40,593 research outputs found
A new approach to multi-frequency synthesis in radio interferometry
We present a new approach to multi-frequency synthesis in radio astronomy.
Using Bayesian inference techniques, the new technique estimates the sky
brightness and the spectral index simultaneously. In principle, the bandwidth
of a wide-band observation can be fully exploited for sensitivity and
resolution, currently only limited by higher order effects like spectral
curvature. Employing this new approach, we further present a multi-frequency
extension to the imaging algorithm RESOLVE. In simulations, this new algorithm
outperforms current multi-frequency imaging techniques like MS-MF-CLEAN.Comment: 13 pages, 5 fugures, submitted to Astronomy and Astrophysic
Leavitt path algebras satisfying a polynomial identity
Leavitt path algebras L of an arbitrary graph E over a field K satisfying a
polynomial identity are completely characterized both in graph-theoretic and
algebraic terms. When E is a finite graph, L satisfying a polynomial identity
is shown to be equivalent to the Gelfand-Kirillov dimension of L being at most
one, though this is no longer true for infinite graphs. It is shown that, for
an arbitrary graph E, the Leavitt path algebra L has Gelfand-Kirillov dimension
zero if and only if E has no cycles. Likewise, L has Gelfand-Kirillov dimension
one if and only if E contains at least one cycle, but no cycle in E has an
exit.Comment: 12 page
Electromagnetic Magic: The Relativistically Rotating Disk
A closed form analytic solution is found for the electromagnetic field of the
charged uniformly rotating conducting disk for all values of the tip speed
up to . For it becomes the Magic field of the Kerr-Newman black hole
with set to zero.
The field energy, field angular momentum and gyromagnetic ratio are
calculated and compared with those of the electron.
A new mathematical expression that sums products of 3 Legendre functions each
of a different argument, is demonstrated.Comment: 10 pages, one figur
Doppler effects of inertial currents on subsurface temperature measurements
In the presence of inertial oscillations, temperature measurements from a moored sensor will be contaminated by advected spatial structure. The degree of contamination is comparable to that induced by steady currents of the same magnitude, and its effect resembles that of a discrete linear filter acting on the spectrum..
Cosmic ray acceleration to ultrahigh energy in radio galaxies
The origin of ultrahigh energy cosmic rays (UHECRs) is an open question. In
this proceeding, we first review the general physical requirements that a
source must meet for acceleration to 10-100 EeV, including the consideration
that the shock is not highly relativistic. We show that shocks in the backflows
of radio galaxies can meet these requirements. We discuss a model in which
giant-lobed radio galaxies such as Centaurus A and Fornax A act as
slowly-leaking UHECR reservoirs, with the UHECRs being accelerated during a
more powerful past episode. We also show that Centaurus A, Fornax A and other
radio galaxies may explain the observed anisotropies in data from the Pierre
Auger Observatory, before examining some of the difficulties in associating
UHECR anisotropies with astrophysical sources.Comment: 6 pages, 4 figures. Proceedings of UHECR 2018, 8-12 October 2018,
Paris, Franc
Visual Search for Galaxies near the Northern Crossing of the Supergalactic plane by the Milky Way
We have visually examined twelve Palomar red Plates for galaxies at low
Galactic latitude b, where the Supergalactic Plane (SGP) is crossed by the
Galactic Plane (GP), at Galactic longitude l ~135 degrees. The catalogue
consists of 2575 galaxy candidates, of which 462 have major axis diameters d >=
0.8 arc min (uncorrected for extinction). Galaxy candidates can be identified
down to |b| ~ 0 degrees. One of our galaxy candidates (J24 = Dwingeloo 1) has
recently been discovered independently in 21cm by Kraan-Korteweg et al. (1994)
as a nearby galaxy. Comparisons with the structures seen in the IRAS and UGC
catalogues are made. We compare the success rate of identifying galaxies using
the IRAS Point Source Catalogue under different colour selection criteria. The
criteria that require both the 60 micron and 100 micron fluxes to be of high
quality, have the highest probability of selecting a galaxy (with d >= 0.6 arc
min), but at the expense of selecting a smaller number of galaxies in total.Comment: uuencoded compressed postscript, without figures. The figures are
available at http://www.ast.cam.ac.uk/preprint/PrePrint.htm
Amplification of perpendicular and parallel magnetic fields by cosmic ray currents
Cosmic ray (CR) currents through magnetised plasma drive strong instabilities
producing amplification of the magnetic field. This amplification helps explain
the CR energy spectrum as well as observations of supernova remnants and radio
galaxy hot spots. Using magnetohydrodynamic (MHD) simulations, we study the
behaviour of the non-resonant hybrid (NRH) instability (also known as the Bell
instability) in the case of CR currents perpendicular and parallel to the
initial magnetic field. We demonstrate that extending simulations of the
perpendicular case to 3D reveals a different character to the turbulence from
that observed in 2D. Despite these differences, in 3D the perpendicular NRH
instability still grows exponentially far into the non-linear regime with a
similar growth rate to both the 2D perpendicular and 3D parallel situations. We
introduce some simple analytical models to elucidate the physical behaviour,
using them to demonstrate that the transition to the non-linear regime is
governed by the growth of thermal pressure inside dense filaments at the edges
of the expanding loops. We discuss our results in the context of supernova
remnants and jets in radio galaxies. Our work shows that the NRH instability
can amplify magnetic fields to many times their initial value in parallel and
perpendicular shocks.Comment: Published in MNRAS. 14 pages, 12 figures, 2 tables. Replacement
corrects some typesetting error
Efficiency of feedback process in cavity quantum electrodynamics
Utilizing the continuous frequency mode quantization scheme, we study from
first principle the efficiency of a feedback scheme that can generate maximally
entangled states of two atoms in an optical cavity through their interactions
with a single input photon. The spectral function of the photon emitted from
the cavity, which will be used as the input of the next round in the feedback
process, is obtained analytically. We find that the spectral function of the
photon is modified in each round and deviates from the original one. The
efficiency of the feedback scheme consequently deteriorates gradually after
several rounds of operation.Comment: 11 pages, 5 figures, accepted for publication in Journal of Physics
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