70 research outputs found
Intraday variability of AGNs in the southern hemisphere
Understanding of the spectral and polarimetric characteristics of rapidly scintillating blazars is fundamental in order to describe both the innermost (sub-pc) regions of these compact objects and the interstellar medium responsible for the scintillation. A multi frequency analysis of the intraday variability in PMN J1326-5256, based on the combination of Australia Telescope Compact Array observations with the data from the monitoring projects at the University of Tasmania, will be described. Some implications concerning the structure of compact radio cores and the properties of the interstellar medium will be discussed
A Compact Extreme Scattering Event Cloud Towards AO 0235+164
We present observations of a rare, rapid, high amplitude Extreme Scattering
Event toward the compact BL-Lac AO 0235+164 at 6.65 GHz. The ESE cloud is
compact; we estimate its diameter between 0.09 and 0.9 AU, and is at a distance
of less than 3.6 kpc. Limits on the angular extent of the ESE cloud imply a
minimum cloud electron density of ~ 4 x 10^3 cm^-3. Based on the amplitude and
timescale of the ESE observed here, we suggest that at least one of the
transients reported by Bower et al. (2007) may be attributed to ESEs.Comment: 11 pages, 2 figure
The Micro-Arcsecond Scintillation-Induced Variability (MASIV) Survey II: The First Four Epochs
We report on the variability of 443 flat spectrum, compact radio sources
monitored using the VLA for 3 days in 4 epochs at ~ 4 month intervals at 5 GHz
as part of the Micro-Arcsecond Scintillation-Induced Variability (MASIV)
survey. Over half of these sources exhibited 2-10% rms variations on timescales
over 2 days. We analyzed the variations by two independent methods, and find
that the rms variability amplitudes of the sources correlate with the emission
measure in the ionized Interstellar Medium along their respective lines of
sight. We thus link the variations with interstellar scintillation of
components of these sources, with some (unknown) fraction of the total flux
density contained within a compact region of angular diameter in the range
10-50 micro-arcseconds. We also find that the variations decrease for high mean
flux density sources and, most importantly, for high redshift sources. The
decrease in variability is probably due either to an increase in the apparent
diameter of the source, or a decrease in the flux density of the compact
fraction beyond z ~ 2. Here we present a statistical analysis of these results,
and a future paper will the discuss the cosmological implications in detail.Comment: 62 pages, 13 figures. Accepted for publication in the Astrophysical
Journa
Decrease in the orbital period of dwarf nova OY Carinae
We have measured the orbital light curve of dwarf nova OY Carinae on 8
separate occasions between 1997 September and 2005 December. The measurements
were made in white light using CCD photometers on the Mt Canopus 1 m telescope.
The time of eclipse in 2005 December was 168 +- 5 s earlier than that predicted
by the Wood et al.(1989) ephemeris. Using the times of eclipse from our
measurements and the compilation of published measurements by Pratt et al
(1999) we find that the observational data are inconsistent with a constant
period and indicate that the orbital period is decreasing by 5+-1 X 10^-12 s/s.
This is too fast to be explained by gravitational radiation emission. It is
possible that the change is cyclic with a period greater than about 80 years.
This is much longer than typical magnetic activity cycles and may be due to the
presence of a third object in the system. Preliminary estimates suggest that
this is a brown dwarf with mass about 0.016 Msun and orbital radius >= 17 AU.Comment: 4 pages 2 figures. MNRAS submitted Final proofread version.
