862 research outputs found
Aerodynamic challenges of ALT
The approach and landing test (ALT) of the Space Shuttle Orbiter presented a number of unique challenges in the area of aerodynamics. The purpose of the ALT program was both to confirm the use of the Boeing 747 as a transport vehicle for ferrying the Orbiter across the country and to demonstrate the flight characteristics of the Orbiter in its approach and landing phase. Concerns for structural fatigue and performance dictated a tailcone be attached to the Orbiter for ferry and for the initial landing tests. The Orbiter with a tailcone attached presented additional challenges to the normal aft sting concept of wind tunnel testing. The landing tests required that the Orbiter be separated from the 747 at approximately 20,000 feet using aerodynamic forces to fly the vehicles apart. The concept required a complex test program to determine the relative effects of the two vehicles on each other. Also of concern, and tested, was the vortex wake created by the 747 and the means for the Orbiter to avoid it following separation
MOJAVE: Monitoring of Jets in Active Galactic Nuclei with VLBA Experiments. VI. Kinematics Analysis of a Complete Sample of Blazar Jets
We discuss the jet kinematics of a complete flux-density-limited sample of
135 radio-loud active galactic nuclei (AGN) resulting from a 13 year program to
investigate the structure and evolution of parsec-scale jet phenomena. Our
analysis is based on new 2 cm Very Long Baseline Array (VLBA) images obtained
between 2002 and 2007, but includes our previously published observations made
at the same wavelength, and is supplemented by VLBA archive data. In all, we
have used 2424 images spanning the years 1994-2007 to study and determine the
motions of 526 separate jet features in 127 jets. The data quality and temporal
coverage (a median of 15 epochs per source) of this complete AGN jet sample
represents a significant advance over previous kinematics surveys. In all but
five AGNs, the jets appear one-sided, most likely the result of differential
Doppler boosting. In general the observed motions are directed along the jet
ridge line, outward from the optically thick core feature. We directly observe
changes in speed and/or direction in one third of the well-sampled jet
components in our survey. While there is some spread in the apparent speeds of
separate features within an individual jet, the dispersion is about three times
smaller than the overall dispersion of speeds among all jets. This supports the
idea that there is a characteristic flow that describes each jet, which we have
characterized by the fastest observed component speed. The observed maximum
speed distribution is peaked at ~10c, with a tail that extends out to ~50c.
This requires a distribution of intrinsic Lorentz factors in the parent
population that range up to ~50. We also note the presence of some rare
low-pattern speeds or even stationary features in otherwise rapidly flowing
jets... (abridged)Comment: 19 pages, 10 figures, 2 tables, accepted by the Astronomical Journal;
online only material is available from
http://www.cv.nrao.edu/2cmVLBA/pub/MOJAVE_VI_suppl.zi
MOJAVE: Monitoring of Jets in AGN with VLBA Experiments. VII. Blazar Jet Acceleration
We discuss acceleration measurements for a large sample of extragalactic
radio jets from the MOJAVE program which studies the parsec-scale jet structure
and kinematics of a complete, flux-density-limited sample of Active Galactic
Nuclei (AGN). Accelerations are measured from the apparent motion of individual
jet features or "components" which may represent patterns in the jet flow. We
find that significant accelerations are common both parallel and perpendicular
to the observed component velocities. Parallel accelerations, representing
changes in apparent speed, are generally larger than perpendicular acceleration
that represent changes in apparent direction. The trend for larger parallel
accelerations indicates that a significant fraction of these changes in
apparent speed are due to changes in intrinsic speed of the component rather
than changes in direction to the line of sight. We find an overall tendency for
components with increasing apparent speed to be closer to the base of their
jets than components with decreasing apparent speed. This suggests a link
between the observed pattern motions and the underlying flow which, in some
cases, may increase in speed close to the base and decrease in speed further
out; however, common hydro-dynamical processes for propagating shocks may also
play a role. About half of the components show "non-radial" motion, or a
misalignment between the component's structural position angle and its velocity
direction, and these misalignments generally better align the component motion
with the downstream emission. Perpendicular accelerations are closely linked
with non-radial motion. When observed together, perpendicular accelerations are
usually in the correct direction to have caused the observed misalignment.Comment: 17 pages, 11 figures, 1 table, accepted by the Astrophysical Journa
Full-Stokes polarimetry with circularly polarized feeds - Sources with stable linear and circular polarization in the GHz regime
We present a pipeline that allows recovering reliable information for all
four Stokes parameters with high accuracy. Its novelty relies on the treatment
of the instrumental effects already prior to the computation of the Stokes
parameters contrary to conventional methods, such as the M\"uller matrix one.
