10,214 research outputs found
Radio Observations of Supernova Remnants
Supernovae release an enormous amount of energy into the interstellar medium.
Their remnants can observationally be traced up to several ten-thousand years.
So far more than 230 Galactic supernova remnants (SNRs) have been identified in
the radio range. Detailed studies of the different types of SNRs give insight
into the interaction of the blast wave with the interstellar medium. Shock
accelerated particles are observed, but also neutron stars left from the
supernova explosion make their contribution. X-ray observations in conjunction
with radio data constrain models of supernova evolution.
A brief review of the origin and evolution of SNRs is given, which are
compared with supernova statistics and observational limitations. In addition
the morphology and characteristics of the different types of SNRs are
described, including some recent results and illustrated by SNRs images mostly
obtained with the Effelsberg 100-m telescope.Comment: 11 pages, 15 figures. To appear in the Proceedings of the 270.
WE-Heraeus Seminar on Neutron Stars, Pulsars and Supernova Remnants, Jan.
21-25, 2002, Physikzentrum Bad Honnef, eds W. Becker, H. Lesch & J. Truemper.
Proceedings are available as MPE-Report 27
G181.1+9.5, a new high-latitude low-surface brightness supernova remnant
More than 90% of the known Milky Way supernova remnants are within 5 degrees
of the Galactic Plane. We present the discovery of the supernova remnant
G181.1+9.5, a new high-latitude SNR, serendipitously discovered in an ongoing
survey of the Galactic Anti-centre High-Velocity Cloud complex, observed with
the DRAO Synthesis Telescope in the 21~cm radio continuum and HI spectral line.
We use radio continuum observations (including the linearly polarized
component) at 1420~MHz (observed with the DRAO ST) and 4850~MHz (observed with
the Effelsberg 100-m radio telescope) to map G181.1+9.5 and determine its
nature as a SNR. High-resolution 21~cm HI line observations and HI emission and
absorption spectra reveal the physical characteristics of its local
interstellar environment. Finally, we estimate the basic physical parameters of
G181.1+9.5 using models for highly-evolved SNRs. G181.1+9.5 has a circular
shell-like morphology with a radius of about 16~pc at a distance of 1.5 kpc
some 250 pc above the mid-plane. The radio observations reveal highly linearly
polarized emission with a non-thermal spectrum. Archival ROSAT X-ray data
reveal high-energy emission from the interior of G181.1+9.5 indicative of the
presence of shock-heated ejecta. The SNR is in the advanced radiative phase of
SNR evolution, expanding into the HVC inter-cloud medium with a density of
1^{-3}$. Basic physical attributes of G181.1+9.5 calculated with radiative
SNR models show an upper-limit age of 16,000 years, a swept-up mass of more
than 300 solar masses, and an ambient density in agreement with that estimated
from HI observations. G181.1+9.5 shows all characteristics of a typical mature
shell-type SNR, but its observed faintness is unusual and requires further
study.Comment: A&A accepted, 11 pages, 13 figure
Magnetic fields of the W4 superbubble
Superbubbles and supershells are the channels for transferring mass and
energy from the Galactic disk to the halo. Magnetic fields are believed to play
a vital role in their evolution. We study the radio continuum and polarized
emission properties of the W4 superbubble to determine its magnetic field
strength. New sensitive radio continuum observations were made at 6 cm, 11 cm,
and 21 cm. The total intensity measurements were used to derive the radio
spectrum of the W4 superbubble. The linear polarization data were analysed to
determine the magnetic field properties within the bubble shells. The
observations show a multi-shell structure of the W4 superbubble. A flat radio
continuum spectrum that stems from optically thin thermal emission is derived
from 1.4 GHz to 4.8 GHz. By fitting a passive Faraday screen model and
considering the filling factor fne , we obtain the thermal electron density ne
= 1.0/\sqrt{fne} (\pm5%) cm^-3 and the strength of the line-of-sight component
of the magnetic field B// = -5.0/\sqrt{fne} (\pm10%) {\mu}G (i.e. pointing away
from us) within the western shell of the W4 superbubble. When the known tilted
geometry of the W4 superbubble is considered, the total magnetic field Btot in
its western shell is greater than 12 {\mu}G. The electron density and the
magnetic field are lower and weaker in the high-latitude parts of the
superbubble. The rotation measure is found to be positive in the eastern shell
but negative in the western shell of the W4 superbubble, which is consistent
with the case that the magnetic field in the Perseus arm is lifted up from the
plane towards high latitudes. The magnetic field strength and the electron
density we derived for the W4 superbubble are important parameters for
evolution models of superbubbles breaking out of the Galactic plane.Comment: 13 pages, 8 figures, accepted for publication in Astronomy &
Astrophysic
A Sino-German 6cm polarisation survey of the Galactic plane IX. HII regions
Large-scale radio continuum surveys provide data to get insights into the
physical properties of radio sources. HII regions are prominent radio sources
produced by thermal emission of ionised gas around young massive stars. We
identify and analyse HII regions in the Sino-German 6cm polarisation survey of
the Galactic plane. Objects with flat radio continuum spectra together with
infrared and/or Halpha emission were identified as HII regions. For HII regions
with small apparent sizes, we cross-matched the 6cm small-diameter source
catalogue with the radio HII region catalogue compiled by Paladini and the
infrared HII region catalogue based on the WISE data. Extended HII regions were
identified by eye by overlaying the Paladini and the WISE HII regions onto the
