“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.