Witnessing the birth of a radio supernova.

Supernova 1987A is on the air again with a new, intriguing radio message. After a lengthy interval of quiet following a brief, initial outburst of radio waves associated with the violent explosion of a blue supergiant star, the region surrounding the star's remains has resumed emitting at radio frequencies.

Radio astronomers in Australia first detected these renewed -- but then faint -- signals in the summer of 1990, more than three years after the initial explosion. Since then, the signals have grown stronger, but this increase in intensity has occurred unevenly across the monitored frequencies.

"This is the first time the birth of a nearby radio supernova remnant has been witnessed, and future observations will allow the structure of the remnant to be compared with the many other known radio remnants," researchers at the Australia Telescope National Facility and the University of Sydney write in the Jan. 9 NATURE.

The stellar explosion that created supernova 1987A generated prodigious amounts of electromagnetic radiation over a wide range of wavelengths. It also hurled vast quantities of matter -- electrons and ions -- into space.

Astronomers expected the impact of the ejected material, traveling at roughly one-tenth the speed of light, to cause tremendous shock waves in any gas clouds surrounding the exploded star. This interaction would accelerate electrons to nearly the speed of light. As these relativistic electrons spiraled down magnetic field lines in a cloud of gas, they would emit radio waves at particular frequencies.

The fact that radio waves from supernova 1987A remained undetectable at the monitored frequency of 843 megahertz until July 6, 1990, and at higher frquencies until Aug. 16, 1990, suggested the absence of gas clouds in the region immediately surrounding the central object. This scenario fits with the notion that a fierce stellar wind from the blue supergiant had scoured out the region just before the star finally exploded. Radio emissions resumed when ejected material eventually encountered sufficiently dense clumps of gas.

Having carefully examining the supernova's belated radio emissions, the Australian team concludes that the signals come from an extended source surrounding the explosion site, rather than from a compact object such as a central pulsar. The bulk of the radio waves appears to emanate from clumps of material that lie between the extinct star and an outlying, well-defined ring of dense gas about 1.37 light-years across, the researchers report.

"This hypothesis is consistent with the absence so far of soft X-ray emission from the region," they note. "Strong X-ray emission is expected when the blast wave encounters the relatively dense ring material.

The apprent width of the radio-emitting region indicates that the supernova blast wave is moving at roughly 30,000 kilometers per second. At this rate, the shock wave's first impact with the outlying ring may occur within a few years, the researchers suggest.

Fluctuations in the intensity of the radio wave emissions at various frequencies provide indirect clues to the geometry and distribution of the gas clumps surrounding the exploded star. Last summer, a rapid increase in the intensity of radiation at 843 megahertz wihout a corresponding increase at higher frequencies indicated that the blast wave may have reached a new clump of circumstellar gas, the researchers say. As electrons in the new region are accelerated to higher energies, the higher-frequency emissions will probably catch up.

"Further observations will help to establish parameters of the electron acceleration process and the relationship between the radio and [visible-light] emitting regions," the team concludes.