278 research outputs found
The anatomy of a quadruply imaged gravitational lens system
The key to using a strong gravitational lens system to measure the Hubble
constant is to obtain an accurate model of the lens potential. In this paper,
we investigate the properties of gravitational lens B1608+656, a
quadruply-imaged lens system with an extended source intensity distribution.
Our analysis is valid for generic quadruply-lensed systems. Limit curves and
isophotal separatrices are defined for such systems, and we show that the
isophotal separatrices must intersect at the critical curves and the satellite
isophotes must be tangent to the limit curves. The current model of B1608+656
(Koopmans et al. 2003) satisfies these criteria for some, but not all, of the
isophotal separatrices within the observational uncertainty. We study a
non-parametric method of potential reconstruction proposed by Blandford, Surpi
& Kundic (2001) and demonstrate that although the method works in principle and
elucidates image formation, the initial potential only converges to the true
model when it is within ~ 1 percent of the true model.Comment: 12 pages, 12 figures. Minor revisions based on referee's comments
after initial submission to MNRA
Linkage between Accretion Disks and Blazars
The magnetic field in an accretion disk is estimated assuming that all of the
angular momentum within prescribed accretion disk radii is removed by a jet.
The magnetic field estimated at the base of the jet is extrapolated to the
blazar emission region using a model for a relativistic axisymmetric jet
combined with some simplifying assumptions based on the relativistic nature of
the flow. The extrapolated magnetic field is compared with estimates based upon
the synchrotron and inverse Compton emission from three blazars, MKN 501, MKN
421 and PKS 2155-304. The magnetic fields evaluated from pure synchrotron self-
Compton models are inconsistent with the magnetic fields extrapolated in this
way. However, in two cases inverse Compton models in which a substantial part
of the soft photon field is generated locally agree well, mainly because these
models imply magnetic field strengths which are closer to being consistent with
Poynting flux dominated jets. This comparison is based on estimating the mass
accretion rate from the jet energy flux. Further comparisons along these lines
will be facilitated by independent estimates of the mass accretion rate in
blazars and by more detailed models for jet propagation near the black hole.Comment: Submiteed to the Astrophysics & Space Science special issue on the
5th Stromlo Symposiu
The Point of Origin of the Radio Radiation from the Unresolved Cores of Radio-Loud Quasars
Locating the exact point of origin of the core radiation in active galactic
nuclei (AGN) would represent important progress in our understanding of
physical processes in the central engine of these objects. However, due to our
inability to resolve the region containing both the central compact object and
the jet base, this has so far been difficult. Here, using an analysis in which
the lack of resolution does not play a significant role, we demonstrate that it
may be impossible even in most radio loud sources for more than a small
percentage of the core radiation at radio wavelengths to come from the jet
base. We find for 3C279 that percent of the core flux at 15 GHz must
come from a separate, reasonably stable, region that is not part of the jet
base, and that then likely radiates at least quasi-isotropically and is
centered on the black hole. The long-term stability of this component also
suggests that it may originate in a region that extends over many Schwarzschild
radii.Comment: 7 pages with 3 figures, accepted for publication in Astrophysics and
Space Scienc
Particle Acceleration at Relativistic Shocks
I review the current status of Fermi acceleration theory at relativistic
shocks. I first discuss the relativistic shock jump conditions, then describe
the non-relativistic Fermi mechanism and the differences introduced by
relativistic flows. I present numerical calculations of the accelerated
particle spectrum, and examine the maximum energy attainable by this process. I
briefly consider the minimum energy for Fermi acceleration, and a possible
electron pre-acceleration mechanism.Comment: 17 pages, 4 figures. To appear in "Relativistic Flows in
Astrophysics", A.W. Guthmann, M. Georganopoulos, A. Marcowith and K.
