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Going Not Gentle Into That Good Night
The Mystery of SN 2006gy, the Most Luminous Supernova Ever Recorded

by WKT

May 7, 2007 ::
Do not go gentle into that good night,
Old age should burn and rave at close of day;
Rage, rage against the dying of the light.
-Dylan Thomas


Astronomers have long recognized that massive stars do not have a gentle demise. Rather they explode as supernovas, explosions that can blaze with the light of a billion or more suns.

Which stars make the most luminous supernovas? The recent discovery of Supernova 2006gy (SN 2006gy), a supernova that produced more light than any previously recorded supernova, has stimulated a lively discussion of this issue, and may provide insight into how very massive stars die.

SN 2006gy was first detected by an optical robotic telescope as part of the Texas Supernova Search project on September 18, 2006. It brightened slowly for about 70 days, peaked at a luminosity or intrinsic brightness equal to that of 50 billion suns - ten times brighter than its host galaxy - and began a slow decline.

The peak luminosity, the very gradual rise and decay of the brightness, and the total energy radiated put SN 2006gy in a class by itself. One possible explanation for this behavior is that a very massive star exploded into a dense gas cloud. Debris from such an explosion would collide with the gas cloud and produce a shock wave that would convert the explosive energy into light.

Additional observations of SN 2006gy with optical telescopes in California and Hawaii determined that the bulk of the debris is moving outward at around 15 million kilometers per hour (kph) into a circumstellar cloud that is coasting along at a leisurely 700,000 kph. This cloud was presumably ejected by the doomed star prior to the explosion.

So far so good. Until the X-ray data from Chandra came in. The Chandra data, taken 56 days after the explosion of SN 2006gy, revealed that SN 2006gy was a relatively paltry X-ray emitter. Although a collision of the supernova debris with the surrounding cloud is occurring, the cloud is not dense enough to explain the optical brilliance of the supernova. The weak X-ray emission also rules out any type of gamma-ray burst event.

Another way to make an ultra-bright supernova is for the initial explosion to produce a large amount of radioactive nickel. Radioactive decay of the nickel into cobalt and other nuclei could feed energy into the expanding debris for several months, heightening the luminosity of the supernova. This happens when a white dwarf star becomes unstable and is disrupted in a thermonuclear explosion that produces, among other heavy elements, a fraction of a solar mass of radioactive nickel.

About 50 times this much radioactive nickel would be required to account for the extreme luminosity of SN 2006gy. This rules out the possibility that the explosion of a white dwarf star, with a maximum mass of about 1.4 solar masses, is responsible.

Simply cranking up the mass of the pre-supernova star fiftyfold will not work either. Theoretical calculations indicate that stars more massive than about 40 solar masses will collapse directly to a black hole without a supernova explosion, unless they manage to shed most of their mass and leave behind a neutron star when they explode. However, none of these scenarios produces much nickel.

The solution of the mystery of SN 2006gy may lie in an obscure corner of the theory of massive stars. According to the theory, temperatures rise to several billion degrees in the central regions of stars with masses between 140 and 260 suns. The usual process of converting mass into energy (E = mc2) is reversed, and energy is converted into mass in the form of pairs of electrons and antielectrons, or positrons.

The production of electron-positron pairs saps energy from the core of the star, disturbing the equilibrium and precipitates a collapse. This so-called "pair instability" would cause violent pulsations that eject a large fraction of the outer layers of the star, and eventually disrupt the star completely.

Pair-instability supernovas, if they exist, would be the most energetic thermonuclear explosions in the universe. In stars with masses greater than about 260 suns, the pulsations would be overwhelmed by gravity and the star would collapse to form a black hole without an explosion.

For stars with initial masses above about 200 suns, pair-instability supernovas would produce an abundance of radioactive nickel. So, it would seem that the mystery of SN 2006gy has an intriguing, even spectacular solution. The outburst represents the first detected example of a long-predicted (40 years ago) but never observed pair-instability supernova. At the same time it would establish that these very massive stars can exist.

Maybe. A previous calculation of the expected light output from pair-instability supernovas showed that their peak luminosity would be about the same as that produced by the explosion of a white dwarf. This surprising, and disappointing, result was attributed to absorption of energy by the massive outer envelope of the star which is ejected in a pair-instability supernova.

So, is it back to the hunt for other suspects? Not yet. The earlier calculations of peak luminosities made assumptions about the state of the pre-supernova star which may not be valid. In particular, it will be interesting to see new calculations for very massive stars with a different chemical composition (more carbon, for example) and somewhat smaller diameters prior to explosion.

As SN 2006gy continues to evolve it will reveal more clues to its true nature. If its luminosity continues to decline smoothly from the peak as predicted from the known decay rates of radioactive nickel and cobalt, then the likelihood that astronomers have sighted a rare astronomical bird will be greatly strengthened. If not, there will be more raging against the dying of the light.

REFERENCE: N. Smith et al. 2007, astro-ph/0612617v2 and references cited therein.

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