When stars go hyper

Different kind of nova ends
not with a whimper, but with a bang

October 21, 1998: Revolutions often start with a bang. Such a blast may have been delivered this spring by a supernova that may force scientists to rethink how stars die. For now, observers have a classic situation - evidence that points in different directions and dares anyone to find a single answer.

"All of this may lead to a revolution in our thinking about how core-collapse supernovae are produced," wrote Dr. Eddie Baron of the University of Oklahoma in the Oct. 15 issue of the prestigious science journal, Nature. Questions to be answered include what causes "ordinary" supernovae, is there a limit in their energy release, and when does core collapse cause a gamma-ray burst?

Before and during: In a May 15, 1985 image of galaxy ESO 184-G82 shows nothing unusual in a 120-minute, red light exposure by the United Kingdom Schmidt Telescope in Australia. The image at right is a color composite of three short-exposure green, red, and near-infrared images obtained with the multi-mode instrument at the ESO's 3.58-m New Technology Telescope (NTT) at La Silla on May 4, 1998. SN1998bw is the very bright, bluish star at the center (arrow), located on an arm of spiral galaxy ESO 184-G82. Several other galaxies are in the field. In both photos, the field of view measures 3.6 x 3.6 arcmin; north is up and east is left. Note that while the brighter objects are more prominent on the older, long-exposure Schmidt photo (right), considerably more details are in the NTT image (left). Images courtesy European Southern Observatory. Links to 463x365-pixel, 44KB JPG.

Baron was commenting on three Nature papers describing, as the first paper is titled, "An unusual supernova in the error box of the gamma-ray burst of 25 April 1998." The lead author is Titus J. Galama, a graduate student at the Astronomical Institute at the University of Amsterdam, The Netherlands. Among his many co-authors are Dr. Jan van Paradijs of the University of Amsterdam and the University of Alabama in Huntsville (UAH), Drs. Chryssa Kouveliotou, Craig Robinson and Thomas Koshut of the Universities Space Research Association at Marshall Space Flight Center, and Dr. Marc Kippen, also of UAH.

The excitement started about 140 million years ago, near the end of the Jurassic period when dinosaurs were in their prime, but the news just arrived on April 25, 1998, when instruments aboard several spacecraft recorded a strong flash of gamma radiation. Optical astronomers pointing powerful ground-based telescopes to the direction of the X-rays from the gamma-ray burst discovered a supernova in a distant galaxy. Scientists estimate that the blast, perhaps 10 times greater than an ordinary supernova, collapsed the core of the dying star into a black hole. At the same time, it slammed enough matter - mostly radioactive nickel 56 weighing 70 percent as much as our sun - outward to create a shock wave of gamma radiation.

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This raises the possibility that "hypernovae" might be a significant cause of gamma-ray bursts. Scientists have puzzled over gamma-ray bursts since the late 1960's when they were discovered. Not until the last couple of years, though, were they able to conclude that the bursts were from cosmological distances, that is, from far outside our own galaxy.

Two cross-section images depict the likely explanations for the April 25, 1998 supernova. It could have blown up in all directions (left) - called isotropic - in which case it's almost impossibly powerful in order to account for the energy output. Or, it could have squirted most of the matter in jets long the magnetic poles - in which case it's most unlikely that we would be lined up to see it just right. In either case, the blast produces a black hole surrounded by a swirling disk of matter crowding its way into the hole and oblivion. Links to 1000x658-pixel, 59KB JPG. Also available: 2821x1857-pixel, 259KB JPG. Credit: NASA/Marshall Space Flight Center, after Baron (Nature, 395: 635, 1998).

The Burst and Transient Source Experiment (BATSE) aboard the Compton Gamma Ray Observatory has recorded more than 2,000 bursts - about one a day - since its launch in 1991. A key finding is that the bursts are randomly distributed across the sky. This means they have to be peppered at random throughout the universe since a link to our galaxy would show them clustered along the galactic plane, where most of the stars of our Milky Way are located.

The discovery of an optical companion to a burst recorded on Feb. 28, 1997, linked bursts with distant galaxies. The distances of the GRB host galaxies were subsequently measured in several cases. Just what causes them, though, remains open to debate. Supernovae had been ruled out as being not quite powerful enough, especially given the distances involved.

