4,019 research outputs found
The observational evidence pertinent to possible kick mechanisms in neutron stars
We examine available observations on pulsars for evidence pertaining to
mechanisms proposed to explain the origin of their velocities. We find that
mechanisms predicting a correlation between the rotation axis and the pulsar
velocity are ruled out. Also, that there is no significant correlation between
pulsar magnetic field strengths and velocities. With respect to recent
suggestions postulating asymmetric impulses at birth being solely responsible
for both the spins and velocities of pulsars, single impulses of any duration
and multiple extended duration impulses appear ruled out.Comment: 7 pages. Accepted for publication in Astronomy & Astrophysic
Impact of Tandem Repeats on the Scaling of Nucleotide Sequences
Techniques such as detrended fluctuation analysis (DFA) and its extensions
have been widely used to determine the nature of scaling in nucleotide
sequences. In this brief communication we show that tandem repeats which are
ubiquitous in nucleotide sequences can prevent reliable estimation of possible
long-range correlations. Therefore, it is important to investigate the presence
of tandem repeats prior to scaling exponent estimation.Comment: 14 Pages, 3 Figure
Robust Randomness Amplifiers: Upper and Lower Bounds
A recent sequence of works, initially motivated by the study of the nonlocal
properties of entanglement, demonstrate that a source of
information-theoretically certified randomness can be constructed based only on
two simple assumptions: the prior existence of a short random seed and the
ability to ensure that two black-box devices do not communicate (i.e. are
non-signaling). We call protocols achieving such certified amplification of a
short random seed randomness amplifiers.
We introduce a simple framework in which we initiate the systematic study of
the possibilities and limitations of randomness amplifiers. Our main results
include a new, improved analysis of a robust randomness amplifier with
exponential expansion, as well as the first upper bounds on the maximum
expansion achievable by a broad class of randomness amplifiers. In particular,
we show that non-adaptive randomness amplifiers that are robust to noise cannot
achieve more than doubly exponential expansion. Finally, we show that a wide
class of protocols based on the use of the CHSH game can only lead to (singly)
exponential expansion if adversarial devices are allowed the full power of
non-signaling strategies. Our upper bound results apply to all known
non-adaptive randomness amplifier constructions to date.Comment: 28 pages. Comments welcom
Coupled nonlinear Schrodinger equations with cubic-quintic nonlinearity: integrability and soliton interaction in non-kerr media
We propose an integrable system of coupled nonlinear Schrödinger equations with cubic-quintic terms describing the effects of quintic nonlinearity on the ultrashort optical soliton pulse propagation in non-Kerr media. Lax pairs, conserved quantities and exact soliton solutions for the proposed integrable model are given. The explicit form of two solitons are used to study soliton interaction showing many intriguing features including inelastic (shape changing or intensity redistribution) scattering. Another system of coupled equations with fifth-degree nonlinearity is derived, which represents vector generalization of the known chiral-soliton bearing system
Pulsar Radio Emission Altitude from Curvature Radiation
We assume that the relativistic sources moving along the dipolar magnetic
field lines emit curvature radiation. The beamed emission occurs in the
direction of tangents to the field lines, and to receive it, the sight line
must align with the tangent within the beaming angle 1/gamma, where gamma is
the particle Lorentz factor. By solving the viewing geometry in an inclined and
rotating dipole magnetic field, we show that, at any given pulse phase,
observer tends to receive radiation only from the specific heights allowed by
the geometry. We find outer conal components are emitted at higher altitudes
compared to inner components including the core. At any pulse phase, low
frequency emission comes from higher altitudes than high frequency emission. We
have modeled the emission heights of pulse components of PSR B0329+54, and
estimated field line curvature radii and particle Lorentz factors in the
emission regions.Comment: 14 pages, 3 figures. Accepted for Astrophysical Journal, 200
On the Origin of the Wide HI Absorption Line Toward Sgr A*
We have imaged a region of about 5' extent surrounding Sgr A* in the HI 21
cm-line absorption using the Very Large Array. A Gaussian decomposition of the
optical depth spectra at positions within about 2' (approx. 5 pc at 8.5 kpc) of
Sgr A* detects a wide line underlying the many narrow absorption lines. The
wide line has a mean peak optical depth of 0.32 +/- 0.12 centered at a mean
velocity of V(lsr) = -4 +/- 15 km/s. The mean full width at half maximum is 119
+/- 42 km/s. Such a wide line is absent in the spectra at positions beyond
about 2' from Sgr A*. The position-velocity diagrams in optical depth reveal
that the wide line originates in various components of the circumnuclear disk
(radius approx. 1.3') surrounding Sgr A*. These components contribute to the
optical depth of the wide line in different velocity ranges. The
position-velocity diagrams do not reveal any diffuse feature which could be
attributed to a large number of HI clouds along the line of sight to Sgr A*.
