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August 5, 2005

It's An Exceptional Supernova, But Is It a GRB?
The Still-Missing Link Between Gamma-Ray Bursts and Type Ic Supernovae

Nobody knows what causes gamma-ray bursts (GRBs), which astronomer Brian Schmidt, writing in Science magazine, says have been proclaimed "the largest bangs in the universe since the big one."

Some gamma-ray bursts may be associated with Type Ic supernovae, which occur when a massive star collapses to form a black hole or neutron star.

What is known is that some of these enormous flashes of energy are associated, in some way, with some Type Ic supernovae. In 1998, for the first time, a Type Ic supernova tagged 1998bw was directly associated with a gamma-ray burst. Five years later a brighter GRB was associated with Type Ic supernova 2003dh. "It is known that supernovae underlie some GRBs," Schmidt wrote in Science, "but how some supernovae produce GRBs remains unclear."

Supernovae are categorized by elemental lines in their spectra; the spectrum of a Type Ic shows no hydrogen and little or no helium. Presumably the outer gaseous envelopes of the supernova's massive precursor — a star 10 to 100 times as massive as the sun — are stripped away in a stellar collision, or through some other mechanism. What's left is a core, mostly of carbon and oxygen, that soon collapses into a black hole or neutron star.

"The conventional picture of a normal supernova is of a very spherical object," says Lifan Wang of Berkeley Lab's Physics Division. If one could imagine that the precursor of a Type Ic supernova were not rapidly spinning, then its subsequent blaze of energy might be perfectly symmetrical, shining equally brightly in every direction.

But in fact, says Wang, "most supernovae are probably asymmetric." This is the clear message of the data Wang has been gathering since the mid-1990s, as a pioneer in the measurement of supernova polarimetry. Even though supernovae are visible only as point sources, any slight polarization of that light betrays an aspherical shape. Wang first found net polarization in many Type II supernovae, which like Types Ib and Ic are core-collapse supernovae, and later in some Type Ia's, which explode by quite a different mechanism.

The collapsar model of a gamma-ray burst posits an event very like a Type Ic supernova. When a massive star collapses into a black hole surrounded by a disk of accreting matter, streaming particle jets along the rotation axis could give rise to the supernova and the GRB. (From SWIFT Satellite animation, Jet Propulsion Laboratory and NASA)

Wang is pursuing programs to observe both core-collapse and Type Ia supernovae at the 8.2-meter Subaru telescope on Mauna Kea in Hawaii, operated by the National Astronomical Observatory of Japan; in the summer of 2004 many of his colleagues there were hunting for a supernova connection with GRBs.

Any link between immensely powerful gamma-ray bursts and even the most energetic supernova explosions depends crucially on the asymmetry of the system, because only asymmetry provides a mechanism for focusing an exploding star's radiation into a beam tight enough to be seen halfway across the universe. The connection is explained by the collapsar model of GRBs — also called the hypernova model — which starts with a precursor star at least 30 times as massive as the sun and results in an extraordinarily powerful, high-velocity explosion.

When a rapidly spinning object starts to come apart, most of the wreckage is swept into a whirling equatorial disk. The oxygen in the core of an asymmetric Type Ic supernova's precursor star ends up mostly in such a disk, for example.

Just before it goes supernova, the massive naked core will have collapsed into a spinning black hole, converting spin energy to hard radiation (gamma rays and x-rays) while gobbling matter as fast as it can. Most of the matter is in the disk, with relatively little at the poles. Along the axis of spin the star's magnetic fields collimate remnant matter and hard radiation into a pair of tightly focused, near-light-speed jets.

A distant observer looking into one of these jets would see a brilliant burst of gamma rays lasting a few seconds or minutes (SN 1998bw and the brighter SN 2003dh were presumably seen from this head-on angle). As time passes, fast-moving heavy elements like iron and nickel, expelled when the black hole formed — and which from the side of the supernova would be obscured by overlying layers of lighter carbon and oxygen — also appear.

An observer looking along the equatorial plane, however, sees only an aspherical supernova.

Whether a Type Ic supernova is seen as a gamma-ray burst could depend upon how the asymmetric object is viewed. SN 1998bw may have been viewed along the axis of the jets; its spectrum showed a strong, single peak in the oxygen emission line. SN 2003jd may have been viewed from the side, through a rapidly rotating disk that caused the oxygen line to split.

"If you don't see the gamma-ray burst, it may be because the jets are pointed vertically from your point of view. There are two ways to test whether a Type Ic supernova fits this model of a GRB," says Wang. "At an early phase of the explosion, when the ejecta are still opaque, you can judge how rapidly the disk is spinning from polarimetry. In the late or 'nebular' phase the supernova becomes transparent, everything is revealed, and you can measure the element lines in the spectrum."

Oxygen in particular: if the peak of the oxygen emission line is doubled, it's because you're looking at the disk more nearly edge-on. One of the spectral lines is blue-shifted because one side of the disk is spinning toward you, while the line from the part that's rotating away is red-shifted.

"This is what made supernova 2003jd a particularly interesting Type Ic," says Wang. "It was the most highly polarized supernova I've seen. And it was the first Type Ic to be found with a double oxygen peak in the nebular phase."

SN 2003jd was found on October 25, 2003, by the KAIT automated supernova-search telescope at the University of California's Lick Observatory. A year later, well into the nebular phase, it was studied with the Subaru telescope and UC's Keck telescope on Mauna Kea. The researchers wrote up their observations and published them in Science with a title that promised "a link to gamma ray bursts."

Many features of this unusually bright Type Ic supernova fit the collapsar model. Even the lack of evidence of an actual gamma-ray burst was argued as proof of the model ("Absence of evidence is not necessarily evidence of absence," the authors wrote). Assuming that a highly asymmetric supernova was being viewed sideways, at an angle of 60 degrees or more off the vertical orientation of the presumed jet, not only gamma rays but other radiation in the x-ray and radio wavelengths — produced by the jet piercing the star's gaseous remains — would be undetectable as well.

Supernova 2003jd, seen here as observed by the Australian National Observatory.

The Science paper was received with some skepticism, despite its authors' concluding plea that "additional sensitive radio and x-ray observations ... are strongly encouraged." After publication some of the contributors acknowledged among themselves the desirability of observing SN 2003jd over a longer time interval to strengthen their case.

"To say that we found an off-axis GRB would not be accurate," says Wang. "We found an off-axis Type Ic supernova." GRB or not, Wang is happy that SN 2003jd affirms the argument he has been making since he began his studies of supernova polarimetry a decade ago — an often uphill struggle against the opinion of many astronomers — that more often than not, supernovae are asymmetric. "This Science paper shows that when you look at Type Ic supernovae from different angles, they look different."

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