Study of the Local Interstellar Medium using Pulsar Scintillation

We present here the results from an extensive scintillation study of twenty pulsars in the dispersion measure (DM) range 3 - 35 pc cm^-3 carried out using the Ooty Radio Telescope, to investigate the distribution of ionized material in the local interstellar medium (LISM). Our analysis reveals several anomalies in the scattering strength, which suggest that the distribution of scattering material in the Solar neighborhood is not uniform. Our model suggests the presence of a low density bubble surrounded by a shell of much higher density fluctuations. We are able to put some constraints on geometrical and scattering properties of such a structure, and find it to be morphologically similar to the Local Bubble known from other studies.Comment: 5 pages, 3 figure

On the peculiarities in the rotational frequency evolution of isolated neutron stars

The measurements of pulsar frequency second derivatives have shown that they are $10^2-10^6$ times larger than expected for standard pulsar spin-down law, and are even negative for about half of pulsars. We explain these paradoxical results on the basis of the statistical analysis of the rotational parameters $\nu$, $\dot \nu$ and $\ddot \nu$ of the subset of 295 pulsars taken mostly from the ATNF database. We have found a strong correlation between $\ddot \nu$ and $\dot \nu$ for both $\ddot\nu > 0$ and $\ddot\nu < 0$, as well as between $\nu$ and $\dot\nu$. We interpret these dependencies as evolutionary ones due to $\dot\nu$ being nearly proportional to the pulsars' age. The derived statistical relations as well as "anomalous" values of $\ddot\nu$ are well described by assuming the long-time variations of the spin-down rate. The pulsar frequency evolution, therefore, consists of secular change of $\nu_{ev}(t)$, $\dot\nu_{ev}(t)$ and $\ddot\nu_{ev}(t)$ according to the power law with $n \approx 5$, the irregularities, observed within a timespan as a timing noise, and the variations on the timescale larger than that timespan -- several tens of years.Comment: 4 pages, 3 figures. Accepted for publication in ApSS, in the proceedings of the conference "Isolated Neutron Stars: from the Interior to the Surface", London, April 2006; eds. S. Zane, R. Turolla and D. Pag

Plasma Turbulence in the Local Bubble

Turbulence in the Local Bubble could play an important role in the thermodynamics of the gas that is there. The best astronomical technique for measuring turbulence in astrophysical plasmas is radio scintillation. Measurements of the level of scattering to the nearby pulsar B0950+08 by Philips and Clegg in 1992 showed a markedly lower value for the line-of-sight averaged turbulent intensity parameter $$than is observed for other pulsars, consistent with radio wave propagation through a highly rarefied plasma. In this paper, we discuss the observational progress that has been made since that time. At present, there are four pulsars (B0950+08, B1133+16, J0437-4715, and B0809+74) whose lines of sight seem to lie mainly within the local bubble. The mean densities and line of sight components of the interstellar magnetic field along these lines of sight are smaller than nominal values for pulsars, but not by as much expected. Three of the four pulsars also have measurements of interstellar scintillation. The value of the parameter$$ is smaller than normal for two of them, but is completely nominal for the third. This inconclusive status of affairs could be improved by measurements and analysis of ``arcs'' in ``secondary spectra'' of pulsars.Comment: Submitted to Space Science Reviews as contribution to Proceedings of ISSI (International Space Science Institute) workshop "From the Heliosphere to the Local Bubble". Refereed version accepted for publicatio

The Microarcsecond Sky and Cosmic Turbulence

Radio waves are imprinted with propagation effects from ionized media through which they pass. Owing to electron density fluctuations, compact sources (pulsars, masers, and compact extragalactic sources) can display a wide variety of scattering effects. These scattering effects, particularly interstellar scintillation, can be exploited to provide *superresolution*, with achievable angular resolutions (<~ 1 microarcsecond) far in excess of what can be obtained by very long baseline interferometry on terrestrial baselines. Scattering effects also provide a powerful sub-AU probe of the microphysics of the interstellar medium, potentially to spatial scales smaller than 100 km, as well as a tracer of the Galactic distribution of energy input into the interstellar medium through a variety of integrated measures. Coupled with future gamma-ray observations, SKA observations also may provide a means of detecting fainter compact gamma-ray sources. Though it is not yet clear that propagation effects due to the intergalactic medium are significant, the SKA will either detect or place stringent constraints on intergalactic scattering.Comment: 20 pages, 8 figures in 8 PostScript files, to appear in "Science with the Square Kilometer Array," eds. C. Carilli and S. Rawlings, New Astronomy Reviews (Elsevier: Amsterdam

