X-Ray Studies of Radio-Loud AGN
Abstract
In this thesis I use X-ray observations to study the cores and extended
structures of radio-loud AGN, to determine their structure, accretion properties
and the impact they have on their surroundings.
I use new Chandra data and archival XMM-Newton observations ofMarkarian
6 to look for evidence of emission from shocked gas around the external
radio bubbles, using spatially resolved regions in Chandra and spectral
analysis of the XMM data. The results show that the bubbles in Mrk 6 are
indeed driving a shock into the halo of the host galaxy, with a Mach number
of 3.9. I also find that the spectrum of the AGN has a variable absorbing
column, which changes from 8 × 1021 atoms cm−2 to 3 × 1023
atoms cm−2 on short timescales (2-6 years). This is probably caused by a
clump of gas close to the central AGN, passing in front of us at the moment
of the observation.
Using new and archival Chandra observations of the Circinus galaxy, I
match them to pre-existing radio, infrared and optical data to study the
kpc-scale emission. As for Mrk 6, I find that the radio bubbles in Circinus
are driving a shock into the interstellar medium of the host galaxy,
with Mach numbers M 2.7–3.6 and M 2.8–5.3 for the W and E
shells respectively. Comparing the results with those we previously obtained
for Centaurus A, NGC 3801 and Mrk 6, I show that the total energy
in the lobes (thermal+kinetic) scales approximately with the radio
power of the parent AGN. The spatial coincidence between the X-ray and
edge-brightened radio emission in Circinus resembles the morphology of
some SNR shocks, a parallel that has been expected for AGN, but has
never been observed before. I investigate what underlying mechanisms
both types of systems may have in common, arguing that, in Circinus,
the edge-brightening in the shells may be accounted for by a B field enhancement
caused by shock compression, but do not preclude some local
particle acceleration.
I also carry out a systematic study of the X-ray emission from the cores in
the 0.02 < z < 0.7 2Jy sample, using Chandra and XMM-Newton observations.
I combine the results with the mid-IR, optical emission line and
radio luminosities, and compare them with those of the 3CRR sources, to
show that the low-excitation objects in our sample show all the signs of radiatively
inefficient accretion. I study the effect of the jet-related emission
on the various luminosities, confirming that it is the main source of soft
X-ray emission for our sources. I also find strong correlations between the
accretion-related luminosities, and identify several sources whose optical
classification is incompatible with their accretion properties. I derive the
bolometric and jet kinetic luminosities for the sample and find a difference
in the total Eddington rate between the low and high-excitation populations,
with the former peaking at 1 per cent and the latter at 20 per
cent Eddington. There is, however, an overlap between the two, indicating
that a simple Eddington switch may not be possible. The apparent
independence of jet kinetic power and radiative luminosity in the highexcitation
population in our plots allows us to test the hypothesis in which
jet production and radiatively efficient accretion are in fact independent
processes that can coexist in high-excitation objects.
Publication date
2013-05-24Published version
https://doi.org/10.18745/th.10921https://doi.org/10.18745/th.10921