124 research outputs found
Resolution of the Compact Radio Continuum Sources in Arp220
We present 2 cm and 3.6 cm wavelength very long baseline interferometry
images of the compact radio continuum sources in the nearby ultra-luminous
infrared galaxy Arp220. Based on their radio spectra and variability
properties, we confirm these sources to be a mixture of supernovae (SNe) and
supernova remnants (SNRs). Of the 17 detected sources we resolve 7 at both
wavelengths. The SNe generally only have upper size limits. In contrast all the
SNRs are resolved with diameters {\geq} 0.27 pc. This size limit is consistent
with them having just entered their Sedov phase while embedded in an
interstellar medium (ISM) of density 10^4 cm^{-3} . These objects lie on the
diameter-luminosity correlation for SNRs (and so also on the diameter-surface
brightness relation) and extend these correlations to very small sources. The
data are consistent with the relation L {\propto} D^{-9/4}. Revised
equipartition arguments adjusted to a magnetic field to relativistic particle
energy density ratio of 1% combined with a reasonable synchrotron-emitting
volume filling factor of 10% give estimated magnetic field strengths in the SNR
shells of ~ 15-50 mG. The SNR shell magnetic fields are unlikely to come from
compression of ambient ISM fields and must instead be internally generated. We
set an upper limit of 7 mG for the ISM magnetic field. The estimated energy in
relativistic particles, 2%-20% of the explosion kinetic energy, is consistent
with estimates from models that fit the IR-radio correlation in compact
starburst galaxies.Comment: 16 pages, 5 figure
Multiwavelength radio observations of the compact starburst in Arp 220
We report the first detection at multiple radio wavelengths (13, 6, and 3.6 cm) of 18 compact
sources within both nuclei of the Ultra Luminous Infra-Red Galaxy (ULIRG) Arp 220. In just
over half of the sources we find that the observed spectra are consistent with the standard model
of powerful Type IIn supernovae interacting with their pre-explosion stellar wind. The rate of appearance
of new radio sources ascribed to these supernova events suggests that a large fraction of
core-collapse supernovae in Arp 220 are highly luminous, possibly implying a radically different
stellar initial mass function (IMF) or stellar evolution compared to galactic disks. A second group
of sources, consisting of the brightest and longest monitored sources at 18 cm, do not easily fit
the radio supernova model. We propose that these are young supernova remnants that have just
begun interacting with their surrounding dense ISM
SIMULTANEOUS OBSERVATIONS OF GIANT PULSES FROM THE CRAB PULSAR, WITH THE MURCHISON WIDEFIELD ARRAY AND PARKES RADIO TELESCOPE: IMPLICATIONS FOR THE GIANT PULSE EMISSION MECHANISM
We report on observations of giant pulses from the Crab pulsar performed simultaneously with the Parkes radio telescope and the incoherent combination of the Murchison Widefield Array (MWA) antenna tiles. The observations were performed over a duration of approximately one hour at a center frequency of 1382 MHz with 340 MHz bandwidth at Parkes, and at a center frequency of 193 MHz with 15 MHz bandwidth at the MWA. Our analysis has led to the detection of 55 giant pulses at the MWA and 2075 at Parkes above a threshold of 3.5σ and 6.5σ, respectively. We detected 51% of the MWA giant pulses at the Parkes radio telescope, with spectral indices in the range of -3.6 > α > -4.9 (S[subscript v] ∝ v[superscript α]). We present a Monte Carlo analysis supporting the conjecture that the giant pulse emission in the Crab is intrinsically broadband, the less than 100% correlation being due to the relative sensitivities of the two instruments and the width of the spectral index distribution. Our observations are consistent with the hypothesis that the spectral index of giant pulses is drawn from normal distribution of standard deviation 0.6, but with a mean that displays an evolution with frequency from −3.00 at 1382 MHz, to −2.85 at 192 MHz
Multiwavelength radio observations of the compact starburst in Arp 220
We report the first detection at multiple radio wavelengths (13, 6, and 3.6 cm) of 18 compact
sources within both nuclei of the Ultra Luminous Infra-Red Galaxy (ULIRG) Arp 220. In just
over half of the sources we find that the observed spectra are consistent with the standard model
of powerful Type IIn supernovae interacting with their pre-explosion stellar wind. The rate of appearance
of new radio sources ascribed to these supernova events suggests that a large fraction of
core-collapse supernovae in Arp 220 are highly luminous, possibly implying a radically different
stellar initial mass function (IMF) or stellar evolution compared to galactic disks. A second group
of sources, consisting of the brightest and longest monitored sources at 18 cm, do not easily fit
the radio supernova model. We propose that these are young supernova remnants that have just
begun interacting with their surrounding dense ISM
Analytic approximations of scattering effects on beam chromaticity in 21-cm global experiments
Scattering from objects near an antenna produce correlated signals from
strong compact radio sources in a manner similar to those used by the Sea
Interferometer to measure the radio source positions using the fine frequency
structure in the total power spectrum of a single antenna. These fringes or
ripples due to correlated signal interference are present at a low level in the
