63 research outputs found
Magnetic Confinement, MHD Waves, and Smooth Line Profiles in AGN
In this paper, we show that if the broad line region clouds are in
approximate energy equipartition between the magnetic field and gravity, as
hypothesized by Rees, there will be a significant effect on the shape and
smoothness of broad emission line profiles in active galactic nuclei. Line
widths of contributing clouds or flow elements are much wider than their
thermal widths, due to the presence of non-dissipative MHD waves, and their
collective contribution produce emission line profiles broader and smoother
than would be expected if a magnetic field were not present. As an
illustration, a simple model of isotropically emitting clouds, normally
distributed in velocity, is used to show that smoothness can be achieved for
less than 80,000 clouds and may even be as low as a few hundred. We conclude
that magnetic confinement has far reaching consequences for observing and
modeling active galactic nuclei.Comment: to appear in MNRA
Spherically symmetric relativistic MHD simulations of pulsar wind nebulae in supernova remnants
Pulsars, formed during supernova explosions, are known to be sources of
relativistic magnetized winds whose interaction with the expanding supernova
remnants (SNRs) gives rise to a pulsar wind nebula (PWN). We present
spherically symmetric relativistic magnetohydrodynamics (RMHD) simulations of
the interaction of a pulsar wind with the surrounding SNR, both in particle and
magnetically dominated regimes. As shown by previous simulations, the evolution
can be divided in three phases: free expansion, a transient phase characterized
by the compression and reverberation of the reverse shock, and a final Sedov
expansion. The evolution of the contact discontinuity between the PWN and the
SNR (and consequently of the SNR itself) is almost independent of the
magnetization of the nebula as long as the total (magnetic plus particle)
energy is the same. However, a different behaviour of the PWN internal
structure is observable during the compression-reverberation phase, depending
on the degree of magnetization=2E The simulations were performed using the
third order conservative scheme by Del Zanna et al. (2003).Comment: 11 pages, Latex, 22 Encapsulated PostScript figures, accepted f or
publication on A&
The effects of spin-down on the structure and evolution of pulsar wind nebulae
We present high resolution spherically symmetric relativistic
magnetohydrodynamical simulations of the evolution of a pulsar wind nebula
inside the free expanding ejecta of the supernova progenitor. The evolution is
followed starting from a few years after the supernova explosion and up to an
age of the remnant of 1500 years. We consider different values of the pulsar
wind magnetization parameter and also different braking indices for the
spin-down process. We compare the numerical results with those derived through
an approximate semi-analytical approach that allows us to trace the time
evolution of the positions of both the pulsar wind termination shock and the
contact discontinuity between the nebula and the supernova ejecta. We also
discuss, whenever a comparison is possible, to what extent our numerical
results agree with former self-similar models, and how these models could be
adapted to take into account the temporal evolution of the system. The inferred
magnetization of the pulsar wind could be an order of magnitude lower than that
derived from time independent analytic models.Comment: 11 pages, 7 figures, Accepted for publication on A&
Ubiquitous equatorial accretion disc winds in black hole soft states
High resolution spectra of Galactic Black Holes (GBH) reveal the presence of
highly ionised absorbers. In one GBH, accreting close to the Eddington limit
for more than a decade, a powerful accretion disc wind is observed to be
present in softer X-ray states and it has been suggested that it can carry away
enough mass and energy to quench the radio jet. Here we report that these
winds, which may have mass outflow rates of the order of the inner accretion
rate or higher, are an ubiquitous component of the jet-free soft states of all
GBH. We furthermore demonstrate that these winds have an equatorial geometry
with opening angles of few tens of degrees, and so are only observed in sources
in which the disc is inclined at a large angle to the line of sight. The
decrease in Fe XXV / Fe XXVI line ratio with Compton temperature, observed in
the soft state, suggests a link between higher wind ionisation and harder
spectral shapes. Although the physical interaction between the wind, accretion
flow and jet is still not fully understood, the mass flux and power of these
winds, and their presence ubiquitously during the soft X-ray states suggests
they are fundamental components of the accretion phenomenon.Comment: Accepted for publication in MNRAS Letter
Multi-dimensional modelling of X-ray spectra for AGN accretion-disk outflows III: application to a hydrodynamical simulation
We perform multi-dimensional radiative transfer simulations to compute
spectra for a hydrodynamical simulation of a line-driven accretion disk wind
from an active galactic nucleus. The synthetic spectra confirm expectations
from parameterized models that a disk wind can imprint a wide variety of
spectroscopic signatures including narrow absorption lines, broad emission
lines and a Compton hump. The formation of these features is complex with
contributions originating from many of the different structures present in the
hydrodynamical simulation. In particular, spectral features are shaped both by
gas in a successfully launched outflow and in complex flows where material is