The instrumental linear polarization is corrected across the whole telescope
beam and significant Stokes and can be recovered even when the recorded
signals are severely corrupted. The accuracy we reach in terms of polarization
degree is of the order of 0.1-0.2 %. The polarization angles are determined
with an accuracy of almost 1. The presented methodology was applied
to recover the linear and circular polarization of around 150 Active Galactic
Nuclei. The sources were monitored from July 2010 to April 2016 with the
Effelsberg 100-m telescope at 4.85 GHz and 8.35 GHz with a cadence of around
1.2 months. The polarized emission of the Moon was used to calibrate the
polarization angle. Our analysis showed a small system-induced rotation of
about 1 at both observing frequencies. Finally, we identify five
sources with significant and stable linear polarization; three sources remain
constantly linearly unpolarized over the period we examined; a total of 11
sources have stable circular polarization degree and four of
them with non-zero . We also identify eight sources that maintain
a stable polarization angle over the examined period. All this is provided to
the community for polarization observations reference. We finally show that our
analysis method is conceptually different from the traditionally used ones and
performs better than the M\"uller matrix method. Although it was developed for
a system equipped with circularly polarized feeds it can easily be modified for
systems with linearly polarized feeds as well.Comment: 19 pages, 17 figures, accepted for publication in Astronomy &
Astrophysics on May 30, 201
Examining the divergent effects of perceived inclusion of ethnic minorities on majority and minority groups’ inter-ethnic responses
This study examines the paradoxical effects of a perceived inclusive environment for ethnic minorities. We argue that while perceptions of an inclusive environment may be associated with more positive intergroup attitudes and affect among minority groups, they may instill a sense of threat among the majority group, resulting in negative intergroup sentiments and attitudes towards minorities. We analyzed data from two waves of a nationally representative survey conducted in the Netherlands (ntotal = 11,897) comprising minority and majority groups. We find support for the proposed paradoxical relationship between the perceived inclusionary climate towards minorities and the attitudes of the majority and minority groups. The results indicate that when perceiving the national climate to be more inclusive towards minorities, the majority group tends to report higher levels of ethnocentrism, avoid direct inter-ethnic contact, and oppose ethnic diversity in general. Among minority groups, a perceived inclusive climate is linked to lower levels of ethnocentrism and a higher willingness to engage in inter-ethnic interactions with the majority group. The results unexpectedly also show that the perception of an inclusionary climate is positively related to opposition to increased ethnic diversity among minority groups. We discuss theoretical and societal implications, while also considering the contextual relevance and limitations of our approach.</p
A transient high-coherence oscillation in 4U 1820-30
We re-analyzed two Rossi X-Ray Timing Explorer archival observations of the
atoll source 4U 1820-30 in order to investigate the detailed time-frequency
properties of the source during the intervals when a ~7 Hz QPO was detected by
Wijnands et al. (1999, ApJ, 512, L39). We find that in both observations, in
addition to a QPO signal lasting a couple of minutes as previously reported,
there is a much narrower transient oscillation with a life time of only a few
seconds. Within this time, the oscillation is consistent with being coherent.