Observations of an optical component to the April 25, 1998, burst (GRB980425) indicate that a special set of supernovae - the hypernova - might be a contributor.

GRB980425 was detected by BATSE, the Dutch-Italian BeppoSAX (X-ray astronomy) satellite, and instruments on other spacecraft. With BeppoSAX scientists were able to image the part of the sky where the burst came from. Within this image, both the European Southern Observatory (ESO) at La Silla (about 600 km north of Santiago, Chile), the National Radio Astronomy Observatory in Socorro, N.M., and others went hunting.

They soon found their quarry.

Circling the drain Trying to figure out what happens inside a black hole is a bit like Alice trying to make sense of what she found down the rabbit's hole. Nothing works the way we think it should. When a star is massive enough, at the end of its life it can generate enough inward force to compress the core beyond the stage of a super-dense neutron star and seemingly erase the laws of physics that let subatomic particles maintain their identities. Left: An artist's concept depicts a black hole at the center of a warped disk of material that is swirling into the disk. Whether SN1998b blew out in all directions or beamed its energy like a lighthouse, the end result most likely is similar to this. Illustration by James Gitlin, Space Telescope Science Institute. Click here for details and a high-resolution copy of the image. All that's left is a deep gravity well strong enough to keep light from escaping. What happens at the center is anyone's guess, literally, because the laws of physics, as we understand them, do not work under such extreme conditions. What we can see and measure are the effects of the black hole on space and matter around the hole. Materials swirl inward, like water circling the drain. As they near the event horizon - the point of no return - they run into each other, heating the gas to glow in radio, then visible light, ultraviolet, and finally, X-rays.

The UK Schmidt Telescope in Australia had surveyed the area as one of its first tasks in the 1970s, so it had a good set of "before" images in hand. In an arm of the spiral galaxy tagged ESO 184-G82, scientists found a brilliant star that was not in previous images. This was dubbed SN1998bw.

At such close range, GRB980425 could only have about 1 percent of 1 percent (0.0001) of the raw power of more distant bursts. But its light curve in radio waves was striking, indicating that the shock wave from the blast was moving very close to the speed of light. Indeed, it's the brightest supernova ever seen in radio waves.

"There is a significant chance that the GRB and the supernova explosion are associated", said Kouveliotou. "SN1998bw is a rare type of supernova, both in its optical and in its radio properties". However, "Nobody would have picked up GRB980425 as somehow special on the basis of its gamma-ray properties alone", said van Paradijs.

If GRB980425 and SN1998bw are associated, the scientists wrote, then GRB980425 is a rare type of burster, and SN1998bw is a rare type of supernova.

The rarity that might fit this pigeonhole is the hypernova, an idea that has been around for a few years but not yet confirmed.

"The extremely large energy suggests the existence of a new mechanism of massive star explosion that can also produce the relativistic shocks necessary to generate the observed gamma rays," wrote a team led by K. Iwatomo of the University of Tokyo, and including Kouveliotou. (The hypernova bears no relation to the magnetars which Kouveliotou discovered earlier this year. Magnetars are highly magnetized neutron stars. A hypernova would leave behind nothing to form a magnetar, just a black hole in space.)

Models of the light curves indicate that it started with the core collapse of a star about 40 times as massive as our sun. It was spinning rapidly - possibly due to a binary companion spiraling into the star - and had a strong magnetic field.

It had already burned through all its nuclear fuel, converting hydrogen into helium "ash," and the ash into heavier elements until all that was left was silicon ash. In a final fury, it burned this into nickel 56 and collapsed on itself, compacting the core into a black hole and blasting the outer layers into space.

The authors outlined two possible paths at this point. First, the star blasted outward in all directions, making for a phenomenally powerful blast, about 30 times greater than any recorded supernova. Or, it focused most of the energy into one direction, which happened to be aimed at Earth - a most unlikely event.

Either way, astronomers now have a new mystery to study.

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Author: Dave Dooling
Curator: Linda Porter
NASA Official: Gregory S. Wilson