6cm survey images for coincidences. The TT-plot method was employed for
spectral index verification. A total of 401 HII regions were identified and
their flux densities were determined with the Sino-German 6cm survey data. In
the surveyed area, 76 pairs of sources are found to be duplicated in the
Paladini HII region catalogue, mainly due to the non-distinction of previous
observations with different angular resolutions, and 78 objects in their
catalogue are misclassified as HII regions, being actually planetary nebulae,
supernova remnants or extragalactic sources that have steep spectra. More than
30 HII regions and HII region candidates from our 6cm survey data, especially
extended ones, do not have counterparts in the WISE HII region catalogue, of
which 9 are identified for the first time. Based on the newly derived radio
continuum spectra and the evidence of infrared emission, the previously
identified SNRs G11.1-1.0, G20.4+0.1 and G16.4-0.5 are believed to be HII
regions.Comment: version after some minor corrections and language editing, full Table
2 - 5 will appear in CDS, accepted for publication in A&
The Fragmenting Superbubble Associated with the HII Region W4
New observations at high latitudes above the HII region W4 show that the
structure formerly identified as a chimney candidate, an opening to the
Galactic halo, is instead a superbubble in the process of fragmenting and
possibly evolving into a chimney. Data at high Galactic latitudes (b > 5
degrees) above the W3/W4 star forming region at 1420 and 408 MHz Stokes I
(total power) and 1420 MHz Stokes Q and U (linear polarization) reveal an
egg-shaped structure with morphological correlations between our data and the
H-alpha data of Dennison, Topasna, & Simonetti. Polarized intensity images show
depolarization extending from W4 up the walls of the superbubble, providing
strong evidence that the radio continuum is generated by thermal emission
coincident with the H-alpha emission regions. We conclude that the parts of the
HII region hitherto known as W4 and the newly revealed thermal emission are all
ionized by the open cluster OCl 352. Assuming a distance of 2.35 kpc, the ovoid
structure is 164 pc wide and extends 246 pc above the mid-plane of the Galaxy.
The shell's emission decreases in total-intensity and polarized intensity in
various locations, appearing to have a break at its top and another on one
side. Using a geometric analysis of the depolarization in the shell's walls, we
estimate that a magnetic field line-of-sight component of 3 to 5 uG exists in
the shell. We explore the connection between W4 and the Galactic halo,
considering whether sufficient radiation can escape from the fragmenting
superbubble to ionize the kpc-scale H-alpha loop discovered by Reynolds,
Sterling & Haffner.Comment: 42 pages, 14 figures; Accepted for publication in Ap
Synchrotron Polarization at High Galactic Latitude
We present preliminary results from mapping the high-latitude Galactic
polarization with the Effelsberg Telescope at 21 cm. Structures on the
resulting maps are mostly on the scale of several degrees. The results show
detection of polarization over most of the field, at the level of tens of
percent of the synchrotron emission. The evidence of more structure in Stokes Q
and U rather than in suggests the existence of Faraday
rotation.Comment: To be published in the proceedings of "The Cosmic Microwave
Background and its Polarization", New Astronomy Reviews, (eds. S. Hanany and
K.A. Olive
A radio continuum survey of the southern sky at 1420 MHz. Observations and data reduction
We describe the equipment, observational method and reduction procedure of an
absolutely calibrated radio continuum survey of the South Celestial Hemisphere
at a frequency of 1420 MHz. These observations cover the area 0h < R.A. < 24h
for declinations less than -10 degree. The sensitivity is about 50 mK T_B (full
beam brightness) and the angular resolution (HPBW) is 35.4', which matches the
existing northern sky survey at the same frequency.Comment: 9 pages with 9 figures, A&A, in pres
A Sino-German 6cm polarisation survey of the Galactic plane - VIII. Small-diameter sources
Information of small-diameter sources is extracted from the Sino-German 6cm
polarisation survey of the Galactic plane carried out with the Urumqi 25-m
telescope. We performed two-dimensional elliptical Gaussian fits to the 6cm
maps to obtain a list of sources with total-intensity and polarised flux
densities. The source list contains 3832 sources with a fitted diameter smaller
than 16 arcmin and a peak flux density exceeding 30 mJy, so about 5 times the
rms noise, of the total-intensity data. The cumulative source count indicates
completeness for flux densities exceeding about 60 mJy. We identify 125
linearly polarised sources at 6cm with a peak polarisation flux density greater
than 10 mJy, so about 3 times the rms noise, of the polarised-intensity data.
Despite lacking compact steep spectrum sources, the 6cm catalogue lists about
20 percent more sources than the Effelsberg 21cm source catalogue at the same
angular resolution and for the same area. Most of the faint 6cm sources must
have a flat spectrum and are either HII regions or extragalactic. When compared
with the Green Bank 6cm (GB6) catalogue, we obtain higher flux densities for a
number of extended sources with complex structures. Polarised 6cm sources
density are uniformly distributed in Galactic latitude. Their number density
decreases towards the inner Galaxy. More than 80 percent of the polarised
sources are most likely extragalactic. With a few exceptions, the sources have
a higher percentage polarisation at 6cm than at 21cm. Depolarisation seems to
occur mostly within the sources with a minor contribution from the Galactic
foreground emission.Comment: A&A accepted, 9 pages, 5 figures, Tables 1 and 2 are accessible from
http://zmtt.bao.ac.cn/6cm
- …