Supernova's light curve tells its tail.

Supernova's Light Curve Tells Its Tail

Since reaching its maximum brightness 85 days after its core explosively collapsed almost 22 months ago, supernova 1987 A has gotten steadily fainter. But that may be changing. Recent observations of the total light coming from the supernova indicate a possible slowing in the rate at which the light is dimming. If that trend continues, the leveling off in the supernova's "light curve" would be the first hint of a hidden source of energy -- perhaps a pulsar -- buried at the supernova's center.

Initially, astronomers at the Cerror Tololo Inter-American Observatory in Chile reported that, starting near the end of October, the supernova's brightness began to decline less sharply than it had during the previous 300 days. Data from the International Ultraviolet Explorer satellite and observations made at a South African observatory confirmed this "inflection" in the light curve. But the most recent South African observations show the brightness decline is again close to its previous faster rate.

"Everyone agrees that something is going on, but it's too soon to say that [the light curve] is leveling off," says Stanford E. Woosley of the University of California, Santa Cruz. "If anything, it looks like the light curve is wiggling more than leveling out."

Astronomers have seen wiggles in the tails of light curves from other supernovas. Such wiggles may occur if the gas cloud around a supernova becomes lumpy, and the amount of matter between an observer and the supernova's center changes, causing the brightness to vary.

On the other hand, the light-curve shift might be the first sign of an energy source at the supernova's center. Until now, the supernova's major light-producing energy source has been the radioactive decay of the isotope cobalt-56.

The new energy source could be radiation emitted by a neutron star when ejected matter settles back onto its hot surface. A more tantalizing possibility is that the neutron star is rotating fast enough and has a sufficiently large magnetic field to be a pulsar. In that case, the pulsar's energy would be captured by surrounding gas an reradiated as visible light. Because the pulsar itself would be hidden in the cloud of gas, the chances of seeing optical pulses at this stage would be small.

"It is true that there is a little more light coming from the supernova than you'd expect just from radioactive decay," says Robert P. Kirshner of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. "What we see is consistent witht he formation of a neutron star that's a pulsar, but it doesn't prove it."

Nevertheless, according to predictions made by numerous astronomers, the timing of the light-curve shift is close to what would be expected it the pulsar at the supernova's center were somewhat less luminous than the pulsar in the Crab Nebula and similar in energy to a pulsar found in a supernova remnant in the same region of the Large Magellanic Cloud as supernova 1987 A. The timing is also consistent with estimates of how much energy would be produced by matter falling onto a neutron star. At the same time, nothing in recent observations totally rules out the unlikely possibility that a black hole sits in the middle of supernova 1987 A.

"At the present time, we can't be absolutely sure just what is giving rise to the change in the light curve," says Roger A. Chevalier of the University of Virginia in Charlottesville. The trouble is, says Woosley, "you're just starting to see the last few hairs on the tail of the elephant, and you're trying to talk about its trunk."

With no clear view directly into the center of the supernova, astronomers will keep a close eye on the supernova's overall brightness to see if the light curve really does flatten out. The trend should become clearer during the next few weeks, Woosley says. "I think next year will be the year of the pulsar. One way or another, we should see some evidence for it."