295 research outputs found
The remnant of SN1987A revealed at (sub-)mm wavelengths
Context: Supernova 1987A (SN1987A) exploded in the Large Magellanic Cloud
(LMC). Its proximity and rapid evolution makes it a unique case study of the
early phases in the development of a supernova remnant. One particular aspect
of interest is the possible formation of dust in SN1987A, as SNe could
contribute significantly to the dust seen at high redshifts. Aims: We explore
the properties of SN1987A and its circumburst medium as seen at mm and sub-mm
wavelengths, bridging the gap between extant radio and infrared (IR)
observations of respectively the synchrotron and dust emission. Methods:
SN1987A was observed with the Australia Telescope Compact Array (ATCA) at 3.2
mm in July 2005, and with the Atacama Pathfinder EXperiment (APEX) at 0.87 mm
in May 2007. We present the images and brightness measurements of SN1987A at
these wavelengths for the first time. Results: SN1987A is detected as an
unresolved point source of 11.2 +/- 2.0 mJy at 3.2 mm (5" beam) and 21 +/- 4
mJy at 0.87 mm (18" beam). These flux densities are in perfect agreement with
extrapolations of the powerlaw radio spectrum and modified-blackbody dust
emission, respectively. This places limits on the presence of free-free
emission, which is similar to the expected free-free emission from the ionized
ejecta from SN1987A. Adjacent, fainter emission is observed at 0.87 mm
extending ~0.5' towards the south-west. This could be the impact of the
supernova progenitor's wind when it was still a red supergiant upon a dense
medium. Conclusions: We have established a continuous spectral energy
distribution for the emission from SN1987A and its immediate surroundings,
linking the IR and radio data. This places limits on the contribution from
ionized plasma. Our sub-mm image reveals complexity in the distribution of cold
dust surrounding SN1987A, but leaves room for freshly synthesized dust in the
SN ejecta.Comment: Accepted for publication in Astronomy and Astrophysics Letters on 28
April 2011. A better quality figure 1 can be had from
http://www.astro.keele.ac.uk/~jacco/research/SN1987A087mm.ep
Subtraction of Bright Point Sources from Synthesis Images of the Epoch of Reionization
Bright point sources associated with extragalactic AGN and radio galaxies are
an important foreground for low frequency radio experiments aimed at detecting
the redshifted 21cm emission from neutral hydrogen during the epoch of
reionization. The frequency dependence of the synthesized beam implies that the
sidelobes of these sources will move across the field of view as a function of
observing frequency, hence frustrating line-of-sight foreground subtraction
techniques. We describe a method for subtracting these point sources from dirty
maps produced by an instrument such as the MWA. This technique combines matched
filters with an iterative centroiding scheme to locate and characterize point
sources in the presence of a diffuse background. Simulations show that this
technique can improve the dynamic range of EOR maps by 2-3 orders of magnitude.Comment: 11 pages, 8 figures, 1 table, submitted to PAS
LISA Data Analysis using MCMC methods
The Laser Interferometer Space Antenna (LISA) is expected to simultaneously
detect many thousands of low frequency gravitational wave signals. This
presents a data analysis challenge that is very different to the one
encountered in ground based gravitational wave astronomy. LISA data analysis
requires the identification of individual signals from a data stream containing
an unknown number of overlapping signals. Because of the signal overlaps, a
global fit to all the signals has to be performed in order to avoid biasing the
solution. However, performing such a global fit requires the exploration of an
enormous parameter space with a dimension upwards of 50,000. Markov Chain Monte
Carlo (MCMC) methods offer a very promising solution to the LISA data analysis
problem. MCMC algorithms are able to efficiently explore large parameter
spaces, simultaneously providing parameter estimates, error analyses and even
model selection. Here we present the first application of MCMC methods to
simulated LISA data and demonstrate the great potential of the MCMC approach.