lifted out of the disk plane but ultimately falls back. We also confirm that
the strong Fe Kalpha line can develop a weak, red-skewed line wing as a result
of Compton scattering in the outflow. In addition, we demonstrate that X-ray
radiation scattered and reprocessed in the flow has a pivotal part in both the
spectrum formation and determining the ionization conditions in the wind. We
find that scattered radiation is rather effective in ionizing gas which is
shielded from direct irradiation from the central source. This effect likely
makes the successful launching of a massive disk wind somewhat more challenging
and should be considered in future wind simulations.Comment: 14 pages, 8 figures. Accepted for publication by MNRA
The formation of broad emission line regions in supernova-QSO wind interactions: II. 2D calculations
One aspect of supernova remnant evolution that is relatively unstudied is the influence of an AGN environment. A high density ambient medium and a nearby powerful continuum source will assist the cooling of shocked ejecta and swept-up gas. Motion of the surrounding medium relative to the remnant will also affect the remnant morphology. In an extension to previous work we have performed 2D hydrodynamical calculations of SNR evolution in an AGN environment, and have determined the evolutionary behaviour of cold gas in the remnant. The cold gas will contribute to the observed broad line emission in AGNs, and we present preliminary theoretical line profiles from our calculations. A more detailed comparison with observations will be performed in future work. The SNR-AGN interaction may be also useful as a diagnostic of AGN winds
The theory of pulsar winds and nebulae
We review current theoretical ideas on pulsar winds and their surrounding
nebulae. Relativistic MHD models of the wind of the aligned rotator, and of the
striped wind, together with models of magnetic dissipation are discussed. It is
shown that the observational signature of this dissipation is likely to be
point-like, rather than extended, and that pulsed emission may be produced. The
possible pulse shapes and polarisation properties are described. Particle
acceleration at the termination shock of the wind is discussed, and it is
argued that two distinct mechanisms must be operating, with the first-order
Fermi mechanism producing the high-energy electrons (above 1 TeV) and either
magnetic annihilation or resonant absorption of ion cyclotron waves responsible
for the 100 MeV to 1 TeV electrons. Finally, MHD models of the morphology of
the nebula are discussed and compared with observation.Comment: 33 pages, to appear in Springer Lecture Notes on "Neutron stars and
pulsars, 40 years after the discovery", ed W.Becke
A Variable-Density Absorption Event in NGC 3227 mapped with Suzaku and Swift
The morphology of the circumnuclear gas accreting onto supermassive black
holes in Seyfert galaxies remains a topic of much debate. As the innermost
regions of Active Galactic Nuclei (AGN) are spatially unresolved, X-ray
spectroscopy, and in particular line-of-sight absorption variability, is a key
diagnostic to map out the distribution of gas. Observations of variable X-ray
absorption in multiple Seyferts and over a wide range of timescales indicate
the presence of clumps/clouds of gas within the circumnuclear material. Eclipse
events by clumps transiting the line of sight allow us to explore the
properties of the clumps over a wide range of radial distances from the
optical/UV Broad Line Region (BLR) to beyond the dust sublimation radius.
Time-resolved absorption events have been extremely rare so far, but suggest a
range of density profiles across Seyferts. We resolve a weeks-long absorption
event in the Seyfert NGC 3227. We examine six Suzaku and twelve Swift
observations from a 2008 campaign spanning 5 weeks. We use a model accounting
for the complex spectral interplay of three differently-ionized absorbers. We
perform time-resolved spectroscopy to discern the absorption variability
behavior. We also examine the IR-to-X-ray spectral energy distribution (SED) to
test for reddening by dust. The 2008 absorption event is due to
moderately-ionized () gas covering 90% of the line of
sight. We resolve the density profile to be highly irregular, in contrast to a
previous symmetric and centrally-peaked event mapped with RXTE in the same
object. The UV data do not show significant reddening, suggesting that the
cloud is dust-free. The 2008 campaign has revealed a transit by a filamentary,
moderately-ionized cloud of variable density that is likely located in the BLR,
and possibly part of a disk wind.Comment: Accepted for publication by A&
Supermassive Black Holes in Galactic Nuclei: Past, Present and Future Research
This review discusses the current status of supermassive black hole research,
as seen from a purely observational standpoint. Since the early '90s, rapid
technological advances, most notably the launch of the Hubble Space Telescope,
the commissioning of the VLBA and improvements in near-infrared speckle imaging
techniques, have not only given us incontrovertible proof of the existence of
supermassive black holes, but have unveiled fundamental connections between the
mass of the central singularity and the global properties of the host galaxy.
It is thanks to these observations that we are now, for the first time, in a
position to understand the origin, evolution and cosmic relevance of these
fascinating objects.Comment: Invited Review, 114 pages. Because of space requirements, this
version contains low resolution figures. The full resolution version can be
downloaded from http://www.physics.rutgers.edu/~lff/publications.htm
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