arXiv subjects

C. R. Gwinn

Publications and source records attributed to C. R. Gwinn.

24 records · Page 2Linked to original sources

arXiv1999

Size of the Vela Pulsar's Emission Region at 13 cm Wavelength

We present measurements of the size of the Vela pulsar in 3 gates across the pulse, from observations of the distribution of intensity. We calculate the effects on this distribution of noise in the observing system, and measure and remove it using observations of a strong continuum source. We also calculate and remove the expected effects of averaging in time and frequency. We find that effects of variations in pulsar flux density and instrumental gain, self-noise, and one-bit digitization are undetectably small. Effects of normalization of the correlation are detectable, but do not affect the fitted size. The size of the pulsar declines from 440 +/- 90 km (FWHM of best-fitting Gaussian distribution) to less than 200 km across the pulse. We discuss implications of this size for theories of pulsar emission.

astro-ph↗

arXiv1998

Coexisting conical bipolar and equatorial outflows from a high-mass protostar

The BN/KL region in the Orion molecular cloud is an archetype in the study of the formation of stars much more massive than the Sun. This region contains luminous young stars and protostars, but it is difficult to study because of overlying dust and gas. Our basic expectations are shaped to some extent by the present theoretical picture of star formation, the cornerstone of which is that protostars acrete gas from rotating equatorial disks, and shed angular momentum by ejecting gas in bipolar outflows. The main source of the outflow in the BN/KL region may be an object known as radio source I, which is commonly believed to be surrounded by a rotating disk of molecular material. Here we report high-resolution observations of silicon monoxide (SiO) and water maser emission from the gas surrounding source I; we show that within 60 AU (about the size of the Solar System), the region is dominated by a conical bipolar outflow, rather than the expected disk. A slower outflow, close to the equatorial plane of the protostellar system, extends to radii of 1,000 AU.

astro-ph↗

arXiv1997

VLBA Imaging of the OH Maser in IIIZw35

We present a parsec-scale image of the OH maser in the nucleus of the active galaxy IIIZw35, made using the Very Long Baseline Array at a wavelength of 18 cm. We detected two distinct components, with a projected separation of 50 pc (for D=110 Mpc) and a separation in Doppler velocity of 70 km/s, which contain 50% of the total maser flux. Velocity gradients within these components could indicate rotation of clouds with binding mass densities of ~7000 solar masses per cubic parsec, or total masses of more than 500,000 solar masses. Emission in the 1665-MHz OH line is roughly coincident in position with that in the 1667-MHz line, although the lines peak at different Doppler velocities. We detected no 18 cm continuum emission; our upper limit implies a peak apparent optical depth greater than 3.4, assuming the maser is an unsaturated amplifier of continuum radiation.

astro-ph↗

arXiv1996

Angular Broadening of Nearby Pulsars

We conducted a very long baseline interferometric observation of 5 nearby pulsars and did not resolve the scattering disks of any of these sources. Using our upper limits on the angular diameters of these scattering disks and published values of the broadening times and proper motion velocities, we constrain the possible distributions of scattering material. The material responsible for scattering these sources is neither uniformly distributed nor concentrated at the surface of the Local Bubble. We argue that these pulsars themselves influence their environments to produce this scattering material.

astro-ph↗

arXiv1996

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 from the systemic velocity. Images with submilliarcsecond angular resolution show that the red-shifted emission lies along an arc to the northwest of the systemic emission. (The blue-shifted 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 about 1e7 Msun 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 (T_B< 5e6 K on size scales of about 0.1 pc), so that the observed flat-spectrum core cannot be direct self-absorbed synchrotron radiation.

astro-ph↗

arXiv1996

Size of the Vela Pulsar's Radio Emission Region: 500 km

We use interstellar scattering of the Vela pulsar to determine the size of its emission region. From interferometric phase variations on short baselines, we find that radio-wave scattering broadens the source by 3.4+/-0.3 milliarcseconds along the major axis at position angle 81+/-3 degrees. The ratio of minor axis to major axis is 0.51+/-0.03. Comparison of angular and temporal broadening indicates that the scattering material lies in the Vela-X supernova remnant surrounding the pulsar. From the modulation of the pulsar's scintillation on very short baselines, we infer a size of 500 km for the pulsar's emission region. We suggest that radio-wave refraction within the pulsar's magnetosphere may plausibly explain this size.

astro-ph↗