9,360 research outputs found
Baryon Loading of AGN Jets Mediated by Neutrons
Plasmas of geometrically thick, black hole (BH) accretion flows in active
galactic nuclei (AGNs) are generally collisionless for protons, and involve
magnetic field turbulence. Under such conditions a fraction of protons can be
accelerated stochastically and create relativistic neutrons via nuclear
collisions. These neutrons can freely escape from the accretion flow and decay
into protons in dilute polar region above the rotating BH to form relativistic
jets. We calculate geometric efficiencies of the neutron energy and mass
injections into the polar region, and show that this process can deposit
luminosity as high as L_j ~ 2e-3 dot{M} c^2 and mass loading dot{M}_j ~ 6e-4
dot{M} for the case of the BH mass M ~ 1e8 M_sun, where dot{M} is mass
accretion rate. The terminal Lorentz factors of the jets are Gamma ~ 3, and
they may explain the AGN jets having low luminosities. For higher luminosity
jets, which can be produced by additional energy inputs such as Poynting flux,
the neutron decay still can be a dominant mass loading process, leading to
e.g., Gamma ~ 50 for L_{j,tot} ~ 3e-2 dot{M}c^2.Comment: 7 pages, 6 figures; accepted for publication in Ap
Transient Emission From Dissipative Fronts in Magnetized, Relativistic Outflows. II. Synchrotron Flares
The time dependent synchrotron emission from relativistic jets, and the
relation between the synchrotron and ERC emission is considered within the
framework of the radiative front model. The timescale and profile of the
optically thin emission are shown to be determined, in this model, by the shock
formation radius, the thickness of expelled fluid slab and the variation of the
front's parameters due to its transverse expansion. For a range of reasonable
conditions, a variety of flare shapes can be produced, varying from roughly
symmetric with exponential rises and decays, as often seen in blazars, to
highly asymmetric with a fast rise and a much slower, power law decay, as seen
in GRB afterglows. The onset, duration, and fluence of low-frequency (below the
initial turnover frequency) and hard gamma-ray (above the initial gamma-spheric
energy) outbursts are limited by opacity effects; the emission at these
energies is quite generally delayed and, in the case of sufficiently short
length outbursts, severely attenuated. The observational consequences are
discussed. One distinctive prediction of this model is that in a single,
powerful source, the upper cutoff of the gamma-ray spectrum should be
correlated with the timescale of the outburst and with the amplitude of
variations at long wavelengths (typically radio to millimeter).Comment: AAS LaTex, 14 pgs, accepted to A
Probing Micro-quasars with TeV Neutrinos
The jets associated with Galactic micro-quasars are believed to be ejected by
accreting stellar mass black-holes or neutron stars. We show that if the energy
content of the jets in the transient sources is dominated by electron-proton
plasma, then a several hour outburst of 1--100 TeV neutrinos produced by photo-
meson interactions should precede the radio flares associated with major
ejection events. Several neutrinos may be detected during a single outburst by
a 1km^2 detector, thereby providing a powerful probe of micro-quasars jet
physics.Comment: Accepted to PRL. More detailed discussion of particle acceleratio
Entanglement, measurement, and conditional evolution of the Kondo singlet interacting with a mesoscopic detector
We investigate various aspects of the Kondo singlet in a quantum dot (QD)
electrostatically coupled to a mesoscopic detector. The two subsystems are
represented by an entangled state between the Kondo singlet and the
charge-dependent detector state. We show that the phase-coherence of the Kondo
singlet is destroyed in a way that is sensitive to the charge-state information
restored both in the magnitude and in the phase of the scattering coefficients
of the detector. We also introduce the notion of the `conditional evolution' of
the Kondo singlet under projective measurement on the detector. Our study
reveals that the state of the composite system is disentangled upon this
measurement. The Kondo singlet evolves into a particular state with a fixed
number of electrons in the quantum dot. Its relaxation time is shown to be
sensitive only to the QD-charge dependence of the transmission probability in
the detector, which implies that the phase information is erased in this
conditional evolution process. We discuss implications of our observations in
view of the possible experimental realization.Comment: Focus issue on "Interference in Mesoscopic Systems" of New J. Phy
Adiabatic transport in nanostructures
A confined system of non-interacting electrons, subject to the combined
effect of a time-dependent potential and different external
chemical-potentials, is considered. The current flowing through such a system
is obtained for arbitrary strengths of the modulating potential, using the
adiabatic approximation in an iterative manner. A new formula is derived for
the charge pumped through an un-biased system (all external chemical potentials
are kept at the same value); It reproduces the Brouwer formula for a
two-terminal nanostructure. The formalism presented yields the effect of the
chemical potential bias on the pumped charge on one hand, and the modification
of the Landauer formula (which gives the current in response to a constant
chemical-potential difference) brought about by the modulating potential on the
other. Corrections to the adiabatic approximation are derived and discussed.Comment: 8 pages, 2 figure
The significance of silence. Long gaps attenuate the preference for ‘yes’ responses in conversation.
In conversation, negative responses to invitations, requests, offers and the like more often occur with a delay – conversation analysts talk of them as dispreferred. Here we examine the contrastive cognitive load ‘yes’ and ‘no’ responses make, either when given relatively fast (300 ms) or delayed (1000 ms). Participants heard minidialogues, with turns extracted from a spoken corpus, while having their EEG recorded. We find that a fast ‘no’ evokes an N400-effect relative to a fast ‘yes’, however this contrast is not present for delayed responses. This shows that an immediate response is expected to be positive – but this expectation disappears as the response time lengthens because now in ordinary conversation the probability of a ‘no’ has increased. Additionally, however, 'No' responses elicit a late frontal positivity both when they are fast and when they are delayed. Thus, regardless of the latency of response, a ‘no’ response is associated with a late positivity, since a negative response is always dispreferred and may require an account. Together these results show that negative responses to social actions exact a higher cognitive load, but especially when least expected, as an immediate response
Students' Forms of Dialogue When Engaged with Contemporary Biological Research: Insights from University and High School Students' Group Discussions
Classroom dialogues have special educational value because they allow students to engage critically but constructively with each other’s ideas, solve scientific problems jointly and develop their scientific understanding. The present study focuses on how groups of twelfth-grade high school and university students communicate and co-operate through dialogue to solve a biological problem they have not encountered before. The specific research questions are as follows: (a) What are the dialogic structures that help students construct scientific explanations? (b) How does prior scientific knowledge support student dialogue in constructing explanations? A coding scheme was developed inductively for the analysis of participants’ utterances. We use illustrative exemplars from participants’ dialogues to discuss those aspects which might support explanatory reasoning. We focus on reasoned attention for contending opinions and striving for consensus that characterise cases of constructive dialogue. We also discuss observed objections and disagreements as triggering factors for constructive alternative explanations. Finally, we discuss the evidence showing that while prior knowledge supports student reasoning it can also hinder the ability of students to think in a creative way
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