Exoplanet-hunting telescope focuses on more explosive fare

“Wouldn’t it be fantastic if Kepler ends its journey by finding out how type Ia supernovae happen?” asks NASA astronomer Geert Barentsen, who oversees guest use of the space telescope. Kepler, launched by NASA in 2009 to hunt for exoplanets, has widened its range of studies since it was repurposed in 2014 following the failure of two of its four gyroscopes (see Physics Today, March 2015, page 19 ).

Even as the second phase of the mission, dubbed K2, continues to search for exoplanets, since December the focus has been supernovae. Specifically, astronomers are optimistic about sorting out a decades-old mystery: Do type Ia supernovae occur when two white dwarfs merge? Or does a single white dwarf accrete material from a companion object and then explode? The answer has implications for understanding not only supernovae, but also cosmology: Type Ia supernovae are used as distance markers to estimate the Hubble constant, a measurement of cosmic expansion that has proved difficult to pin down (see the article by Mario Livio and Adam Riess, Physics Today, October 2013, page 41 ).

Using Kepler, which is designed to continually examine the brightness of thousands of stars, to observe supernovae suffered an inauspicious start. A pilot program in the original mission looked at 100 galaxies in search of supernovae and failed to spot a single one. Then scientists who had been studying active galactic nuclei with Kepler showed some light curves to Armin Rest of the Space Telescope Science Institute. “I started to drool,” he says. They had stumbled on six supernovae. “My colleagues with the 100 galaxies had had bad luck,” Rest says. The light curves showed the supernovae in great detail, with data collected every 30 minutes over many weeks. Previous data had provided snapshots with a much lower cadence—some twice a day, but most only every few days.

With those spectacular data in hand, Rest and colleagues won time on K2 to search for supernovae. They have kept an eye on some 25 000 galaxies. In the first three-month campaign, they recorded more than a dozen supernovae, and they expect a similar number from the second campaign, for which the data were downlinked in mid May and are now being analyzed.

In the first iteration of the Kepler mission, the telescope pointed continuously at one field. But now that the telescope relies on solar pressure for stability, it has to point either toward or away from Earth. It had been pointed away until the supernova campaigns started. “That switch was key for following supernovae,” says Barentsen. That’s because when Kepler looks toward Earth, it can monitor the same events that other telescopes have tracked from the ground.

By coordinating with ground-based telescopes, astronomers can complement the light curves from K2 with spectral and detailed photometric data. The combination makes possible the classification of supernovae and provides tools to better understand their physical properties and evolution. In addition, the K2 light curves serve as a bridge to greater understanding of thousands of other supernovae for which only ground-based data exist.

“We are probing early times at a new level of detail and resolution,” says Dan Kasen, a theorist at the University of California, Berkeley, and Lawrence Berkeley National Laboratory. By analyzing the moments right after a supernova explodes, “we can get more information about what triggers the explosion.” Rest says that so far, astronomers are finding that white-dwarf mergers appear to be the dominant mechanism behind type Ia supernovae. But some of the explosions feature a surge of flux early on, which could point to a white dwarf exploding after absorbing material from a companion star.

The existence of more than one triggering mechanism raises the question of whether some cosmological distance measurements are flawed. The brightness from each scenario may differ a bit, says Rest, which would mean that not every type Ia supernova could be treated the same for the purposes of estimating the distance to the explosion. It’s also likely that the ratio of supernovae due to the two triggers has changed over time, “so the populations at high and low redshift may differ systematically.” Rest says that to better constrain cosmic expansion and better understand dark energy, “we need to know the fractions” of supernovae triggered by white dwarf mergers versus a white dwarf accreting material from a companion star.

Kasen says K2 is also revealing new kinds of supernovae. One “weird” sighting was a fast-evolving luminous transient, for which the brightness rose and fell in just a couple of days instead of the weeks or months characteristic of supernovae. “We are still debating the cause,” he says. “It may be the signature of a massive star that experienced a violent mini-eruption just before it exploded completely.”

K2‘s days are numbered. The spacecraft is 152 million kilometers away, and its fuel is running out. The supernova team’s slogan for the mission: “Going out with a bang!”