Discussion modified with figure showing fits and residuals to models,
statistical significance of fits added and minor typographical edit
Scintillation Surveys, Serendipitous, Systematic and MASIV: What do they tell us
A variety of surveys, both serendipitous and systematic, have revealed the dramatic phenomenon of cm-wavelength refractive inter-stellar scintillation. Throughout these discoveries, the presence of accurate and reliable flux density measurements has been an essential component of progress, as have the various surveys both serendipitous and systemati
Interstellar scintillation, AGN physics and the SKA
A large fraction of compact, extragalactic radio sources exhibit rapid variability at centimetre wavelengths as their radio emission is scattered by electron density fluctuations in the interstellar medium of the Galaxy. Next-generation wide-field radio telescopes will have to account for this in forming deep images of the radio sky. Interstellar scintillation offers a unique probe of very small-scale structure in both the ionized interstellar medium and the compact jets of the radio sources themselves. The effective resolution is two orders of magnitude higher than achievable with very long baseline interferometry. The recent Micro-Arcsecond Scintillation-Induced Variability Survey revealed a reduction in ISS at 4.9 GHz with increasing source redshift, implying either an increase in the apparent angular size of high-redshift radio cores beyond that expected due to a cosmological decrease in brightness, or a decrease in the microarcsecond-scale core dominance towards high redshift. The result could be due either to source-intrinsic evolution in the selected sample, or to scatter-broadening in the intergalactic medium
100 Microarcsecond Resolution VLBI Imaging of Anisotropic Interstellar Scattering towards Pulsar B0834+06
We have invented a novel technique to measure the radio image of a pulsar
scattered by the interstellar plasma with 0.1 mas resolution. We extend the
"secondary spectrum" analysis of parabolic arcs by Stinebring et al. (2001) to
very long baseline interferometry and, when the scattering is anisotropic, we
are able to map the scattered brightness astrometrically with much higher
resolution than the diffractive limit of the interferometer. We employ this
technique to measure an extremely anisotropic scattered image of the pulsar
B0834+06 at 327 MHz. We find that the scattering occurs in a compact region
about 420 pc from the Earth. This image has two components, both essentially
linear and nearly parallel. The primary feature, which is about 16 AU long and
less than 0.5 AU in width, is highly inhomogeneous on spatial scales as small
as 0.05 AU. The second feature is much fainter and is displaced from the axis
of the primary feature by about 9 AU. We find that the velocity of the
scattering plasma is 16+-10 km/s approximately parallel to the axis of the
linear feature. The origin of the observed anisotropy is unclear and we discuss
two very different models. It could be, as has been assumed in earlier work,
that the turbulence on spatial scales of ~1000 km is homogeneous but
anisotropic. However it may be that the turbulence on these scales is
homogeneous and isotropic but the anisotropy is produced by highly elongated
(filamentary) inhomogeneities of scale 0.05-16 AU.Comment: 18 pages, 7 figures, accepted for publication in Astrophysical
Journa
The Allen Telescope Array Pi GHz Sky Survey I. Survey Description and Static Catalog Results for the Bootes Field
The Pi GHz Sky Survey (PiGSS) is a key project of the Allen Telescope Array.
PiGSS is a 3.1 GHz survey of radio continuum emission in the extragalactic sky
with an emphasis on synoptic observations that measure the static and
time-variable properties of the sky. During the 2.5-year campaign, PiGSS will
twice observe ~250,000 radio sources in the 10,000 deg^2 region of the sky with
b > 30 deg to an rms sensitivity of ~1 mJy. Additionally, sub-regions of the
sky will be observed multiple times to characterize variability on time scales
of days to years. We present here observations of a 10 deg^2 region in the
Bootes constellation overlapping the NOAO Deep Wide Field Survey field. The
PiGSS image was constructed from 75 daily observations distributed over a
4-month period and has an rms flux density between 200 and 250 microJy. This
represents a deeper image by a factor of 4 to 8 than we will achieve over the
entire 10,000 deg^2. We provide flux densities, source sizes, and spectral
indices for the 425 sources detected in the image. We identify ~100$ new flat
spectrum radio sources; we project that when completed PiGSS will identify 10^4
flat spectrum sources. We identify one source that is a possible transient
radio source. This survey provides new limits on faint radio transients and
variables with characteristic durations of months.Comment: Accepted for publication in ApJ; revision submitted with extraneous
figure remove
Morphological analysis of cerium oxide stabilized nanoporous gold catalysts by soft X-ray ASAXS
Nanoporous (np) gold is a promising catalyst material for selective oxidation reactions. Especially the addition of oxide deposits like ceria (CeO2) promises enhanced morphological stability for high temperature applications. Describing such temperature induced morphological changes in porous materials is challenging. Here, X-ray nanoanalysis is particularly promising due to the high penetration depth that allows studying of the bulk properties with high spatial sensitivity. We applied soft X-ray small angle scattering (SAXS) to determine temperature induced structural changes in nanoporous gold catalysts. The results show that CeO2 deposits enhance the temperature stability of the nanoporous gold catalyst. Moreover, we demonstrate the ability of soft X-rays to selectively provide structural information on the stabilizing cerium oxide deposits via resonant, anomalous SAXS (ASAXS) measurements at the cerium M-edge, revealing no growth of the ceria particles
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