9,017 research outputs found
Characteristics of EGRET Blazars in the VLBA Imaging and Polarimetry Survey (VIPS)
We examine the radio properties of EGRET-detected blazars observed as part of
the VLBA Imaging and Polarimetry Survey (VIPS). VIPS has a flux limit roughly
an order of magnitude below the MOJAVE survey and most other samples that have
been used to study the properties of EGRET blazars. At lower flux levels, radio
flux density does not directly correlate with gamma-ray flux density. We do
find that the EGRET-detected blazars tend to have higher brightness
temperatures, greater core fractions, and possibly larger than average jet
opening angles. A weak correlation is also found with jet length and with
polarization. All of the well-established trends can be explained by
systematically larger Doppler factors in the gamma-ray loud blazars, consistent
with the measurements of higher apparent velocities found in monitoring
programs carried out at radio frequencies above 10 GHz.Comment: 20 pages, 7 figures, accepted to Ap
An excess of emission in the dark cloud LDN 1111 with the Arcminute Microkelvin Imager
We present observations of the Lynds' dark nebula LDN 1111 made at microwave
frequencies between 14.6 and 17.2 GHz with the Arcminute Microkelvin Imager
(AMI). We find emission in this frequency band in excess of a thermal
free--free spectrum extrapolated from data at 1.4 GHz with matched uv-coverage.
This excess is > 15 sigma above the predicted emission. We fit the measured
spectrum using the spinning dust model of Drain & Lazarian (1998a) and find the
best fitting model parameters agree well with those derived from Scuba data for
this object by Visser et al. (2001).Comment: accepted MNRA
Circular Polarization Induced by Scintillation in a Magnetized Medium
A new theory is presented for the development of circular polarization as
radio waves propagate through the turbulent, birefringent interstellar medium.
The fourth order moments of the wavefield are calculated and it is shown that
unpolarized incident radiation develops a nonzero variance in circular
polarization. A magnetized turbulent medium causes the Stokes parameters to
scintillate in a non-identical manner. A specific model for this effect is
developed for the case of density fluctuations in a uniform magnetic field.Comment: 16 pages, 1 figure, Phys. Rev. E, accepte
The IDV source J1128+5925, a new candidate for annual modulation?
Short time-scale radio variations of compact extragalactic radio sources,
known as IntraDay Variability, can be explained in at least some sources by a
source-extrinsic effect, in which the variations are interpreted as
scintillation of radio waves caused by the turbulent ISM of the Milky Way. One
of the most convincing observational arguments in favour of propagation-induced
variability is the so called annual modulation of the characteristic
variability time-scale, which is due to the orbital motion of the Earth. Data
for the recently discovered and highly variable IDV source J1128+5925 are
presented. We study the frequency and time dependence of the IDV in this
compact quasar. We measure the characteristic variability time-scale of the IDV
throughout the year, and analyze whether the observed changes in the
variability time-scale are consistent with annual modulation. We monitored the
flux density variability of J1128+5925 with dense time sampling between 2.7 and
10.45GHz with the 100m Effelsberg radio telescope of the MPIfR and with the 25m
Urumqi radio telescope. From ten observing sessions, we determine the
variability characteristics and time-scales. The observed pronounced changes of
the variability time-scale of J1128+5925 are modelled with an anisotropic
annual modulation model. The observed frequency dependence of the variation is
in good agreement with the prediction from interstellar scintillation. Adopting
a simple model for the annual modulation model and using also the frequency
dependence of the IDV, we derive a lower limit to the distance of the
scattering screen and an upper limit to the scintillating source size. The
latter is found to be consistent with the measured core size from VLBI.Comment: 15 pages, 9 figures Accepted for publication in Astronomy and
Astrophysic
VLBA Imaging Polarimetry of Active Galactic Nuclei - An Automated Approach
We present full polarization Very Long Baseline Array (VLBA) observations at
5 GHz and 15 GHz of 24 compact active galactic nuclei (AGN). These sources were
observed as part of a pilot project to demonstrate the feasibility of
conducting a large VLBI survey to further our understanding of the physical
properties and temporal evolution of AGN jets. The sample is drawn from the
Cosmic Lens All-Sky Survey (CLASS) where it overlaps with the Sloan Digital Sky
Survey at declinations north of 15 degrees. There are 2100 CLASS sources
brighter than 50 mJy at 8.4 GHz, of which we have chosen 24 for this pilot
study. All 24 sources were detected and imaged at 5 GHz with a typical dynamic
range of 500:1, and 21 of 24 sources were detected and imaged at 15 GHz. Linear
polarization was detected in 8 sources at both 5 and 15 GHz, allowing for the
creation of Faraday rotation measure (RM) images. The core RMs for the sample
