194 research outputs found
A Model of Polarized X-ray Emission from Twinkling Synchrotron Supernova Shells
Synchrotron X-ray emission components were recently detected in many young
supernova remnants (SNRs). There is even an emerging class - SN1006,
RXJ1713.72-3946, Vela Jr, and others - that is dominated by non-thermal
emission in X-rays, also probably of synchrotron origin. Such emission results
from electrons/positrons accelerated well above TeV energies in the spectral
cut-off regime. In the case of diffusive shock acceleration, which is the most
promising acceleration mechanism in SNRs, very strong magnetic fluctuations
with amplitudes well above the mean magnetic field must be present. Starting
from such a fluctuating field, we have simulated images of polarized X-ray
emission of SNR shells and show that these are highly clumpy with high
polarizations up to 50%. Another distinct characteristic of this emission is
the strong intermittency, resulting from the fluctuating field amplifications.
The details of this "twinkling" polarized X-ray emission of SNRs depend
strongly on the magnetic-field fluctuation spectra, providing a potentially
sensitive diagnostic tool. We demonstrate that the predicted characteristics
can be studied with instruments that are currently being considered. These can
give unique information on magnetic-field characteristics and high-energy
particle acceleration in SNRs.Comment: 7 pages, 8 figures, MNRAS (in press
The Radio Structure of the Supernova Remnant MSH14-63
G315.4-2.3 is an extended shell supernova remnant (SNR) with some
characteristics of evolutionarily young remnants and some of older ones. To
further elucidate some of its characteristics, we present imaging and
polarimetry of this SNR at a frequency of 1.34 GHz with a resolution of 8
arcsec made with the Australia Telescope Compact Array.
The indicators of youth are: Morphologically, the radio emission arises in a
smooth shell without the fine scale filaments seen in the optical. Many of the
optical filaments are Balmer dominated. Where measurable, the orientation of
the magnetic field appears to be radial with respect to the center of the
remnant. There may have been a supernova in that region in AD185.
Indications of older age include: Particularly in RCW86, the bright optical
nebula in the southwestern corner of this extended SNR, but also in other
locations there are several filaments with bright [S II] emission
representative of older shocked filaments in radiative equilibrium. If the
remnant lies at the kinematical distance of 2.8 kpc, it has a diameter of 37 pc
which would be large for a remnant less than two thousand years old.
The remnant seems to be expanding inside a cavity outlined by infrared
emission and so it could well be young and large. Where it is encountering the
walls of the cavity it is slowing rapidly and we observe the radiative
filaments. RCW86 itself is encountering a dense clump of material but may also
be the remains of a more compact lump of ejecta ploughing into the
surroundings.Comment: 15 pages in AAS LaTeX 5.0, 5 figures (2 parts in JPEG, 6 in GIF, 4 in
eps), accepted by Ap
The Relation Between the Surface Brightness and the Diameter for Galactic Supernova Remnants
In this work, we have constructed a relation between the surface brightness
() and diameter (D) of Galactic C- and S-type supernova remnants
(SNRs). In order to calibrate the -D dependence, we have carefully
examined some intrinsic (e.g. explosion energy) and extrinsic (e.g. density of
the ambient medium) properties of the remnants and, taking into account also
the distance values given in the literature, we have adopted distances for some
of the SNRs which have relatively more reliable distance values. These
calibrator SNRs are all C- and S-type SNRs, i.e. F-type SNRs (and S-type SNR
Cas A which has an exceptionally high surface brightness) are excluded. The
Sigma-D relation has 2 slopes with a turning point at D=36.5 pc: (at 1
GHz)=8.4 D
WmHzster (for
WmHzster and D36.5 pc) and (at 1
GHz)=2.7 10 D
WmHzster (for
WmHzster and D36.5 pc). We discussed the theoretical
basis for the -D dependence and particularly the reasons for the change
in slope of the relation were stated. Added to this, we have shown the
dependence between the radio luminosity and the diameter which seems to have a
slope close to zero up to about D=36.5 pc. We have also adopted distance and
diameter values for all of the observed Galactic SNRs by examining all the
