C. R. Gwinn

VLBI Imaging of Water Maser Emission from the Nuclear Torus of NGC 1068

We have made the first VLBI synthesis images of the H2O maser emission associated with the central engine of the Seyfert galaxy NGC 1068. Emission extends about ±300 km s-1 from the systemic velocity. Images with sub-milliarcsecond angular resolution show that the redshifted emission lies along an arc to the northwest of the systemic emission. (The blueshifted emission has not yet been imaged with VLBI.) Based on the maser velocities and the relative orientation of the known radio jet, we propose that the maser emission arises on the surface of a nearly edge-on torus, where physical conditions are conducive to maser action. The visible part of the torus is axially thick, with comparable height and radius. The velocity field indicates sub-Keplerian differential rotation around a central mass of ~1 × 107 M☉ that lies within a cylindrical radius of about 0.65 pc. The estimated luminosity of the central engine is about 0.5 of the Eddington limit. There is no detectable compact radio continuum emission near the proposed center of the torus (TB < 5 × 106 K on size scales of ~0.1 pc), so that the observed flat-spectrum core cannot be direct self-absorbed synchrotron radiation.

The distance to the center of the Galaxy: H2O maser proper motions in sagittarius B2(N)

Onsala Space Observatory The distance to a star forming region can be determined by measuring the proper motions within H 2 0 maser clusters. If the motions of the maser spots are random, the distance can be determined by applying the technique known as statistical parallax. Alternatively, if organized motions are evident in the proper motions, one can model the source to estimate its the distance. Both methods rely on a comparison of the radial component of the motion (in km/s) and the proper motion on the plane of the sky (in milli-arcseconds/year).

Evidence for an inner scale to the density turbulence in the interstellar medium

The form of the density power spectrum is studied. The relationship between the visibility and the properties of density turbulence is discussed. Observational determinations of the power-law index of the interstellar density power spectrum are presented. An inner scale is derived, and the effect of an inner scale in the interstellar plasma turbulence is examined. It is concluded that the host plasma for the density irregularities must be similar to the warm ionized medium in the Mckee-Ostriker model (1979) of the interstellar medium or the low-density envelopes of H II regions. 32 refs.

Quasar proper motions and low-frequency gravitational waves

We report observational upper limits on the mass-energy of the cosmological gravitational wave background, from limits on proper motions of quasars. Gravitational waves with periods longer than the time span of observations produce a simple pattern of apparent proper motions over the sky, composed primarily of second-order transverse vector spherical harmonics. A fit of such harmonics to measured motions yields a 95% confidence limit on the mass-energy of gravitational waves with frequencies ν < 2 × 10-9 Hz, of less than 0.11 h-2 times the closure density of the universe.

The multicomponent nature of the Vela pulsar nonthermal X-ray spectrum

We report on our analysis of a 274 ks observation of the Vela pulsar with the Rossi X-Ray Timing Explorer (RXTE). The double-peaked, pulsed emission at 2-30 keV, which we had previously detected during a 93 ks observation, is confirmed with much-improved statistics. There is now clear evidence, in both the spectrum and the light curve, that the emission in the RXTE band is a blend of two separate nonthermal components. The spectrum of the harder component connects smoothly with the OSSE, COMPTEL, and EGRET spectra, and the peaks in the light curve are in phase coincidence with those of the high-energy light curve. The spectrum of the softer component is consistent with an extrapolation to the pulsed optical flux, and the second RXTE pulse is in phase coincidence with the second optical peak. In addition, we see a peak in the 2-8 keV RXTE pulse profile at the radio phase.

Imaging an event horizon: Mitigation of scattering toward Sagittarius a

The image of the emission surrounding the black hole in the center of the Milky Way is predicted to exhibit the imprint of general relativistic (GR) effects, including the existence of a shadow feature and a photon ring of diameter ~50 microarcseconds. Structure on these scales can be resolved by millimeter-wavelength very long baseline interferometry (VLBI). However, strong-field GR features of interest will be blurred at lambda >= 1.3 mm due to scattering by interstellar electrons. The scattering properties are well understood over most of the relevant range of baseline lengths, suggesting that the scattering may be (mostly) invertible. We simulate observations of a model image of Sgr A* and demonstrate that the effects of scattering can indeed be mitigated by correcting the visibilities before reconstructing the image. This technique is also applicable to Sgr A* at longer wavelengths.

Gravitational radiation and very long baseline interferometry

Gravitational waves affect the observed direction of light from distant sources. At telescopes, this change in direction appears as periodic variations in the apparent positions of these sources on the sky; that is, as proper motion. A wave of a given phase, traveling in a given direction, produces a characteristic pattern of proper motions over the sky. Comparison of observed proper motions with this pattern serves to test for the presence of gravitational waves. A stochastic background of waves induces apparent proper motions with specific statistical properties, and so, may also be sought. In this paper we consider the effects of a cosmological background of gravitational radiation on astrometric observations. We derive an equation for the time delay measured by two antennae observing the same source in an Einstein-de Sitter spacetime containing gravitational radiation. We also show how to obtain similar expressions for curved Friedmann-Robertson-Walker spacetimes.

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.

SCINTILLATIONS AND LEVY FLIGHTS THROUGH THE INTERSTELLAR MEDIUM

Temporal broadening of pulsar signals results from electron density fluctuations in the interstellar medium that cause the radiation to travel along paths of different lengths. The Gaussian theory of fluctuations predicts that the pulse temporal broadening should scale with the wavelength as lambda^4, and with the dispersion measure (corresponding to distance to the pulsar) as DM^2. For large dispersion measure, DM > 20 pc/cm^3, the observed scaling is lambda^4 DM^4, contradicting the conventional theory. Although the problem has existed for 30 years, there has been no resolution to this paradox. We suggest that scintillations for distant pulsars are caused by non-Gaussian, spatially intermittent density fluctuations with a power-like probability distribution. This probability distribution does not have a second moment in a large range of density fluctuations, and therefore the previously applied conventional Fokker-Planck theory does not hold. Instead, we propose to apply the theory of Levy distributions (so-called Levy flights). Using the scaling analysis (confirmed by numerical simulations of ray propagation) we show that the observed scaling is recovered for large DM, if the density differences, delta N, have Levy distribution decaying as |delta N|^{-5/3}.

DISCOVERY OF SUBSTRUCTURE IN THE SCATTER-BROADENED IMAGE OF SGR A*

We have detected substructure within the smooth scattering disk of the celebrated Galactic Center radio source Sagittarius A* (SgrA*). We observed this structure at 1.3 cm wavelength with the Very Long Baseline Array together with the Green Bank Telescope, on baselines of up to 3000 km, long enough to completely resolve the average scattering disk. Such structure is predicted theoretically, as a consequence of refraction by large-scale plasma fluctuations in the interstellar medium. Along with the much-studied