388 research outputs found
On the Spatial Distribution of Hard X-Rays from Solar Flare Loops
The aim of this paper is to investigate the spatial structure of the
impulsive phase hard X-ray emission from solar flares. This work is motivated
by the YOHKOH and the forthcoming HESSI observations. Summarizing past results,
it is shown that the transport effects can account for the observations by
inhomogeneous loops where there is a strong field convergence and/or density
enhancement at the top of the flaring loop. Scattering by plasma turbulence at
the acceleration site or pancake type pitch angle distribution of the
accelerated electrons can also give rise to enhanced emission at the loop tops.
These could be a natural consequence of acceleration by plasma waves. This
paper considers a general case of stochastic scattering and acceleration that
leads to an isotropic pitch angle distribution and an enhanced emission from
the loop tops or the acceleration site.
Following the formalism developed in earlier papers the strength and the
spectrum of the radiation expected from the acceleration site and the foot
points are evaluated and their dependence on the parameters describing the
acceleration process and the flare plasma are determined. The theoretical ratio
of these two intensities and relative values of their spectral indices are
compared with the YOHKOH observations, demonstrating that the above mentioned
parameters can be constrained with such observations. It is shown that future
high spatial and spectral resolution observations, for example those expected
from HESSI, can begin to distinguish between different models and constrain
their parameters.Comment: 37 pages with 20 figures. Accepted for publication in ApJ
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Particle Acceleration by Fast Modes in Solar Flares
We address the problem of particle acceleration in solar flares by fast modes
which may be excited during the reconnection and undergo cascade and are
subjected to damping. We extend the calculations beyond quasilinear
approximation and compare the acceleration and scattering by transit time
damping and gyroresonance interactions. We find that the acceleration is
dominated by the so called transit time damping mechanism. We estimate the
total energy transferred into particles, and show that our approach provides
sufficiently accurate results We compare this rate with energy loss rate.
Scattering by fast modes appears to be sufficient to prevent the protons from
escaping the system during the acceleration. Confinement of electrons, on the
other hand, requires the existence of plasma waves. Electrons can be
accelerated to GeV energies through the process described here for solar flare
conditions.Comment: 7 pages, 4 figures, accepted to Ap
The Extraordinary Infrared Spectrum of NGC 1222 (Mkn 603)
The infrared spectra of starburst galaxies are dominated by the
low-excitation lines of [NeII] and [SIII], and the stellar populations deduced
from these spectra appear to lack stars larger than about 35 Msun. The only
exceptions to this result until now were low metallicity dwarf galaxies. We
report our analysis of the mid-infrared spectra obtained with IRS on Spitzer of
the starburst galaxy NGC 1222 (Mkn 603). NGC 1222 is a large spheroidal galaxy
with a starburst nucleus that is a compact radio and infrared source, and its
infrared emission is dominated by the [NeIII] line. This is the first starburst
of solar or near-solar metallicity, known to us, which is dominated by the
high-excitation lines and which is a likely host of high mass stars. We model
the emission with several different assumptions as to the spatial distibution
of the high- and low-excitation lines and find that the upper mass cutoff in
this galaxy is 40-100 Msun.Comment: accepted, Astronomical Journal. 29 pp, 4 figures. In replacement
version an acknowledgment to NRAO is adde
On The Non Thermal Emission and Acceleration of Electrons in Coma and Other Clusters of Galaxies
Some clusters of galaxies in addition to thermal bremsstrahlung (TB), emit
diffuse radiation from the intercluster medium (ICM) at radio, EUV and hard
x-ray (HXR) ranges. The radio radiation is due to synchrotron by relativistic
electrons, and the inverse Compton (IC) scattering by the cosmic microwave
background radiation of the same electrons is the most natural source for the
HXR and perhaps the EUV emissions. However, simple estimates give a weaker
magnetic field than that suggested by Faraday rotation measurements.