available distance values presented in the literature together with the
distances found from our -D relation.Comment: 45 pages, 2 figures, accepted for publication in Astronomical and
Astrophysical Transaction
Magnetic fields in supernova remnants and pulsar-wind nebulae
We review the observations of supernova remnants (SNRs) and pulsar-wind
nebulae (PWNe) that give information on the strength and orientation of
magnetic fields. Radio polarimetry gives the degree of order of magnetic
fields, and the orientation of the ordered component. Many young shell
supernova remnants show evidence for synchrotron X-ray emission. The spatial
analysis of this emission suggests that magnetic fields are amplified by one to
two orders of magnitude in strong shocks. Detection of several remnants in TeV
gamma rays implies a lower limit on the magnetic-field strength (or a
measurement, if the emission process is inverse-Compton upscattering of cosmic
microwave background photons). Upper limits to GeV emission similarly provide
lower limits on magnetic-field strengths. In the historical shell remnants,
lower limits on B range from 25 to 1000 microGauss. Two remnants show
variability of synchrotron X-ray emission with a timescale of years. If this
timescale is the electron-acceleration or radiative loss timescale, magnetic
fields of order 1 mG are also implied. In pulsar-wind nebulae, equipartition
arguments and dynamical modeling can be used to infer magnetic-field strengths
anywhere from about 5 microGauss to 1 mG. Polarized fractions are considerably
higher than in SNRs, ranging to 50 or 60% in some cases; magnetic-field
geometries often suggest a toroidal structure around the pulsar, but this is
not universal. Viewing-angle effects undoubtedly play a role. MHD models of
radio emission in shell SNRs show that different orientations of upstream
magnetic field, and different assumptions about electron acceleration, predict
different radio morphology. In the remnant of SN 1006, such comparisons imply a
magnetic-field orientation connecting the bright limbs, with a non-negligible
gradient of its strength across the remnant.Comment: 20 pages, 24 figures; to be published in SpSciRev. Minor wording
change in Abstrac
Radio emission from Supernova Remnants
The explosion of a supernova releases almost instantaneously about 10^51 ergs
of mechanic energy, changing irreversibly the physical and chemical properties
of large regions in the galaxies. The stellar ejecta, the nebula resulting from
the powerful shock waves, and sometimes a compact stellar remnant, constitute a
supernova remnant (SNR). They can radiate their energy across the whole
electromagnetic spectrum, but the great majority are radio sources. Almost 70
years after the first detection of radio emission coming from a SNR, great
progress has been achieved in the comprehension of their physical
characteristics and evolution. We review the present knowledge of different
aspects of radio remnants, focusing on sources of the Milky Way and the
Magellanic Clouds, where the SNRs can be spatially resolved. We present a brief
overview of theoretical background, analyze morphology and polarization
properties, and review and critical discuss different methods applied to
determine the radio spectrum and distances. The consequences of the interaction
between the SNR shocks and the surrounding medium are examined, including the
question of whether SNRs can trigger the formation of new stars. Cases of
multispectral comparison are presented. A section is devoted to reviewing
recent results of radio SNRs in the Magellanic Clouds, with particular emphasis
on the radio properties of SN 1987A, an ideal laboratory to investigate
dynamical evolution of an SNR in near real time. The review concludes with a
summary of issues on radio SNRs that deserve further study, and analyzing the
prospects for future research with the latest generation radio telescopes.Comment: Revised version. 48 pages, 15 figure
ALL-1/MLL1, a homologue of Drosophila TRITHORAX, modifies chromatin and is directly involved in infant acute leukaemia
Rearrangements of the ALL-1/MLL1 gene underlie the majority of infant acute leukaemias, as well as of therapy-related leukaemias developing in cancer patients treated with inhibitors of topoisomerase II, such as VP16 and doxorubicin. The rearrangements fuse ALL-1 to any of \u3e50 partner genes or to itself. Here, we describe the unique features of ALL-1-associated leukaemias, and recent progress in understanding molecular mechanisms involved in the activity of the ALL-1 protein and of its Drosophila homologue TRITHORAX