Consequently, the wide line has no implications either to a global population
of shocked HI clouds in the Galaxy or to the energetics of the interstellar
medium as was earlier thought.Comment: LaTeX, 12 pages and 9 figures, accepted for publication in J.
Astrophys. Ast
Radio Polarization of the Young High-Magnetic-Field Pulsar PSR J1119-6127
We have investigated the radio polarization properties of PSR J1119-6127, a
recently discovered young radio pulsar with a large magnetic field. Using
pulsar-gated radio imaging data taken at a center frequency of 2496 MHz with
the Australia Telescope Compact Array, we have determined a rotation measure
for the pulsar of +842 +/- 23 rad m^-2. These data, combined with archival
polarimetry data taken at a center frequency of 1366 MHz with the Parkes
telescope, were used to determine the polarization characteristics of PSR
J1119-6127 at both frequencies. The pulsar has a fractional linear polarization
of ~75% and ~55% at 1366 and 2496 MHz, respectively, and the profile consists
of a single, wide component. This pulse morphology and high degree of linear
polarization are in agreement with previously noticed trends for young pulsars
(e.g., PSR J1513-5908). A rotating-vector (RV) model fit of the position angle
(PA) of linear polarization over pulse phase using the Parkes data suggests
that the radio emission comes from the leading edge of a conal beam. We discuss
PSR J1119-6127 in the context of a recent theoretical model of pulsar spin-down
which can in principle be tested with polarization and timing data from this
pulsar. Geometric constraints from the RV fit are currently insufficient to
test this model with statistical significance, but additional data may allow
such a test in the future.Comment: 9 pages, including 6 figures and 1 table. Accepted for publication in
Ap
Analysis of Senior-Subordinated Structures Backed by Private-Label Mortgages
This paper does a valuation analysis of senior-subordinated struc ture tranches backed by non-agency mortgages. The valuation is done using Monte Carlo simulation and employs the CIR interest rate process in conjunction with an empirical model estimated for non-agency mortgage prepayments and defaults. The sensitivity of the value of tranches to a number of variables are analyzed. We find that the interest rate process parameters significantly affect prepayments and defaults but not the relative value of the senior tranche. It is found that with the shifting o f prepayments, the senior tranche does not dominate all the junior tranches at all interest rates. The shifting of prepayments has the unintended effect of providing stability to the junior tranches by making their cashflows less sensitive to prepayments. Our main conclusion is that while the shifting o f prepayments increases protection from default to the senior tranche for a given level of subordination, it has the unwanted effect of lowering its value through increased contraction risk. This value loss should be taken into account in determining the optimum level of subordination
The Sasa-Satsuma higher order nonlinear Schrodinger equation and its bilinearization and multi-soliton solutions
Higher order and multicomponent generalizations of the nonlinear Schrodinger
equation are important in various applications, e.g., in optics. One of these
equations, the integrable Sasa-Satsuma equation, has particularly interesting
soliton solutions. Unfortunately the construction of multi-soliton solutions to
this equation presents difficulties due to its complicated bilinearization. We
discuss briefly some previous attempts and then give the correct
bilinearization based on the interpretation of the Sasa-Satsuma equation as a
reduction of the three-component Kadomtsev-Petvishvili hierarchy. In the
process we also get bilinearizations and multi-soliton formulae for a two
component generalization of the Sasa-Satsuma equation (the
Yajima-Oikawa-Tasgal-Potasek model), and for a (2+1)-dimensional
generalization.Comment: 13 pages in RevTex, added reference
A Pulsational Model for the Orthogonal Polarization Modes in Radio Pulsars
In an earlier paper, we introduced a model for pulsars in which non-radial
oscillations of high spherical degree (\el) aligned to the magnetic axis of a
spinning neutron star were able to reproduce subpulses like those observed in
single-pulse measurements of pulsar intensity. The model did not address
polarization, which is an integral part of pulsar emission. Observations show
that many pulsars emit radio waves that appear to be the superposition of two
linearly polarized emission modes with orthogonal polarization angles. In this
paper, we extend our model to incorporate linear polarization. As before, we
propose that pulsational displacements of stellar material modulate the pulsar
emission, but now we apply this modulation to a linearly-polarized mode of
emission, as might be produced by curvature radiation. We further introduce a
second polarization mode, orthogonal to the first, that is modulated by
pulsational velocities. We combine these modes in superposition to model the
observed Stokes parameters in radio pulsars.Comment: 19 pages, 4 figures accepted Ap
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