807 research outputs found
The late time radio emission from SN 1993J at meter wavelengths
We present the investigations of SN 1993J using low frequency observations
with the Giant Meterwave Radio Telescope. We analyze the light curves of SN
1993J at 1420, 610, 325 and 243 MHz during years since explosion.The
supernova has become optically thin early on in the 1420 MHz and 610 MHz bands
while it has only recently entered the optically thin phase in the 325 MHz
band. The radio light curve in the 235 MHz band is more or less flat. This
indicates that the supernova is undergoing a transition from an optically thick
to optically thin limit in this frequency band. In addition, we analyze the SN
radio spectra at five epochs on day 3000, 3200, 3266, 3460 and 3730 since
explosion. Day 3200 spectrum shows a synchrotron cooling break. SN 1993J is the
only young supernova for which the magnetic field and the size of the radio
emitting region are determined through unrelated methods. Thus the mechanism
that controls the evolution of the radio spectra can be identified. We suggest
that at all epochs, the synchrotron self absorption mechanism is primarily
responsible for the turn-over in the spectra. Light curve models based on free
free absorption in homogeneous or inhomogeneous media at high frequencies
overpredict the flux densities at low frequencies. The discrepancy is
increasingly larger at lower and lower frequencies. We suggest that an extra
opacity, sensitively dependent on frequency, is likely to account for the
difference at lower frequencies. The evolution of the magnetic field
(determined from synchrotron self absorption turn-over) is roughly consistent
with . Radio spectral index in the optically thin part
evolves from at few tens of days to in about
10 years.Comment: 37 pages, 9 figures in LaTex; scheduled for ApJ 10 September 2004,
v612 issue; send comments to: [email protected]
Photometric Variability and Astrometric Stability of the Radio Continuum Nucleus in the Seyfert Galaxy NGC 5548
The NRAO VLA and VLBA were used from 1988 November to 1998 June to monitor
the radio continuum counterpart to the optical broad line region (BLR) in the
Seyfert galaxy NGC 5548. Photometric and astrometric observations were obtained
at 21 epochs. The radio nucleus appeared resolved, so comparisons were limited
to observations spanning 10-60 days and 3-4 yr, and acquired at matched
resolutions of 1210 mas = 640 pc (9 VLA observations), 500 mas = 260 pc (9 VLA
observations), or 2.3 mas = 1.2 pc (3 VLBA observations). The nucleus is
photometrically variable at 8.4 GHz by % and % between VLA
observations separated by 41 days and 4.1 yr, respectively. The 41-day changes
are milder (%) at 4.9 GHz and exhibit an inverted spectrum (, ) from 4.9 to 8.4 GHz. The nucleus
is astrometrically stable at 8.4 GHz, to an accuracy of 28 mas = 15 pc between
VLA observations separated by 4.1 yr and to an accuracy of 1.8 mas = 0.95 pc
between VLBA observations separated by 3.1 yr. Such photometric variability and
astrometric stability is consistent with a black hole being the ultimate energy
source for the BLR, but is problematic for star cluster models that treat the
BLR as a compact supernova remnant and, for NGC 5548, require a new supernova
event every 1.7 yr within an effective radius 42 mas = 22 pc. A deep
image at 8.4 GHz with resolution 660 mas = 350 pc was obtained by adding data
from quiescent VLA observations. This image shows faint bipolar lobes
straddling the radio nucleus and spanning 12 arcsec = 6.4 kpc. These
synchrotron-emitting lobes could be driven by twin jets or a bipolar wind from
the Seyfert 1 nucleus.Comment: with 9 figures, to appear in the Astrophysical Journal, 2000 March
10, volume 53
Evidence for Proportionate Partition Between the Magnetic Field and Hot Gas in Turbulent Cassiopeia A
We present a deep X-ray observation of the young Galactic supernova remnant
Cas A, acquired with the ROSAT High Resolution Imager. This high dynamic range
(232 ks) image reveals low-surface-brightness X-ray structure, which appears
qualitatively similar to corresponding radio features. We consider the
correlation between the X-ray and radio morphologies and its physical
implications. After correcting for the inhomogeneous absorption across the
remnant, we performed a point by point (4" resolution) surface brightness
comparison between the X-ray and radio images. We find a strong (r = 0.75)
log-log correlation, implying an overall relationship of . This is
consistent with proportionate partition (and possibly equipartition) between
the local magnetic field and the hot gas --- implying that Cas A's plasma is
fully turbulent and continuously amplifying the magnetic field.Comment: 8 pages with embedded bitmapped figures, Accepted by ApJ Letters
5/1/9
SN 1986J VLBI. The Evolution and Deceleration of the Complex Source and a Search for a Pulsar Nebula
We report on VLBI observations of supernova 1986J in the spiral galaxy NGC
891 at two new epochs, 1990 July and 1999 February, t=7.4 and 15.9 yr after the
explosion, and on a comprehensive analysis of these and earlier observations
from t~4 yr after the explosion date, which we estimate to be 1983.2 +/- 1.1.
