894 research outputs found
The Cone-Like H-alpha Nebula in NGC 4945: A Galactic Superwind Bow Shock ?
We find that a non-axisymmetric bow shock model, with an appropriate choice
of parameters, could fit the line splitting velocity field of the cone-like
H nebula in NGC 4945 better than a canonical cone model. Meanwhile, the
bow shock model could also reproduce the morphology of the H nebula.
The bow shock results from the interaction of the galactic superwinds with a
giant HII region. It is implied that the starburst ring or disk around the
galactic nucleus should be generating strong winds, and the bright H
knot northwest of the nucleus be suffering an anisotropic mass loss process.Comment: 14 pages, aasms4.sty, 3 figures not included (available upon request)
To appear in ApJ Letters. email chy, [email protected]
The Density Spike in Cosmic-Ray-Modified Shocks: Formation, Evolution, and Instability
We examine the formation and evolution of the density enhancement (density
spike) that appears downstream of strong, cosmic-ray-modified shocks. This
feature results from temporary overcompression of the flow by the combined
cosmic-ray shock precursor/gas subshock. Formation of the density spike is
expected whenever shock modification by cosmic-ray pressure increases strongly.
That occurence may be anticipated for newly generated strong shocks or for
cosmic-ray-modified shocks encountering a region of higher external density,
for example. The predicted mass density within the spike increases with the
shock Mach number and with shocks more dominated by cosmic-ray pressure. We
find this spike to be linearly unstable under a modified Rayleigh-Taylor
instability criterion at the early stage of its formation. We confirm this
instability numerically using two independent codes based on the two-fluid
model for cosmic-ray transport. These two-dimensional simulations show that the
instability grows impulsively at early stages and then slows down as the
gradients of total pressure and gas density decrease. Observational discovery
of this unstable density spike behind shocks, possibly through radio emission
enhanced by the amplified magnetic fields would provide evidence for the
existence of strongly cosmic-ray modified shock structures.Comment: 26 pages in Latex and 6 figures. Accepted to Ap
Radio emission and particle acceleration in plerionic supernova remnants
Plerionic supernova remnants exhibit radio emission with remarkably flat
spectral indices ranging from to . The origin of very
hard particle energy distributions still awaits an explanation, since shock
waves generate particle distributions with synchrotron spectra characterized by
. Acceleration of high energy leptons in magnetohydrodynamic
turbulence instead may be responsible for the observed hard spectra. This
process is studied by means of relativistic test particle calculations using
electromagnetic fields produced by three-dimensional simulations of resistive
magnetohydrodynamical turbulence. The particles receive power-law energy
spectra with ranging from 1.2 to 1.6, i.e.
particle spectra that are required to explain the radio emission of plerions.Comment: 8 pages, 7 Figures, to be published in A&
Gamma-ray emission expected from Kepler's SNR
Nonlinear kinetic theory of cosmic ray (CR) acceleration in supernova
remnants (SNRs) is used to investigate the properties of Kepler's SNR and, in
particular, to predict the gamma-ray spectrum expected from this SNR.
Observations of the nonthermal radio and X-ray emission spectra as well as
theoretical constraints for the total supernova (SN) explosion energy E_sn are
used to constrain the astronomical and particle acceleration parameters of the
system. Under the assumption that Kepler's SN is a type Ia SN we determine for
any given explosion energy E_sn and source distance d the mass density of the
ambient interstellar medium (ISM) from a fit to the observed SNR size and
expansion speed. This makes it possible to make predictions for the expected
gamma-ray flux. Exploring the expected distance range we find that for a
typical explosion energy E_sn=10^51 erg the expected energy flux of TeV
gamma-rays varies from 2x10^{-11} to 10^{-13} erg/(cm^2 s) when the distance
changes from d=3.4 kpc to 7 kpc. In all cases the gamma-ray emission is
dominated by \pi^0-decay gamma-rays due to nuclear CRs. Therefore Kepler's SNR
represents a very promising target for instruments like H.E.S.S., CANGAROO and
GLAST. A non-detection of gamma-rays would mean that the actual source distance
is larger than 7 kpc.Comment: 6 pages, 4 figures. Accepted for publication in Astronomy and
Astrophysics, minor typos correcte
Renewing Criminalized and Hegemonic Cultural Landscapes
The Mafia's long historical pedigree in Mezzogiorno, Southern Italy, has empowered the Mafioso as a notorious, uncontested, and hegemonic figure. The counter-cultural resistance against the mafiosi culture began to be institutionalized in the early 1990s. Today, Libera Terra is the largest civil society organization in the country that uses the lands confiscated from the Mafia as a space of cultural repertoire to realize its ideals. Deploying labor force through volunteer participation, producing biological fruits and vegetables, and providing information to the students on the fields are the principal cultural practices of this struggle. The confiscated lands make the Italian experience of anti-Mafia resistance a unique example by connecting the land with the ideals of cultural change. The sociocultural resistance of Libera Terra conveys a political message through these practices and utters that the Mafia is not invincible. This study draws the complex panorama of the Mafia and anti-Mafia movement that uses the âconfiscated landsâ as cultural and public spaces for resistance and socio-cultural change. In doing so, this article sheds new light on the relationship between rural criminology and crime prevention policies in Southern Italy by demonstrating how community development practice of Libera Terra changes the meaning of landscape through iconographic symbolism and ethnographic performance
A catalogue of damped Lyman alpha absorption systems and radio flux densities of the background quasars
We present a catalogue of the 322 damped Lyman alpha absorbers taken from the
literature. All damped Lyman alpha absorbers are included, with no selection on
redshift or quasar magnitude. Of these, 123 are candidates and await
confirmation using high resolution spectroscopy. For all 322 objects we
catalogue the radio properties of the background quasars, where known. Around
60 quasars have radio flux densities above 0.1 Jy and approximately half of
these have optical magnitudes brighter than V = 18. This compilation should
prove useful in several areas of extragalactic/cosmological research.Comment: 26 Pages, 12 PS tables, 1 embedded table. Accepted by PASA.