were found to have an average absolute value of 390 +/- 100 rad/m^2. We also
present the discovery of a new Compact Symmetric Object, J08553+5751. All data
were processed automatically using pipelines created or adapted for the survey.Comment: 25 pages, 3 figures, 4 tables, accepted to the Astrophysical Journal
Supplement Series Full resolution figures and more available from
http://www.aoc.nrao.edu/~gtaylor/VIP
Genome Evolution of Wolbachia Strain wPip from the Culex pipiens Group
The obligate intracellular bacterium Wolbachia pipientis strain wPip induces cytoplasmic incompatibility (CI), patterns of crossing sterility, in the Culex pipiens group of mosquitoes. The complete sequence is presented of the 1.48-Mbp genome of wPip which encodes 1386 coding sequences (CDSs), representing the first genome sequence of a B-supergroup Wolbachia. Comparisons were made with the smaller genomes of Wolbachia strains wMel of Drosophila melanogaster, an A-supergroup Wolbachia that is also a CI inducer, and wBm, a mutualist of Brugia malayi nematodes that belongs to the D-supergroup of Wolbachia. Despite extensive gene order rearrangement, a core set of Wolbachia genes shared between the 3 genomes can be identified and contrasts with a flexible gene pool where rapid evolution has taken place. There are much more extensive prophage and ankyrin repeat encoding (ANK) gene components of the wPip genome compared with wMel and wBm, and both are likely to be of considerable importance in wPip biology. Five WO-Bâlike prophage regions are present and contain some genes that are identical or highly similar in multiple prophage copies, whereas other genes are unique, and it is likely that extensive recombination, duplication, and insertion have occurred between copies. A much larger number of genes encode ankyrin repeat (ANK) proteins in wPip, with 60 present compared with 23 in wMel, many of which are within or close to the prophage regions. It is likely that this pattern is partly a result of expansions in the wPip lineage, due for example to gene duplication, but their presence is in some cases more ancient. The wPip genome underlines the considerable evolutionary flexibility of Wolbachia, providing clear evidence for the rapid evolution of ANK-encoding genes and of prophage regions. This hostâWolbachia system, with its complex patterns of sterility induced between populations, now provides an excellent model for unraveling the molecular systems underlying host reproductive manipulation
âItâs my dream to work with Olympic athletesâ: Neophyte sport psychologistsâ expectations and initial experiences regarding service delivery
We examined trainee practitioners' initial experiences of applied sport psychology practice. Semi-structured interviews (4) were conducted over 6 months with 7 full-time MSc students before, during, and after the applied sport psychology module, when they were working with clients. Participants also kept reflective diaries over an 8-week period whilst working with clients. Findings included: (a) motivations and expectations of an ASP practice career, (b) perceptions of service delivery, (c) emotional demands, and (d) pivotal experiences. Findings extend previous literature on the initial stages of practitioner development, providing micro-level detail on aspects of the intense development process during this pivotal perio
AMI observations of Lynds Dark Nebulae: further evidence for anomalous cm-wave emission
Observations at 14.2 to 17.9 GHz made with the AMI Small Array towards
fourteen Lynds Dark Nebulae with a resolution of 2' are reported. These sources
are selected from the SCUBA observations of Visser et al. (2001) as small
angular diameter clouds well matched to the synthesized beam of the AMI Small
Array. Comparison of the AMI observations with radio observations at lower
frequencies with matched uv-plane coverage is made, in order to search for any
anomalous excess emission which can be attributed to spinning dust. Possible
emission from spinning dust is identified as a source within a 2' radius of the
Scuba position of the Lynds dark nebula, exhibiting an excess with respect to
lower frequency radio emission. We find five sources which show a possible
spinning dust component in their spectra. These sources have rising spectral
indices in the frequency range 14.2--17.9 GHz. Of these five one has already
been reported, L1111, we report one new definite detection, L675, and three new
probable detections (L944, L1103 and L1246). The relative certainty of these
detections is assessed on the basis of three criteria: the extent of the
emission, the coincidence of the emission with the Scuba position and the
likelihood of alternative explanations for the excess. Extended microwave
emission makes the likelihood of the anomalous emission arising as a
consequence of a radio counterpart to a protostar or a proto-planetary disk
unlikely. We use a 2' radius in order to be consistent with the IRAS
identifications of dark nebulae (Parker 1988), and our third criterion is used
in the case of L1103 where a high flux density at 850 microns relative to the
FIR data suggests a more complicated emission spectrum.Comment: submitted MNRA
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