43 research outputs found
A Dust Twin of Cas A: Cool Dust and 21-micron Silicate Dust Feature in the Supernova Remnant G54.1+0.3
We present infrared (IR) and submillimeter observations of the Crab-like
supernova remnant (SNR) G54.1+0.3 including 350 micron (SHARC-II), 870 micron
(LABOCA), 70, 100, 160, 250, 350, 500 micron (Herschel) and 3-40 micron
(Spitzer). We detect dust features at 9, 11 and 21 micron and a long wavelength
continuum dust component. The 21 micron dust coincides with [Ar II] ejecta
emission, and the feature is remarkably similar to that in Cas A. The IRAC 8
micron image including Ar ejecta is distributed in a shell-like morphology
which is coincident with dust features, suggesting that dust has formed in the
ejecta. We create a cold dust map that shows excess emission in the
northwestern shell. We fit the spectral energy distribution of the SNR using
the continuous distributions of ellipsoidal (CDE) grain model of pre-solar
grain SiO2 that reproduces the 21 and 9 micron dust features and discuss grains
of SiC and PAH that may be responsible for the 10-13 micron dust features. To
reproduce the long-wavelength continuum, we explore models consisting of
different grains including Mg2SiO4, MgSiO3, Al2O3, FeS, carbon, and Fe3O4. We
tested a model with a temperature-dependent silicate absorption coefficient. We
detect cold dust (27-44 K) in the remnant, making this the fourth such SNR with
freshly-formed dust. The total dust mass in the SNR ranges from 0.08-0.9 Msun
depending on the grain composition, which is comparable to predicted masses
from theoretical models. Our estimated dust masses are consistent with the idea
that SNe are a significant source of dust in the early Universe.Comment: MNRAS: accepted on June 28, 2018 and published on July 4, 201
Turbulent diffusion and drift in galactic magnetic fields and the explanation of the knee in the cosmic ray spectrum
We reconsider the scenario in which the knee in the cosmic ray spectrum is
explained as due to a change in the escape mechanism of cosmic rays from the
Galaxy from one dominated by transverse diffusion to one dominated by drifts.
We solve the diffusion equations adopting realistic galactic field models and
using diffusion coefficients appropriate for strong turbulence (with a
Kolmogorov spectrum of fluctuations) and consistent with the assumed magnetic
fields. We show that properly taking into account these effects leads to a
natural explanation of the knee in the spectrum, and a transition towards a
heavier composition above the knee is predicted.Comment: 17 pp., 6 figures; revised version with minor changes. To appear in
JHE
Antineutrinos from Earth: A reference model and its uncertainties
We predict geoneutrino fluxes in a reference model based on a detailed
description of Earth's crust and mantle and using the best available
information on the abundances of uranium, thorium, and potassium inside Earth's
layers. We estimate the uncertainties of fluxes corresponding to the
uncertainties of the element abundances. In addition to distance integrated
fluxes, we also provide the differential fluxes as a function of distance from
several sites of experimental interest. Event yields at several locations are
estimated and their dependence on the neutrino oscillation parameters is
discussed. At Kamioka we predict N(U+Th)=35 +- 6 events for 10^{32} proton yr
and 100% efficiency assuming sin^2(2theta)=0.863 and delta m^2 = 7.3 X 10^{-5}
eV^2. The maximal prediction is 55 events, obtained in a model with fully
radiogenic production of the terrestrial heat flow.Comment: 24 pages, ReVTeX4, plus 7 postscript figures; minor formal changes to
match version to be published in PR
Understanding hadronic gamma-ray emission from supernova remnants
We aim to test the plausibility of a theoretical framework in which the
gamma-ray emission detected from supernova remnants may be of hadronic origin,
i.e., due to the decay of neutral pions produced in nuclear collisions
involving relativistic nuclei. In particular, we investigate the effects
induced by magnetic field amplification on the expected particle spectra,
outlining a phenomenological scenario consistent with both the underlying
Physics and the larger and larger amount of observational data provided by the
present generation of gamma experiments, which seem to indicate rather steep
spectra for the accelerated particles. In addition, in order to study to study
how pre-supernova winds might affect the expected emission in this class of
sources, the time-dependent gamma-ray luminosity of a remnant with a massive
progenitor is worked out. Solid points and limitations of the proposed scenario
are finally discussed in a critical way.Comment: 30 pages, 5 figures; Several comments, references and a figure added.
