David J. McConnell

An H I Aperture Synthesis Mosaic of the Large Magellanic Cloud

We present the results of an H I aperture synthesis mosaic of the Large Magellanic Cloud (LMC), made by combining data from 1344 separate pointing centers using the Australia Telescope Compact Array (ATCA). The resolution of the mosaicked images is 10 (15 pc, using a distance to the LMC of 50 kpc). This mosaic, with a spatial resolution 15 times higher than that which had been previously obtained, emphasizes the turbulent and fractal structure of the ISM on the small scale, resulting from the dynamical feedback of the star formation processes with the ISM. The structure of the neutral atomic ISM in the LMC is dominated by H I filaments combined with numerous shells and holes. On the large scale, the H I disk appears to be remarkably symmetric and to have a well-organized and orderly, if somewhat complex, rotational field. The bulk of the H I resides in a disk 7.3 kpc in diameter. The mass of the disk component of the LMC is 2.5 × 109 M☉, and the upper limit to all mass within a radius of 4 kpc is ~3.5 × 109 M☉.

Coherent Radiation Patterns Suggested by Single-Pulse Observations of a Millisecond Pulsar

We have observed intense radiation spikes from the millisecond pulsar PSR J0437-4715 and find their distribution in pulsar spin phase to be quite unlike the integrated pulse profile. The spikes are observed almost exclusively in a 10° phase window centered on the main pulse. Within that window the phase distribution has a periodic variation, producing a pattern reminiscent of the diffraction fringes associated with coherent emission from a finite aperture. We propose a source model in which the spikes are emitted coherently from a region with spatial scale ≈ 100 m.

Radio Emission of PSR J0437–4715 at 327 MHz

More than 412,000 single pulses from the brightest millisecond radio pulsar PSR J0437-4715 were obtained at 327 MHz using the Ooty Radio Telescope. This pulsar displays neither the nulling nor the systematic drifting phenomenon found in several normal (long period) pulsars. A rigorous upper limit to the percentage of nulling in PSR J0437-4715 is 0.0016%. However, its integrated profile varies on timescales ranging from ≈ 500 periods to days; the variation is slow and unlike the mode-changing phenomenon of normal pulsars. About 3% of the intensity of PSR J0437-4715 varies quasi-periodically, particularly at early pulse phases within the integrated profile. Its fluctuation spectrum has a weak feature at the mean value of 0.253 ± 0.002 cycles per period, with an average rms width of 0.067 ± 0.005 cycles per period, but both parameters vary from phase to phase within the integrated profile, which has not been noticed so far in pulsars. Our results are important to resolve the issue whether the radio emission of millisecond pulsars is similar to that of normal pulsars.

A CATALOG OF H i CLOUDS IN THE LARGE MAGELLANIC CLOUD

A 21 cm neutral hydrogen interferometric survey of the Large Magellanic Cloud (LMC) combined with the Parkes multi-beam HI single-dish survey clearly shows that the HI gas is distributed in the form of clumps or clouds. The HI clouds and clumps have been identified using a thresholding method with three separate brightness temperature thresholds (

Identifications from the Parkes-MIT-NRAO Surveys

T_b

Beamforming for the ASKAP Radio Telescope

). Each catalog of HI cloud candidates shows a power law relationship between the sizes and the velocity dispersions of the clouds roughly following the Larson Law scaling

Catalog of LMC HI clouds (Kim+, 2007)

\sigma_v \propto R^{0.5}

Parkes 300 to 900 MHz Rotation Measure Survey

, with steeper indices associated with dynamically hot regions. The clouds in each catalog have roughly constant virial parameters as a function mass suggesting that that the clouds are all in roughly the same dynamical state, but the values of the virial parameter are significantly larger than unity showing that turbulent motions dominate gravity in these clouds. The mass distribution of the clouds is a power law with differential indices between -1.6 and -2.0 for the three catalogs. In contrast, the distribution of mean surface densities is a log-normal distribution.