abd-A Regulation by the iab-8 Noncoding RNA
The homeotic genes in Drosophila melanogaster are aligned on the chromosome in the order of the body segments that they affect. The genes affecting the more posterior segments repress the more anterior genes. This posterior dominance rule must be qualified in the case of abdominal-A (abd-A) repression by Abdominal-B (Abd-B). Animals lacking Abd-B show ectopic expression of abd-A in the epidermis of the eighth abdominal segment, but not in the central nervous system. Repression in these neuronal cells is accomplished by a 92 kb noncoding RNA. This âiab-8 RNAâ produces a micro RNA to repress abd-A, but also has a second, redundant repression mechanism that acts only âin cis.â Transcriptional interference with the abd-A promoter is the most likely mechanism
A Large Fraction of Extragenic RNA Pol II Transcription Sites Overlap Enhancers
A substantial fraction of extragenic Pol II transcription sites coincides with transcriptional enhancers, which may be relevant for functional annotation of mammalian genomes
Sensitivity of the Cherenkov Telescope Array to spectral signatures of hadronic PeVatrons with application to Galactic Supernova Remnants
The local Cosmic Ray (CR) energy spectrum exhibits a spectral softening at
energies around 3~PeV. Sources which are capable of accelerating hadrons to
such energies are called hadronic PeVatrons. However, hadronic PeVatrons have
not yet been firmly identified within the Galaxy. Several source classes,
including Galactic Supernova Remnants (SNRs), have been proposed as PeVatron
candidates. The potential to search for hadronic PeVatrons with the Cherenkov
Telescope Array (CTA) is assessed. The focus is on the usage of very high
energy -ray spectral signatures for the identification of PeVatrons.
Assuming that SNRs can accelerate CRs up to knee energies, the number of
Galactic SNRs which can be identified as PeVatrons with CTA is estimated within
a model for the evolution of SNRs. Additionally, the potential of a follow-up
observation strategy under moonlight conditions for PeVatron searches is
investigated. Statistical methods for the identification of PeVatrons are
introduced, and realistic Monte--Carlo simulations of the response of the CTA
observatory to the emission spectra from hadronic PeVatrons are performed.
Based on simulations of a simplified model for the evolution for SNRs, the
detection of a -ray signal from in average 9 Galactic PeVatron SNRs is
expected to result from the scan of the Galactic plane with CTA after 10 hours
of exposure. CTA is also shown to have excellent potential to confirm these
sources as PeVatrons in deep observations with hours of
exposure per source.Comment: 34 pages, 16 figures, Accepted for publication in Astroparticle
Physic
Sensitivity of the Cherenkov Telescope Array to TeV photon emission from the Large Magellanic Cloud
A deep survey of the Large Magellanic Cloud at âŒ0.1-100 TeV photon energies with the Cherenkov Telescope Array is planned. We assess the detection prospects based on a model for the emission of the galaxy, comprising the four known TeV emitters, mock populations of sources, and interstellar emission on galactic scales. We also assess the detectability of 30 Doradus and SN 1987A, and the constraints that can be derived on the nature of dark matter. The survey will allow for fine spectral studies of N 157B, N 132D, LMC P3, and 30 Doradus C, and half a dozen other sources should be revealed, mainly pulsar-powered objects. The remnant from SN 1987A could be detected if it produces cosmic-ray nuclei with a flat power-law spectrum at high energies, or with a steeper index 2.3-2.4 pending a flux increase by a factor of >3-4 over âŒ2015-2035. Large-scale interstellar emission remains mostly out of reach of the survey if its >10 GeV spectrum has a soft photon index âŒ2.7, but degree-scale 0.1-10 TeV pion-decay emission could be detected if the cosmic-ray spectrum hardens above >100 GeV. The 30 Doradus star-forming region is detectable if acceleration efficiency is on the order of 1â10 per cent of the mechanical luminosity and diffusion is suppressed by two orders of magnitude within <100 pc. Finally, the survey could probe the canonical velocity-averaged cross-section for self-annihilation of weakly interacting massive particles for cuspy Navarro-Frenk-White profiles
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