Our implementation uses a generalized F-statistic to evaluate the likelihoods,
and simulated annealing to speed convergence of the Markov chains. As a final
step we super-cool the chains to extract maximum likelihood estimates, and
estimates of the Bayes factors for competing models. We find that the MCMC
approach is able to correctly identify the number of signals present, extract
the source parameters, and return error estimates consistent with Fisher
information matrix predictions.Comment: 14 pages, 7 figure
Multiple RNA interactions position Mrd1 at the site of the small subunit pseudoknot within the 90S pre-ribosome
Compressed sensing imaging techniques for radio interferometry
Radio interferometry probes astrophysical signals through incomplete and
noisy Fourier measurements. The theory of compressed sensing demonstrates that
such measurements may actually suffice for accurate reconstruction of sparse or
compressible signals. We propose new generic imaging techniques based on convex
optimization for global minimization problems defined in this context. The
versatility of the framework notably allows introduction of specific prior
information on the signals, which offers the possibility of significant
improvements of reconstruction relative to the standard local matching pursuit
algorithm CLEAN used in radio astronomy. We illustrate the potential of the
approach by studying reconstruction performances on simulations of two
different kinds of signals observed with very generic interferometric
configurations. The first kind is an intensity field of compact astrophysical
objects. The second kind is the imprint of cosmic strings in the temperature
field of the cosmic microwave background radiation, of particular interest for
cosmology.Comment: 10 pages, 1 figure. Version 2 matches version accepted for
publication in MNRAS. Changes includes: writing corrections, clarifications
of arguments, figure update, and a new subsection 4.1 commenting on the exact
compliance of radio interferometric measurements with compressed sensin
Ultradeep Ks Imaging in the GOODS-N
We present an ultradeep Ks-band image that covers 0.5*0.5 deg^2 centered on
the Great Observatories Origins Deep Survey-North (GOODS-N). The image reaches
a 5 \sigma depth of Ks(AB) = 24.45 in the GOODS-N region, which is as deep as
the GOODS-N Spitzer Infrared Array Camera (IRAC) 3.6 \mu m image. We present a
new method of constructing IRAC catalogs that uses the higher spatial
resolution Ks image and catalog as priors and iteratively subtracts fluxes from
the IRAC images to estimate the IRAC fluxes. Our iterative method is different
from the \chi^2 approach adopted by other groups. We verified our results using
data taken in two different epochs of observations, as well as by comparing our
colors with the colors of stars and with the colors derived from model spectral
energy distributions (SEDs) of galaxies at various redshifts. We make available
to the community our WIRCam Ks-band image and catalog (94951 objects in 0.25
deg^2), the Interactive Data Language (IDL) pipeline used for reducing the
WIRCam images, and our IRAC 3.6 to 8.0 \mu m catalog (16950 objects in 0.06
deg^2 at 3.6 \mu m). With this improved Ks and IRAC catalog and a large
spectroscopic sample from our previous work, we study the color-magnitude and
color-color diagrams of galaxies. We compare the effectiveness of using Ks and
IRAC colors to select active galactic nuclei (AGNs) and galaxies at various
redshifts. We also study a color selection of z = 0.65--1.2 galaxies using the
Ks, 3.6 \mu m, and 4.5 \mu m bands.Comment: Accepted for publication on ApJS. Online data are availabl
Modelling the Spectral Energy Distribution of Compact Luminous Infrared Galaxies: Constraints from High Frequency Radio Data
We have performed 23 GHz VLA observations of 7 compact, luminous infrared
galaxies, selected to have evidence of starburst activity. New and published
multi-frequency data are combined to obtain the spectral energy distributions
of all 7 galaxies from the near-infrared to the radio (at 1.4 GHz). These SEDs
are compared with new models, for dust enshrouded galaxies, which account for
both starburst and AGN components. In all 7 galaxies the starburst provides the
dominant contribution to the infrared luminosity; in 4 sources no contribution
from an AGN is required. Although AGN may contribute up to 50 percent of the
total far--infrared emission, the starbursts always dominate in the radio. The
SEDs of most of our sources are best fit with a very high optical depth of
(>=50) at 1 micron. The scatter in the far-infrared/radio correlation, found
among luminous IRAS sources, is due mainly to the different evolutionary status
of their starburst components. The short time-scale of the star formation
process amplifies the delay between the far-infrared and radio emission. This
becomes more evident at low radio frequencies (below about 1 GHz) where
synchrotron radiation is the dominant process. In the far-infrared (at
wavelengths shorter than 100 micron) an additional source of scatter is
provided by AGN, where present. AGN may be detected in the near-infrared by the
absence of the knee, typical of stellar photospheres. However, near-infrared
data alone cannot constrain the level at which AGN contribute because the
interpretation of their observed properties, in this wave-band, depends
strongly on model parameters.Comment: 14 pages, accepted for publication in Astronomy and Astrophysic
VLBA monitoring of Mrk 421 at 15 GHz and 24 GHz during 2011
High-resolution radio observations are ideal for constraining the value of
physical parameters in the inner regions of active-galactic-nucleus jets and
complement results on multiwavelength (MWL) observations. This study is part of
a wider multifrequency campaign targeting the nearby TeV blazar Markarian 421
(z=0.031), with observations in the sub-mm (SMA), optical/IR (GASP), UV/X-ray
(Swift, RXTE, MAXI), and gamma rays (Fermi-LAT, MAGIC, VERITAS). We investigate
the jet's morphology and any proper motions, and the time evolution of physical
parameters such as flux densities and spectral index. The aim of our wider
multifrequency campaign is to try to shed light on questions such as the nature
of the radiating particles, the connection between the radio and gamma-ray
emission, the location of the emitting regions and the origin of the flux
variability. We consider data obtained with the Very Long Baseline Array (VLBA)
over twelve epochs (one observation per month from January to December 2011) at
15 GHz and 24 GHz. We investigate the inner jet structure on parsec scales
through the study of model-fit components for each epoch. The structure of Mrk
421 is dominated by a compact (~0.13 mas) and bright component, with a
one-sided jet detected out to ~10 mas. We identify 5-6 components in the jet
that are consistent with being stationary during the 12-month period studied
here. Measurements of the spectral index agree with those of other works: they
are fairly flat in the core region and steepen along the jet length.
