478 research outputs found
Fourier Modeling of the Radio Torus Surrounding Supernova 1987A
We present detailed Fourier modeling of the radio remnant of Supernova 1987A,
using high-resolution 9 GHz and 18 GHz data taken with the Australia Telescope
Compact Array over the period 1992 to 2008. We develop a parameterized
three-dimensional torus model for the expanding radio shell, in which the
emission is confined to an inclined equatorial belt; our model also
incorporates both a correction for light travel-time effects and an overall
east-west gradient in the radio emissivity. By deriving an analytic expression
for the two-dimensional Fourier transform of the projected three-dimensional
brightness distribution, we can fit our spatial model directly to the
interferometric visibility data. This provides robust estimates to the radio
morphology at each epoch. The best-fit results suggest a constant remnant
expansion at 4000 +/- 400 km/s over the 16-year period covered by the
observations. The model fits also indicate substantial mid-latitude emission,
extending to 40 degree on either side of the equatorial plane. This likely
corresponds to the extra-planar structure seen in H and Ly
emission from the supernova reverse shock, and broadly supports hydrodynamic
models in which the complex circumstellar environment was produced by a
progression of interacting winds from the progenitor. Our model quantifies the
clear asymmetry seen in the radio images: we find that the eastern half of the
radio remnant is consistently ~40 brighter than the western half at all epochs,
which may result from an asymmetry in the ejecta distribution between these two
hemispheres.Comment: accepted by ApJ, 11 figures, some have been scaled down in resolutio
A Sino-German 6\ cm polarization survey of the Galactic plane VI. Discovery of supernova remnants G178.2-4.2 and G25.1-2.3
Supernova remnants (SNRs) were often discovered in radio surveys of the
Galactic plane. Because of the surface-brightness limit of previous surveys,
more faint or confused SNRs await discovery. The Sino-German 6\ cm
Galactic plane survey is a sensitive survey with the potential to detect new
low surface-brightness SNRs. We want to identify new SNRs from the 6\
cm survey map of the Galactic plane. We searched for new shell-like objects in
the 6\ cm survey maps, and studied their radio emission, polarization,
and spectra using the 6\ cm maps together with the 11\ cm and
21\ cm Effelsberg observations. Extended polarized objects with
non-thermal spectra were identified as SNRs. We have discovered two new, large,
faint SNRs, G178.2-4.2 and G25.1-2.3, both of which show shell structure.
G178.2-4.2 has a size of 72 arcmin x 62 arcmin with strongly polarized emission
being detected along its northern shell. The spectrum of G178.2-4.2 is
non-thermal, with an integrated spectral index of . Its
surface brightness is , which makes G178.2-4.2 the second faintest known Galactic SNR.
G25.1-2.3 is revealed by its strong southern shell which has a size of 80
arcmin x 30\arcmin. It has a non-thermal radio spectrum with a spectral index
of . Two new large shell-type SNRs have been detected at
6\ cm in an area of 2200 deg^2 along the the Galactic plane. This
demonstrates that more large and faint SNRs exist, but are very difficult to
detect.Comment: 8 pages, 8 figures, accepted by Astronomy and Astrophysics. For the
version with high resolution figures, please go to
http://zmtt.bao.ac.cn/6cm/papers/2newSNR.pd
A high-resolution radio survey of the Vela supernova remnant
This paper presents a high-resolution radio continuum (843 MHz) survey of the
Vela supernova remnant. The contrast between the structures in the central
pulsar-powered nebula of the remnant and the synchrotron radiation shell allows
the remnant to be identified morphologically as a member of the composite
class. The data are the first of a composite remnant at spatial scales
comparable with those available for the Cygnus Loop and the Crab Nebula, and
make possible a comparison of radio, optical and soft X-ray emission from the
resolved shell filaments. The survey, made with the Molonglo Observatory
Synthesis Telescope, covers an area of 50 square degrees at a resolution of
43'' x 60'', while imaging structures on scales up to 30'.Comment: 18 pages, 7 jpg figures (version with ps figures at
http://astro.berkeley.edu/~dbock/papers/); AJ, in pres
XMMU J0541.8-6659, a new supernova remnant in the Large Magellanic Cloud
The high sensitivity of the XMM-Newton instrumentation offers the opportunity
to study faint and extended sources in the Milky Way and nearby galaxies such
as the Large Magellanic Cloud (LMC) in detail. The ROSAT PSPC survey of the LMC
has revealed more than 700 X-ray sources, among which there are 46 supernova
remnants (SNRs) and candidates. We have observed the field around one of the
most promising SNR candidates in the ROSAT PSPC catalogue, labelled [HP99] 456
with XMM-Newton, to determine its nature. We investigated the XMM-Newton data
along with new radio-continuum, near infrared and optical data. In particular,
spectral and morphological studies of the X-ray and radio data were performed.
