By John Matson
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.
“Right away I got a perfect match to a type Ia supernova,” Quimby says. “But it was way too bright.” A type Ia supernova—the variety of stellar explosion that provided the evidence for an accelerating expansion of the universe in the late 1990s—is thought to mark the demise of a compact white dwarf star that has accreted enough material to have swelled beyond its maximum stable mass.
In a paper appearing in the May 1 issue of the ApJ Letters, Quimby and his colleagues argue that the supernova was a fairly ordinary type Ia supernova that has been magnified by the lensing of some unseen yet massive object between the supernova and Earth. One possibility for the lens is a small galaxy that has so far escaped detection.
“I think that’s the most likely explanation—there’s two galaxies,” Quimby says. “There’s just a small lensing galaxy, and then you have a background host galaxy” where the supernova occurred. A more exotic possibility is that a free-floating black hole magnified the supernova.
Chornock and his colleagues do not view the lensing mechanism as a likely explanation. “This was a hypothesis that we actually considered prior to his paper,” Chornock says. But the team rejected it based on a number of factors, including the fact that no object has been found that fits the bill for a possible gravitational lens. “Based on our knowledge of the universe, which is of course imperfect, that kind of lensing is usually produced by clusters of galaxies. That’s clearly not the case here because there’s no cluster of galaxies,” he adds, noting that the explanation favored by Quimby and his colleagues “does require some sort of unexpected or unlikely alignment.”
Quimby says that he and his colleagues have applied for observing time on the Hubble Space Telescope to take a closer look. If the light from the supernova’s host galaxy was indeed bent by a gravitational lens, that distortion should still be apparent. “That’s one of the beautiful things about this result: it’s testable,” he says.