6,139 research outputs found
Vela, its X-ray nebula, and the polarization of pulsar radiation
The recent identification of the perpendicular mode of radio polarization as
the primary one in the Vela pulsar by Lai et al. (2001) is interpreted in terms
of the maser mechanism proposed by Luo & Melrose (1995). We suggest that such a
mechanism may also be operative for the parallel mode which opens up the
possibility of accounting for all types of polarization observed in pulsars. We
propose an alternative interpretation of the arcs in the nebular X-radiation
observed by Pavlov et al.(2000) & Helfand et al. (2001) with the Chandra
Observatory, and interpreted by the latter as an equatorial wind. We interpret
the arcs as traces of the particle beams from the two magnetic poles at the
shock front. We also propose that the alignment with the rotation axis of the
jet-like feature bisecting the arcs is an effect of projection on the sky plane
and that there is no physical jet along the axis of rotation.Comment: 7 pages, 3 figures; version 2; accepted for publication in A&
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
Considerations concerning fatigue life of metal matrix composites
Since metal matrix composites (MMC) are composed from two very distinct materials each having their own physical and mechanical properties, it is feasible that the fatigue resistance depends on the strength of the weaker constituent. Based on this assumption, isothermal fatigue lives of several MMC's were analyzed utilizing a fatigue life diagram approach. For each MMC, the fatigue life diagram was quantified using the mechanical properties of its constituents. The fatigue life regions controlled by fiber fracture and matrix were also quantitatively defined
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
On the Excess Dispersion in the Polarization Position Angle of Pulsar Radio Emission
The polarization position angles (PA) of pulsar radio emission occupy a
distribution that can be much wider than what is expected from the average
linear polarization and the off-pulse instrumental noise. Contrary to our
limited understanding of the emission mechanism, the excess dispersion in PA
implies that pulsar PAs vary in a random fashion. An eigenvalue analysis of the
measured Stokes parameters is developed to determine the origin of the excess
PA dispersion. The analysis is applied to sensitive, well-calibrated
polarization observations of PSR B1929+10 and PSR B2020+28. The analysis
clarifies the origin of polarization fluctuations in the emission and reveals
that the excess PA dispersion is caused by the isotropic inflation of the data
point cluster formed by the measured Stokes parameters. The inflation of the
cluster is not consistent with random fluctuations in PA, as might be expected
from random changes in the orientation of the magnetic field lines in the
emission region or from stochastic Faraday rotation in either the pulsar
magnetosphere or the interstellar medium. The inflation of the cluster, and
thus the excess PA dispersion, is attributed to randomly polarized radiation in
the received pulsar signal. The analysis also indicates that orthogonal
polarization modes (OPM) occur where the radio emission is heavily modulated.
In fact, OPM may only occur where the modulation index exceeds a critical value
of about 0.3.Comment: Accepted for publication in Ap
A Note on the Intermediate Region in Turbulent Boundary Layers
We demonstrate that the processing of the experimental data for the average
velocity profiles obtained by J. M. \"Osterlund
(www.mesh.kth.se/jens/zpg/) presented in [1] was incorrect. Properly
processed these data lead to the opposite conclusion: they confirm the
Reynolds-number-dependent scaling law and disprove the conclusion that the flow
in the intermediate (`overlap') region is Reynolds-number-independent.Comment: 8 pages, includes 1 table and 3 figures, broken web link in abstract
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Pair Multiplicities and Pulsar Death
Through a simple model of particle acceleration and pair creation above the
polar caps of rotation-powered pulsars, we calculate the height of the
pair-formation front (PFF) and the dominant photon emission mechanism for the
pulsars in the Princeton catalog. We find that for most low- and moderate-field
pulsars, the height of the pair formation front and the final Lorentz factor of
the primary beam is set by nonresonant inverse Compton scattering (NRICS), in
the Klein-Nishina limit. NRICS is capable of creating pairs over a wide range
of pulsar parameters without invoking a magnetic field more complicated than a
centered dipole, although we still require a reduced radius of curvature for
most millisecond pulsars. For short-period pulsars, the dominant process is
curvature radiation, while for extremely high-field pulsars, it is resonant
inverse Compton scattering (RICS). The dividing point between NRICS dominance
and curvature dominance is very temperature-dependent; large numbers of pulsars
dominated by NRICS at a stellar temperature of K are dominated by
curvature at K. We apply these results to pulsar death-line calculations
and to the issue of particle injection into the Crab Nebula.Comment: 14 pages, 7 figures, to appear in Ap
Do Public Equity Markets Matter in Emerging Economies? Evidence From India
Do public equity markets serve an unique role that is not easily served by other forms of financing in emerging economies? We analyze this question using the collapse of India’s equity market in 1997, which provides an exogenous shock to firms’ ability to issue equity. We find that both public and private firms exhibit higher bankruptcy rates and lower growth after 1997. The decline in growth is greater among firms with more external finance needs and fewer tangible assets. Overall, the evidence suggests that public equity markets are an important, not easily replaced, source of finance in emerging economies
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