2,171 research outputs found
Flat Cosmology with Coupled Matter and Dark Energies
Three models of a flat universe of coupled matter and dark energies with
different low-redshift parameterizations of the dark energy equation of state
are considered. The dark energy is assumed to vary with time like the trace of
the energy-momentum tensor of cosmic matter. In the radiation-dominated era the
models reduce to standard cosmology. In the matter-dominated era they are, for
modern values of the cosmological parameters, consistent with data from SNe Ia
searches and with the data of Gurvits et al.(1999)for angular sizes of ultra
compact radio sources. We find that the angular size-redshift tests for our
models offer a higher statistical confidence than that based on SNe Ia data. A
comparison of our results with a recent revised analysis of angular
size-redshift legacy data is made,and the implications of our models with
optimized relativistic beaming in the radio sources is discussed. In particular
we find that relativistic beaming implies a Lorentz factor less than 6,in
agreement with its values for powerful Active Galactic Nuclei.Comment: Version to appear in The Astronomical Journal, with a modified name-
Flat Cosmology with Coupled Matter and Dark Energies. Expanded and Modified
conten
Synchrotron radiation of self-collimating relativistic MHD jets
The goal of this paper is to derive signatures of synchrotron radiation from
state-of-the-art simulation models of collimating relativistic
magnetohydrodynamic (MHD) jets featuring a large-scale helical magnetic field.
We perform axisymmetric special relativistic MHD simulations of the jet
acceleration region using the PLUTO code. The computational domain extends from
the slow magnetosonic launching surface of the disk up to 6000^2 Schwarzschild
radii allowing to reach highly relativistic Lorentz factors. The Poynting
dominated disk wind develops into a jet with Lorentz factors of 8 and is
collimated to 1 degree. In addition to the disk jet, we evolve a thermally
driven spine jet, emanating from a hypothetical black hole corona. Solving the
linearly polarized synchrotron radiation transport within the jet, we derive
VLBI radio and (sub-) mm diagnostics such as core shift, polarization
structure, intensity maps, spectra and Faraday rotation measure (RM), directly
from the Stokes parameters. We also investigate depolarization and the
detectability of a lambda^2-law RM depending on beam resolution and observing
frequency. We find non-monotonic intrinsic RM profiles which could be detected
at a resolution of 100 Schwarzschild radii. In our collimating jet geometry,
the strict bi-modality in polarization direction (as predicted by Pariev et
al.) can be circumvented. Due to relativistic aberration, asymmetries in the
polarization vectors across the jet can hint to the spin direction of the
central engine.Comment: Submitted to Ap
Cosmic-Ray and Neutrino Emission from Gamma-Ray Bursts with a Nuclear Cascade
We discuss neutrino and cosmic-ray emission from Gamma-Ray Bursts (GRBs) with
the injection of nuclei, where we take into account that a nuclear cascade from
photo-disintegration can fully develop in the source. One of our main
objectives is to test if recent results from the IceCube and the Pierre Auger
Observatory can be accommodated with the paradigm that GRBs are the sources of
Ultra-High Energy Cosmic Rays (UHECRs). While our key results are obtained
using an internal shock model, we discuss how the secondary emission from a GRB
shell can be interpreted in terms of other astrophysical models. It is
demonstrated that the expected neutrino flux from GRBs weakly depends on the
injection composition, which implies that prompt neutrinos from GRBs can
efficiently test the GRB-UHECR paradigm even if the UHECRs are nuclei. We show
that the UHECR spectrum and composition, as measured by the Pierre Auger
Observatory, can be self-consistently reproduced in a combined
source-propagation model. In an attempt to describe the energy range including
the ankle, we find tension with the IceCube bounds from the GRB stacking
analyses. In an alternative scenario, where only the UHECRs beyond the ankle
originate from GRBs, the requirement for a joint description of cosmic-ray and
neutrino observations favors lower luminosities, which does not correspond to
the typical expectation from {\gamma}-ray observations.Comment: 36 pages, 25 figure
Neutrinos and Ultra-High-Energy Cosmic-Ray Nuclei from Blazars
We discuss the production of ultra-high-energy cosmic ray (UHECR) nuclei and
neutrinos from blazars. We compute the nuclear cascade in the jet for both BL
Lac objects and flat-spectrum radio quasars (FSRQs), and in the ambient
radiation zones for FSRQs as well. By modeling representative spectral energy
distributions along the blazar sequence, two distinct regimes are identified,
which we call "nuclear survival" -- typically found in low-luminosity BL Lacs,
and "nuclear cascade" -- typically found in high-luminosity FSRQs. We quantify
how the neutrino and cosmic-ray (CR) emission efficiencies evolve over the
blazar sequence, and demonstrate that neutrinos and CRs come from very
different object classes. For example, high-frequency peaked BL Lacs (HBLs)
tend to produce CRs, and HL-FSRQs are the more efficient neutrino emitters.
This conclusion does not depend on the CR escape mechanism, for which we
discuss two alternatives (diffusive and advective escape). Finally, the
neutrino spectrum from blazars is shown to significantly depend on the
injection composition into the jet, especially in the nuclear cascade case:
Injection compositions heavier than protons lead to reduced neutrino production
at the peak, which moves at the same time to lower energies. Thus, these
sources will exhibit better compatibility with the observed IceCube and UHECR
data.Comment: 23 pages, 20 figure
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