36,448 research outputs found
A Shotgun Model for Gamma Ray Bursts
We propose that gamma ray bursts (GRBs) are produced by a shower of heavy
blobs running into circumstellar material at highly relativistic speeds. The
gamma ray emission is produced in the shocks these bullets drive into the
surrounding medium. The short term variability seen in GRBs is set by the
slowing-down time of the bullets while the overall duration of the burst is set
by the lifetime of the central engine. A requirement of this model is that the
ambient medium be dense, consistent with a strong stellar wind. The efficiency
of the burst can be relatively high.Comment: 4 pages, 2 figures, revised version accepted by ApJ Letter
Photon Bubbles and the Vertical Structure of Accretion Disks
We consider the effects of "photon bubble" shock trains on the vertical
structure of radiation pressure-dominated accretion disks. These density
inhomogeneities are expected to develop spontaneously in radiation-dominated
accretion disks where magnetic pressure exceeds gas pressure, even in the
presence of magnetorotational instability. They increase the rate at which
radiation escapes from the disk, and may allow disks to exceed the Eddington
limit by a substantial factor. We first generalize the theory of photon bubbles
to include the effects of finite optical depths and radiation damping.
Modifications to the diffusion law at low optical depth tend to fill in the
low-density regions of photon bubbles, while radiation damping inhibits the
formation of photon bubbles at large radii, small accretion rates, and small
heights above the equatorial plane. Accretion disks dominated by photon bubble
transport may reach luminosities of 10 to >100 times the Eddington limit (L_E),
depending on the mass of the central object, while remaining geometrically
thin. However, photon bubble-dominated disks with alpha-viscosity are subject
to the same thermal and viscous instabilities that plague standard radiation
pressure-dominated disks, suggesting that they may be intrinsically unsteady.
Photon bubbles can lead to a "core-halo" vertical disk structure. In
super-Eddington disks the halo forms the base of a wind, which carries away
substantial energy and mass, but not enough to prevent the luminosity from
exceeding L_E. Photon bubble-dominated disks may have smaller color corrections
than standard accretion disks of the same luminosity. They remain viable
contenders for some ultraluminous X-ray sources and may play a role in the
rapid growth of supermassive black holes at high redshift.Comment: 38 pages, 2 figures, accepted for publication in The Astrophysical
Journa
Radiatively-Driven Outflows and Avoidance of Common-Envelope Evolution in Close Binaries
Recent work on Cygnus X-2 suggests that neutron-star or black-hole binaries
survive highly super-Eddington mass transfer rates without undergoing
common-envelope evolution. We suggest here that the accretion flows in such
cases are radiation pressure-dominated versions of the "ADIOS" picture proposed
by Blandford and Begelman (1999), in which almost all the mass is expelled from
large radii in the accretion disk. We estimate the maximum radius from which
mass loss is likely to occur, and show that common-envelope evolution is
probably avoided in any binary in which a main-sequence donor transfers mass on
a thermal timescale to a neutron star or black hole, even though the mass
transfer rate may reach values of 0.001 solar masses per year. This conclusion
probably applies also to donors expanding across the Hertzsprung gap, provided
that their envelopes are radiative. SS433 may be an example of a system in this
state.Comment: 4 pages, submitted to Astrophysical Journal Letters, 26 March 199
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An Examination of Motivation to Change and Neural Alcohol Cue Reactivity Following a Brief Intervention.
