7,579 research outputs found
Hadronic Gamma Rays from Supernova Remnants
A gas cloud near a supernova remnant (SNR) provides a target for
pp-collisions leading to subsequent gamma-ray emission through neutral pion
decay. The assumption of a power-law ambient spectrum of accelerated particles
with index near -2 is usually built into models predicting the spectra of
very-high energy (VHE) gamma-ray emission from SNRs. However, if the gas cloud
is located at some distance from the SNR shock, this assumption is not
necessarily correct. In this case, the particles which interact with the cloud
are those leaking from the shock and their spectrum is approximately
monoenergetic with the injection energy gradually decreasing as the SNR ages.
In the GLAST energy range the gamma-ray spectrum resulting from particle
interactions with the gas cloud will be flatter than expected, with the cutoff
defined by the pion momentum distribution in the laboratory frame. We evaluate
the flux of particles escaping from a SNR shock and apply the results to the
VHE diffuse emission detected by the HESS at the Galactic centre.Comment: 4 pages, 3 figures. Contribution to the 30th ICRC, Merida, Mexico,
2007 (final version
Multivariate p-dic L-function
We construct multivariate p-adic L-function in the p-adic number fild by
using Washington method.Comment: 9 page
Parkinson\u27s disease and multiple system atrophy have distinct α-synuclein seed characteristics
Considering Fluctuation Energy as a Measure of Gyrokinetic Turbulence
In gyrokinetic theory there are two quadratic measures of fluctuation energy,
left invariant under nonlinear interactions, that constrain the turbulence. The
recent work of Plunk and Tatsuno [Phys. Rev. Lett. 106, 165003 (2011)] reported
on the novel consequences that this constraint has on the direction and
locality of spectral energy transfer. This paper builds on that work. We
provide detailed analysis in support of the results of Plunk and Tatsuno but
also significantly broaden the scope and use additional methods to address the
problem of energy transfer. The perspective taken here is that the fluctuation
energies are not merely formal invariants of an idealized model
(two-dimensional gyrokinetics) but are general measures of gyrokinetic
turbulence, i.e. quantities that can be used to predict the behavior of the
turbulence. Though many open questions remain, this paper collects evidence in
favor of this perspective by demonstrating in several contexts that constrained
spectral energy transfer governs the dynamics.Comment: Final version as published. Some cosmetic changes and update of
reference
Nonlinear shock acceleration beyond the Bohm limit
We suggest a physical mechanism whereby the acceleration time of cosmic rays
by shock waves can be significantly reduced. This creates the possibility of
particle acceleration beyond the knee energy at ~10^15eV. The acceleration
results from a nonlinear modification of the flow ahead of the shock supported
by particles already accelerated to the knee momentum at p ~ p_*. The particles
gain energy by bouncing off converging magnetic irregularities frozen into the
flow in the shock precursor and not so much by re-crossing the shock itself.
The acceleration rate is thus determined by the gradient of the flow velocity
and turns out to be formally independent of the particle mean free path
(m.f.p.). The velocity gradient is, in turn, set by the knee-particles at p ~
p_* as having the dominant contribution to the CR pressure. Since it is
independent of the m.f.p., the acceleration rate of particles above the knee
does not decrease with energy, unlike in the linear acceleration regime. The
reason for the knee formation at p ~ p_* is that particles with are
effectively confined to the shock precursor only while they are within limited
domains in the momentum space, while other particles fall into
``loss-islands'', similar to the ``loss-cone'' of magnetic traps. This
structure of the momentum space is due to the character of the scattering
magnetic irregularities. They are formed by a train of shock waves that
naturally emerge from unstably growing and steepening magnetosonic waves or as
a result of acoustic instability of the CR precursor. These losses steepen the
spectrum above the knee, which also prevents the shock width from increasing
with the maximum particle energy.Comment: aastex, 13 eps figure
Reconnection in a Weakly Stochastic Field
We examine the effect of weak, small scale magnetic field structure on the
rate of reconnection in a strongly magnetized plasma. This affects the rate of
reconnection by reducing the transverse scale for reconnection flows, and by
allowing many independent flux reconnection events to occur simultaneously.
