Improving Pulsar Distances by Modelling Interstellar Scattering

We present here a method to study the distribution of electron density fluctuations in pulsar directions as well as to estimate pulsar distances. The method, based on a simple two-component model of the scattering medium discussed by Gwinn et al. (1993), uses scintillation & proper motion data in addition to the measurements of angular broadening & temporal broadening to solve for the model parameters, namely, the fractional distance to a discrete scatterer and the ascociated relative scattering strength. We show how this method can be used to estimate pulsar distances reliably, when the location of a discrete scatterer (e.g. an HII region), if any, is known. Considering the specific example of PSR B0736-40, we illustrate how a simple characterization of the Gum nebula region (using the data on the Vela pulsar) is possible and can be used along with the temporal broadening measurements to estimate pulsar distances.Comment: To be published in MNRAS, 7 pages, 3 figure

Upper Limits on the Pulsed Radio Emission from the Geminga Pulsar at 35 & 327 MHz

We report here our observations at 35 MHz and 327 MHz made in the direction of the Gamma Ray pulsar Geminga. Based on the observed absence of any significant pulsed emission from this source above our detection thresholds at the two frequencies, we obtain useful upper-limits for the average flux to be 75-100 mJy at 35 MHz, and 0.2-0.3 mJy at 327 MHz. We discuss a few possible reasons for the ``radio-quiet'' nature of this pulsar at frequencies other than around 100 MHz.Comment: Accepted for publication in Astronomy & Astrophysic

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

Non-Leptonic B Decays into K-Resonances

We estimate the non-leptonic B decays $B \rightarrow (\psi ,\psi^\prime , \chi_{1c})+K^i$, where $K^i$ are various K-meson resonances. We use the model of Isgur, Wise, Scora and Grinstein in the context of heavy quark effective theory, to calculate the hadronic matrix elements. Our estimates show that a substantial fraction of $B \rightarrow X_s \psi$ results in higher resonances of K-meson and besides $B \rightarrow K(K^*) \psi$, a considerable fraction of $B \rightarrow X_s (c\bar c)$ goes to $B \rightarrow (K,K^*)+( \psi^{\prime},\chi_{1c})$.Comment: 9 pages, late

Search for Discrete Refractive Scattering Events

We have searched for discrete refractive scattering events (including effects due to possible non-multiple diffractive scattering) at meter wavelengths in the direction of two close by pulsars B0950+08 and B1929+10, where we looked for spectral signatures associated with the multiple imaging of pulsars due to scattering in the interstellar medium. We do not find any signatures of such events in the direction of either source over a spectral periodicity range of 50 KHz to 5 MHz. Our analysis puts strong upper limits on the column density contrast associated with a range of spatial scales of the interstellar electron density irregularities along these lines of sight.Comment: Accepted for publication in Astronomy & Astrophysic

Rail transit fare collection: Policy and technology assessment

The impact of fare policies and fare structure on the selection of equipment was investigated, fare collection systems are described, hardware and technology related problems are documented, and the requirements of a fare collection simulation model are outlined. Major findings include: (1) a wide variation in the fare collection systems and equipment, caused primarily by historical precedence; (2) the reliability of AFC equipment used at BART and WMATA discouraged other properties from considering use of similar equipment; (3) existing equipment may not meet the fare collection needs of properties in the near future; (4) the cost of fare collection operation and maintenance is high; and (5) the relatively small market in fare collection equipment discourages new product development by suppliers. Recommendations for fare collection R&D programs include development of new hardware to meet rail transit needs, study of impacts of alternate fare policies increased communication among policymakers, and consensus on fare policy issues

Small-Scale Variations of HI Spectra from Interstellar Scintillatio

I suggest that radio-wave scattering by the interstellar plasma, in combination with subsonic gradients in the Doppler velocity of interstellar HI, is responsible for the observed small-scale variation in HI absorption spectra of pulsars. Velocity gradients on the order of 0.05 to 0.3 km/s across 1 AU can produce the observed variations. I suggest observational tests to distinguish between this model and the traditional picture of small-scale opacity variations from cloudlets.Comment: 24 pages, 2 figures, Latex, uses AASTe