Background: Brief interventions represent a promising psychological intervention targeting individuals with heavy alcohol use. Motivation to change represents an individual's openness to engage in a behavior change strategy and is thought to be a crucial component of brief interventions. Neuroimaging techniques provide a translational tool to investigate the neurobiological mechanisms underlying potential mediators of treatment response, including motivation to change. Therefore, this study aimed to examine the effect of a brief intervention on motivation to change drinking behavior and neural alcohol taste cue reactivity. Methods: Non-treatment-seeking heavy drinkers were randomized to receive a brief drinking intervention (n = 22) or an attention-matched control (n = 24). Three indices of motivation to change were assessed at baseline and after the intervention or control session: importance, confidence, and readiness. Immediately following the intervention or control session, participants also underwent an functional magnetic resonance imaging (fMRI) during which they completed an alcohol taste cues paradigm. Results: There was a significant effect of the brief intervention on increasing ratings of importance of changing drinking behavior, but not on ratings of confidence or readiness to change. Ratings of importance after the intervention or control session were associated with neural alcohol taste cue reactivity, but notably, this effect was only significant for participants who received the intervention. Individuals in the intervention condition showed a positive association between ratings of importance and activation in the precuneus, posterior cingulate, and insula. Conclusions: The brief drinking intervention was successful at improving one dimension of motivation to change among non-treatment-seeking heavy drinkers. The brief intervention moderated the relationship between ratings of importance and brain activation in circuitry associated with interoceptive awareness and self-reflection. Together, findings represent an initial step toward understanding the neurobiological mechanisms through which a brief intervention may improve motivation to change
Discovery of a transient radiation belt at Saturn
Radiation belts have been detected in situ at five planets. Only at Earth however has any variability in their intensity been heretofore observed, in indirect response to solar eruptions and high altitude nuclear explosions. The Cassini spacecraft's MIMI/LEMMS instrument has now detected systematic radiation belt variability elsewhere. We report three sudden increases in energetic ion intensity around Saturn, in the vicinity of the moons Dione and Tethys, each lasting for several weeks, in response to interplanetary events caused by solar eruptions. However, the intensifications, which could create temporary satellite atmospheres at the aforementioned moons, were sharply restricted outside the orbit of Tethys. Unlike Earth, Saturn has almost unchanging inner ion radiation belts: due to Saturn's near-symmetrical magnetic field, Tethys and Dione inhibit inward radial transport of energetic ions, shielding the planet's main, inner radiation belt from solar wind influences
The Clash of the Paradigms: Tracking, Cooperative Learning, and the Demolition of the Zone of Proximal Development
This report describes the effect of cooperative learning in low-track and regular classrooms, using the dimensions of student alienation, academic self-esteem, willingness to cooperate, and academic supportiveness. The investigators examined the influence of student agency in choosing peers for the cooperative groups in a high school science classroom. Results indicate that the ability for low-track students to choose group mates can result in greater feelings of alienation, have no effect on bolstering a lower academic self-esteem, and reduce willingness to cooperate with peers. Regardless of stream, students did not feel academically supported by peers in the classroom. Recommendations for using cooperative learning and coping with educational reform are discussed.Ce rapport décrit l'effet de l'apprentissage coopératif dans des salles de classe d'éducation spécialisée et des salles de classe régulières et ce, sous l'optique de la marginalisation des élèves, l'estime de soi sur le plan académique, la volonté de coopérer et le soutien académique. Les chercheurs ont examiné l'influence de l'action par les étudiants dans le choix de pairs pour les groupes coopératifs dans un cours de science à l'école secondaire. Les résultats indiquent que la capacité qu'ont les élèves de la voie éducation spécialisée à choisir des partenaires de groupe peut entrainer les effets suivants : augmentation du sentiment de marginalisation; aucun rehaussement du faible estime de soi sur le plan académique; et diminution de la volonté de collaborer avec les pairs. Les élèves dans les deux voies ne sentaient pas d'appui académique de la part de leurs pairs en classe. Nous discutons de recommandations pour l'emploi de l'apprentissage coopératif et pour la gestion de la réforme de l'enseignement
Imaging of the equatorial ionosphere
The equatorial anomaly is the dominant structure in free electron concentration in the tropical ionosphere. Due
to its edges (crests) which are characterised by steep latitudinal gradients in TEC and are temporally and spatially
variable, it is one of the ionospheric regions most difficult to image with inversion methods. In this paper
we reconstruct an International Reference Ionosphere model of the equatorial ionosphere by employing a grid
of virtual ground GPS receivers and actual GPS satellite positions. The MIDAS algorithm, an inversion method
for reconstructing the ionosphere as a linear composition of given empirical orthogonal functions, is used. Comparing
the true model ionosphere with the resulting images a fine tuning of the basis functions (vertical profile
contraints) in the inversion is realised
Super-Eddington Atmospheres that Don't Blow Away
We show that magnetized, radiation dominated atmospheres can support steady
state patterns of density inhomogeneity that enable them to radiate at far
above the Eddington limit, without suffering mass loss. The inhomogeneities
consist of periodic shock fronts bounding narrow, high-density regions,
interspersed with much broader regions of low density. The radiation flux
avoids the regions of high density, which are therefore weighed down by
gravity, while gas in the low-density regions is slammed upward into the shock
fronts by radiation force. As the wave pattern moves through the atmosphere,
each parcel of matter alternately experiences upward and downward forces, which
balance on average. Magnetic tension shares the competing forces between
regions of different densities, preventing the atmosphere from blowing apart.
We calculate the density structure and phase speed of the wave pattern, and
relate these to the wavelength, the density contrast, and the factor by which
the net radiation flux exceeds the Eddington limit. In principle, this factor
can be as large as the ratio of magnetic pressure to mean gas pressure, or the
ratio of radiation pressure to gas pressure, whichever is smaller. Although the
magnetic pressure must be large compared to the mean gas pressure in order to
support a large density contrast, it need not be large compared to the
radiation pressure. These highly inhomogeneous flows could represent the
nonlinear development of the "photon bubble" instability discovered by Gammie.
We briefly discuss the applicability of these solutions to astrophysical
systems.Comment: 11 pages, 1 figure, accepted for publication in The Astrophysical
Journa
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