Vela's clouds have a silver lining




Using a shell of gas around the Vela pulsar as a fuzzy magnifying glass,
a team led by C. Gwinn (Uni. California, Santa Barbara) has made an image
of the pulsar at an angular resolution of about 10 nanoarcseconds. This
is probably the highest-resolution image ever made of a cosmic object &shyp;
and equivalent to resolving a virus at the distance of the Moon.



The gas in question was the Vela supernova remnant, irregular wisps
of ionised material swept up and heated by the the shock wave of a supernova
about 10 000 years ago. Fluctuations in the density of the gas scatter the
radio emission from the pulsar, blurring the image we can make from it.
But the scattering process does not always destroy information in the signal:
rather, it can add to it. The interstellar plasma acts like an imperfect
lens, convolving ('smearing') the ideal image of the pulsar with a point-spread
function. In principle, measuring this convolved image can yield not only
a magnified image of the source but also complete information on the 'scattering
screen' &shyp; the gas itself.



The deconvolution technique was applied to observations of the Vela
pulsar made with a VLBI (very long baseline interferometry) network of telescopes
at Tidbinbinbilla, Parkes,
Hobart and
Hartebeesthoek (South Africa)
and the VLBA (Very
Long Baseline Array) in the USA. The result was the first image of a pulsar's
radiosphere &shyp; a region perhaps 1 000 km in size from which the radio
pulses emanate.



C. Gwinn (Uni. California, Santa Barbara); D. Jauncey, J. Reynolds (ATNF);


P. McCulloch, E. King (Uni. Tasmania); C. Flanagan (Hartebeesthoek RAO)




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