1,680 research outputs found
Confinement and diffusion time-scales of CR hadrons in AGN-inflated bubbles
While rich clusters are powerful sources of X-rays, gamma-ray emission from
these large cosmic structures has not been detected yet. X-ray radiative energy
losses in the central regions of relaxed galaxy clusters are so strong that one
needs to consider special sources of energy, likely AGN feedback, to suppress
catastrophic cooling of the gas. We consider a model of AGN feedback that
postulates that the AGN supplies the energy to the gas by inflating bubbles of
relativistic plasma, whose energy content is dominated by cosmic-ray (CR)
hadrons. If most of these hadrons can quickly escape the bubbles, then
collisions of CRs with thermal protons in the intracluster medium (ICM) should
lead to strong gamma-ray emission, unless fast diffusion of CRs removes them
from the cluster. Therefore, the lack of detections with modern gamma-ray
telescopes sets limits on the confinement time of CR hadrons in bubbles and CR
diffusive propagation in the ICM.Comment: 8 pages, 2 figures, accepted for publication in MNRA
Clipping in Neurocontrol by Adaptive Dynamic Programming
In adaptive dynamic programming, neurocontrol, and reinforcement learning, the objective is for an agent to learn to choose actions so as to minimize a total cost function. In this paper, we show that when discretized time is used to model the motion of the agent, it can be very important to do clipping on the motion of the agent in the final time step of the trajectory. By clipping, we mean that the final time step of the trajectory is to be truncated such that the agent stops exactly at the first terminal state reached, and no distance further. We demonstrate that when clipping is omitted, learning performance can fail to reach the optimum, and when clipping is done properly, learning performance can improve significantly. The clipping problem we describe affects algorithms that use explicit derivatives of the model functions of the environment to calculate a learning gradient. These include backpropagation through time for control and methods based on dual heuristic programming. However, the clipping problem does not significantly affect methods based on heuristic dynamic programming, temporal differences learning, or policy-gradient learning algorithms
An Equivalence Between Adaptive Dynamic Programming With a Critic and Backpropagation Through Time
We consider the adaptive dynamic programming technique called Dual Heuristic Programming (DHP), which is designed to learn a critic function, when using learned model functions of the environment. DHP is designed for optimizing control problems in large and continuous state spaces. We extend DHP into a new algorithm that we call Value-Gradient Learning, VGL(λ), and prove equivalence of an instance of the new algorithm to Backpropagation Through Time for Control with a greedy policy. Not only does this equivalence provide a link between these two different approaches, but it also enables our variant of DHP to have guaranteed convergence, under certain smoothness conditions and a greedy policy, when using a general smooth nonlinear function approximator for the critic. We consider several experimental scenarios including some that prove divergence of DHP under a greedy policy, which contrasts against our proven-convergent algorithm
ROSAT X-ray sources and exponential field decay in isolated neutron stars
In this paper we semianalyticaly evaluate influence of the exponential decay
of magnetic field on the fate of isolated neutron stars. The fact of ROSAT
observations of several X-ray sources, which can be accreting old isolated
neutron stars gives us an opportunity to put some limits on the parameters of
the exponential decay.
We argue, that, if most part of neutron stars have approximately the same
decay and initial parameters, then the combinations of the bottom magnetic
momentum, , in the range and
characteristic time scale, , in the range for
standard initial magnetic momentum, , can be
excluded, because for that sets of parameters neutron stars never come to the
stage when accretion of the interstellar medium on their surfaces is possible
even for low velocity of neutron stars and relatively high density of the
interstellar medium. The region of excluded parameters increases with
decreasing.Comment: 5 pages, 4 PostScript figures (uses A&A style
YBCO microwave resonators for strong collective coupling with spin ensembles
Coplanar microwave resonators made of 330 nm-thick superconducting YBCO have
been realized and characterized in a wide temperature (, 2-100 K) and
magnetic field (, 0-7 T) range. The quality factor exceeds 10
below 55 K and it slightly decreases for increasing fields, remaining 90 of
for T and K. These features allow the coherent coupling
of resonant photons with a spin ensemble at finite temperature and magnetic
field. To demonstrate this, collective strong coupling was achieved by using
DPPH organic radical placed at the magnetic antinode of the fundamental mode:
the in-plane magnetic field is used to tune the spin frequency gap splitting
across the single-mode cavity resonance at 7.75 GHz, where clear anticrossings
are observed with a splitting as large as MHz at K. The
spin-cavity collective coupling rate is shown to scale as the square root of
the number of active spins in the ensemble.Comment: to appear in Appl. Phys. Let
Constrains on parameters of magnetic field decay for accreting isolated neutron stars
The influence of exponential magnetic field decay (MFD) on the spin evolution
of isolated neutron stars is studied. The ROSAT observations of several X-ray
sources, which can be accreting old isolated neutron stars, are used to
constrain the exponential and power-law decay parameters. We show that for the
exponential decay the ranges of minimum value of magnetic moment, , and
the characteristic decay time, , , are excluded assuming the
standard initial magnetic moment, . For these
parameters, neutron stars would never reach the stage of accretion from the
interstellar medium even for a low space velocity of the stars and a high
density of the ambient plasma. The range of excluded parameters increases for
lower values of .
We also show, that, contrary to exponential MFD, no significant restrictions
can be made for the parameters of power-law decay from the statistics of
isolated neutron star candidates in ROSAT observations.
Isolated neutron stars with constant magnetic fields and initial values of
them less than never come to the stage
of accretion.
We briefly discuss the fate of old magnetars with and without MFD, and
describe parameters of old accreting magnetars.Comment: 18 pages, 6 PostScript figures, to be published in the Proceedings of
the XXVIII ITEP Winter Schoo
Evolution of isolated neutron stars in globular clusters: number of Accretors
With a simple model from the point of view of population synthesis we try to
verify an interesting suggestion made by Pfahl & Rappaport (2001) that dim
sources in globular clusters (GCs) can be isolated accreting neutron stars
(NSs). Simple estimates show, that we can expect about 0.5-1 accreting isolated
NS per typical GC with in correspondence with observations.
Properties of old accreting isolated NSs in GCs are briefly discussed. We
suggest that accreting NSs in GCs experienced significant magnetic field decay.Comment: 6 pages, no figures. Submitted to Astronomical and Astrophysical
Transactions (style included
Uncertainty relations in curved spaces
Uncertainty relations for particle motion in curved spaces are discussed. The
relations are shown to be topologically invariant. New coordinate system on a
sphere appropriate to the problem is proposed. The case of a sphere is
considered in details. The investigation can be of interest for string and
brane theory, solid state physics (quantum wires) and quantum optics.Comment: published version; phase space structure discussion adde
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