arXiv
(422.153 recursos)
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Mostrando recursos 1 - 20 de 87.451
1.
Improved Cosmological Constraints from Gravitational Lens Statistics - Mitchell, J. L.; Keeton, C. R.; Frieman, J. A; Sheth, R. K.
We combine the Cosmic Lens All-Sky Survey (CLASS) with new Sloan Digital Sky
Survey (SDSS) data on the local velocity dispersion distribution function of
E/S0 galaxies, $\phi(\sigma)$, to derive lens statistics constraints on
$\Omega_\Lambda$ and $\Omega_m$. Previous studies of this kind relied on a
combination of the E/S0 galaxy luminosity function and the Faber-Jackson
relation to characterize the lens galaxy population. However, ignoring
dispersion in the Faber-Jackson relation leads to a biased estimate of
$\phi(\sigma)$ and therefore biased and overconfident constraints on the
cosmological parameters. The measured velocity dispersion function from a large
sample of E/S0 galaxies provides a more reliable method for probing cosmology
with strong lens statistics. Our...
2.
Hyperbolic Universes with a Horned Topology and the CMB Anisotropy - Aurich, Ralf; Lustig, Sven; Steiner, Frank; Then, Holger
We analyse the anisotropy of the cosmic microwave background (CMB) in
hyperbolic universes possessing a non-trivial topology with a fundamental cell
having an infinitely long horn. The aim of this paper is twofold. On the one
hand, we show that the horned topology does not lead to a flat spot in the CMB
sky maps in the direction of the horn as stated in the literature. On the other
hand, we demonstrate that a horned topology having a finite volume could
explain the suppression of the lower multipoles in the CMB anisotropy as
observed by COBE and WMAP.
3.
High Energy Emission and Cosmic Rays from Gamma-Ray Bursts - Gialis, Denis; Pelletier, Guy
The paper is devoted to the analysis of particle acceleration in Gamma-Ray
Bursts and its radiative consequences. Therefore we get on one hand constraints
on the physics and on the other hand possible signatures of particle
acceleration that could be recorded by the new gamma ray instruments. We have
previously shown that UHECRs can be generated in GRBs even with conservative
assumptions on the magnetic field and the scattering capability of its
perturbations, provided that a suitable relativistic Fermi process is at work
during the so-called "internal shock" phase. We extend here the analysis of the
consequences of these assumptions to the whole prompt emission of both
electrons and...
4.
New views of the solar wind with the Lambert W function - Cranmer, Steven R.
This paper presents closed-form analytic solutions to two illustrative
problems in solar physics that have been considered not solvable in this way
previously. Both the outflow speed and the mass loss rate of the solar wind of
plasma particles ejected by the Sun are derived analytically for certain
illustrative approximations. The calculated radial dependence of the flow speed
applies to both Parker's isothermal solar wind equation and Bondi's equation of
spherical accretion. These problems involve the solution of transcendental
equations containing products of variables and their logarithms. Such equations
appear in many fields of physics and are solvable by use of the Lambert W
function, which is briefly described....
5.
Calibration of cameras of the H.E.S.S. detector - Aharonian, H. E. S. S. Collaboration: F.
H.E.S.S. - the High Energy Stereoscopic System- is a new system of large
atmospheric Cherenkov telescopes for GeV/TeV astronomy. Each of the four
telescopes of 107 m^2 mirror area is equipped with a 960-pixel
photomulitiplier-tube camera. This paper describes the methods used to convert
the photomultiplier signals into the quantities needed for Cherenkov image
analysis. Two independent calibration techniques have been applied in parallel
to provide an estimation of uncertainties. Results on the long-term stability
of the H.E.S.S. cameras are also presented.
6.
The Cooling Behavior of Thermal Pulses in Gamma-Ray Bursts - Ryde, Felix
We discuss gamma-ray bursts that have very hard spectra, consistent with
black-body radiation, throughout their duration. We find that the temperature
decay during a pulse can be well described by a broken power-law in time, with
an initially constant or weak decay. After the break, most cases are consistent
with a decay with index -2/3. Some pulses have a weak non-thermal component
overlayed the thermal one, and are better modelled with a combination of a
thermal and a non-thermal component. Such a two-component model can explain the
whole time-evolution of other bursts, that are found to be only initially
thermal and later become non-thermal. The relative strengths between...
7.