Consequently, non-thermal bremsstrahlung (NTB) and TB have also been suggested
as sources of these emissions. We show that NTB cannot be the source of the
HXRs and that the difficulty with the low magnetic field in the IC model is
alleviated if we take into account the effects of observational bias,
nonisotropic pitch angle distribution and spectral breaks. We derive a spectrum
for the radiating electrons and discuss acceleration scenarios. We show that
continuous and in situ acceleration in the ICM of the background thermal
electrons requires unreasonably high energy input and acceleration of injected
relativistic electrons gives rise to a much flatter spectrum than desired,
unless a large fraction of electrons escape the ICM, in which case one obtains
EUV and HXR emissions extending well beyond the boundaries of the cluster. A
continuous emission by a cooling spectrum resulting from interaction with ICM
of electrons accelerated elsewhere also suffers from similar shortcomings. The
most likely scenario appears to be an episodic injection-acceleration model,
whereby one obtains a time dependent spectrum that for certain phases of its
evolution satisfies all the requirements.Comment: 27 pages, one Table, Four Figures. Latex AAS v5.0. Accepted by Ap
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The Radio and Optical Luminosity Evolution of Quasars II -- The SDSS Sample
We determine the radio and optical luminosity evolutions and the true distribution of the radio-loudness parameter R, defined as the ratio of the radio to optical luminosity, for a set of more than 5000 quasars combining Sloan Digital Sky Survey optical and Faint Images of the Radio Sky at Twenty cm (FIRST) radio data. We apply the method of Efron and Petrosian to access the intrinsic distribution parameters, taking into account the truncations and correlations inherent in the data. We find that the population exhibits strong positive evolution with redshift in both wavebands, with somewhat greater radio evolution than optical. With the luminosity evolutions accounted for, we determine the density evolutions and local radio and optical luminosity functions. The intrinsic distribution of the radio-loudness parameter R is found to be quite different from the observed one and is smooth with no evidence of a bimodality in radio loudness for log R ≥ −1. The results we find are in general agreement with the previous analysis of Singal et al., which used POSS-I optical and FIRST radio data
Supernovae and their host galaxies - V. The vertical distribution of supernovae in disc galaxies
We present an analysis of the height distributions of the different types of
supernovae (SNe) from the plane of their host galaxies. We use a well-defined
sample of 102 nearby SNe appeared inside high-inclined (i > 85 deg),
morphologically non-disturbed S0-Sd host galaxies from the Sloan Digital Sky
Survey. For the first time, we show that in all the subsamples of spirals, the
vertical distribution of core-collapse (CC) SNe is about twice closer to the
plane of host disc than the distribution of SNe Ia. In Sb-Sc hosts, the
exponential scale height of CC SNe is consistent with those of the younger
stellar population in the Milky Way (MW) thin disc, while the scale height of
SNe Ia is consistent with those of the old population in the MW thick disc. We
show that the ratio of scale lengths to scale heights of the distribution of CC
SNe is consistent with those of the resolved young stars with ages from ~ 10
Myr up to ~ 100 Myr in nearby edge-on galaxies and the unresolved stellar
population of extragalactic thin discs. The corresponding ratio for SNe Ia is
consistent with the same ratios of the two populations of resolved stars with
ages from a few 100 Myr up to a few Gyr and from a few Gyr up to ~ 10 Gyr, as
well as with the unresolved population of the thick disc. These results can be
explained considering the age-scale height relation of the distribution of
stellar population and the mean age difference between Type Ia and CC SNe
progenitors.Comment: 11 pages, 6 figures, 6 tables, accepted for publication in MNRA
The impact of bars on the radial distribution of supernovae in disc galaxies
We present an analysis of the impact of bars on the radial distributions of
the different types of supernovae (SNe) in the stellar discs of host galaxies
with various morphologies. We find that in Sa-Sbc galaxies, the radial
distribution of core-collapse (CC) SNe in barred hosts is inconsistent with
that in unbarred ones, while the distributions of SNe Ia are not significantly
different. At the same time, the radial distributions of both types of SNe in
Sc-Sm galaxies are not affected by bars. We propose that the additional
mechanism shaping the distributions of Type Ia and CC SNe can be explained
within the framework of substantial suppression of massive star formation in
the radial range swept by strong bars, particularly in early-type spirals. The
radial distribution of CC SNe in unbarred Sa-Sbc galaxies is more centrally
peaked and inconsistent with that in unbarred Sc-Sm hosts, while the
distribution of SNe Ia in unbarred galaxies is not affected by host morphology.
These results can be explained by the distinct distributions of massive stars
in the discs of early-and late-type spirals.Comment: 3 pages, 1 figure. This is a brief summary of arXiv:1511.08896,
written for a short contribution in the EWASS-2016 Symposium 16 "Frontiers of
massive-star evolution and core-collapse supernovae
Numerical simulations of chromospheric hard X-ray source sizes in solar flares
X-ray observations are a powerful diagnostic tool for transport,
acceleration, and heating of electrons in solar flares. Height and size
measurements of X-ray footpoints sources can be used to determine the
chromospheric density and constrain the parameters of magnetic field
convergence and electron pitch-angle evolution. We investigate the influence of
the chromospheric density, magnetic mirroring and collisional pitch-angle
scattering on the size of X-ray sources. The time-independent Fokker-Planck
equation for electron transport is solved numerically and analytically to find
the electron distribution as a function of height above the photosphere. From
this distribution, the expected X-ray flux as a function of height, its peak
height and full width at half maximum are calculated and compared with RHESSI
observations. A purely instrumental explanation for the observed source size
was ruled out by using simulated RHESSI images. We find that magnetic mirroring
and collisional pitch-angle scattering tend to change the electron flux such
that electrons are stopped higher in the atmosphere compared with the simple
case with collisional energy loss only. However, the resulting X-ray flux is
dominated by the density structure in the chromosphere and only marginal
increases in source width are found. Very high loop densities (>10^{11}
cm^{-3}) could explain the observed sizes at higher energies, but are
unrealistic and would result in no footpoint emission below about 40 keV,
contrary to observations. We conclude that within a monolithic density model
the vertical sizes are given mostly by the density scale-height and are
predicted smaller than the RHESSI results show.Comment: 19 pages, 9 figures, accepted for publication in Ap
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