Some typos correcte
Soft gamma-ray constraints on a bright flare from the Galactic Center supermassive black hole
Sagittarius A* (Sgr A*) is the supermassive black hole residing at the center
of the Milky Way. It has been the main target of an extensive multiwavelength
campaign we carried out in April 2007. Herein, we report the detection of a
bright flare from the vicinity of the horizon, observed simultaneously in
X-rays (XMM/EPIC) and near infrared (VLT/NACO) on April 4th for 1-2 h. For the
first time, such an event also benefitted from a soft gamma-rays
(INTEGRAL/ISGRI) and mid infrared (VLT/VISIR) coverage, which enabled us to
derive upper limits at both ends of the flare spectral energy distribution
(SED). We discuss the physical implications of the contemporaneous light curves
as well as the SED, in terms of synchrotron, synchrotron self-Compton and
external Compton emission processes.Comment: 30 pages, 5 figures, accepted for publication in AS
A self-consistent determination of the temperature profile and the magnetic field geometry in winds of late-type stars
Cool giant and supergiant stars generally present low velocity winds with
high mass loss rates. Several models have been proposed to explain the
acceleration process of these winds. Although dust is known to be present in
these objects, the radiation pressure on these particles is uneffective in
reproducing the observed physical parameters of the wind. The most promising
acceleration mechanism cited in the literature is the transference of momentum
and energy from Alfven waves to the gas. Usually, these models consider the
wind to be isothermal. We present a stellar wind model in which the Alfven
waves are used as the main acceleration mechanism, and determine the
temperature profile by solving the energy equation taking into account both the
radiative losses and the wave heating. We also determine self-consistently the
magnetic field geometry as the result of the competition between the magnetic
field and the thermal pressures gradient. As main result, we show that the
magnetic geometry present a super-radial index in the region where the gas
pressure is increasing. However, this super-radial index is greater than that
observed for the solar corona.Comment: Accepted for publication in Space Science Reviews. Presented at the
World Space Environment Forum 2005, Austria. 8 pages, 2 figure
Extragalactic Sources for Ultra High Energy Cosmic Ray Nuclei
In this article we examine the hypothesis that the highest energy cosmic rays
are complex nuclei from extragalactic sources. Under reasonable physical
assumptions, we show that the nearby metally rich starburst galaxies (M82 and
NGC 253) can produce all the events observed above the ankle. This requires
diffusion of particles below eV in extragalactic magnetic fields nG. Above eV, the model predicts the presence of
significant fluxes of medium mass and heavy nuclei with small rate of change of
composition. Notwithstanding, the most salient feature of the
starburst-hypothesis is a slight anisotropy induced by iron debris just before
the spectrum-cutoff.Comment: To appear in Phys. Rev. D, reference adde
Primary proton spectrum between 200 TeV and 1000 TeV observed with the Tibet burst detector and air shower array
Since 1996, a hybrid experiment consisting of the emulsion chamber and burst
detector array and the Tibet-II air-shower array has been operated at
Yangbajing (4300 m above sea level, 606 g/cm^2) in Tibet. This experiment can
detect air-shower cores, called as burst events, accompanied by air showers in
excess of about 100 TeV. We observed about 4300 burst events accompanied by air
showers during 690 days of operation and selected 820 proton-induced events
with its primary energy above 200 TeV using a neural network method. Using this
data set, we obtained the energy spectrum of primary protons in the energy
range from 200 to 1000 TeV. The differential energy spectrum obtained in this
energy region can be fitted by a power law with the index of -2.97 0.06,
which is steeper than that obtained by direct measurements at lower energies.
We also obtained the energy spectrum of helium nuclei at particle energies
around 1000 TeV.Comment: 25 pages, 22 figures, Accepted for publication in Phys. Rev.
Author correction: outflows from the youngest stars are mostly molecular
Correction to: Nature https://doi.org/10.1038/s41586-023-06551-1 Published online 24 August 2023Interstellar matter and star formatio
Topical Issues for Particle Acceleration Mechanisms in Astrophysical Shocks
Particle acceleration at plasma shocks appears to be ubiquitous in the
universe, spanning systems in the heliosphere, supernova remnants, and
relativistic jets in distant active galaxies and gamma-ray bursts. This review
addresses some of the key issues for shock acceleration theory that require
resolution in order to propel our understanding of particle energization in
astrophysical environments. These include magnetic field amplification in shock
ramps, the non-linear hydrodynamic interplay between thermal ions and their
extremely energetic counterparts possessing ultrarelativistic energies, and the
ability to inject and accelerate electrons in both non-relativistic and
relativistic shocks. Recent observational developments that impact these issues
are summarized. While these topics are currently being probed by
astrophysicists using numerical simulations, they are also ripe for
investigation in laboratory experiments, which potentially can provide valuable
insights into the physics of cosmic shocks.Comment: 13 pages, no figures. Invited review, accepted for publication in
Astrophysics and Space Science, as part of the HEDLA 2006 conference
proceeding