Allowing only for the first effect and using Goldreich and Sridhar's model of
strong turbulence in a magnetized plasma with negligible intermittency, we find
that the lower limit for the reconnection speed is the Alfven speed times the
Lundquist number to the power (-3/16). The upper limit on the reconnection
speed is typically a large fraction of Alfven speed. We argue that generic
reconnection in turbulent plasmas will normally occur at close to this upper
limit. The fraction of magnetic energy that goes directly into electron heating
scales as Lundquist number to the power (-2/5) and the thickness of the current
sheet scales as the Lundquist number to the power (-3/5). A significant
fraction of the magnetic energy goes into high frequency Alfven waves. We claim
that the qualitative sense of these conclusions, that reconnection is fast even
though current sheets are narrow, is almost independent of the local physics of
reconnection and the nature of the turbulent cascade. As the consequence of
this the Galactic and Solar dynamos are generically fast, i.e. do not depend on
the plasma resistivity.Comment: Extended version accepted to ApJ, 44pages, 2 figure
Widespread Tau Seeding Activity at Early Braak Stages
Transcellular propagation of tau aggregates may underlie the progression of pathology in Alzheimer\u27s disease (AD) and other tauopathies. Braak staging (B1, B2, B3) is based on phospho-tau accumulation within connected brain regions: entorhinal cortex (B1); hippocampus/limbic system (B2); and frontal and parietal lobes (B3). We previously developed a specific and sensitive assay that uses flow cytometry to quantify tissue seeding activity based on fluorescence resonance energy transfer (FRET) in cells that stably express tau reporter proteins. In a tauopathy mouse model, we have detected seeding activity far in advance of histopathological changes. It remains unknown whether individuals with AD also develop seeding activity prior to accumulation of phospho-tau. We measured tau seeding activity across four brain regions (hippocampus, frontal lobe, parietal lobe, and cerebellum) in 104 fresh-frozen human AD brain samples from all Braak stages. We observed widespread seeding activity, notably in regions predicted to be free of phospho-tau deposition, and in detergent-insoluble fractions that lacked tau detectable by ELISA. Seeding activity correlated positively with Braak stage and negatively with MMSE. Our results are consistent with early transcellular propagation of tau seeds that triggers subsequent development of neuropathology. The FRET-based seeding assay may also complement standard neuropathological classification of tauopathies
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Biases in the perceived timing of perisaccadic perceptual and motor events
Subjects typically experience the temporal interval immediately following a saccade as longer than a comparable control interval. One explanation of this effect is that the brain antedates the perceptual onset of a saccade target to around the time of saccade initiation. This could explain the apparent continuity of visual perception across eye movements. Thisantedating account was tested in three experiments in which subjects made saccades of differing extents and then judged either the duration or the temporal order of key events. Postsaccadic stimuli underwent subjective temporal lengthening and had early perceived onsets. A temporally advanced awareness of saccade completion was also found, independently of antedating effects. These results provide convergent evidence supporting antedating and differentiating it from other temporal biases
Renminbi Internationalisation: Precedents and Implications
While it is commonly assumed that there are no known precedents against which to benchmark the internationalisation of the Renminbi (RMB), this paper argues that the PRCs own development experience provides a useful perspective on the internationalisation debate. In particular it indicates that lessons can be learnt from both the successes and the shortcomings of efforts to internationalise the RMB in the 1970s. During this period state-owned banks in Hong Kong played a central role in mobilising finance for foreign trade. Access to Hong Kong’s developed financial institutions allowed the PRC to maximise receipts from foreign trade as well as minimise the risks of undue swings in capital flows. The paper shows that although China no longer faces foreign exchange scarcity, economic reforms have not yet resolved vulnerabilities in China’s financial institutions and as such Hong Kong’s role in mitigating the risk of undue capital swings remains
A transient relativistic radio jet from Cygnus X-1
We report the first observation of a transient relativistic jet from the
canonical black hole candidate, Cygnus X-1, obtained with the Multi-Element
Radio-Linked Interferometer Network (MERLIN). The jet was observed in only one
of six epochs of MERLIN imaging of the source during a phase of repeated X-ray
spectral transitions in 2004 Jan--Feb, and this epoch corresponded to the
softest 1.5-12 keV X-ray spectrum. With only a single epoch revealing the jet,
we cannot formally constrain its velocity. Nevertheless, several lines of
reasoning suggest that the jet was probably launched 0.5-4.0 days before this
brightening, corresponding to projected velocities of 0.2c < v_app < 1.6c, and
an intrinsic velocity of > 0.3c. We also report the occurrence of a major radio
flare from Cyg X-1, reaching a flux density of ~120 mJy at 15 GHz, and yet not
associated with any resolvable radio emission, despite a concerted effort with
MERLIN. We discuss the resolved jet in terms of the recently proposed 'unified
model' for the disc-jet coupling in black hole X-ray binaries, and tentatively
identify the 'jet line' for Cyg X-1. The source is consistent with the model in
the sense that a steady jet appears to persist initially when the X-ray
spectrum starts softening, and that once the spectral softening is complete the
core radio emission is suppressed and transient ejecta / shock observed.
However, there are some anomalies, and Cyg X-1 clearly does not behave like a
normal black hole transient in progressing to the canonical soft / thermal
state once the ejection event has happened.Comment: Accepted for publication in MNRA
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