878 research outputs found
From Prosumer to âProdesignerâ: Participatory News Consumption
New democratic participation forms and collaborative productions of diverse audiences have emerged as a result of digital innovations in the online access to and consumption of news. The aim of this paper is to propose a conceptual framework based on the possibilities of Web 2.0. Outlining the construction of a âsocial logicâ, which combines computer and communicative logics, the conceptual framework is theoretically built to explore the evolution of news consumption from a pure circulation of designed products towards a global conversation of proactive news designers. Then, the framework was tested using an empirical database built by the Pew Research Centre, which investigates the future of the news industry, through a large-scale survey with adults. Results show significant differences (by age, gender and educational level) in the forms of participation, access and consumption of news. However, whilst immersed in the culture of Web 2.0 there is a low-level of user participation in news production; far from being proactive news designers, findings suggest that citizens still located in the lower participatory levels of our conceptual framework. Conclusions suggest there is a need for media education providers to carry out training initiatives according to the social logic possibilities through proposed guidelines
21cm Absorption by Compact Hydrogen Disks Around Black Holes in Radio-Loud Nuclei of Galaxies
The clumpy maser disks observed in some galactic nuclei mark the outskirts of
the accretion disk that fuels the central black hole and provide a potential
site of nuclear star formation. Unfortunately, most of the gas in maser disks
is currently not being probed; large maser gains favor paths that are
characterized by a small velocity gradient and require rare edge-on
orientations of the disk. Here we propose a method for mapping the atomic
hydrogen distribution in nuclear disks through its 21cm absorption against the
radio continuum glow around the central black hole. In NGC 4258, the 21cm
optical depth may approach unity for high angular-resolution (VLBI) imaging of
coherent clumps which are dominated by thermal broadening and have the column
density inferred from X-ray absorption data, ~10^{23}/cm^2. Spreading the 21cm
absorption over the full rotation velocity width of the material in front of
the narrow radio jets gives a mean optical depth of ~0.1. Spectroscopic
searches for the 21cm absorption feature in other galaxies can be used to
identify the large population of inclined gaseous disks which are not masing in
our direction. Follow-up imaging of 21cm silhouettes of accelerating clumps
within these disks can in turn be used to measure cosmological distances.Comment: 4 page
High Angular Resolution Mid-infrared Imaging of Young Stars in Orion BN/KL
We present Keck LWS images of the Orion BN/KL star forming region obtained in
the first multi-wavelength study to have 0.3-0.5" resolution from 4.7 to 22
microns. The young stellar objects designated infrared source-n and radio
source-I are believed to dominate the BN/KL region. We have detected extended
emission from a probable accretion disk around source-n but infer a stellar
luminosity on the order of only 2000 Lsun. Although source-I is believed to be
more luminous, we do not detect an infrared counterpart even at the longest
wavelengths. However, we resolve the closeby infrared source, IRc2, into an arc
of knots ~1000 AU long at all wavelengths. Although the physical relation of
source-I to IRc2 remains ambiguous, we suggest these sources mark a high
density core (10^7-10^8 pc^-3 over 1000 AU) within the larger BN/KL star
forming cluster. The high density may be a consequence of the core being young
and heavily embedded. We suggest the energetics of the BN/KL region may be
dominated by this cluster core rather than one or two individual sources.Comment: 13 pages including 3 color figures. Accepted to The Astrophysical
Journal Letters pending slight reduction in length. High resolution figures
(jpeg) may be found at
http://cfa-www.harvard.edu/~lincoln/keck.bnkl.midir.ppr
A Technique for Narrowband Time Series Photometry: the X-ray Star V2116 Oph
We have used innovative features of the Taurus Tunable Filter instrument on
the 3.9-m Anglo-Australian Telescope to obtain nearly-continuous,
high-throughput, linear photometry of V2116 Oph in a 7 Angstrom bandpass at the
center of the O I 8446 emission line. This instrumental technique shows promise
for applications requiring precise, rapid, narrowband photometry of faint
objects. The spectrum of V2116 Oph, the counterpart of GX 1+4 (=X1728-247), is
exotic, even among the unusual spectra of other optical counterparts of compact
Galactic X-ray sources. The second strongest emission line is an unusual one,
namely extremely prominent O I 8446, which is likely to result from pumping by
an intense Ly beta radiation field. As the X-radiation from GX 1+4 is steadily
pulsed, with typical pulsed fractions of 0.4, the O I 8446 emission in V2116
Oph may also be strongly modulated with the current 127 s period of the X-ray
source. If so, this may well allow us to obtain high signal-to-noise radial
velocity measurements and thus to determine the system parameters. However, no
such pulsations are detected, and we set an upper limit of ~1% (full-amplitude)
on periodic 8446 oscillations at the X-ray frequency. This value is comparable
to the amplitude of continuum oscillations observed on some nights by other
workers. Thus we rule out an enhancement of the pulsation amplitude in O I
emission, at least at the time of our observations.Comment: 9 pages including 4 figures and no tables. Accepted for publication
in PASP; to appear in Volume 110, August 199
A 4% Geometric Distance to the Galaxy NGC4258 from Orbital Motions in a Nuclear Gas Disk
The water maser in the mildly active nucleus in the nearby galaxy NGC4258