Manolokou, eds., Lecture Notes in Pysics, Springer Verla
A Bayesian analysis of regularised source inversions in gravitational lensing
Strong gravitational lens systems with extended sources are of special
interest because they provide additional constraints on the models of the lens
systems. To use a gravitational lens system for measuring the Hubble constant,
one would need to determine the lens potential and the source intensity
distribution simultaneously. A linear inversion method to reconstruct a
pixellated source brightness distribution of a given lens potential model was
introduced by Warren & Dye. In the inversion process, a regularisation on the
source intensity is often needed to ensure a successful inversion with a
faithful resulting source. In this paper, we use Bayesian analysis to determine
the optimal regularisation constant (strength of regularisation) of a given
form of regularisation and to objectively choose the optimal form of
regularisation given a selection of regularisations. We consider and compare
quantitatively three different forms of regularisation previously described in
the literature for source inversions in gravitational lensing: zeroth-order,
gradient and curvature. We use simulated data with the exact lens potential to
demonstrate the method. We find that the preferred form of regularisation
depends on the nature of the source distribution.Comment: 18 pages, 10 figures; Revisions based on referee's comments after
initial submission to MNRA
Jet disc coupling in black hole binaries
In the last decade multi-wavelength observations have demonstrated the
importance of jets in the energy output of accreting black hole binaries. The
observed correlations between the presence of a jet and the state of the
accretion flow provide important information on the coupling between accretion
and ejection processes. After a brief review of the properties of black hole
binaries, I illustrate the connection between accretion and ejection through
two particularly interesting examples. First, an INTEGRAL observation of Cygnus
X-1 during a 'mini-' state transition reveals disc jet coupling on time scales
of orders of hours. Second, the black hole XTEJ1118+480 shows complex
correlations between the X-ray and optical emission. Those correlations are
interpreted in terms of coupling between disc and jet on time scales of seconds
or less. Those observations are discussed in the framework of current models.Comment: Invited talk at the Fifth Stromlo Symposium: Disks, Winds & Jets -
from Planets to Quasars. Accepted for publication in Astrophysics & Space
Scienc
The parallel lives of supermassive black holes and their host galaxies
We compare all the available observational data on the redshift evolution of
the total stellar mass and star formation rate density in the Universe with the
mass and accretion rate density evolution of supermassive black holes,
estimated from the hard X-ray selected luminosity function of quasars and
active galactic nuclei. We find that on average black hole mass must have been
higher at higher redshift for given spheroid stellar mass. Moreover, we find
negative redshift evolution of the disk/irregulars to spheroid mass ratio. The
total accretion efficiency is constrained to be between 0.06 and 0.12,
depending on the exact value of the local SMBH mass density, and on the
critical accretion rate below which radiatively inefficient accretion may take
place.Comment: 5 pages, 2 color figures. To appear in the proceedings of
"Relativistic Astrophysics and Cosmology - Einstein's Legacy" (Eds.: B.
Aschenbach, V. Burwitz, G. Hasinger, and B. Leibundgut), 7 - 11 November
2005, Munich, German
Two-flow magnetohydrodynamical jets around young stellar objects
We present the first-ever simulations of non-ideal magnetohydrodynamical
(MHD) stellar winds coupled with disc-driven jets where the resistive and
viscous accretion disc is self-consistently described. The transmagnetosonic,
collimated MHD outflows are investigated numerically using the VAC code. Our
simulations show that the inner outflow is accelerated from the central object
hot corona thanks to both the thermal pressure and the Lorentz force. In our
framework, the thermal acceleration is sustained by the heating produced by the
dissipated magnetic energy due to the turbulence. Conversely, the outflow
launched from the resistive accretion disc is mainly accelerated by the
magneto-centrifugal force. We also show that when a dense inner stellar wind
occurs, the resulting disc-driven jet have a different structure, namely a
magnetic structure where poloidal magnetic field lines are more inclined
because of the pressure caused by the stellar wind. This modification leads to
both an enhanced mass ejection rate in the disc-driven jet and a larger radial
extension which is in better agreement with the observations besides being more
consistent.Comment: Accepted for publication in Astrophysics & Space Science. Referred
proceeding of the fifth Mont Stromlo Symposium Dec. 1-8 2006, Canberra,
Australia. 5 pages, 3 figures. For high resolution version of the paper,
please click here http://www.apc.univ-paris7.fr/~fcasse/publications.htm
From Quasars to Extraordinary N-body Problems
We outline reasoning that led to the current theory of quasars and look at
George Contopoulos's place in the long history of the N-body problem. Following
Newton we find new exactly soluble N-body problems with multibody forces and
give a strange eternally pulsating system that in its other degrees of freedom
reaches statistical equilibrium.Comment: 13 pages, LaTeX with 1 postscript figure included. To appear in
Proceedings of New York Academy of Sciences, 13th Florida Workshop in
Nonlinear Astronomy and Physic
Extragalactic jets on subpc and large scales
Jets can be probed in their innermost regions (d~0.1 pc) through the study of
the relativistically-boosted emission of blazars. On the other extreme of
spatial scales, the study of structure and dynamics of extragalactic
relativistic jets received renewed impulse after the discovery, made by
Chandra, of bright X-ray emission from regions at distances larger than
hundreds of kpc from the central engine. At both scales it is thus possible to
infer some of the basic parameters of the flow (speed, density, magnetic field
intensity, power). After a brief review of the available observational
evidence, I discuss how the comparison between the physical quantities
independently derived at the two scales can be used to shed light on the global
dynamics of the jet, from the innermost regions to the hundreds of kpc scale.Comment: Proceedings of the 5th Stromlo Symposium: Disks, Winds, and Jets -
from Planets to Quasars. Accepted, to be published in Astrophysics & Space
Scienc
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