49,002 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
Montane lakes (lagoons) of the New England Tablelands Bioregion
The vegetation of montane lagoons of the New England Tablelands Bioregion, New South Wales is examined using flexible UPGMA analysis of frequency scores on all vascular plant taxa, charophytes and one liverworts. Seven communities are described: 1. Hydrocotyle tripartita – Isotoma fluviatilis – Ranunculus inundatus – Lilaeopsis polyantha herbfield; 2. Eleocharis sphacelata – Potamogeton tricarinatus sedgeland; 3. Eleocharis sphacelata – Utricularia australis – Isolepis fluitans, herbfield; 4. Utricularia australis – Nitella sonderi herbfield; 5. Eleocharis sphacelata – Utricularia australis – Ricciocarpus natans sedgeland; 6. Carex gaudichaudiana – Holcus lanatus – Stellaria angustifolia sedgeland; 7. Cyperus sphaeroides – Eleocharis gracilis – Schoenus apogon – Carex gaudichaudiana sedgeland. 58 lagoons were located and identified, only 28% of which are considered to be intact and in good condition. Two threatened species (Aldovandra vesiculosa and Arthaxon hispidus) and three RoTAP-listed taxa were encountered during the survey
Relativistic Poynting Jets from Accretion Disks
A model is developed for relativistic Poynting jets from the inner region of
a disk around a rotating black hole. The disk is initially threaded by a
dipole-like magnetic field. The model is derived from the special relativistic
equation for a force-free electromagnetic field. The ``head'' of the Poynting
jet is found to propagate outward with a velocity which may be relativistic.
The Lorentz factor of the head (Gamma) is found to be dependent on the magnetic
field strength close to the black hole, B_0, the density of the external medium
n_ext, and on the ratio R=r_0/r_g >1, where r_g is the gravitational radius of
the black hole, and r_0 is the radius of the O-point of the initial dipole
field threading the disk. For conditions pertinent to an active galactic
nuclei, Gamma is approximately equal to 8 (10/R)^(1/3) (B_0/10^3 Gauss)^(1/3)
(1/cm^3/n_ext)^(1/6). This model offers an explanation for the observed Lorentz
factors which are of the order of 10 for the parsec-scale radio jets measured
with very long baseline interferometry.Comment: 4 pages, 1 figur
Evidence that particle acceleration in hotspots of FR II galaxies is not constrained by synchrotron cooling
We study the hotspots of powerful radiogalaxies, where electrons accelerated
at the jet termination shock emit synchrotron radiation. The turnover of the
synchrotron spectrum is typically observed between infrared and optical
frequencies, indicating that the maximum energy of non-thermal electrons
accelerated at the shock is ~TeV for a canonical magnetic field of ~100 micro
Gauss. We show that this maximum energy cannot be constrained by synchrotron
losses as usually assumed, unless the jet density is unreasonably large and
most of the jet upstream energy goes to non-thermal particles. We test this
result by considering a sample of hotspots observed at radio, infrared and
optical wavelengths.Comment: 7 pages, 2 figures. To be appear in the proceedings of the conference
"Cosmic ray origin - beyond the standard models" (San Vito di Cadore, Italy,
September 2016
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