6,237 research outputs found
The Dirac point electron in zero-gravity Kerr--Newman spacetime
Dirac's wave equation for a point electron in the topologically nontrivial
maximal analytically extended electromagnetic Kerr--Newman spacetime is studied
in a zero-gravity limit; here, "zero-gravity" means , where is
Newton's constant of universal gravitation. The following results are obtained:
the formal Dirac Hamiltonian on the static spacelike slices is essentially
self-adjoint; the spectrum of the self-adjoint extension is symmetric about
zero, featuring a continuum with a gap about zero that, under two smallness
conditions, contains a point spectrum. Some of our results extend to a
generalization of the zero- Kerr--Newman spacetime with different
electric-monopole-to-magnetic-dipole-moment ratio.Comment: 49 pages, 17 figures; referee's comments implemented; the endnotes in
the published version appear as footnotes in this preprin
A Note on Tsallis Holographic Dark Energy
We explore the effects of considering various infrared (IR) cutoffs,
including the particle horizon, Ricci horizon and Granda-Oliveros (GO) cutoffs,
on the properties of Tsallis holographic dark energy (THDE) model, proposed
inspired by Tsallis generalized entropy formalism \cite{THDE}. Interestingly
enough, we find that for the particle horizon as IR cutoff, the obtained THDE
model can describe the accelerated universe. This is in contrast to the usual
HDE model which cannot lead to an accelerated universe, if one consider the
particle horizon as IR cutoff. We also investigate the cosmological
consequences of THDE under the assumption of a mutual interaction between the
dark sectors of the Universe. It is shown that the evolution history of the
Universe can be described by these IR cutoffs and thus the current cosmic
acceleration can also been realized. The sound instability of THDE models for
each cutoff are also explored, separately.Comment: 12 pages, 31 figure
The Variability of Polarized Radiation from Sgr A*
Sgr A* is variable at radio and submillimeter wavelengths on hourly time
scales showing time delays between the peaks of flare emission as well as
linearly polarized emission at millimeter and sub-mm wavelengths. To determine
the polarization characteristics of this variable source at radio frequencies,
we present VLA observations of Sgr A* and report the detection of polarized
emission at a level of 0.77\pm0.01% and 0.2\pm0.01% at 43 and 22 GHz,
respectively. The change in the time averaged polarization angle between 22 and
43 GHz corresponds to a RM of -2.5\pm0.6 x10^3 rad m{-2} with no phase wrapping
(or \sim 5x10^4 rad m^2 with 2\pi phase wrap). We also note a rise and fall
time scale of 1.5 -- 2 hours in the total polarized intensity. The light curves
of the degree of linearly polarized emission suggests a a correlation with the
variability of the total intensity at 43 GHz. The available polarization data
at radio and sub-mm wavelengths suggest that the rotation measure decreases
with decreasing frequency. This frequency dependence, and observed changes in
polarization angle during flare events, may be caused by the reduction in
rotation measure associated with the expansion of synchrotron-emitting blobs.Comment: 11 pages, 3 figures, ApJL (in press
An Inverse Compton Scattering Origin of X-ray Flares from Sgr A*
The X-ray and near-IR emission from Sgr A* is dominated by flaring, while a
quiescent component dominates the emission at radio and sub-mm wavelengths. The
spectral energy distribution of the quiescent emission from Sgr A* peaks at
sub-mm wavelengths and is modeled as synchrotron radiation from a thermal
population of electrons in the accretion flow, with electron temperatures
ranging up to \,MeV. Here we investigate the mechanism by which
X-ray flare emission is produced through the interaction of the quiescent and
flaring components of Sgr A*. The X-ray flare emission has been interpreted as
inverse Compton, self-synchrotron-Compton, or synchrotron emission. We present
results of simultaneous X-ray and near-IR observations and show evidence that
X-ray peak flare emission lags behind near-IR flare emission with a time delay
ranging from a few to tens of minutes. Our Inverse Compton scattering modeling
places constraints on the electron density and temperature distributions of the
accretion flow and on the locations where flares are produced. In the context
of this model, the strong X-ray counterparts to near-IR flares arising from the
inner disk should show no significant time delay, whereas near-IR flares in the
outer disk should show a broadened and delayed X-ray flare.Comment: 22 pages, 6 figures, 2 tables, AJ (in press
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