Astronomers puzzle over exploding star's `mystery companion'

It's known as the supernova's ``mystery companion.'' The object lies about 2 light weeks (about 22.5 billion miles) away from Supernova 1987A and is about one-tenth as brilliant, making it much brighter than the supernova's progenitor. The Large Magellenic Cloud, the galaxy in which the supernova, or exploding star, appears, is very well studied, researchers say, and no image that bright has ever appeared in the mystery companion's position.

``This thing is so unbelievable!'' says Boston University astrophysicist Kenneth Bresher. ``There are no plausible explanations for it.''

Peter Nisenson and Costas Papaliolios, discovered the object after analyzing images they'd taken on March 25 and April 2 at the Cerro Tololo Interamerican Observatory in Chile. Both astronomers are with the Harvard-Smithsonian Center for Astrophysics. The sighting was confirmed by a group of astronomers at London's Imperial College using the Anglo-Australian telescope located some 200 miles west of Sydney.

Dr. Nisenson and Margarita Karovska, also of the Center for Astrophysics, returned to Chile to make more observations that would give them an idea of the object's size and composition. Hopes ran high at a recent joint meeting of the Canadian and American Astronomical Societies here that the new data would be presented. But those hopes were dashed by a balky computer at the Center for Astrophysics.

Richard A. McCray, with the Joint Institute for Laboratory Astrophysics at the University of Colorado at Boulder, points out that if the object is real, there should be some fireworks when the shell of the supernova slams into it sometime next month. The result: ``a blaze of x-rays, gamma rays, and ultraviolet light,'' he says.

While scientists puzzle over the mystery companion, another mystery has been cleared up. After some initial confusion, the progenitor has been identified as a blue supergiant star known as Sanduleak -69 202.

Identifying the progenitor as a blue supergiant seems to explain other oddities of the supernova: its faintness relative to other supernovae in its class and the fact that its brightness peaked some 3 months after it went off, rather than immediately. Scientists have explained the quick peak in other supernovae of the same class by saying their progenitors are red supergiant stars, instead of the smaller, blue supergiants. This is a tidy way to account for the difference, says astrophysicist Stanford Woosley at the University of California at Santa Cruz.

At 153,000 light years away, the 1987A supernova is some 10,000 times brighter than those seen in other galaxies. Hence, astronomers say, we wouldn't be likely to see the more distant supernovae until they had peaked. The inability to observe the early stages of distant supernovae would lead scientists to conclude that blue supergiants weren't involved.

Now that the supernova's shell is thinning, astronomers are eager to see what has been energizing the shell for so long. The supernova's neutrinos confirm that the Sanduleak's core collapsed into a dense neutron star. Depending on the star's temperature and rate of spin, it could be a pulsar, emitting radiation in regular spurts. Moreover, radioactive decay from elements formed in the stellar collapse would also light up the shell.

Dr. McCray says that both processes are likely to be present - the question is, which process dominates? The first clues should come from x-ray radiation, which should start to leak through the shell this fall, he says.