58 research outputs found
Identification of Shocks in the Spectra from Black Holes
We study the spectral properties of a low angular momentum flow as a function
of the shock strength, compression ratio, accretion rate and flow geometry. In
the absence of a satisfactory description of magnetic fields inside the
advective disk, we consider the presence of only stochastic fields and use the
ratio of the field energy to the gravitational energy density as a parameter.
We not only include `conventional' synchrotron emission and Comptonization by
Maxwell-Bolzmann electrons in the gas, but we also compute these effects due to
power-law electrons. For strong shocks, a bump is produced due to the
post-shock flow. A power-law spectral components due to the thermal and
non-thermal electrons appear after this bump.Comment: 8 pages, 5 figures, Astronomy and Space Science (in press),
Proceedings of the Hong Kong Conference (2004) Edited by Cheng and Romer
A cosmological dust model with extended f(chi) gravity
Introducing a fundamental constant of nature with dimensions of acceleration
into the theory of gravity makes it possible to extend gravity in a very
consistent manner. At the non-relativistic level a MOND-like theory with a
modification in the force sector is obtained, which is the limit of a very
general metric relativistic theory of gravity. Since the mass and length scales
involved in the dynamics of the whole universe require small accelerations of
the order of Milgrom's acceleration constant a_0, it turns out that the
relativistic theory of gravity can be used to explain the expansion of the
universe. In this work it is explained how to use that relativistic theory of
gravity in such a way that the overall large-scale dynamics of the universe can
be treated in a pure metric approach without the need to introduce dark matter
and/or dark energy components.Comment: 7 pages, 1 figure. Accepted for publication in the European Physical
Journal
Infrared Magnitude-Redshift Relations for Luminous Radio Galaxies
Infrared magnitude-redshift relations for the 3CR and 6C samples of radio
galaxies are presented for a wide range of plausible cosmological models,
including those with non-zero cosmological constant OmegaLambda. Variations in
the galaxy formation redshift, metallicity and star formation history are also
considered. The results of the modelling are displayed in terms of magnitude
differences between the models and no-evolution tracks, illustrating the amount
of K-band evolution necessary to account for the observational data. Given a
number of plausible assumptions, the results of these analyses suggest that:
(i) cosmologies which predict T_0xH_0>1 (where T_0 denotes the current age of
the universe) can be excluded; (ii) the star formation redshift should lie in
the redshift interval 5<z<20, values towards the lower end of the range being
preferred in cosmologies with larger values of T_0xH_0; (iii) the Einstein-de
Sitter model provides a reasonable fit to the data; (iv) models with finite
values of OmegaLambda can provide good agreement with the observations only if
appropriate adjustments of other parameters such as the galaxy metallicities
and star-formation histories are made. Without such modifications, even after
accounting for stellar evolution, the high redshift radio galaxies are more
luminous (ie. more massive) than those nearby in models with finite
OmegaLambda, including the favoured model with Omega=0.3, OmegaLambda=0.7. For
cosmological models with larger values of T_0xH_0, the conclusions are the same
regardless of whether any adjustments are made or not. The implications of
these results for cosmology and models of galaxy formation are discussed.Comment: 14 pages, LaTeX, 9 figures, accepted for publication in MNRAS.
Replacement corrects some annoying typo
Stress effects in structure formation
Residual velocity dispersion in cold dark matter induces stresses which lead
to effects that are absent in the idealized dust model. A previous Newtonian
analysis showed how this approach can provide a theoretical foundation for the
phenomenological adhesion model. We develop a relativistic kinetic theory
generalization which also incorporates the anisotropic velocity dispersion that
will typically be present. In addition to density perturbations, we consider
the rotational and shape distortion properties of clustering. These quantities
together characterize the linear development of density inhomogeneity, and we
find exact solutions for their evolution. As expected, the corrections are
small and arise only in the decaying modes, but their effect is interesting.
One of the modes for density perturbations decays less rapidly than the
standard decaying mode. The new rotational mode generates precession of the
axis of rotation. The new shape modes produce additional distortion that
remains frozen in during the subsequent (linear) evolution, despite the rapid
decay of the terms that caused it.Comment: significantly improved discussion of kinetic theory of CDM velocity
dispersion; to appear Phys. Rev.
Gamma rays from dark matter annihilation in the Draco and observability at ARGO
The CACTUS experiment recently observed a gamma ray excess above 50 GeV from
the direction of the Draco dwarf spheroidal galaxy. Considering that Draco is
dark matter dominated the gamma rays may be generated through dark matter
annihilation in the Draco halo. In the framework of the minimal supersymmetric
extension of the standard model we explore the parameter space to account for
the gamma ray signals at CACTUS. We find that the neutralino mass is
constrained to be approximately in the range between 100 GeV ~ 400 GeV and a
sharp central cuspy of the dark halo profile in Draco is necessary to explain
the CACTUS results. We then discuss further constraints on the supersymmetric
parameter space by observations at the ground based ARGO detector. It is found
that the parameter space can be strongly constrained by ARGO if no excess from
Draco is observed above 100 GeV.Comment: 15 pages, 4 figure
What is 3C 324?