The Missing Luminous Blue Variables and the Bistability Jump - Smith, Nathan; Vink, Jorick S.; de Koter, Alex
We discuss an interesting feature of the distribution of luminous blue
variables on the H-R diagram, and we propose a connection with the bistability
jump in the winds of early-type supergiants. There appears to be a deficiency
of quiescent LBVs on the S Dor instability strip at luminosities between log
L/Lsun = 5.6 and 5.8. The upper boundary, is also where the
temperature-dependent S Dor instability strip intersects the bistability jump
at about 21,000 K. Due to increased opacity, winds of early-type supergiants
are slower and denser on the cool side of the bistability jump, and we
postulate that this may trigger optically-thick winds that inhibit quiescent
LBVs from...
8.
The spectral evolution of impulsive solar X-ray flares - Grigis, Paolo C.; Benz, Arnold O.
The time evolution of the spectral index and the non-thermal flux in 24
impulsive solar hard X-ray flares of GOES class M was studied in RHESSI
observations. The high spectral resolution allows for a clean separation of
thermal and non-thermal components in the 10-30 keV range, where most of the
non-thermal photons are emitted. Spectral index and flux can thus be determined
with much better accuracy than before. The spectral soft-hard-soft behavior in
rise-peak-decay phases is discovered not only in the general flare development,
but even more pronounced in subpeaks. An empirically found power-law dependence
between the spectral index and the normalization of the non-thermal flux holds
during the...
9.
Gamma-ray burst internal shocks with magnetization - Fan, Y. Z.; Wei, D. M.; Zhang, Bing
(Please note that the abstract has been significantly shorted) We investigate
Gamma-ray Burst (GRB) internal shocks with moderate magnetization, with the
magnetization parameter $\sigma$ ranging from 0.001 to 10. Possible magnetic
dissipation in the stripped magnetized shells is also taken into account
through introducing a parameter $k$ ($0
10.
Cosmogenic Neutrinos from Ultra-High Energy Nuclei - Ave, Maximo; Busca, N.; Olinto, A. V.; Watson, A. A.; Yamamoto, T.
We calculate the flux of neutrinos generated by the propagation of ultra-high
energy iron over cosmological distances and show that even if ultra-high energy
cosmic rays are composed of heavy nuclei, a significant flux of high-energy
neutrinos should be present throughout the universe. The resulting neutrino
flux has a new peak at $\sim 10^{14} eV$ generated by neutron decay and
reproduces the double peak structure due to photopion production at higher
energies ($\sim 10^{18}$ eV). Depending on the maximum energy and cosmological
evolution of extremely high energy cosmic accelerators the generated neutrino
flux can be detected by future experiments.
11.
On the structure of line-driven winds near black holes - Dorodnitsyn, A. V.; Novikov, I. D.
A general physical mechanism of the formation of line-driven winds at the
vicinity of strong gravitational field sources is investigated in the frame of
General Relativity. We argue that gravitational redshifting should be taken
into account to model such outflows. The generalization of the Sobolev
approximation in the frame of General Relativity is presented. We consider all
processes in the metric of a nonrotating (Schwarzschild) black hole. The
radiation force that is due to absorbtion of the radiation flux in lines is
derived. It is demonstrated that if gravitational redshifting is taken into
account, the radiation force becomes a function of the local velocity gradient
(as in the standard...
12.
Is Dark Energy Dynamical? Prospects for an Answer - Linder, Eric V.; Miquel, Ramon
Recent data advances offer the exciting prospect of a first look at whether
dark energy has a dynamical equation of state or not. While formally theories
exist with a constant equation of state, they are nongeneric -- Einstein's
cosmological constant is a notable exception. So limits on the time variation,
w', directly tell us crucial physics. Two recent improvements in supernova data
from the Hubble Space Telescope allow important steps forward in constraining
the dynamics of dark energy, possessing the ability to exclude models with
w'\ga 1, if the universe truly has a cosmological constant. These data bring us
much closer to the ``systematics'' era, where further improvements...
13.
The Ultraviolet flash accompanying GRBs from neutron-rich internal shocks - Fan, Y. Z.; Wei, D. M.
In the neutron-rich internal shocks model for Gamma-ray Burts (GRBs), the
Lorentz factors (LFs) of ions shells are variable, so are the LFs of
accompanying neutron shells. For slow neutron shells with a typical LF tens,
the typical beta-decay radius reads R_{\beta,s} several 10^{14} cm, which is
much larger than the typical internal shocks radius 10^{13} cm, so their impact
on the internal shocks may be unimportant. However, as GRBs last long enough
(T_{90}>20(1+z) s), one earlier but slower ejected neutron shell will be swept
successively by later ejected ion shells in the range 10^{13}-10^{15} cm, where
slow neutrons have decayed significantly. We show in this work that...
