Theory and Simulations of Refractive Substructure in Resolved Scatter-Broadened Images

At radio wavelengths, scattering in the interstellar medium distorts the appearance of astronomical sources. Averaged over a scattering ensemble, the result is a blurred image of the source. However, Narayan & Goodman (1989) and Goodman & Narayan (1989) showed that for an incomplete average, scattering introduces refractive substructure in the image of a point source that is both persistent and wideband. We show that this substructure is quenched but not smoothed by an extended source. As a result, when the scatter-broadening is comparable to or exceeds the unscattered source size, the scattering can introduce spurious compact features into images. In addition, we derive efficient strategies to numerically compute realistic scattered images, and we present characteristic examples from simulations. Our results show that refractive substructure is an important consideration for ongoing missions at the highest angular resolutions, and we discuss specific implications for RadioAstron and the Event Horizon Telescope.Comment: Equation numbering in appendix now matches published version. Two minor typos correcte

The history of Sutter\u27s Fort, 1839-1931

Of all the stories and articles written about Captain J.A. Sutter in recent years, not one of them has adequately covered the history of his famous Fort. To the layman the fort itself may seem unimportant, but when we stop to consider that it was an outpost of civilization penetrating the wilderness of Central California, offering shelter to those who pioneered before us; we must confess that its existence was necessary to protect and usher into full City-hood, the infant Sacramento, then known as New Helvetia

Optimal Correlation Estimators for Quantized Signals

Using a maximum-likelihood criterion, we derive optimal correlation strategies for signals with and without digitization. We assume that the signals are drawn from zero-mean Gaussian distributions, as is expected in radio-astronomical applications, and we present correlation estimators both with and without a priori knowledge of the signal variances. We demonstrate that traditional estimators of correlation, which rely on averaging products, exhibit large and paradoxical noise when the correlation is strong. However, we also show that these estimators are fully optimal in the limit of vanishing correlation. We calculate the bias and noise in each of these estimators and discuss their suitability for implementation in modern digital correlators.Comment: 8 Pages, 3 Figures, Submitted to Ap

Size of the Vela Pulsar's Emission Region at 18 cm Wavelength

We present measurements of the linear diameter of the emission region of the Vela pulsar at observing wavelength lambda=18 cm. We infer the diameter as a function of pulse phase from the distribution of visibility on the Mopra-Tidbinbilla baseline. As we demonstrate, in the presence of strong scintillation, finite size of the emission region produces a characteristic W-shaped signature in the projection of the visibility distribution onto the real axis. This modification involves heightened probability density near the mean amplitude, decreased probability to either side, and a return to the zero-size distribution beyond. We observe this signature with high statistical significance, as compared with the best-fitting zero-size model, in many regions of pulse phase. We find that the equivalent full width at half maximum of the pulsar's emission region decreases from more than 400 km early in the pulse to near zero at the peak of the pulse, and then increases again to approximately 800 km near the trailing edge. We discuss possible systematic effects, and compare our work with previous results

Ultra-High Resolution Intensity Statistics of a Scintillating Source

We derive the distribution of flux density of a compact source exhibiting strong diffractive scintillation. Our treatment accounts for arbitrary spectral averaging, spatially-extended source emission, and the possibility of intrinsic variability within the averaging time, as is typical for pulsars. We also derive the modulation index and present a technique for estimating the self-noise of the distribution, which can be used to identify amplitude variations on timescales shorter than the spectral accumulation time. Our results enable a for direct comparison with ultra-high resolution observations of pulsars, particularly single-pulse studies with Nyquist-limited resolution, and can be used to identify the spatial emission structure of individual pulses at a small fraction of the diffractive scale.Comment: 14 Pages, 4 Figures, accepted for publication in Ap

On the Ionisation of Warm Opaque Interstellar Clouds and the Intercloud Medium

In this paper we use a number of observations to construct an integrated picture of the ionisation in the interiors of quiescent warm opaque interstellar clouds and in the intercloud medium (ICM) outside dense HII regions and hot dilute bubbles. Our main conclusion is that within $\sim$ 1kpc of the sun the ionisation rate of hydrogen per unit volume in both the interiors of such clouds and in the ICM is independent of the local density of neutral hydrogen, and varies with position by less than $\sim$ 20 per cent. These conclusions strongly favour the decaying neutrino hypothesis for the ionisation of the interstellar medium in these regions. Our analysis is based on a variety of observations, of which the most remarkable is the discovery by Spitzer and Fitzpatrick (1993) that, in the four slowly moving clouds along the line of sight to the halo star HD93521, the column densities of both SII and CII$^*$, which individually range over a factor $\sim$4, are proportional to the column density of HI to within $\sim$20 per cent. This proportionality is used to show that the free electrons exciting the CII to CII$^*$ are located mainly in the interiors of the clouds, rather than in their skins, despite the large opacity of the clouds to Lyman continuum radiation. The same conclusion also follows more unambiguously from the low value of the H$\alpha$ flux in this direction which was found by Reynolds (1996) in unpublished observations. These results are then used, in conjunction with observations of three pulsar parallaxes and dispersion measures, and with data on HeI, NII and OI line emissions, to constrain the ionisation of H, He, N and O and the flux of Lyman continuum photons from O stars in the ICM.Comment: 16 pages, no figures, Latex fil