We report ground based and HST observations of the z=1.206 radio galaxy 3C
324, a prototypical example of the radio-optical ``alignment effect.'' While
infrared images shows a simple, round object reminiscent of a giant elliptical
galaxy, the HST images reveal a spectacular, linear chain of UV-bright
subcomponents closely aligned with the radio axis. In light of the available
data, we consider various scenarios to explain the properties of 3C 324, as
well as evidence for the presence of dust which may obscure the central active
nucleus and scatter its light to produce the polarized, aligned continuum seen
in the rest-frame UV.Comment: 9 pages, uuencoded gzipped postscript. To appear in ``Galaxies in the
Young Universe,'' ed. H. Hippelein, Springer Verlag. Revised version
(hopefully) corrects postscript error which garbled the last pag
Pulsars as the Sources of High Energy Cosmic Ray Positrons
Recent results from the PAMELA satellite indicate the presence of a large
flux of positrons (relative to electrons) in the cosmic ray spectrum between
approximately 10 and 100 GeV. As annihilating dark matter particles in many
models are predicted to contribute to the cosmic ray positron spectrum in this
energy range, a great deal of interest has resulted from this observation.
Here, we consider pulsars (rapidly spinning, magnetized neutron stars) as an
alternative source of this signal. After calculating the contribution to the
cosmic ray positron and electron spectra from pulsars, we find that the
spectrum observed by PAMELA could plausibly originate from such sources. In
particular, a significant contribution is expected from the sum of all mature
pulsars throughout the Milky Way, as well as from the most nearby mature
pulsars (such as Geminga and B0656+14). The signal from nearby pulsars is
expected to generate a small but significant dipole anisotropy in the cosmic
ray electron spectrum, potentially providing a method by which the Fermi
gamma-ray space telescope would be capable of discriminating between the pulsar
and dark matter origins of the observed high energy positrons.Comment: 10 pages, 5 figures; updated to include published Pamela results;
version to appear in JCA
Dark matter and Colliders searches in the MSSM
We study the complementarity between dark matter experiments (direct
detection and indirect detections) and accelerator facilities (the CERN LHC and
a TeV Linear Collider) in the framework of the
constrained Minimal Supersymmetric Standard Model (MSSM). We show how
non--universality in the scalar and gaugino sectors can affect the experimental
prospects to discover the supersymmetric particles. The future experiments will
cover a large part of the parameter space of the MSSM favored by WMAP
constraint on the relic density, but there still exist some regions beyond
reach for some extreme (fine tuned) values of the supersymmetric parameters.
Whereas the Focus Point region characterized by heavy scalars will be easily
probed by experiments searching for dark matter, the regions with heavy
gauginos and light sfermions will be accessible more easily by collider
experiments. More informations on both supersymmetry and astrophysics
parameters can be thus obtained by correlating the different signals.Comment: 25 pages, 10 figures, corrected typos and reference adde
Observing Supermassive Black Holes across cosmic time: from phenomenology to physics
In the last decade, a combination of high sensitivity, high spatial
resolution observations and of coordinated multi-wavelength surveys has
revolutionized our view of extra-galactic black hole (BH) astrophysics. We now
know that supermassive black holes reside in the nuclei of almost every galaxy,
grow over cosmological times by accreting matter, interact and merge with each
other, and in the process liberate enormous amounts of energy that influence
dramatically the evolution of the surrounding gas and stars, providing a
powerful self-regulatory mechanism for galaxy formation. The different
energetic phenomena associated to growing black holes and Active Galactic
Nuclei (AGN), their cosmological evolution and the observational techniques
used to unveil them, are the subject of this chapter. In particular, I will
focus my attention on the connection between the theory of high-energy
astrophysical processes giving rise to the observed emission in AGN, the
observable imprints they leave at different wavelengths, and the methods used
to uncover them in a statistically robust way. I will show how such a combined
effort of theorists and observers have led us to unveil most of the SMBH growth
over a large fraction of the age of the Universe, but that nagging
uncertainties remain, preventing us from fully understating the exact role of
black holes in the complex process of galaxy and large-scale structure
formation, assembly and evolution.Comment: 46 pages, 21 figures. This review article appears as a chapter in the
book: "Astrophysical Black Holes", Haardt, F., Gorini, V., Moschella, U and
Treves A. (Eds), 2015, Springer International Publishing AG, Cha
Time-Dependent Models for a decade of SN 1993J
A classical and a relativistic law of motion for a supernova remnant (SNR)
are deduced assuming an inverse power law behavior for the density of the
interstellar medium and applying the thin layer approximation. A third equation
of motion is found in the framework of relativistic hydrodynamics with
pressure, applying momentum conservation. These new formulas are calibrated
against a decade of observations of \snr. The existing knowledge of the
diffusive processes of ultrarelativistic electrons is reviewed in order to
explain the behavior of the `U' shaped profile of intensity versus distance
from the center of SN 1993J.Comment: 20 pages 19 figures, Accepted for pubblication in Astrophysics and
Space Science 201
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