14.
Ultra-Luminous Sources in Nearby Galaxies - Mushotzky, Richard
I briefly review much of the X-ray and optical data on the nature of the ULXs
in nearby galaxies. I present new results on radio emission, finding that the
radio is usually rather luminous and extended. I review the X-ray data on
timing and spectra. There is no direct evidence in the X-ray data for either
geometric or relativistic beaming and in 4 objects direct evidence against
beaming. I argue that the X-ray timing and spectral properties of these objects
are, in general, not good analogs of AGN or galactic black holes and that the
ULX may represent a new mode of accretion only rarely seen in...
15.
Faraday Rotation Measures through the Cores of Southern Galaxy Clusters - Johnston-Hollitt, M.; Ekers, R. D.
We present a study of the rotation measure (RM) obtained from a sample of
extra-galactic radio sources either embedded in or seen in projection through a
sample of seven southern galaxy clusters (declination less than 30 degrees). We
compare our results with those obtained previously through similar statistical
studies and conclude there is a statistically significant broadening of the RM
signal in galaxy clusters when compared to a control sample. Further, we concur
with the findings of Clarke (2000) that the typical influence of the cluster on
the RM extends to around 800 kpc from the cluster core and that the RMs
determined are on average consistent with...
16.
Particle Acceleration at shocks: some modern aspects of an old problem - Blasi, Pasquale
The acceleration of charged particles at astrophysical collisionless shock
waves is one of the best studied processes for the energization of particles to
ultrarelativistic energies, required by multifrequency observations in a
variety of astrophysical situations. In this paper we discuss some work aimed
at describing one of the main progresses made in the theory of shock
acceleration, namely the introduction of the non-linear backreaction of the
accelerated particles onto the shocked fluid. The implications for the
investigation of the origin of ultra high energy cosmic rays will be discussed.
17.
Gravitational collapse of polytropic, magnetized, filamentary clouds - Shadmehri, Mohsen
When the gas of a magnetized filamentary cloud obeys a polytropic equation of
state, gravitational collapse of the cloud is studied using a simplified model.
We concentrate on the radial distribution and restrict ourselves to the purely
toroidal magnetic field. If the axial motions and poloidal magnetic fields are
sufficiently weak, we could reasonably expect our solutions to be a good
approximation. We show that while the filament experiences gravitational
condensation and the density at the center increases, the toroidal flux-to-mass
ratio remains constant. A series of spatial profiles of density, velocity and
magnetic field for several values of the toroidal flux-to-mass ratio and the
polytropic index, is obtained...
18.
Spurious contribution to CR scattering calculations - Shalchi, A.; Yan, H.; Lazarian, A.
The quasilinear theory for cosmic ray propagation is a well known and widely
accepted theory. In this paper, we discuss the different contributions to the
pitch-angle Fokker-Planck coefficient from large and small scales for slab
geometry using the damping model of dynamical turbulence. These examinations
will give us a hint on the limitation range where quasilinear approximation is
a good approximation.
19.
Eighth-Order Image Masks for Terrestrial Planet Finding - Kuchner, Marc J.; Crepp, Justin; Ge, Jian
We describe a new series of band-limited image masks for coronagraphy that
are insensitive to pointing errors and other low-spatial-frequency optical
aberrations. For a modest cost in throughput, these ``eighth-order'' masks
would allow the Terrestrial Planet Finder Coronagraph (TPF-C) to operate with a
pointing accuracy no better than that of the Hubble Space Telescope. We also
provide eighth-order notch filter masks that offer the same robustness to
pointing errors combined with more manageable construction tolerances: binary
masks and graded masks with moderate optical density requirements.
20.
Kinetic Luminosity and Composition of Active Galactic Nuclei Jets - Hirotani, Kouichi
We present a new method how to discriminate the matter content of
parsec-scale jets of active galactic nuclei. By constraining the kinetic
luminosity of a jet from the observed core size at a single very long baseline
interferometry frequency, we can infer the electron density of a radio-emitting
component as a function of the composition. Comparing this density with that
obtained from the theory of synchrotron self-absorption, we can determine the
composition. We apply this procedure to the five components in the 3C~345 jet
and find that they are likely pair-plasma dominated at 11 epochs out of the
total 21 epochs, provided that the bulk Lorentz factor is...
1.
