3,454 research outputs found
Envisioning the Future of Mosaic Landscapes:Actor Perceptions in a Mixed Cocoa/Oil-Palm Area in Ghana
Kinematics of parsec-scale structures in AGN: the 2cm VLBA Survey
We are investigating the kinematics of jets in active galactic nuclei on
parsec scales by studying a representative population of sources. This study is
being carried out using the Very Long Baseline Array at 15 GHz, with more than
800 images taken since 1994. In this contribution we present an overview of the
diversity of kinematics for a complete sample of sources.Comment: Proceedings of the 6th European VLBI Network Symposium, Ros E.,
Porcas R.W., Lobanov, A.P., & Zensus, J.A. (eds), MPIfR, Bonn, Germany. 2
pages, 3 figures, needs evn2002.cls style fil
Intrinsic Brightness Temperatures of AGN Jets
We present a new method for studying the intrinsic brightness temperatures of
the parsec-scale jet cores of Active Galactic Nuclei (AGN). Our method uses
observed superluminal motions and observed brightness temperatures for a large
sample of AGN to constrain the characteristic intrinsic brightness temperature
of the sample as a whole. To study changes in intrinsic brightness temperature,
we assume that the Doppler factors of individual jets are constant in time as
justified by their relatively small changes in observed flux density. We find
that in their median-low brightness temperature state, the sources in our
sample have a narrow range of intrinsic brightness temperatures centered on a
characteristic temperature, T_int = 3 x 10^10 K, which is close to the value
expected for equipartition, when the energy in the radiating particles equals
the energy stored in the magnetic fields. However, in their maximum brightness
state, we find that sources in our sample have a characteristic intrinsic
brightness temperature greater than 2 x 10^11 K, which is well in excess of the
equipartition temperature. In this state, we estimate the energy in radiating
particles exceeds the energy in the magnetic field by a factor of ~ 10^5. We
suggest that the excess of particle energy when sources are in their maximum
brightness state is due to injection or acceleration of particles at the base
of the jet. Our results suggest that the common method of estimating jet
Doppler factors by using a single measurement of observed brightness
temperature and/or the assumption of equipartition may lead to large scatter or
systematic errors in the derived values.Comment: 4 pages, 2 figures, Accepted to Appear in ApJ Letter
Variability and Velocity of Superluminal Sources
We investigate the relation between the Doppler factor determined from
variations in total flux at 22 and 37 GHz, and the apparent transverse velocity
determined from VLBA observations at 2 cm. The data are consistent with the
relativistic beaming theory for compact radio sources, in that the distribution
of beta_{app}/delta_{var}, for 30 quasars, is roughly consistent with a Monte
Carlo simulation. The intrinsic temperature appears to be ~2x10^{10} K, close
to the "equipartition value" calculated by Readhead (1994). We deduce the
distribution of Lorentz factors for a group of 48 sources; the values range up
to about gamma=40.Comment: To be published in "Radio Astronomy at the Fringe", ASP Conf. Ser.
Vol. 300, J. A. Zensus, M. H. Cohen, & E. Ros (eds.), 8 pages, 3 figures,
needs rafringe.st
The EVN view of the highly variable TeV active galaxy IC 310
Very-high-energy -ray observations of the active galaxy IC 310 with
the MAGIC telescopes have revealed fast variability with doubling time scales
of less than 4.8min. This implies that the emission region in IC 310 is smaller
than 20% of the gravitational radius of the central supermassive black hole
with a mass of , which poses serious questions on the
emission mechanism and classification of this enigmatic object. We report on
the first quasi-simultaneous multi-frequency VLBI observations of IC 310
conducted with the EVN. We find a blazar-like one-sided core-jet structure on
parsec scales, constraining the inclination angle to be less than but very small angles are excluded to limit the de-projected length
of the large-scale radio jet.Comment: 4 pages, proceedings of the 12th European VLBI Network Symposium and
Users Meeting - EVN 2014, 7-10 October 2014, Cagliari, Italy. Published
online in PoS, ID.10
An intrinsic timer specifies distal structures of the vertebrate limb
How the positional values along the proximo-distal axis (stylopod-zeugopod-autopod) of the limb are specified is intensely debated. Early work suggested that cells intrinsically change their proximo-distal positional values by measuring time. Recently, however, it is suggested that instructive extrinsic signals from the trunk and apical ectodermal ridge specify the stylopod and zeugopod/autopod, respectively. Here, we show that the zeugopod and autopod are specified by an intrinsic timing mechanism. By grafting green fluorescent protein-expressing cells from early to late chick wing buds, we demonstrate that distal mesenchyme cells intrinsically time Hoxa13 expression, cell cycle parameters and the duration of the overlying apical ectodermal ridge. In addition, we reveal that cell affinities intrinsically change in the distal mesenchyme, which we suggest results in a gradient of positional values along the proximo-distal axis. We propose a complete model in which a switch from extrinsic signalling to intrinsic timing patterns the vertebrate limb
1.6 GHz VLBI Observations of SN 1979C: almost-free expansion
We report on 1.6 GHz Very-Long-Baseline-Interferometry (VLBI) observations of
supernova SN 1979C made on 18 November 2002. We derive a model-dependent
supernova size. We also present a reanalysis of VLBI observations made by us on
June 1999 and by other authors on February 2005. We conclude that, contrary to
our earlier claim of strong deceleration in the expansion, SN 1979C has been
undergoing almost-free expansion (; ) for over
25 years.Comment: 4 pages, 4 figures; submitted to A&A on 14 May 2009. Accepted on 7
Jul 200
Water sorption and diffusion in cellulose acetate: The effect of plasticisers
The conservation of cellulose acetate plastics in museum collections presents a significant challenge, due to the material's instability. Several studies have led to an understanding of the role of relative humidity (RH) and temperature in the decay process. It is well established that the first decay mechanism in cellulose acetate museum objects is the loss of plasticiser, and that the main decay mechanism of the polymer chain involves hydrolysis reactions. This leads to the loss of sidechain groups and the breakdown of the main polymer backbone. However, interactions between these decay mechanisms, specifically the way in which the loss of plasticiser can modify the interaction between cellulose acetate and water, has not yet been investigated. This research addresses the role of RH, studying the sorption and diffusion of water in cellulose acetate and how this interaction can be affected by plasticiser concentration using Dynamic Vapour Sorption (DVS)
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