259 research outputs found
Radio emission of SN1993J. The complete picture: II. Simultaneous fit of expansion and radio light curves
We report on a simultaneous modelling of the expansion and radio light curves
of SN1993J. We have developed a simulation code capable of generating synthetic
expansion and radio light curves of supernovae by taking into consideration the
evolution of the expanding shock, magnetic fields, and relativistic electrons,
as well as the finite sensitivity of the interferometric arrays used in the
observations. Our software successfully fits all the available radio data of SN
1993J with an standard emission model for supernovae extended with some
physical considerations, as an evolution in the opacity of the ejecta material,
a radial drop of the magnetic fields inside the radiating region, and a
changing radial density profile of the circumstellar medium beyond day 3100
after explosion.Comment: 12 pages, 12 figures, accepted for publication in A&
VLBI observations of SN2011dh: imaging of the youngest radio supernova
We report on the VLBI detection of supernova SN2011dh at 22GHz using a subset
of the EVN array. The observations took place 14 days after the discovery of
the supernova, thus resulting in a VLBI image of the youngest radio-loud
supernova ever. We provide revised coordinates for the supernova with
milli-arcsecond precision, linked to the ICRF. The recovered flux density is a
factor 2 below the EVLA flux density reported by other authors at the same
frequency and epoch of our observations. This discrepancy could be due to
extended emission detected with the EVLA or to calibration problems in the VLBI
and/or EVLA observations.Comment: Letter. Accepted in A&
The population of SNe/SNRs in the starburst galaxy Arp 220. A self-consistent analysis of 20 years of VLBI monitoring
The nearby ultra-luminous infrared galaxy (ULIRG) Arp 220 is an excellent
laboratory for studies of extreme astrophysical environments. For 20 years,
Very Long Baseline Interferometry (VLBI) has been used to monitor a population
of compact sources thought to be supernovae (SNe), supernova remnants (SNRs)
and possibly active galactic nuclei (AGNs). Using new and archival VLBI data
spanning 20 years, we obtain 23 high-resolution radio images of Arp 220 at
wavelengths from 18 cm to 2 cm. From model-fitting to the images we obtain
estimates of flux densities and sizes of all detected sources. We detect radio
continuum emission from 97 compact sources and present flux densities and sizes
for all analysed observation epochs. We find evidence for a LD-relation within
Arp 220, with larger sources being less luminous. We find a compact source LF
with , similar to SNRs in normal
galaxies. Based on simulations we argue that there are many relatively large
and weak sources below our detection threshold. The observations can be
explained by a mixed population of SNe and SNRs, where the former expand in a
dense circumstellar medium (CSM) and the latter interact with the surrounding
interstellar medium (ISM). Nine sources are likely luminous, type IIn SNe. This
number of luminous SNe correspond to few percent of the total number of SNe in
Arp 220 which is consistent with a total SN-rate of 4 yr as inferred
from the total radio emission given a normal stellar initial mass function
(IMF). Based on the fitted luminosity function, we argue that emission from all
compact sources, also below our detection threshold, make up at most 20\% of
the total radio emission at GHz frequencies.Comment: Accepted for publication in Astronomy and Astrophysic
Absolute kinematics of radio source components in the complete S5 polar cap sample. III. First wide-field high-precision astrometry at 15.4 GHz
We report on the first wide-field, high-precision astrometric analysis of the
13 extragalactic radio sources of the complete S5 polar cap sample at 15.4 GHz.
We describe new algorithms developed to enable the use of differenced phase
delays in wide-field astrometric observations and discuss the impact of using
differenced phase delays on the precision of the wide-field astrometric
analysis. From this global fit, we obtained estimates of the relative source
positions with precisions ranging from 14 to 200 as at 15.4 GHz, depending
on the angular separation of the sources (from 1.6 to 20.8
degrees). These precisions are 10 times higher than the achievable
precisions using the phase-reference mapping technique.Comment: 9 pages, 7 figure
Radio emission of SN1993J: the complete picture. I. Re-analysis of all the available VLBI data
We have performed a complete re-calibration and re-analysis of all the
available VLBI observations of supernova SN1993J, following an homogeneous and
well-defined methodology. Observations of SN1993J at 69 epochs, spanning 13
years, were performed by two teams, which used different strategies and
analysis tools. The results obtained by each group are similar, but their
conclusions on the supernova expansion and the shape and evolution of the
emitting region differ significantly. From our analysis of the combined set of
observations, we have obtained an expansion curve with unprecedented time
resolution and coverage. We find that the data from both teams are compatible
when analyzed with the same methodology. One expansion index () is enough to model the expansion observed at 1.7\,GHz, while two
expansion indices ( and ), separated
by a break time, days, are needed to model the data, at
frequencies higher than 1.7\,GHz, up to day 4000 after explosion. We thus
confirm the wavelength dependence of the size of the emitting region reported
by one of the groups. We also find that all sizes measured at epochs later than
day 4000 after explosion are systematically smaller than our model predictions.
