M.Sc Thesis

M.Sc StudentRaginski Igor
SubjectEmission from Magnetized Hot Coronae above AGN
Accretion Discs
DepartmentDepartment of Physics
Supervisor PROF. Ari Laor
Full Thesis textFull thesis text - English Version


In this work we focus on the spectra of Radio Quiet (RQ) Active Galactic

Nuclei (AGN) in the radio and X-ray ranges. AGN consists of an accretion

disc surrounding a massive black hole, which is covered by a hot and dilute

corona, which is analogous to coronae of active stars. The Gudel-Benz relation

of LR/LX ≈ 10−5 appears in both stars and RQ AGN, which suggests that the

radio and the X ray emission in both systems share a common origin. In this

work we follow the idea that the radio emission originates in the accretion disc

corona, rather than in jets, thus has the same source as the X-ray radiation.

The main goal of this work is to understand the origin of the Gudel-Benz

relation, using numerical tools for calculation of the radio and the X-ray


We show that the radio emission by a thermal distribution of electrons

is low comparing to the emission by a power law population. The coronal

gas needs to have T 5 ? 1010K for thermal synchrotron to produce the

observed luminosity, in contrast with the X-ray observations which suggest

T 5 ? 109K, but it may still be valid if the magnetic field is well above

equipartition. We give some predictions on the radio spectrum in the high frequencies

(around 100 GHz), based on models which explain the low-frequency

radio spectrum (around ν = 5 GHz). We find that the gas should extend out

to pc scale to produce a flat spectral slope down to a frequency of 1 GHz.

We also develop an analytical expression for the radio spectral slope. We

find that in order to get a flat spectrum, the energy density should scale as

R−1.3 in the case of disk like configuration, and as R−2 in the case of

spherical. Moreover, we investigate the inclination dependence of the radio

emission and find that in the case of synchrotron emission by power law electrons, the emission is close to isotropic, while for thermal electrons it scales

with the cosine of the inclination angle.

The X-ray emission originates in Compton scattering of the disk photons

on electrons within the central parts of the AGN corona. For equipartition

between the thermal and power law populations, a power index greater than

2 produces hard X-ray emission which is stronger than the one produced

by thermally distributed electrons. The spectral slope in the X-ray range

should equal the spectral slope of the radio emission at high frequencies,

where it is optically thin. A mixed population of both power law and thermal

electrons, can fit both the soft and hard X-ray spectral shapes. We show that

LR/LX ≈ 10−5 is obtained for equipartition conditions, using the observed

typical disk temperature, and the observed coronal temperature and optical

depth. However, this does not explain the similarity to the observed LR/LX

in stars.