The source is a shell or composite, and continues to show a complex morphology
with large brightness modulations along the ridge and with protrusions. The
supernova is moderately to strongly decelerated. The average outer radius
expands as t^(0.71 +/- 0.11), and the expansion velocity has slowed to 6000
km/s at t=15.9 yr from an extrapolated 20,000 km/s at t=0.25 yr. The structure
changes significantly with time, showing that the evolution is not
self-similar. The shell structure is best visible at the latest epoch, when the
protrusions have diminished somewhat in prominence and a new, compact component
has appeared. The radio spectrum shows a clear inversion above 10 GHz. This
might be related to a pulsar nebula becoming visible through the debris of the
explosion. The radio flux density between 1.5 and 23 GHz decreases strongly
with time, with the flux density proportional to t^(-2.94 +/- 0.24) between
t~15 to 19 yr. This decrease is much more rapid than that found in earlier
measurements up to t~6 yr.Comment: 24 pages, 9 Figures, LaTeX Accepted for Publication in the
Astrophysical Journa
Correlated radio--X-ray variability of Galactic Black Holes: A radio--X-ray flare in Cygnus X-1
We report on the first detection of a quasi-simultaneous radio-X-ray flare of
Cygnus X-1. The detection was made on 2005 April 16 with pointed observations
by the Rossi X-ray Timing Explorer and the Ryle telescope, during a phase where
the black hole candidate was close to a transition from the its soft into its
hard state. The radio flare lagged the X-rays by approximately 7 minutes,
peaking at 3:20 hours barycentric time (TDB 2453476.63864). We discuss this lag
in the context of models explaining such flaring events as the ejection of
electron bubbles emitting synchrotron radiation.Comment: 4 pages, 4 figure
EARLINET Single Calculus Chain – overview on methodology and strategy
In this paper we describe the EARLINET Single Calculus Chain (SCC), a tool for
the automatic analysis of lidar measurements. The development of this
tool started in the framework of EARLINET-ASOS (European Aerosol
Research Lidar Network – Advanced Sustainable Observation System); it was extended within ACTRIS (Aerosol, Clouds and
Trace gases Research InfraStructure Network), and it is continuing within
ACTRIS-2. The main idea was to develop a data processing chain that allows all
EARLINET stations to retrieve, in a fully automatic way, the aerosol
backscatter and extinction profiles starting from the raw lidar data
of the lidar systems they operate. The calculus subsystem of the SCC
is composed of two modules: a pre-processor module which handles the
raw lidar data and corrects them for instrumental effects and an optical
processing module for the retrieval of aerosol optical products from
the pre-processed data. All input parameters needed to perform the
lidar analysis are stored in a database to keep track of all changes which may occur for any
EARLINET lidar system over the time. The two calculus modules are coordinated and synchronized by an additional module (daemon)
which makes the whole analysis process fully automatic. The end user
can interact with the SCC via a user-friendly web interface.
All SCC modules are developed using open-source and freely available
software packages.
The final products retrieved by the SCC fulfill all requirements of the EARLINET quality assurance
programs on both instrumental and algorithm levels. Moreover, the
manpower needed to provide aerosol optical products is greatly reduced
and thus the near-real-time availability of lidar data is improved.