The X-ray images obtained in different energy bands reveal two different
structures. Below 1.0 keV the X-ray emission shows the shell-like morphology of
an SNR with a diameter of ~73 pc, one of the largest known in the LMC. For its
thermal spectrum we estimate an electron temperature of (0.49 +/- 0.12)keV
assuming non-equilibrium ionisation. The X-ray images above 1.0 keV reveal a
less extended source within the SNR emission, located ~1' west of the centre of
the SNR and coincident with bright point sources detected in radio-continuum.
This hard component has an extent of 0.9' (i.e. ~13 pc at a distance of ~50
kpc) and a non-thermal spectrum. The hard source coincides in position with the
ROSAT source [HP99] 456 and shows an indication for substructure. We firmly
identify a new SNR in the LMC with a shell-like morphology and a thermal
spectrum. Assuming the SNR to be in the Sedov phase yields an age of ~23 kyr.
We explore possible associations of the hard non-thermal emitting component
with a pulsar wind nebula (PWN) or background active galactic nuclei (AGN).Comment: 8 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
Tidal Heating: Lessons from Io and the Jovian System (Report from the KISS Workshop)
Summary of the Keck Institute for Space Studies workshop entitled "Tidal Heating: Lessons from Io and the Jovian System," held on October 15-19, 2018
Titan's cold case files - Outstanding questions after Cassini-Huygens
The entry of the Cassini-Huygens spacecraft into orbit around Saturn in July 2004 marked the start of a golden era in the exploration of Titan, Saturn's giant moon. During the Prime Mission (2004â2008), ground-breaking discoveries were made by the Cassini orbiter including the equatorial dune fields (flyby T3, 2005), northern lakes and seas (T16, 2006), and the large positive and negative ions (T16 & T18, 2006), to name a few. In 2005 the Huygens probe descended through Titan's atmosphere, taking the first close-up pictures of the surface, including large networks of dendritic channels leading to a dried-up seabed, and also obtaining detailed profiles of temperature and gas composition during the atmospheric descent. The discoveries continued through the Equinox Mission (2008â2010) and Solstice Mission (2010â2017) totaling 127 targeted flybys of Titan in all. Now at the end of the mission, we are able to look back on the high-level scientific questions from the start of the mission, and assess the progress that has been made towards answering these. At the same time, new scientific questions regarding Titan have emerged from the discoveries that have been made. In this paper we review a cross-section of important scientific questions that remain partially or completely unanswered, ranging from Titan's deep interior to the exosphere. Our intention is to help formulate the science goals for the next generation of planetary missions to Titan, and to stimulate new experimental, observational and theoretical investigations in the interim
Protecting eyewitness evidence: Examining the efficacy of a self-administered interview tool
Given the crucial role of eyewitness evidence, statements should be obtained as soon as possible after an incident. This is not always achieved due to demands on police resources. Two studies trace the development of a new tool, the Self-Administered Interview (SAI), designed to elicit a comprehensive initial statement. In Study 1, SAI participants reported more correct details than participants who provided a free recall account, and performed at the same level as participants given a Cognitive Interview. In Study 2, participants viewed a simulated crime and half recorded their statement using the SAI. After a delay of 1 week, all participants completed a free recall test. SAI participants recalled more correct details in the delayed recall task than control participants
Titan's cold case files - Outstanding questions after Cassini-Huygens
Abstract The entry of the Cassini-Huygens spacecraft into orbit around Saturn in July 2004 marked the start of a golden era in the exploration of Titan, Saturn's giant moon. During the Prime Mission (2004â2008), ground-breaking discoveries were made by the Cassini orbiter including the equatorial dune fields (flyby T3, 2005), northern lakes and seas (T16, 2006), and the large positive and negative ions (T16 & T18, 2006), to name a few. In 2005 the Huygens probe descended through Titan's atmosphere, taking the first close-up pictures of the surface, including large networks of dendritic channels leading to a dried-up seabed, and also obtaining detailed profiles of temperature and gas composition during the atmospheric descent. The discoveries continued through the Equinox Mission (2008â2010) and Solstice Mission (2010â2017) totaling 127 targeted flybys of Titan in all. Now at the end of the mission, we are able to look back on the high-level scientific questions from the start of the mission, and assess the progress that has been made towards answering these. At the same time, new scientific questions regarding Titan have emerged from the discoveries that have been made. In this paper we review a cross-section of important scientific questions that remain partially or completely unanswered, ranging from Titan's deep interior to the exosphere. Our intention is to help formulate the science goals for the next generation of planetary missions to Titan, and to stimulate new experimental, observational and theoretical investigations in the interim
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
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