​Gamma-ray bursts are enormous cosmic explosions and are one of the brightest and most energetic events in the Universe. Their brightness changes over time, illuminating deep space like a flashlight shining into a dark room. Intense radiation emitted from most observed gamma-ray bursts is predicted to be released during a supernova as a star implodes to form a neutron star or a black hole.

In the recently observed gamma-ray burst event called GRB 160203A, remains of the explosion started glowing much brighter than expected, according to standard scientific models, even several hours after the initial flash. We now believe that this “rebrightening” was caused by the main body of the burst crashing through shells of material ejected by the source star, or interstellar “knots”. Both theories suggest that the standard gamma-ray burst model needs to be re-examined, and perhaps the surrounding space isn’t as smooth and uniform as originally predicted.

In our study, we began collecting reports from all over the world that observed the gamma-ray burst event, including the archives of the Zadko research telescope. By carefully calibrating the data from different sources and comparing the different brightness over time, we unpacked the surrounding galaxy and defined key characteristics of the burst: the temporal index (how quickly it fades over time), the spectral index (the overall colour of the burst), and the extinction (how much light is absorbed by the matter between here, on Earth, and the burst). One surprising finding was that the density of the burst’s host galaxy is unusually dense – about the same as our own galaxy, the Milky Way.

The next step was to see how and when the data moved away from the model. With further calculations, we identified three interesting time periods that indicated significant brightness differences compared to the model’s prediction. Although the third period was probably a coincidence, the first and second periods were too large to ignore. Normally, rebrightening is caused by something happening to the host galaxy(?), such as suddenly collapsing into a black hole; however, these kinds of events normally happen within the first few minutes of a gamma-ray burst – in this event, the first rebrightening didn’t start until three hours after the initial explosion.

As a result, we decided to expand the conventional model of gamma-ray bursts to explain this unusual event. One of the properties of such events is the relationship between the density of the medium and the intensity of radiation emitted from the explosion. What’s particularly convincing about this explanation is its applicability to many contexts. As stars prepare to explode into supernovas and gamma-ray bursts, they eject their outer shells into the surrounding space. For bursts that don’t come from supernovas, these changes in brightness could be the result of turbulence in the interstellar medium. In either case, the change in brightness gives us a new tool to probe the structure of distant space, and we are now eagerly anticipating another burst with similar features to put our new model to the test.

​Written by OzGrav PhD student Hayden Crisp, University of Western Australia 

​What happens if a supernova explosion goes off right beside another star? The star swells up which scientists predict as a frequent occurrence in the Universe. Supernova explosions are the dramatic deaths of massive stars that are about 8 times heavier than our Sun.
 
Most of these massive stars are found in binary systems, where two stars closely orbit each other, so many supernovae occur in binaries. The presence of a companion star can also greatly influence how stars evolve and explode. For this reason, astronomers have long been searching for companion stars after supernovae-- a handful have been discovered over the past few decades and some were found to have unusually low temperatures.

When a star explodes in a binary system, the debris from the explosion violently strikes the companion star. Usually there’s not enough energy to damage the whole star, but it heats up the star’s surface instead. The heat then causes the star to swell up, like having a huge burn blister on your skin. This star blister can be 10 to 100 times larger than the star itself.
 
The swollen star appears very bright and cool, which might explain why some discovered companion stars had low temperatures. Its inflated state only lasts for an ‘astronomically’ short while--after a few years or decades, the blister can “heal” and the star shrinks back to its original form.
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In their recently published study by a team of scientists led by OzGrav postdoctoral researcher Dr Ryosuke Hirai (Monash University), the team carried out hundreds of computer simulations to investigate how companion stars inflate, or swell up, depending on its interaction with a nearby supernova. It was found that the luminosity of inflated stars is only correlated to its mass and doesn’t depend on the strength of the interaction with supernova. The duration of the swelling is also longer when the two stars are closer in distance.

“We applied our results to a supernova called SN2006jc, which has a companion star with a low-temperature. If this is in fact an inflated star as we believe, we expect it should rapidly shrink in the next few years,” explains Hirai

The number of companion stars detected after supernovae are steadily growing over the years. If scientists can observe an inflated companion star and its contraction, these data correlations can measure the properties of the binary system before the explosion—these insights are extremely rare and important for understanding how massive stars evolve.
 
“We think it’s important to not only find companion stars after supernovae, but to monitor them for a few years to decades to see if it shrinks back,” says Hirai.

As featured on Phys.org.

​Have you heard the joke about how many stars it takes to create a merging binary black hole? Hundreds of thousands in the real world… but only a few, if they’re OzStars and you’re using the latest COMPAS version with machine learning tools.