2,580 research outputs found
Anatomy of helical relativistic jets: The case of S5 0836+710
Helical structures are common in extragalactic jets. They are usually
attributed in the literature to periodical phenomena in the source (e.g.,
precession). In this work, we use VLBI data of the radio-jet in the quasar S5
0836+710 and hypothesize that the ridge-line of helical jets like this
corresponds to a pressure maximum in the jet and assume that the helically
twisted pressure maximum is the result of a helical wave pattern. For our
study, we use observations of the jet in S5 0836+710 at different frequencies
and epochs. The results show that the structures observed are physical and not
generated artificially by the observing arrays. Our hypothesis that the
observed intensity ridge-line can correspond to a helically twisted pressure
maximum is confirmed by our observational tests. This interpretation allows us
to explain jet misalignment between parsec and kiloparsec scales when the
viewing angle is small, and also brings us to the conclusion that
high-frequency observations may show only a small region of the jet flow
concentrated around the maximum pressure ridge-line observed at low
frequencies. Our work provides a potential explanation for the apparent
transversal superluminal speeds observed in several extragalactic jets by means
of transversal shift of an apparent core position with time.Comment: Accepted for publication in the Astrophysical Journa
On the nature of an ejection event in the jet of 3C111
We present a possible scenario for the ejection of a superluminal component
in the jet of the Broad Line Radio Galaxy 3C111 in early 1996. VLBI
observations at 15 GHz discovered the presence of two jet features on scales
smaller than one parsec. The first component evolves downstream, whereas the
second one fades out after 1 parsec. We propose the injection of a perturbation
of dense material followed by a decrease in the injection rate of material in
the jet as a plausible explanation. This scenario is supported by 1D
relativistic hydrodynamics and emission simulations. The perturbation is
modeled as an increase in the jet density, without modifying the original
Lorentz factor in the initial conditions. We show that an increase of the
Lorentz factor in the material of the perturbation fails to reproduce the
observed evolution of this flare. We are able to estimate the lifetime of the
ejection event in 3C111 to be 36\pm7 days.Comment: Accepted for publication in Astronomy & Astrophysics Letter
An infrared study of galactic OH/IR stars. I. An optical/near-IR atlas of the Arecibo sample
In this paper we present optical and near-infrared finding charts, accurate
astrometry (~1") and single-epoch near-infrared photometry for 371 IRAS
sources, 96% of those included in the so-called Arecibo sample of OH/IR stars
(Eder et al. 1988; Lewis et al. 1990a; Chengalur et al. 1993). The main
photometric properties of the stars in the sample are presented and discussed
as well as the problems found during the process of identification of the
optical/near-infrared counterparts. In addition, we also identify suitable
reference stars in each field to be used for differential photometry purposes
in the future.
We find that 39% of the sources (144 in number) have no optical counterpart,
8 of them being invisible even at near infrared wavelengths. The relative
distribution of sources with and without optical counterpart in the IRAS
two-colour diagram and their characteristic near infrared colours are
interpreted as the consequence of the increasing thickness of their
circumstellar shells. Among the objects not detected at near infrared
wavelengths four non-variable sources are proposed to be heavily obscured
post-AGB stars which have just very recently left the AGB. Eight additional
objects with unusually bright and/or blue near-infrared colours are identified
as candidate post-AGB stars and/or proto-planetary nebulae.Comment: 28 pages, 9 figures, for associated finding charts see:
http://www.edpsciences.org/articles/aa/full/2005/08/aa1709/FINDING_CHARTS/are
cibo_index.htm
Ultrasonic triggering of giant magnetocaloric effect in MnAs thin films
Mechanical control of magnetic properties in magnetostrictive thin films
offers the unexplored opportunity to employ surface wave acoustics in such a
way that acoustic triggers dynamic magnetic effects. The strain-induced
modulation of the magnetic anisotropy can play the role of a high frequency
varying effective magnetic field leading to ultrasonic tuning of electronic and
magnetic properties of nanostructured materials, eventually integrated in
semiconductor technology. Here, we report about the opportunity to employ
surface acoustic waves to trigger magnetocaloric effect in
MnAs(100nm)/GaAs(001) thin films. During the MnAs magnetostructural phase
transition, in an interval range around room temperature (0{\deg}C -
60{\deg}C), ultrasonic waves (170 MHz) are strongly attenuated by the phase
coexistence (up to 150 dB/cm). We show that the giant magnetocaloric effect of
