Exploring the heart of a supernova

An average of two massive stars in our Milky Way galaxy explode each century, producing magnificent supernovae when they detonate. 

These stellar explosions send fundamental, uncharged particles known neutrinos streaming our way and generate ripples called gravitational waves in the fabric of space-time. 

Astronomers are eagerly awaiting neutrinos and gravitational waves from approximately 1,000 supernovae that have already exploded at distant locations in the Milky Way to reach us.

Here on Earth, large, sensitive neutrino and gravitational-wave detectors are capable of detecting such signals, which are expected to provide critical information about what happens in the core of collapsing massive stars just before they explode.

However, for astronomers to understand the data, researchers will need to know in advance how to interpret the massive amounts of information collected by the detectors.

As such, scientists at the California Institute of Technology (Caltech) recently ran a computer simulation to determine what they believe will be an unmistakable signature of such an event. 

Essentially, if the interior of the dying star is spinning rapidly just before it explodes, the emitted neutrino and gravitational-wave signals will oscillate together at the same frequency.

“We saw this correlation in the results from our simulations and were completely surprised,” explained Christian Ott, an assistant professor of theoretical astrophysics at Caltech. 

”In the gravitational-wave signal alone, you get this oscillation even at slow rotation. But if the star is very rapidly spinning, you see the oscillation in the neutrinos and in the gravitational waves, which very clearly proves that the star was spinning quickly – that’s your smoking-gun evidence.”

As Ott notes, scientists do not yet know all the details that prompt a massive star – one that is at least 10 times as massive as the Sun – to become a supernova. What they do know is that when such a star runs out of fuel, it can no longer support itself against gravity’s pull, and the star begins to collapse in upon itself, forming what is referred to as a proto-neutron star. 

They are also aware that another force, called the strong nuclear force, takes over and leads to the formation of a shock wave which begins to tear the stellar core apart. But this shock wave is not energetic enough to completely explode the star as it stalls part way through its destructive work.

Clearly, there needs to be some mechanism – what scientists refer to as the “supernova mechanism” – to complete the explosion. But what could revive the shock?

Current theory suggests several possibilities. Neutrinos could do the trick if they were absorbed just below the shock, re-energizing it. The proto-neutron star could also rotate rapidly enough, like a dynamo, to produce a magnetic field that could force the star’s material into an energetic outflow, called a jet, through its poles, thereby reviving the shock and leading to explosion. Alternatively, it could be a combination of the above-mentioned effects. 

Fortunately, the new correlation identified by Ott’s team provides a method of determining whether the core’s spin rate played a role in creating any detected supernova.

It would be difficult to glean such information from observations using a telescope, for example, because those provide only information from the surface of the star, not its interior. Neutrinos and gravitational waves, on the other hand, are emitted from inside the stellar core and barely interact with other particles as they zip through space at the speed of light. This means they carry unaltered information about the core with them.

The ability neutrinos have to pass through matter, interacting only ever so weakly, also makes them notoriously difficult to detect. Nonetheless, neutrinos have been detected: twenty neutrinos from Supernova 1987a in the Large Magellanic Cloud were positively identified in February 1987. 

If a supernova went off in the Milky Way, it is estimated that current neutrino detectors would be able to pick up about 10,000 neutrinos.

In addition, scientists and engineers now have detectors – such as the Laser Interferometer Gravitational-Wave Observatory, or LIGO, a collaborative project supported by the National Science Foundation and managed by Caltech and MIT – in place to detect and measure gravitational waves for the first time.

Ott’s team happened across the correlation between the neutrino signal and the gravitational-wave signal when analyzing data from a recent simulation. Previous simulations focusing on the gravitational-wave signal had not included the effect of neutrinos after the formation of a proto-neutron star. This time around, they wanted to specifically examine that particular effect.

“To our big surprise, it wasn’t that the gravitational-wave signal changed significantly,” Ott says. “The big new discovery was that the neutrino signal has these oscillations that are correlated with the gravitational-wave signal.” 

The correlation was  observed when the proto-neutron star reached high rotational velocities – spinning about 400 times per second. 

Future simulation studies are expected to closely analyze the range of rotation rates over which the correlated oscillations between the neutrino signal and the gravitational-wave signal occur.