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/$\sim$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 remove

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 $10^6$ K are dominated by curvature at $10^5$ 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