traces a thin, nearly edge-on, subparsec-scale Keplerian disk. Using the
technique of very long baseline interferometry, we have detected the proper
motions of these masers as they sweep in front of the central black hole at an
orbital velocity of about 1100 km/s. The average maser proper motion of 31.5
microarcseconds per year is used in conjunction with the observed acceleration
of the masers to derive a purely geometric distance to the galaxy of 7.2 +- 0.3
Mpc. This is the most precise extragalactic distance measured to date, and,
being independent of all other distance indicators, is likely to play an
important role in calibrating the extragalactic distance scale.Comment: 11 pages, 3 figures. Accepted for publication in Natur
Antenna beam characterisation for the global 21cm experiment LEDA and its impact on signal model parameter reconstruction
Cosmic Dawn, the onset of star formation in the early universe, can in
principle be studied via the 21cm transition of neutral hydrogen, for which a
sky-averaged absorption signal, redshifted to MHz frequencies, is predicted to
be {\it O}(10-100)\,mK. Detection requires separation of the 21cm signal from
bright chromatic foreground emission due to Galactic structure, and the
characterisation of how it couples to instrumental response. In this work, we
present characterisation of antenna gain patterns for the Large-aperture
Experiment to detect the Dark Ages (LEDA) via simulations, assessing the
effects of the antenna ground-plane geometries used, and measured soil
properties. We then investigate the impact of beam pattern uncertainties on the
reconstruction of a Gaussian absorption feature. Assuming the pattern is known
and correcting for the chromaticity of the instrument, the foregrounds can be
modelled with a log-polynomial, and the 21cm signal identified with high
accuracy. However, uncertainties on the soil properties lead to
\textperthousand\ changes in the chromaticity that can bias the signal
recovery. The bias can be up to a factor of two in amplitude and up to few \%
in the frequency location. These effects do not appear to be mitigated by
larger ground planes, conversely gain patterns with larger ground planes
exhibit more complex frequency structure, significantly compromising the
parameter reconstruction. Our results, consistent with findings from other
antenna design studies, emphasise the importance of chromatic response and
suggest caution in assuming log-polynomial foreground models in global signal
experiments.Comment: Accepted for publication in MNRA
Proper Motion of Water Masers Associated with IRAS 21391+5802: Bipolar Outflow and an AU-Scale Dusty Circumstellar Shell
We present VLBA observations of water maser emission associated with the
star-forming region IRAS 21391+5802, which is embedded in a bright rimmed
cometary globule in IC1396. The angular resolution of the maps is about 0.8
mas, corresponding to a spatial resolution of about 0.6 AU, at an estimated
distance of 750 pc. Proper motions are derived for 10 maser features identified
consistently over three epochs, which were separated by intervals of about one
month. The masers appear in four groups, which are aligned linearly on the sky,
roughly along a northeast-southwest direction, with a total separation of about
520 AU (about 0.7 arcseconds). The 3-D velocities of the masers have a maximum
value of about 42 km/s (about 9 AU/yr). The average error on the derived proper
motions is about 4 km/s. The overall pattern of proper motions is indicative of
a bipolar outflow. Proper motions of the masers in a central cluster, with a
projected extent of about 20 AU, show systematic deviations from a radial
outflow. However, we find no evidence of Keplerian rotation, as has been
claimed elsewhere. A nearly circular loop of masers lies near the middle of the
cluster. The radius of this loop is 1 AU and the line-of-sight velocities of
the masers in the loop are within 2 km/s of the systemic velocity of the
region. These masers presumably exist at the radial distance where significant
dust condensation occurs in the outflow emanating from the star.Comment: 15 pages, 4 figures. Accepted for publication in the Astrophysical
Journal. Version 2.12.00: Astrometric coordinates of maser revise
Radio Continuum Jet in NGC 7479
The barred galaxy NGC 7479 hosts a remarkable jet-like radio continuum
feature: bright, 12-kpc long in projection, and hosting an aligned magnetic
field. The degree of polarization is 6%-8% along the jet, and remarkably
constant, which is consistent with helical field models. The radio brightness
of the jet suggests strong interaction with the ISM and hence a location near
the disk plane. We observed NGC 7479 at four wavelengths with the VLA and
Effelsberg radio telescopes. The equipartition strength is 35-40 micro-G for
the total and >10 micro-G for the ordered magnetic field in the jet. The jet
acts as a bright, polarized background. Faraday rotation between 3.5 and 6 cm
and depolarization between 6 and 22 cm can be explained by magneto-ionic gas in
front of the jet, with thermal electron densities of ~0.06 cm**(-3) in the bar
and ~0.03 cm**(-3) outside the bar. The regular magnetic field along the bar
points toward the nucleus on both sides. The regular field in the disk reveals
multiple reversals, probably consisting of field loops stretched by a shearing
gas flow in the bar. The projection of the jet bending in the sky plane is in
the sense opposite to that of the underlying stellar and gaseous spiral
structure. The bending in 3-D is most easily explained as a precessing jet,
with an age less than 10**6 years. Our observations are consistent with very
recent triggering, possibly by a minor merger. NGC 7479 provides a unique
opportunity to study interaction-triggered 15-kpc scale radio jets within a
spiral galaxy.Comment: 18 pages, 21 figures, accepted for publication in the Astrophysical
Journa
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