6,302 research outputs found
Optical and Gamma-Ray Variability Behaviors of 3C 454.3 from 2006 to 2011
We present our photometric monitoring of a flat spectrum radio quasar (FSRQ)
3C 454.3 at Yunnan observatories from 2006 to 2011. We find that the optical
color of 3C 454.3 shows obvious redder-when-brighter trend, which reaches a
saturation stage when the source is brighter than 15.15 mag at V band. We
perform a simulation with multiple values of disk luminosity and spectral index
to reproduce the magnitude-color diagram. The results show that the
contamination caused by the disk radiation alone is difficult to produce the
observed color variability. The variability properties during the outburst in
December 2009 are also compared with -ray data derived from Fermi
-ray space telescope. The flux variation of these two bands follow a
linear relation with , which provides an
observational evidence for external Compton process in 3C 454.3. Meanwhile,
this flux correlation indicates that electron injection is the main mechanism
for variability origin. We also explore the variation of the flux ratio
and the detailed structures in the lightcurves, and discuss
some possible origins for the detailed variability behaviors.Comment: accepted for publication in The Astrophysical Journal, 5 figures, 2
table
Superradiant anomaly magnification in evolution of vector bosonic condensates bounded by a Kerr black hole with near-horizon reflection
Ultralight vector particles can form evolving condensates around a Kerr black
hole (BH) due to superradiant instability. We study the effect of near-horizon
reflection on the evolution of this system; by matching three pieces of
asymptotic expansions of the Proca equation in Kerr metric and considering the
leading order in the electric mode, we present explicit analytical expressions
for the corrected energy level shifts and the superradiant instability rates.
Particularly, in high-spin BH cases, we identify an anomalous situation where
the superadiance rate is temporarily increased by the reflection parameter
, which also occurs in the scalar scenario, but is largely
magnified in vector condensates due to a faster growth rate in dominant mode;
we constructed several featured quantities to illustrate this anomaly, and
formalized the magnification with relevant correction factors, which may be of
significance in future studies of gravitational waveforms of this monochromatic
type. In addition, the duration of superradiance for the whole evolution is
prolonged with a delay factor, which is calculated to be
approximately
A Three-Dimensional Tight-Binding Model and Magnetic Instability of KFe2e2
For a newly discovered iron-based high T_c superconducting parent material
KFe2Se2, we present an effective three-dimensional five-orbital tight-binding
model by fitting the band structures. The three t2g-symmetry orbitals of the
five Fe 3d orbitals mainly contribute to the electron-like Fermi surface, in
agreement with recent angle-resolved photoemission spectroscopy experiments. To
understand the groundstate magnetic structure, the two- and three-dimensional
dynamical spin susceptibilities within the random phase approximation are
investigated. It obviously shows a sharp peak at wave vector
(, ), indicating the magnetic instability of {\it
Nel}-type antiferromagnetic rather than (/2, /2)-type
antiferromagnetic ordering. While along \emph{c} axis, it exhibits a
ferromagnetic coupling between the nearest neighboring FeSe layers. The
difference between the present results and the experimental observation in
KxFe2-ySe2 is attributed to the presence of Fe vacancy in the latter.Comment: 14 pages, 8 figure
Phase evolution of Ce-based heavy-fermion superconductors under pressure: a combined DFT+DMFT and effective-model description
In typical Ce-based heavy-fermion superconductors, superconducting (SC)
phases emerge or can be tuned in proximity to the antiferromagnetic (AF)
quantum critical point (QCP), but so far the explicit phase-evolution process
and the coexistence of superconductivity and AF order near the QCP remain lack
of understanding. Here, by combing DFT+DMFT with effective-model calculations,
we provide a theoretical description for Ce-based SC compounds under pressure.
Firstly, DFT+DMFT calculations for the normal states reveal that the Kondo
hybridizations are significantly enhanced, while the initially localized
electrons eventually become fully itinerant via a localized-itinerant
crossover. In this context, we construct an effective model with tunable
parameters under pressure, and show that the interplay of magnetic correlation
and Kondo hybridization can drive successive transitions, from AF phase to
AF+SC coexisting phase, then to paramagnetic SC phase via an AF transition
which corresponds to the QCP, and finally to Kondo paramagnetic phase through a
SC transition driven by localized-itinerant crossover. Our study gives a proper
explanation for the pressure-induced magnetic QCP and SC transition, and for
the phase-evolution process under pressure in typical Ce-based superconductors,
and may also help to understand the SC states emerging around the ferromagnetic
quantum transition points in uranium-based superconductors.Comment: 13 pages, 11 figure
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