Continuously updated online catalogue available at
http://www.phys.unsw.edu.au/~sjc/dl
Radiation hydrodynamics of SN 1987A: I. Global analysis of the light curve for the first 4 months
The optical/UV light curves of SN 1987A are analyzed with the multi-energy
group radiation hydrodynamics code STELLA. The calculated monochromatic and
bolometric light curves are compared with observations shortly after shock
breakout, during the early plateau, through the broad second maximum, and
during the earliest phase of the radioactive tail. We have concentrated on a
progenitor model calculated by Nomoto & Hashimoto and Saio, Nomoto, & Kato,
which assumes that 14 solar masses of the stellar mass is ejected. Using this
model, we have updated constraints on the explosion energy and the extent of
mixing in the ejecta. In particular, we determine the most likely range of E/M
(explosion energy over ejecta mass) and R_0 (radius of the progenitor). In
general, our best models have energies in the range E = (1.1 +/- 0.3) x 10^{51}
ergs, and the agreement is better than in earlier, flux-limited diffusion
calculations for the same explosion energy. Our modeled B and V fluxes compare
well with observations, while the flux in U undershoots after about 10 days by
a factor of a few, presumably due to NLTE and line transfer effects. We also
compare our results with IUE observations, and a very good quantitative
agreement is found for the first days, and for one IUE band (2500-3000 A) as
long as for 3 months. We point out that the V flux estimated by McNaught &
Zoltowski should probably be revised to a lower value.Comment: 27 pages AASTeX v.4.0 + 35 postscript figures. ApJ, accepte
MHD models of Pulsar Wind Nebulae
Pulsar Wind Nebulae (PWNe) are bubbles or relativistic plasma that form when
the pulsar wind is confined by the SNR or the ISM. Recent observations have
shown a richness of emission features that has driven a renewed interest in the
theoretical modeling of these objects. In recent years a MHD paradigm has been
developed, capable of reproducing almost all of the observed properties of
PWNe, shedding new light on many old issues. Given that PWNe are perhaps the
nearest systems where processes related to relativistic dynamics can be
investigated with high accuracy, a reliable model of their behavior is
paramount for a correct understanding of high energy astrophysics in general. I
will review the present status of MHD models: what are the key ingredients,
their successes, and open questions that still need further investigation.Comment: 18 pages, 5 figures, Invited Review, Proceedings of the "ICREA
Workshop on The High-Energy Emission from Pulsars and their Systems", Sant
Cugat, Spain, April 12-16, 201
SN 1993J VLBI (III): The Evolution of the Radio Shell
We present two sequences of VLBI images of SN1993J in M81, with 24 images at
8.4 GHz and 19 images at 5.0 GHz, These sequences, from 50 d to ~9 yr after
shock breakout, show the evolution of the expanding radio shell of an exploded
star in detail. The images are all phase-referenced to the stable reference
point of the core of M81, allowing us to display them relative to the supernova
explosion center. At 50 d, SN1993J is almost unresolved with a radius of 520
AU. The shell structure becomes discernible at 175 d. The brightness of the
ridge of the projected shell is not uniform, but rather varies by a factor of
two, having a distinct maximum to the south-east and a minimum to the west.
Over the next ~350 d, this pattern appears to rotate counter-clockwise. After
two years, the structure becomes more complex with hot spots developing in the
east, south, and west. The pattern of modulation continues to change, and after
five years the three hot spots have shifted somewhat. After nine years, the
radio shell has expanded to a radius of 19,000 AU. The brightness in the center
of the images is lower than expected for an optically thin, spherical shell.
Absorption in the center is favored over a thinner shell in the back and/or
front. Allowing for absorption, we find that the thickness of the shell is
(25+/-3)% of its outer radius. We find no compact source in the central region
and conclude that any pulsar nebula in the center of SN 1993J is either much
fainter than the Crab or affected by remaining significant internal radio
absorption.Comment: Accepted for publication in the Astrophysical Journal. 34 Pages, 14
Figures (Figure 3 in color
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