Its integrated fractional rms is around 10% and its frequency 7.3 Hz and 5.7 Hz
in the two observations. We discuss the possible association of this QPO with
other oscillations known both in Neutron-Star and Black-Hole systems,
concentrating on the similarities with the narrow 5-7 Hz oscillations observed
at high flux in Black-Hole Candidates.Comment: 5 pages, 3 figures, accepted for publication in A&A. FIgure 1 is
reduced in resolution, full-resolution version of this text available at
http://www.merate.mi.astro.it/~belloni/ms0335.ps.g
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
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
Evidence for a Link Between Fe K-alpha Emission Line Strength and QPO Phase in a Black Hole
In X-ray binaries, the frequencies revealed in X-ray quasi-periodic
oscillations (QPOs) are often interpreted as characteristic frequencies in the
inner accretion disk, though the exact oscillation mechanism is unknown at
present. Broadened Fe K-alpha lines are also excellent diagnostics of the inner
accretion flow, if their broadening is indeed due to inner disk reflection.
Herein, we present two cases where the flux and equivalent width of the Fe
K-alpha emission lines in spectra of the Galactic black hole GRS 1915+105 vary
with the phase of strong 1 Hz and 2 Hz QPOs in the X-ray flux. These results
provide strong evidence that both QPOs and the Fe-alpha lines originate in the
inner disk. If the 1 Hz QPO is only a Keplerian orbital frequency, the QPO
comes from a distance of 84 +/- 26 R_Schw from the black hole; the 2 Hz QPO
implies a radius of 50 +/- 15 R_Schw. At these radii, relativistic shaping of a
disk line is inevitable. Moreover, the link holds in radio-bright and
radio-faint phases, signaling that in systems like GRS 1915+105, the Fe K-alpha
line is a disk line and not a jet line as per SS 433. A particularly
interesting possibility is that a stable warp in the inner disk, e.g. due to
Lense-Thirring precession, may produce the observed QPOs and line modulations.
More broadly, the FeK-QPO link provides an unprecedented mechanism for
revealing the inner accretion flow and relativistic regime in accreting
systems, in that it gives two measures of radius: for a given disk QPO model,
the frequency translates into a specific radius, and relativistic line models
yield radii directly.Comment: 4 pages, 4 color figures, accepted for publication in ApJ Letters.
For a related animation see
http://www.nasa.gov/centers/goddard/universe/blackhole_surfing.htm
Wind morphology around cool evolved stars in binaries: the case of slowly accelerating oxygen-rich outflows
The late stellar evolutionary phases of low and intermediate-mass stars are
strongly constrained by their mass-loss rates. The wind surrounding cool
evolved stars frequently shows non-spherical features, thought to be due to an
unseen companion orbiting the donor star. We study the morphology of the
circumbinary envelope, in particular around oxygen-rich asymptotic giant branch
(AGB) stars. We run a grid of 70 3D hydrodynamics simulations of a
progressively accelerating wind propagating in the Roche potential formed by a
mass-loosing evolved star in orbit with a main sequence companion. We resolve
the flow structure both in the immediate vicinity of the secondary, where bow
shocks, outflows and wind-captured disks form, and up to 40 orbital
separations, where spiral arms, arcs and equatorial density enhancements
develop. When the companion is deeply engulfed in the wind, the lower terminal
wind speeds and more progressive wind acceleration around oxygen-rich AGB stars
make them more prone than carbon-rich AGB stars to display more disturbed
outflows, a disk-like structure around the companion and a wind concentrated in
the orbital plane. In these configurations, a large fraction of the wind is
captured by the companion which leads to a significant shrinking of the orbit
over the mass-loss timescale, if the donor star is at least a few times more
massive than its companion. Provided the companion has a mass of at least a
tenth of the mass of the donor star, it can compress the wind in the orbital
plane up to large distances. Our grid of models covers a wide scope of
configurations function of the dust chemical content, the terminal wind speed
relative to the orbital speed, the extension of the dust condensation region
around the cool evolved star and the mass ratio. It provides a frame of
reference to interpret high-resolution maps of the outflows surrounding cool
evolved stars
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