spectrum of any single antenna and are a major source of systematics in systems
used to measure the global redshifted 21-cm signal from the early universe. In
the Sea Interferometer a single antenna on a cliff above the sea is used to add
the signal from the direct path to the signal from the path reflected from the
sea thereby forming an interferometer. This was used for mapping radio sources
with a single antenna by Bolton and Slee in the 1950s. In this paper we derive
analytic expressions to determine the level of these ripples and compare these
results in a few simple cases with electromagnetic modeling software to verify
that the analytic calculations are sufficient to obtain the magnitude of the
scattering effects on the measurements of the global 21-cm signal. These
analytic calculations are needed to evaluate the magnitude of the effects in
cases that are either too complex or take too much time to be modeled using
software
The radio spectra of the compact sources in Arp 220: A mixed population of supernovae and supernova remnants
We report the first detection at multiple radio wavelengths (13, 6 and 3.6
cm) of the compact sources within both nuclei of the Ultra Luminous Infra-Red
Galaxy Arp 220. We present the radio spectra of the 18 detected sources. In
just over half of the sources we find that these spectra and other properties
are consistent with the standard model of powerful Type IIn supernovae
interacting with their pre-explosion stellar wind. The rate of appearance of
new radio sources identified with these supernova events suggests that an
unusually large fraction of core collapse supernovae in Arp 220 are highly
luminous; possibly implying a radically different stellar initial mass function
or stellar evolution compared to galactic disks. Another possible explanation
invokes very short (~3 x 10^5 year) intense (~10^3 M_Sol year^-1) star
formation episodes with a duty cycle of ~10%. A second group of our detected
sources, consisting of the brightest and longest monitored sources at 18 cm do
not easily fit the radio supernova model. These sources show a range of
spectral indexes from -0.2 to -1.9. We propose that these are young supernova
remnants which have just begun interacting with a surrounding ISM with a
density between 10^4 and 10^5 cm^-3. One of these sources is probably resolved
at 3.6 cm wavelength with a diameter 0.9 pc. In the western nucleus we estimate
that the ionized component of the ISM gives rise to foreground free-free
absorption with opacity at 18 cm of <0.6 along the majority of lines of sight.
Other sources may be affected by absorption with opacity in the range 1 to 2.
These values are consistent with previous models as fitted to the radio
recombination lines and the continuum spectrum.Comment: 44 pages, 9 figures, 2 tables. Accepted for publication in Ap
Realisation of a low frequency SKA Precursor: The Murchison Widefield Array
The Murchison Widefield Array is a low frequency (80-300 MHz) SKA Precursor, comprising 128 aperture array elements distributed over an area of 3 km diameter. The MWA is located at the extraordinarily radio quiet Murchison Radioastronomy Observatory in the mid-west of Western Australia, the selected home for the Phase 1 and Phase 2 SKA low frequency arrays. The MWA science goals include: 1) detection of fluctuations in the brightness temperature of the diffuse redshifted 21 cm line of neutral hydrogen from the epoch of reionisation; 2) studies of Galactic and extragalactic processes based on deep, confusion-limited surveys of the full sky visible to the array; 3) time domain astrophysics through exploration of the variable radio sky; and 4) solar imaging and characterisation of the heliosphere and ionosphere via propagation effects on background radio source emission. This paper will focus on a brief discussion of the as-built MWA system, highlighting several novel characteristics of the instrument, and a brief progress report (as of June 2012) on the final construction phase. Practical completion of the MWA is expected in November 2012, with commissioning commencing from approximately August 2012 and operations commencing near mid 2013. A brief description of recent science results from the MWA prototype instrument is given
Serendipitous discovery of a dying Giant Radio Galaxy associated with NGC 1534, using the Murchison Widefield Array
Recent observations with the Murchison Widefield Array at 185 MHz have serendipitously unveiled a heretofore unknown giant and relatively nearby (z = 0.0178) radio galaxy associated with NGC 1534. The diffuse emission presented here is the first indication that NGC 1534 is one of a rare class of objects (along with NGC 5128 and NGC 612) in which a galaxy with a prominent dust lane hosts radio emission on scales of ∼700 kpc. We present details of the radio emission along with a detailed comparison with other radio galaxies with discs. NGC 1534 is the lowest surface brightness radio galaxy known with an estimated scaled 1.4-GHz surface brightness of just 0.2 mJy arcmin[superscript −2]. The radio lobes have one of the steepest spectral indices yet observed: α = −2.1 ± 0.1, and the core to lobe luminosity ratio is <0.1 per cent. We estimate the space density of this low brightness (dying) phase of radio galaxy evolution as 7 × 10[superscript −7] Mpc[superscript −3] and argue that normal AGN cannot spend more than 6 per cent of their lifetime in this phase if they all go through the same cycle
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