We estimate the fractional shell width (, average of all epochs
and frequencies) and the level of opacity to the radio emission by the ejecta.
We find evidence of a spectral-index radial gradient in the supernova shell,
which is indicative of a frequency-dependent ejecta opacity. Finally, we study
the distribution and evolution of the azimuthal anisotropies (hot spots) found
around the radio shell during the expansion. These anisotropies have
intensities of % of the mean flux density of the shell, and appear to
systematically evolve during the expansion.Comment: 13 pages, 9 figures, accepted for publication in A&
8.4GHz VLBI observations of SN2004et in NGC6946
We report on 8.4GHz Very Long Baseline Interferometry (VLBI) observations of
the type II-P supernova SN2004et in the spiral galaxy NGC 6946, made on 20
February 2005 (151 days after explosion). The Very Large Array (VLA) flux
density was 1.230.07 mJy, corresponding to an isotropic luminosity at
8.4GHz of (4.450.3) erg s Hz and a brightness
temperature of (1.30.3) K. We also provide an improved
source position, accurate to about 0.5 mas in each coordinate. The VLBI image
shows a clear asymmetry. From model fitting of the size of the radio emission,
we estimate a minimum expansion velocity of 15,7002,000 km s. This
velocity is more than twice the expected mean expansion velocity estimated from
a synchrotron self-absorbed emission model, thus suggesting that synchrotron
self-absorption is not relevant for this supernova. With the benefit of an
optical spectrum obtained 12 days after explosion, we favor an emission model
which consists of two hot spots on an underlying expanding shell of width
comparable to that of SN 1993J.Comment: Accepted for publication in A&A (22/05/07
Feasibility of using rural waste products to increase the denitrification efficiency in a surface flow constructed wetland
A surface flow constructed wetland (CW) was set in the Lerma gully to decrease nitrate (NO3−) pollution from agricultural runoff water. The water flow rate and NO3− concentration were monitored at the inlet and the outlet, and sampling campaigns were performed which consisted of collecting six water samples along the CW flow line. After two years of operation, the NO3− attenuation was limited at a flow rate of ~2.5 L/s and became negligible at ~5.5 L/s. The present work aimed to assess the feasibility of using rural waste products (wheat hay, corn stubble, and animal compost) to induce denitrification in the CW, to assess the effect of temperature on this process, and to trace the efficiency of the treatment by using isotopic tools. In the first stage, microcosm experiments were performed. Afterwards, the selected waste material was applied in the CW, and the treatment efficiency was evaluated by means of a chemical and isotopic characterization and using the isotopic fractionation (ε) values calculated from laboratory experiments to avoid field-scale interference. The microcosms results showed that the stubble was the most appropriate material for application in the CW, but the denitrification rate was found to decrease with temperature. In the CW, biostimulation in autumn-winter promoted NO3− attenuation between two weeks and one month (a reduction in NO3− between 1.2 and 1.5 mM was achieved). After the biostimulation in spring-summer, the attenuation was maintained for approximately three months (NO3− reduction between 0.1 and 1.5 mM). The ε15NNO3/N2 and ε18ONO3/N2 values obtained from the laboratory experiments allowed to estimate the induced denitrification percentage. At an approximate average flow rate of 16 L/s, at least 60% of NO3− attenuation was achieved in the CW. The field samples exhibited a slope of 1.0 for δ18O-NO3− versus δ15N-NO3−, similar to those of the laboratory experiments (0.9-1.2). Plant uptake seemed to play a minor role in NO3− attenuation in the CW. Hence, the application of stubble in the CW allowed the removal of large amounts of NO3− from the Lerma gully, especially when applied during the warm months, but its efficacy was limited to a short time period (up to three months)
On the coherence loss in phase-referenced VLBI observations
Context: Phase referencing is a standard calibration technique in radio
interferometry, particularly suited for the detection of weak sources close to
the sensitivity limits of the interferometers. However, effects from a changing
atmosphere and inaccuracies in the correlator model may affect the
phase-referenced images, leading to wrong estimates of source flux densities
and positions. A systematic observational study of signal decoherence in phase
referencing, and its effects in the image plane, has not been performed yet.