The high-quality of the SCC products is proven by the good
agreement between the SCC analysis, and the corresponding independent manual
retrievals. Finally, the ability of the SCC to provide high-quality
aerosol optical products is demonstrated for an EARLINET intense
observation period
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Depolarization and lidar ratios at 355, 532, and 1064 nm and microphysical properties of aged tropospheric and stratospheric Canadian wildfire smoke
We present spectrally resolved optical and microphysical properties of western Canadian wildfire smoke observed in a tropospheric layer from 5-6.5 km height and in a stratospheric layer from 15-16 km height during a recordbreaking smoke event on 22 August 2017. Three polarization/ Raman lidars were run at the European Aerosol Research Lidar Network (EARLINET) station of Leipzig, Germany, after sunset on 22 August. For the first time, the linear depolarization ratio and extinction-to-backscatter ratio (lidar ratio) of aged smoke particles were measured at all three important lidar wavelengths of 355, 532, and 1064 nm. Very different particle depolarization ratios were found in the troposphere and in the stratosphere. The obviously compact and spherical tropospheric smoke particles caused almost no depolarization of backscattered laser radiation at all three wavelengths ( 500nm). The stratospheric smoke particles formed a pronounced accumulation mode (in terms of particle volume or mass) centered at a particle radius of 350-400 nm. The effective particle radius was 0.32 μm. The tropospheric smoke particles were much smaller (effective radius of 0.17 μm). Mass concentrations were of the order of 5.5 μgm-3 (tropospheric layer) and 40 μgm-3 (stratospheric layer) in the night of 22 August 2017. The single scattering albedo of the stratospheric particles was estimated to be 0.74, 0.8, and 0.83 at 355, 532, and 1064 nm, respectively
VLBI for Gravity Probe B. VII. The Evolution of the Radio Structure of IM Pegasi
We present measurements of the total radio flux density as well as
very-long-baseline interferometry (VLBI) images of the star, IM Pegasi, which
was used as the guide star for the NASA/Stanford relativity mission Gravity
Probe B. We obtained flux densities and images from 35 sessions of observations
at 8.4 GHz (wavelength = 3.6 cm) between 1997 January and 2005 July. The
observations were accurately phase-referenced to several extragalactic
reference sources, and we present the images in a star-centered frame, aligned
by the position of the star as derived from our fits to its orbital motion,
parallax, and proper motion. Both the flux density and the morphology of IM Peg
are variable. For most sessions, the emission region has a single-peaked
structure, but 25% of the time, we observed a two-peaked (and on one occasion
perhaps a three-peaked) structure. On average, the emission region is elongated
by 1.4 +- 0.4 mas (FWHM), with the average direction of elongation being close
to that of the sky projection of the orbit normal. The average length of the
emission region is approximately equal to the diameter of the primary star. No
significant correlation with the orbital phase is found for either the flux
density or the direction of elongation, and no preference for any particular
longitude on the star is shown by the emission region.Comment: Accepted for publication in the Astrophysical Journal Supplement
Serie
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Partial report is the wrong paradigm
Is consciousness independent of the general-purpose information processes known as ‘cognitive access’? The dominantmethodology for supporting this independence hypothesis appeals to partial report experiments as evidence for perceptual consciousness in the absence of cognitive access. Using a standard model of evidential support, and reviewing recent elaborations of the partial report paradigm, this article argues that the paradigm has the wrong structure to support the independence hypothesis. Like reports in general, a subject’s partial report is evidence that she is conscious of information only where that information is cognitively accessed. So partial report experiments could dissociate consciousness from cognitive access only if there were uncontroversial evidence forconsciousness which did not imply reportability. There is no such evidence. An alternative, broadly Marrian methodology for supporting the independence hypothesis is suggested, and some challenges to it outlined. This methodology does not require experimental evidence for consciousness in the absence of cognitive access. Instead it focuses on a function of perceptual consciousness when a stimulus is cognitively accessed. If the processes best suited to implement this function exclude cognitive access, the independence hypothesis will be supported. One relevant function of consciousness may be reflected in reason-based psychological explanations of a subject’s behaviour
Symbiotic Ocean Modeling Using Physics-Controlled Echo State Networks
We introduce a “symbiotic” ocean modeling strategy that leverages data-driven and machine learning methods to allow high- and low-resolution dynamical models to mutually benefit from each other. In this work we mainly focus on how a low-resolution model can be enhanced within a symbiotic model configuration. The broader aim is to enhance the representation of unresolved processes in low-resolution models, while simultaneously improving the efficiency of high-resolution models. To achieve this, we use a grid-switching approach together with hybrid modeling techniques that combine linear regression-based methods with nonlinear echo state networks. The approach is applied to both the Kuramoto–Sivashinsky equation and a single-layer quasi-geostrophic ocean model, and shown to simulate short-term and long-term behavior better than either purely data-based methods or low-resolution models. By maintaining key flow characteristics, the hybrid modeling techniques are also able to provide higher quality initial conditions for high-resolution models, thereby improving their efficiency.</p
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