It Takes Two Stars to Make a Gamma Ray Burst

In 1967, NASA scientists noticed something they had never seen before coming from deep space. In what has come to be known as the “Vela Incident“, multiple satellites registered a Gamma-Ray Burst (GRB) that was so bright, it briefly outshined the entire galaxy. Given their awesome power and the short-lived nature, astronomers have been eager to determine how and why these bursts take place.

Decades of observation have led to the conclusion that these explosions occur when a massive star goes supernova, but astronomers were still unsure why it happened in some cases and not others. Thanks to new research by a team from the University of Warwick, it appears that the key to producing GRBs lies with binary star systems – i.e. a star needs a companion in order to produce the brightest explosion in the Universe.

The research team responsible for the discovery was led by Ashley Chrimes – a Ph.D. student at the University of Warwick Department of Physics. For the sake of their study, the team addressed the central mystery about long-duration GRBs, which is how stars can be spun up fast enough to generate the kind of explosions that have been observed.

To put it succinctly, GRBs occur when massive stars (about ten times the size of our Sun) go supernova and collapse into a neutron star or black hole. In the process, the star’s outer layers are blown off and the ejected material flattens down into a disc around the newly-formed remnant to conserve angular momentum. As this material falls inwards, this momentum launches it in the form of jets emanating from the poles.

These are known as “relativistic jets” because of the way material in them is accelerated to close the speed of light. While GRBs are the brightest events in the Universe, they are only observable from Earth when one of their polar axes is pointed directly at us – which means astronomers can only see around 10-20% of them. They also very brief as astronomical phenomena go, lasting anywhere from a fraction of second to several minutes.

In addition, a star has to be spinning extremely fast in order to launch material along its polar axes at close to the speed of light. This represents a conundrum for astronomers since stars usually lose any spin they acquire very quickly. To address these unresolved questions, the team relied on a collection of stellar evolution models to examine the behavior of massive stars as they collapse.

These models were created by Dr. Jan J. Eldridge from the University of Auckland, New Zealand, with the assistance of researchers from the University of Warwick. Combined with a technique known as binary population synthesis, the scientists simulated a population of thousands of star systems to identify the mechanism whereby the rare explosions that produce GRBs can occur.

From this, the researchers were able to constrain the factors that cause relativistic jets to form from some collapsing stars. What they found was that tidal effects, similar to what occurs between the Earth and the Moon, were the only likely explanation. In other words, long-duration GRBs occur in binary star systems where stars are locked together in their spin, creating a powerful tidal effect that speeds up their rotation.

As Chrimes explained in a recent Warwick press release:

“We’re predicting what kind of stars or systems produce gamma-ray bursts, which are the biggest explosions in the Universe. Until now it’s been unclear what kind of stars or binary systems you need to produce that result.

“The question has been how a star starts spinning, or maintains its spin over time. We found that the effect of a star’s tides on its partner is stopping them from slowing down and, in some cases, it is spinning them up. They are stealing rotational energy from their companion, a consequence of which is that they then drift further away.

“What we have determined is that the majority of stars are spinning fast precisely because they’re in a binary system.”

As Dr. Elizabeth Stanway – a researcher with the University of Warwick Department of Physics and a co-author of the study – pointed out, binary evolution is hardly new to astronomers. However, the kinds of calculations performed by Chrimes and her colleagues have never been done before because of the complicated calculations involved. Hence, this study is the first to consider the physical mechanisms at work within binary models.

“There has also been a big dilemma over the metallicity of stars that produce gamma-ray bursts,” she said. “As astronomers, we measure the composition of stars and the dominant pathway for gamma-ray bursts requires very few iron atoms or other heavy elements in the stellar atmosphere. There’s been a puzzle over why we see a variety of compositions in the stars producing gamma-ray bursts, and this model offers an explanation.”

Thanks to this latest study and the resulting model it provides on binary evolution, astronomers will be able to predict what GRB producing stars should look like in terms of temperature, luminosity, and the properties of their companion star. Looking to the future, Chimes and her colleagues hope to explore and model transient phenomena that remain a mystery to astronomers.

These include Fast Radio Bursts (FRBs) and what causes them (especially the repeating variety) or even rarer events like the transformation of stars into black holes. The study that describes their finding appeared in the January issue of the Monthly Notices of the Royal Astronomical Society and was funded by the Science and Technology Facilities Council at UK Research and Innovation.

Further Reading: University of Warwick, MNRAS