the contribution of non-thermal electrons in the emission region, while following the same photon packets approach as in Schittman and Krolik [27].…”

Testing the Kerr Metric with X-Ray Reflection Spectroscopy of Mrk 335 Suzaku Data

“…In order to avoid pathologies in the spacetime, the following limits on α 13 and α 22 are imposed (Johannsen 2013;Bambi et al 2017): Grier et al 2012). First detected in X-rays by UHURU (Tananbaum et al 1978), Mrk 335 has been observed and studied numerous times by various X-ray observatories like ASCA, Swift, Suzaku, XMM Newton and NuSTAR (e.g., Ballantyne et al 2001;Gondoin et al 2002;Crummy et al 2006;Grupe et al 2007;Longinotti et al 2007;O'Neill et al 2007;Larsson et al 2008;Grupe et al 2008;Patrick et al 2011;Grupe et al 2012;Gallo et al 2013;Walton et al 2013;Longinotti et al 2013;Parker et al 2014;Gallo et al 2015;Wilkins & Gallo 2015;Keek & Ballantyne 2016;Beheshtipour et al 2017;Ballantyne 2017;Gallo et al 2019). It is an extremely variable source, exhibiting more than a factor of 10 fluctuation in the X-ray flux over the past 15 years.…”