Two years ago, Earth received the light from a distant supernova. A star had collapsed into a black hole and triggered an intense gamma-ray emission, known as a gamma-ray burst (GRB). This emission was created by a jet of material released by the dying star. The evolution of that jet was captured in a series of observations by the Atacama Large Millimeter/submillimeter Array, otherwise known as ALMA.
ALMA tracked the jet over a few months and put the images together to create the observatory's first time-lapse. As reported in the Astrophysical Journal, researchers were able to study the reverse shock, which is caused by the jet slamming into the material around the star, creating a shockwave that travels backward through the jet.
“Since ALMA sees in millimeter-wavelength light, which carries information on how the jets interact with the surrounding dust and gas, it is a powerful probe of these violent cosmic explosions,” lead author Dr Tanmoy Laskar, from the National Radio Astronomy Observatory, said in a statement. “With our current understanding of GRBs, we would normally expect a reverse shock to last only a few seconds to a minute at most. This one lasted a good portion of an entire day.”
This is not the only insight the team got from the observations. Millimeter wavelengths of light appear to be ideal for studying the conditions created by the jet, and there’s plenty to learn. The researchers were able to study the geometry of the jet, and by combining this with information about its duration, they estimated the energy released. It was equivalent to the amount of energy our Sun puts out in a billion years.
“This is a fantastical amount of energy, but it is actually one of the least energetic events we have ever seen. Why this is so remains a mystery,” said Kate Alexander, a graduate student at Harvard University who led the Very Large Array (VLA) observations reported in this study. “Though more than 2 billion light-years away, this GRB is actually the nearest such event for which we have measured the detailed properties of the outflow, thanks to the combined power of ALMA and the VLA.”
This is only the fourth gamma-ray burst reverse shock that has been observed convincingly in more than one type of light. The star was in a very low-density environment; the gas around it was 3,000 times less dense than what we normally find around stars in our own galaxy. The researchers suggest that maybe this is the reason why these events are so rare.