28,198 research outputs found
Phase Transitions in a Two-Component Site-Bond Percolation Model
A method to treat a N-component percolation model as effective one component
model is presented by introducing a scaled control variable . In Monte
Carlo simulations on , , and simple cubic
lattices the percolation threshold in terms of is determined for N=2.
Phase transitions are reported in two limits for the bond existence
probabilities and . In the same limits, empirical formulas
for the percolation threshold as function of one
component-concentration, , are proposed. In the limit a new
site percolation threshold, , is reported.Comment: RevTeX, 5 pages, 5 eps-figure
Unusual Non-Fermi Liquid Behavior of CeLaNiGe Analyzed in a Single Impurity Anderson Model with Crystal Field Effects
CeNiGe exhibits unusual non-Fermi liquid behavior with the largest
ever recorded value of the electronic specific heat
JKmol without showing any evidence of magnetic order. Specific
heat measurements show that the logarithmic increase of the Sommerfeld
coefficient flattens off below 200 mK. In marked contrast, the local
susceptibility levels off well above 200 mK and already becomes
constant below 1 K. Furthermore, the entropy reaches 2ln2 below 20 K
corresponding to a four level system. An analysis of and was
performed in terms of an single impurity Anderson model with
additional crystal electric field (CEF) splitting. Numerical renormalization
group calculations point to a possible consistent description of the different
low temperature scales in and stemming from the
interplay of Kondo effect and crystal field splitting.Comment: 2 pages, 2 figure
SN 1996cr: Confirmation of a Luminous Type IIn Supernova in the Circinus Galaxy
We have recently confirmed SN 1996cr as a late-time type IIn supernova (SN)
via VLT spectroscopy and isolated its explosion date to ~1 yr using archival
optical imaging. We briefly touch upon here the wealth of optical, X-ray, and
radio archival observations available for this enigmatic source. Due to its
relative proximity (3.8 +/-0.6 Mpc), SN 1996cr ranks among the brightest X-ray
and radio SNe ever detected and, as such, may offer powerful insights into the
structure and composition of type IIn SNe. We also find that SN 1996cr is
matched to GRB 4B 960202 at a 2-3 sigma confidence level, making it perhaps the
third GRB to be significantly associated with a type II SN. We speculate on
whether SN 1996cr could be an off-axis or ``failed'' GRB.Comment: 3 pages, 1 figure, poster proceeding for "Supernova 1987A: 20 Years
After: Supernovae and Gamma-Ray Bursters" AIP, New York, eds. S. Immler, K.W.
Weiler, and R. McCra
Microwave Scattering and Noise Emission from Afterglow Plasmas in a Magnetic Field
The microwave reflection and noise emission (extraordinary mode) from cylindrical rare‐gas (He, Ne, Ar) afterglow plasmas in an axial magnetic field is described. Reflection and noise emission are measured as a function of magnetic field near electron cyclotron resonance (ω ≈ ω_c) with electron density as a parameter (ω_p < ω). A broad peak, which shifts to lower values of ω_c/ω) as electron density increases, is observed for (ω_c/ω) ≤ 1. For all values of electron density a second sharp peak is found very close to cyclotron resonance in reflection measurements. This peak does not occur in the emission data. Calculations of reflection and emission using a theoretical model consisting of a one‐dimensional, cold plasma slab with nonuniform electron density yield results in qualitative agreement with the observations. Both the experimental and theoretical results suggest that the broad, density‐dependent peak involves resonance effects at the upper hybrid frequency ((ω_h)^2 = (ω_c)^2 + (ω_p)^2) of the plasma
Extending Bauer's corollary to fractional derivatives
We comment on the method of Dreisigmeyer and Young [D. W. Dreisigmeyer and P.
M. Young, J. Phys. A \textbf{36}, 8297, (2003)] to model nonconservative
systems with fractional derivatives. It was previously hoped that using
fractional derivatives in an action would allow us to derive a single retarded
equation of motion using a variational principle. It is proven that, under
certain reasonable assumptions, the method of Dreisigmeyer and Young fails.Comment: Accepted Journal of Physics A at www.iop.org/EJ/journal/JPhys
IC 751: a new changing-look AGN discovered by NuSTAR
We present the results of five NuSTAR observations of the type 2 active
galactic nucleus (AGN) in IC 751, three of which were performed simultaneously
with XMM-Newton or Swift/XRT. We find that the nuclear X-ray source underwent a
clear transition from a Compton-thick () to a Compton-thin () state on timescales of months, which makes
IC 751 the first changing-look AGN discovered by NuSTAR. Changes of the
line-of-sight column density at a level are also found on a
time-scale of hours ().
From the lack of spectral variability on timescales of ks we infer
that the varying absorber is located beyond the emission-weighted average
radius of the broad-line region, and could therefore be related either to the
external part of the broad-line region or a clumpy molecular torus. By adopting
a physical torus X-ray spectral model, we are able to disentangle the column
density of the non-varying absorber () from that of the varying clouds
[], and to constrain that of
the material responsible for the reprocessed X-ray radiation (). We find evidence of significant intrinsic X-ray
variability, with the flux varying by a factor of five on timescales of a few
months in the 2-10 and 10-50 keV band.Comment: Accepted for publication in ApJ, 11 pages, 6 figure
Fact: Many SCUBA galaxies harbour AGNs
Deep SCUBA surveys have uncovered a large population of ultra-luminous
galaxies at z>1. These sources are often assumed to be starburst galaxies, but
there is growing evidence that a substantial fraction host an AGN (i.e., an
accreting super-massive black hole). We present here possibly the strongest
evidence for this viewpoint to date: the combination of ultra-deep X-ray
observations (the 2 Ms Chandra Deep Field-North) and deep optical spectroscopic
data. We argue that upward of 38% of bright (f850um>=5mJy) SCUBA galaxies host
an AGN, a fraction of which are obscured QSOs (i.e., L_X>3x10^{44} erg/s).
However, using evidence from a variety of analyses, we argue that in almost all
cases the AGNs are not bolometrically important (i.e., <20%). Thus, star
formation appears to dominate their bolometric output. A substantial fraction
of bright SCUBA galaxies show evidence for binary AGN activity. Since these
systems appear to be interacting and merging at optical/near-IR wavelengths,
their super-massive black holes will eventually coalesce.Comment: Invited contribution - 10 pages, 4 figures, to appear in the
Proceedings of the ESO/USM/MPE Workshop on "Multiwavelength Mapping of Galaxy
Formation and Evolution", eds. R. Bender and A. Renzin
Semileptonic B Decays and Determination of |Vub|
Semileptonic decays of the B mesons provide an excellent probe for the weak
and strong interactions of the bottom quark. The large data samples collected
at the B Factories have pushed the experimental studies of the semileptonic B
decays to a new height and stimulated significant theoretical developments. I
review recent progresses in this fast-evolving field, with an emphasis on the
determination of the magnitude of the Cabibbo-Kobayashi-Maskawa matrix element
|Vub|.Comment: 16 pages, 4 figures, accepted by Mod. Phys. Lett.
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