"Way Too Bright" Supernova Eludes Astronomers

A 2010 supernova, one of the brightest ever seen, has yielded conflicting interpretations

By John Matson

SPOT ON: Supernova PS1-10afx as it appeared in 2010, with the background light of the sky subtracted. Image: Courtesy R. Chornock et al.

All supernovae are bright. When a star ends its life in a cataclysmic explosion, it emits a burst of energy and light that can outshine the rest of the galaxy in which it resides. But some supernovae are a little too bright?at least from the standpoint of the researchers trying to figure out what caused them.

A supernova discovered in August 2010 at the Pan-STARRS 1 telescope in Hawaii falls into that category. The supernova, PS1-10afx, is so far away that its light has taken nine billion years?more than half the age of the universe?to reach Earth. And at that distance, its apparent glow implies that the supernova shone with the luminosity of 100 billion suns at the source. But whether PS1-10afx is a superluminous cataclysm that defies explanation or a somewhat humdrum supernova that only appears extraordinary because of a chance cosmic alignment depends on whom you ask. In newly published studies, two teams of researchers have taken opposing positions on this question.

?It doesn?t match too well to any of the previous superluminous supernovae,? says Ryan Chornock, an astronomer at the Harvard?Smithsonian Center for Astrophysics. ?In this case we find that it?s hard to explain this object with any of those models.? In a study in the April 20 issue of The Astrophysical Journal (ApJ), Chornock and his colleagues reported the supernova?s unusual attributes, including a redder-than-normal color, its rapid brightening and fading and its extreme luminosity. ?There?s this class of superluminous supernovae that are a factor of 10 to 100 more luminous than a normal supernova,? Chornock says. ?This is comparable to the most luminous examples of those.?

That combination of traits, however, seems to exclude PS1-10afx from any of the explanations that have been floated as an energy source for various extra-bright supernovae: for instance, a supernova exploding into a dense circumstellar medium, which would convert the kinetic energy of the blast into extra radiation or the rapid spin of a highly magnetized neutron star formed in a stellar collapse. Aside from its extreme peak luminosity, the researchers reported in their study, PS1-10afx is an oddball in almost every observable respect.

But what if the supernova was not actually as bright as it seemed? Gravitational lensing, a well-documented consequence of Albert Einstein?s theory of general relativity, can dramatically magnify and brighten the appearance of distant cosmic objects. (In other words, adopting the caveat on a rear-view mirror: ?objects in telescope are dimmer than they appear.?) Such lensing occurs when two celestial bodies fall into alignment as seen from Earth: The gravitational pull of the intervening object bends the light rays from the background object, focusing them toward Earth like a magnifying lens. Studies of gravitational lensing have already been used to infer the presence of invisible dark matter in galaxy clusters and to discover otherwise hidden extrasolar planets orbiting distant stars. Now a group of researchers believes the effect may explain the anomalous supernova PS1-10afx as well.

Having seen Chornock give a talk about PS1-10afx and other supernovae at a scientific conference, and later having read a preprint of the ApJ study, astronomer Robert Quimby began to look into possible explanations himself. ?I think you always have to have some skepticism when someone says, ?We haven?t seen this before,?? says Quimby, who is based at the Kavli Institute for the Physics and Mathematics of the Universe at the University of Tokyo. ?We really have to be sure we haven?t missed anything.? So he plugged the data collected by Chornock and his colleagues into two computer programs that attempt to match supernova spectra?light broken down by wavelength?to the various categories of supernova that astronomers have established: type Ia, type IIn, and so on.

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