First ever gravitationally lensed Type Ia supernovae discovered

Two teams of astronomers using the NASA/ESA Hubble Space Telescope have discovered three distant exploding stars that have been magnified by the immense gravity of foreground galaxy clusters, which act like "cosmic lenses." These supernovae are the first of their type ever to be observed magnified in this way and they offer astronomers a powerful tool to check the prescription of these massive lenses.

Massive clusters of galaxies act as "gravitational lenses" because their powerful gravity bends light passing through them [1]. This lensing phenomenon makes faraway objects behind the clusters appear bigger and brighter -- objects that might otherwise be too faint to see, even with the largest telescopes.

The new findings are the first steps towards the most precise prescription -- or map -- ever made for such a lens. How much a gravitationally lensed object is magnified depends on the amount of matter in a cluster -- including dark matter, which we cannot see directly [2]. Astronomers develop maps that estimate the location and amount of dark matter lurking in a cluster. These maps are the lens prescriptions of a galaxy cluster and predict how distant objects behind a cluster will be magnified when their light passes through it. But how do astronomers know this prescription is accurate?

Now, two independent teams of astronomers from the Supernova Cosmology Project and the Cluster Lensing And Supernova survey with Hubble (CLASH) have found a new method to check the prescription of a gravitational lens. They analysed three supernovae -- nicknamed Tiberius, Didius and Caracalla -- which were each lensed by a different massive galaxy cluster -- Abell 383, RXJ1532.9+3021 and MACS J1720.2+3536, respectively. Luckily, two and possibly all three of these supernovae appeared to be a special type of exploding star that can be used as a standard candle [3].

"Here, for the first time, we have found Type Ia supernovae that can be used like an eye chart for each lensing cluster," explained Saurabh Jha of Rutgers University, USA, a member of the CLASH team. "Because we can estimate the intrinsic brightness of the Type Ia supernovae, we can independently measure the magnification of the lens, which is not possible with other background sources."

The teams measured the brightnesses of the lensed supernovae and compared them to the explosion's intrinsic brightness to calculate how much brighter the exploding stars' were made due to gravitational lensing. One supernova in particular stood out, appearing to be about twice as bright as would have been expected if not for the cluster's magnification power.

The three supernovae were discovered in the CLASH survey, which used Hubble to probe the distribution of dark matter in 25 galaxy clusters. Two of the supernovae were found in 2012; the other in 2010 to 2011.

To perform their analyses, both teams used Hubble observations alongside observations from both space and ground-based telescopes to provide independent estimates of the distances to these exploding stars [4].

In some cases the observations allowed direct confirmation of a Type Ia pedigree. In other cases the supernova spectrum was weak or overwhelmed by the light of its parent galaxy. In those cases the brightening and fading behaviour of the supernovae in different colours was used to help establish the supernova type.

Each team compared its results with independent theoretical models of the clusters' dark matter content. They each came to the same conclusions: that the predictions fit the models.

"It is encouraging that the two independent studies reach quite similar conclusions," explained Supernova Cosmology Project team member Jakob Nordin of the E.O. Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California, Berkeley. "These pilot studies provide very good guidelines for making future observations of lensed supernovae even more accurate." Nordin is the lead author on the team's science paper describing the findings.

The Supernova Cosmology Project's galaxy cluster models were created by team members Johan Richard of the University of Lyon in France, and Jean-Paul Kneib of Ecole Polytechnique Federale de Lausanne in Switzerland. "It's really great to see that these supernovae are behaving in the way we expected," says Kneib. "The more confirmation we get that our complex cluster models are correct, the more we can rely on them, and use them to probe the early Universe."

Read more at Science Daily