MnAs is responsible of the observed phenomenon. By a simple anelastic model we
describe the temperature and the external magnetic field dependence of such a
huge ultrasound attenuation. Strain-manipulation of the magnetocaloric effect
could be a further interesting route for dynamic and static caloritronics and
spintronics applications in semiconductor technology
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THE EFFECT OF PARENTAL ENGAGEMENT ON TREATMENT AND ACADEMIC OUTCOMES AMONG LATINO STUDENTS RECEIVING SCHOOL BASED MENTAL HEALTH SERVICES
Current data reports approximately 46% of children ages 13-18 are affected by a mental health issue. Of those, 21% will experience a serious mental health disorder throughout their life. Due to the high percentage of school aged children that are affected by mental health disorders, school based mental health services have grown in popularity. Despite the positive attributes associated with providing mental health treatment in a school setting, various factors including race and levels of parental engagement have been identified as barriers to successful implementation. This study sought to identify the effects of parental engagement on the treatment, academic, and behavioral outcomes of Latino students receiving school based mental health services. Furthermore, the study utilized a quantitative approach by reviewing available archival and survey data. Findings were analyzed and themes were drawn for the purpose of identifying additional areas of research and ultimately improving the delivery of services
Rapid TeV variability in Blazars as result of Jet-Star Interaction
We propose a new model for the description of ultra-short flares from TeV
blazars by compact magnetized condensations (blobs), produced when red giant
stars cross the jet close to the central black hole. Our study includes a
simple dynamical model for the evolution of the envelope lost by the star in
the jet, and its high energy nonthermal emission through different leptonic and
hadronic radiation mechanisms. We show that the fragmented envelope of the star
can be accelerated to Lorentz factors up to 100 and radiate effectively the
available energy in gamma-rays predominantly through proton synchrotron
radiation or external inverse Compton scattering of electrons. The model can
readily explain the minute-scale TeV flares on top of longer (typical
time-scales of days) gamma-ray variability as observed from the blazar PKS
2155-304. In the framework of the proposed scenario, the key parameters of the
source are robustly constrained. In the case of proton synchrotron origin of
the emission a mass of the central black hole of , a total jet power of and a Doppler factor, of the gamma-ray emitting blobs, of
are required. Whilst for the external inverse Compton model,
parameters of
, and the are required.Comment: 25 pages, 11 figures, Submitted to Ap
The connection between radio and high energy emission in black hole powered systems in the SKA era
Strong evidence exists for a highly significant correlation between the radio
flux density and gamma-ray energy flux in blazars revealed by Fermi. However,
there are central issues that need to be clarified in this field: what are the
counterparts of the about 30% of gamma-ray sources that are as yet
unidentified? Are they just blazars in disguise or they are something more
exotic, possibly associated with dark matter? How would they fit in the
radio-gamma ray connection studied so far?
With their superb sensitivity, SKA1-MID and SKA1-SUR will help to resolve all
of these questions. Even more, while the radio-MeV/GeV connection has been
firmly established, a radio-VHE connection has been entirely elusive so far.
The advent of CTA in the next few years and the expected CTA-SKA1 synergy will
offer the chance to explore this connection, even more intriguing as it
involves the opposite ends of the electromagnetic spectrum and the acceleration
of particles up to the highest energies.
We are already preparing to address these questions by exploiting data from
the various SKA pathfinders and precursors. We have obtained 18 cm European
VLBI Network observations of E>10 GeV sources, with a detection rate of 83%.
Moreover, we are cross correlating the Fermi catalogs with the MWA
commissioning survey: when faint gamma-ray sources are considered, pure
positional coincidence is not significant enough for selecting counterparts and
we need an additional physical criterion to pinpoint the right object. It can
be radio spectral index, variability, polarization, or compactness, needing
high angular resolution in SKA1-MID; timing studies can also reveal pulsars,
which are often found from dedicated searches of unidentified gamma-ray
sources. SKA will be the ideal instrument for investigating these
characteristics in conjunction with CTA.
(abridged)Comment: 12 pages, to be published in the proceedings of "Advancing
Astrophysics with the Square Kilometre Array", PoS(AASKA14)15
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