Lognormal Properties of SGR 1806-20 and Implications for Other SGR Sources

The time interval between successive bursts from SGR 1806-20 and the intensity of these bursts are both consistent with lognormal distributions. Monte Carlo simulations of lognormal burst models with a range of distribution parameters have been investigated. The main conclusions are that while most sources like SGR 1806-20 should be detected in a time interval of 25 years, sources with means about 100 times longer have a probability of about 5\% of being detected in the same interval. A new breed of experiments that operate for long periods are required to search for sources with mean recurrence intervals much longer than SGR 1806-20.Comment: 4 pages, latex with seperate file containing 2 uuencoded, gzip'ed, tarred, .eps figures. Replaced with file that does not use kluwer.sty to allow automatic postscript generation. To appear in proceedings of ESLAB 2

The Contribution of Fermi Gamma-Ray Pulsars to the local Flux of Cosmic-Ray Electrons and Positrons

We analyze the contribution of gamma-ray pulsars from the first Fermi-Large Area Telescope (LAT) catalogue to the local flux of cosmic-ray electrons and positrons (e+e-). We present new distance estimates for all Fermi gamma-ray pulsars, based on the measured gamma-ray flux and pulse shape. We then estimate the contribution of gamma-ray pulsars to the local e+e- flux, in the context of a simple model for the pulsar e+e- emission. We find that 10 of the Fermi pulsars potentially contribute significantly to the measured e+e- flux in the energy range between 100 GeV and 1 TeV. Of the 10 pulsars, 2 are old EGRET gamma-ray pulsars, 2 pulsars were discovered with radio ephemerides, and 6 were discovered with the Fermi pulsar blind-search campaign. We argue that known radio pulsars fall in regions of parameter space where the e+e- contribution is predicted to be typically much smaller than from those regions where Fermi-LAT pulsars exist. However, comparing the Fermi gamma-ray flux sensitivity to the regions of pulsar parameter space where a significant e+e- contribution is predicted, we find that a few known radio pulsars that have not yet been detected by Fermi can also significantly contribute to the local e+e- flux if (i) they are closer than 2 kpc, and if (ii) they have a characteristic age on the order of one mega-year.Comment: 21 pages, 6 figures, accepted for publication in JCA

Twisting gauged non-linear sigma-models

We consider gauged sigma-models from a Riemann surface into a Kaehler and hamiltonian G-manifold X. The supersymmetric N=2 theory can always be twisted to produce a gauged A-model. This model localizes to the moduli space of solutions of the vortex equations and computes the Hamiltonian Gromov-Witten invariants. When the target is equivariantly Calabi-Yau, i.e. when its first G-equivariant Chern class vanishes, the supersymmetric theory can also be twisted into a gauged B-model. This model localizes to the Kaehler quotient X//G.Comment: 33 pages; v2: small additions, published versio

Asymmetric neutrino emission due to neutrino-nucleon scatterings in supernova magnetic fields

We derive the cross section of neutrino-nucleon scatterings in supernova magnetic fields, including weak-magnetism and recoil corrections. Since the weak interaction violates the parity, the scattering cross section asymmetrically depends on the directions of the neutrino momenta to the magnetic field; the origin of pulsar kicks may be explained by the mechanism. An asymmetric neutrino emission (a drift flux) due to neutrino-nucleon scatterings is absent at the leading level of $\mathcal O(\mu_BB/T)$, where $\mu_B$ is the nucleon magneton, $B$ is the magnetic field strength, and $T$ is the matter temperature at a neutrinosphere. This is because at this level the drift flux of the neutrinos are exactly canceled by that of the antineutrinos. Hence, the relevant asymmetry in the neutrino emission is suppressed by much smaller coefficient of $\mathcal O(\mu_BB/m)$, where $m$ is the nucleon mass; detailed form of the relevant drift flux is also derived from the scattering cross section, using a simple diffusion approximation. It appears that the asymmetric neutrino emission is too small to induce the observed pulsar kicks. However, we note the fact that the drift flux is proportional to the deviation of the neutrino distribution function from the value of thermal equilibrium at neutrinosphere. Since the deviation can be large for non-electron neutrinos, it is expected that there occurs cancellation between the deviation and the small suppression factor of $\mathcal O(\mu_BB/m)$. Using a simple parameterization, we show that the drift flux due to neutrino-nucleon scatterings may be comparable to the leading term due to beta processes with nucleons, which has been estimated to give a relevant kick velocity when the magnetic field is sufficiently strong as $10^{15}$--$10^{16}$ G.Comment: 19 pages, 1 figure. Accepted by Physical Review