Aims: We systematically studied how the signal coherence in
Very-Long-Baseline-Interferometry (VLBI) observations is affected by a
phase-reference calibration at different frequencies and for different
calibrator-to-target separations. The results obtained should be of interest
for a correct interpretation of many phase-referenced observations with VLBI.
Methods: We observed a set of 13 strong sources (the S5 polar cap sample) at
8.4 and 15 GHz in phase-reference mode, with 32 different calibrator/target
combinations spanning angular separations between 1.5 and 20.5 degrees. We
obtained phase-referenced images and studied how the dynamic range and peak
flux density depend on observing frequency and source separation.
Results: We obtained dynamic ranges and peak flux densities of the
phase-referenced images as a function of frequency and separation from the
calibrator. We compared our results with models and phenomenological equations
previously reported.
Conclusions: The dynamic range of the phase-referenced images is strongly
limited by the atmosphere at all frequencies and for all source separations.
The limiting dynamic range is inversely proportional to the sine of the
calibrator-to-target separation. We also find that the peak flux densities,
relative to those obtained with the self-calibrated images, decrease with
source separation.Comment: 5 pages, 3 figures. Submitted to A&A on 5.02.2010; accepted on
11.03.2010
VLBI observations of SN 2008iz: I. Expansion velocity and limits on anisotropic expansion
We present observations of the recently discovered supernova 2008iz in M82
with the VLBI High Sensitivity Array at 22 GHz, the Very Large Array at
frequencies of 1.4, 4.8, 8.4, 22 and 43 GHz, and the Chandra X-ray observatory.
The supernova was clearly detected on two VLBI images, separated by 11 months.
The source shows a ring-like morphology and expands with a velocity of ~23000
km/s. The most likely explosion date is in mid February 2008. The measured
expansion speed is a factor of ~2 higher than expected under the assumption
that synchrotron self-absorption dominates the light curve at the peak,
indicating that this absorption mechanism may not be important for the radio
emission. We find no evidence for an asymmetric explosion. The VLA spectrum
shows a broken power law, indicating that the source was still optically thick
at 1.4 GHz in April 2009. Finally, we report upper limits on the X-ray emission
from SN 2008iz and a second radio transient recently discovered by MERLIN
observations.Comment: accepted Astronomy & Astrophysics, 9 pages, 8 figures, also available
at http://www.mpifr-bonn.mpg.de/staff/abrunthaler/pub.shtm
Absolute kinematics of radio source components in the complete S5 polar cap sample. IV. Proper motions of the radio cores over a decade and spectral properties
We have carried out a high-precision astrometric analysis of two very-long-baseline-interferometry (VLBI) epochs of observation of the 13 extragalactic radio sources in the complete S5 polar cap sample. The VLBI epochs span a time baseline of 10 years and enable us to achieve precisions in the proper motions of the source cores up to a few micro-arcseconds per year. The observations were performed at 14.4 GHz and 43.1 GHz, and enable us to estimate the frequency core-shifts in a subset of sources, for which the spectral-index distributions can be computed. We study the source-position stability by analysing the changes in the relative positions of fiducial source points (the jet cores) over a decade. We find motions of 0.1-0.9 mas among close-by sources between the two epochs, which imply drifts in the jet cores of approximately a few tens of micro-as per year. These results have implications for the standard Active Galactic Nucleus (AGN) jet model (where the core locations are supposed to be stable in time). For one of our sources, 0615+820, the morphological and spectral properties in year 2010, as well as the relative astrometry between years 2000 and 2010, suggest the possibility of either a strong parsec-scale interaction of the AGN jet with the ISM, a gravitational lens with ~1 mas diameter, or a resolved massive binary black hole
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