I keep a close eye on gravitational lensing, not only because of the inherent fascination of the subject but also because the prospect of using the Sun’s own lensing to study distant astrophysical phenomena could lead to near-term missions to 550 AU and beyond. And because I’m also intrigued by ‘standard candles,’ those markers of celestial distance so important in the history of astronomy, I was drawn to a new paper on the apparent gravitational lensing of a Type Ia supernova (SNIa). This is the kind of supernova that led to the discovery of the accelerating expansion of the universe by giving us ways to measure the distance to these objects.
The point about Type Ia supernovae is that they are so much alike. We may not fully understand the mechanisms behind their explosions, but we have overwhelming evidence that these supernovae reach nearly standard peak luminosities. There is also a strong correlation between their luminosity and other observables like the shapes of their light-curves, so their value as standard candles — astronomical objects with a known luminosity — is clear. Our studies of dark energy depend crucially on Type Ia supernovae, putting a premium on learning more about them.
The supernova PS1-10afx turned up in data from the Panoramic Survey Telescope & Rapid Response System 1 (Pan-STARRS1). What drew particular attention to it was that it appeared to be extremely bright — with an inferred luminosity about 100 billion times greater than the Sun — but also extremely distant. The luminosity was not itself a problem, as it corresponded to the recently discovered category of superluminous supernovae (SLSNe). But the latter tend to be blue and show a brightness curve that changes slowly with time. Not so PS1-10afx, which was highly luminous, displayed the brightness changes of a normal supernova, and was red.
Ryan Chornock (Harvard–Smithsonian Center for Astrophysics) and colleagues reported on the new supernova’s unusual attributes in a recent paper in The Astrophysical Journal. But Robert Quinby (Kavli Institute for the Physics and Mathematics of the Universe in Tokyo) went to work on matching PS1-10afx to other supernovae. Quinby realized that after correcting for time dilation, the light curve of the new supernova was consistent with a Type Ia supernova despite the observed brightness of the object. Gravitational lensing provides an answer: The supernova is being lensed by an object between it and ourselves. The color and spectra of the supernova remain the same as does its lightcurve over time, but the lensing makes it brighter.
Image: One explanation of PS1-10afx. A massive object between us and the supernova bends light rays much as a glass lens can focus light. As more light rays are directed toward the observer than would be without the lens, the supernova appears magnified. (Credit: Kavli IPMU)
Thus we have the lightcurve of a typical SNIa but an object that is 30 times brighter. Although PS1-10afx would be the first Type Ia supernova found to be magnified by a gravitational lens, one of the co-authors of the paper on this work, Masamune Oguri, was lead author on a paper predicting several years ago that Pan-STARRS1 might discover such an object. The alternative is to assume a new kind of superluminous supernova, an idea that can only be ruled out by further observations. The Quimby paper notes that the magnification of PS1-10afx is independent of the supernova itself and should thus apply to the host environment:
This prediction of a persistent lensing source provides a test of our hypothesis. High spatial resolution images from HST may be able to resolve the Einstein ring from the magniﬁed host galaxy. If the lens is a compact red galaxy then color information should distinguish it from the blue, star-forming galaxy in the background. If a foreground galaxy is not detected, then this would either suggest that more exotic lensing systems, such as free ﬂoating black holes, are required or it could support the hypothesis of C13 [referring to the Chornock paper] that a new class of superluminous supernovae is required to explain PS1-10afx.
The beauty of the gravitational lensing explanation is that no new physics would be needed. The Quimby paper shows that discovering a gravitationally lensed Type Ia supernova is statistically plausible for Pan-STARRS1. The paper also makes the interesting case that some massive red galaxies coincident with ‘dark’ gamma-ray bursts may actually be foreground lenses for higher redshift events. We may, in other words, soon begin to discover more lensing transients like PS1-10afx as new surveys with tools like the Large Synoptic Survey Telescope begin. Such studies should be crucial in developing our ideas about the universe’s expansion.
But back to Quimby’s explanation of PS1-10afx, about which Ryan Chornock remains dubious. Chornock is quoted in a recent Scientific American article as saying he and his team studied the lensing possibility and dismissed it:
“This was a hypothesis that we actually considered prior to his paper… 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 is right to note that this is a testable result, and his team’s application for time on the Hubble Space Telescope could provide an answer between the two explanations. The Chornock paper is “PS1-10afx AT z = 1.388: PAN-STARRS1 Discovery of a New Type of Superluminous Supernova,” The Astrophysical Journal Vol. 767, No. 2 (2013), 162 (abstract). The Quimby paper is “Extraordinary Magnification of the Ordinary Type Ia Supernova PS1-10afx,” The Astrophysical Journal Letters Vol. 768, No. 1 (2013), L20 (abstract).