White dwarf supernovas—also known as type Ia—are both bright enough and consistent enough in their characteristics to be used to measure distances to far-off galaxies. That's why the discovery of an anomalous supernova 400 times brighter than expected gave astronomers indigestion. While researchers have identified other super-bright supernovae that are likely the result of exploding stars, these observations didn't fit easily into any previously described phenomenon.

However, one possibility remained: perhaps the light from this supernova had been magnified by gravitational lensing somewhere between the explosion and Earth. If that's the case, the anomalous explosion could be an ordinary white dwarf supernova that happened to appear much brighter, instead of a fundamentally new type of event. Robert M. Quimby and colleagues found a galaxy that could be doing the lensing by monitoring the supernova as it faded. While lots of gravitational lenses have been identified, this is the first clear example of magnification of a supernova.

The Panoramic Survey Telescope and Rapid Response System 1 (Pan-STARRS1, designed in part to locate transient events) detected the anomalous supernova on August 31, 2010. Based on follow-up observations, astronomers determined that it was more than nine billion light-years away. However, for that to be true, the outburst had to be more than 400 times brighter than a typical exploding star, or 30 times brighter in peak light output than expected from a white dwarf supernova.

One class of explosion can be that bright: superluminous supernovae (SLSN). However, these explosions are typically high temperature or very large in size. By contrast, PS1-10afx was reddish in color—a sign that lower temperatures were involved—and it passed through the cycle of brightening and fading relatively quickly, which shouldn't happen when very large stars blow up. (Another type of very bright supernova is a hypernova, which can produce gamma-ray bursts; these are very directional in character and appear quite different from SLSNs.)

In other words, PS1-10afx appeared to be peculiar even by SLSN standards. However, the shape and colors of the spectrum seemed to correspond to that of a white dwarf supernova, apart from the pesky fact that the total amount of light was way too large.

That problem might seem minor, but white dwarf supernovae are noteworthy because we think they all explode in a very similar way. White dwarfs have a maximum mass (the Chandrasekhar limit) and only explode when they exceed it. As a result, their supernovae share many characteristics: the total light output, the amount of time leading up to the main explosion, and the way in which they fade. For that reason, white dwarf supernovae can be used as standard candles—their consistency means it is possible to measure their distance from Earth. There's little room in theory for anomalous white dwarf supernovas, and finding anomalies would throw a whole host of distance measurements into question.

Not all is lost, though. We only know about many objects, including the most distant galaxies in the Universe, because their light has been magnified by strong gravitational lensing. According to general relativity, when light from a distant source passes near a massive object, gravity can focus it. The result is an image that's much brighter and magnified relative to the original source, which might under ordinary circumstances be too faint to see. Depending on the details of the gravitational distortion, there may even be multiple images or a single image smeared into an Einstein ring.

If a galaxy or other massive object was serving as a lens in the path between PS1-10afx and Earth, the supernova could have been magnified. However, a supernova can be brighter than an entire galaxy, meaning the lens might not be visible (much as a bright camera flash can wash out fainter light sources).

For that reason, the authors of the new paper monitored the region of the sky as PS1-10afx faded, using the Keck observatory in Hawaii. They detected a galaxy in the vicinity, but to show it acted as a gravitational lens, they first had to demonstrate it was in between the supernova and us. That work was made easier when the team figured out how to distinguish the light from the galaxy that hosted the explosion from the light of the lensing galaxy.

This is the first detection of strong gravitational lensing of a supernova, though the data was insufficient to identify an Einstein ring or other distinctive features. However, the authors pointed out that more rapid follow-up observations of anomalous supernova explosions with powerful telescopes (such as the Hubble) could detect those effects, which would lend strong support to the lensing hypothesis and provide a independent way of measuring the expansion rate of the Universe. While most white dwarf supernovas won't provide such a serendipitous alignment with a lensing galaxy, the few that will could be very valuable for cosmology.

Science, 2014. DOI: 10.1126/science.1250903 (About DOIs).