Significant flux-density variations are detected for the core component. From
our results, we draw an overall scenario in which we estimate a viewing angle
2{\deg} < theta < 5{\deg} and a different jet velocity for the radio and the
high-energy emission regions, such that the respective Doppler factors are
{\delta}r ~3 and {\delta}h.e. ~14.Comment: 9 pages, 4 figure
The SuperWASP catalogue of 4963 RR Lyr stars: identification of 983 Blazhko candidates
Aims. We set out to compile a catalogue of RRab pulsating variables in the SuperWASP archive and identify candidate Blazhko effect objects within this catalogue. We analysed their light curves and power spectra for correlations in their common characteristics to further our understanding of the phenomenon. Methods. Pulsation periods were found for each SWASP RRab object using PDM techniques. Low frequency periodic signals detected
in the CLEAN power spectra of RRab stars were matched with modulation sidebands and combined with pairs of sidebands to produce a list of candidate Blazhko periods. A novel technique was used in an attempt to identify Blazhko effect stars by comparing scatter at different parts of the folded light curve. Pulsation amplitudes were calculated based on phase folded light curves.Results. The SuperWASP RRab catalogue consists of 4963 objects of which 3397 are previously unknown. We discovered 983 distinct candidates for Blazhko effect objects, 613 of these being previously unknown in the literature as RR Lyrae stars, and 894 are previously unknown to be Blazhko effect stars. Correlations were investigated between the scatter of points on the light curve, the periods and amplitudes of the objectsâ pulsations, and those of the Blazhko effect.Conclusions. A statistical analysis has been performed on a large population of Blazhko effect stars from the wide-field SuperWASP survey. No correlations were found between the Blazhko period and other parameters including the Blazhko amplitude, although we confirmed a lower rate of occurrence of the Blazhko effect in long pulsation period objects
The ATESP 5 GHz radio survey. III. 4.8, 8.6 and 19 GHz follow-up observations of radio galaxies
[Abridged] Physical and evolutionary properties of the sub-mJy radio
population are not entirely known. The radio/optical analysis of the ATESP 5
GHz sample has revealed a significant class of compact flat/inverted
radio-spectrum sources associated to early-type galaxies up to redshift 2. Such
sources are most plausibly triggered by an AGN, but their observational
properties are not entirely consistent with those of standard radio galaxy
populations. In the present work we aim at a better understanding of the radio
spectra of such sources and ultimately of the nature of AGNs at sub-mJy flux
levels. We used the ATCA to get multi-frequency (4.8, 8.6 and 19 GHz)
quasi-simultaneous observations for a representative sub-sample of ATESP radio
sources associated with early-type galaxies (26 objects with S>0.6 mJy). This
can give us insight into the accretion/radiative mechanism that is at work,
since different regimes display different spectral signatures in the radio
domain. From the analysis of the radio spectra, we find that our sources are
most probably jet-dominated systems. ADAF models are ruled out by the high
frequency data, while ADAF+jet scenarios are still consistent with
flat/moderately inverted-spectrum sources, but are not required to explain the
data. We compared our sample with high (>20 GHz) frequency selected surveys,
finding spectral properties very similar to the ones of much brighter (S>500
mJy) radio galaxies extracted from the Massardi et al. (2008) sample. Linear
sizes of ATESP 5 GHz sources associated with early type galaxies are also often
consistent with the ones of brighter B2 and 3C radio galaxies, with possibly a
very compact component that could be associated at least in part to (obscured)
radio-quiet quasar-like objects and/or low power BL Lacs.Comment: Accepted for publication in Astronomy & Astrophysic
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