GMRT observations question decades-long understanding of Gamma-Ray Bursts
The new source, dubbed GRB 221009A, turned out to be the brightest Gamma-Ray Burst (GRB) ever observed and lasted over 300 seconds
On Sunday, October 9, 2022, a pulse of intense gamma-ray radiation swept through our solar system, saturating detectors on numerous spacecraft, and sending astronomers across the world scurrying to train the fastest and most powerful telescopes on it. The new source, dubbed GRB 221009A, turned out to be the brightest Gamma-Ray Burst (GRB) ever observed and lasted over 300 seconds. In a new study that appears in the ‘Astrophysical Journal Letters’, radio observations of this exceptional and exciting event with the Giant Metrewave Radio Telescope (GMRT) and a slew of other facilities question our decades-long understanding of how these events explode into their multi-coloured fireworks.
The lead author of this study – Tanmoy Laskar, assistant professor of Physics and Astronomy, University of Utah – and his colleagues, quickly compiled radio observations with the GMRT and other facilities such as the MeerKAT Array in South Africa; US National Science Foundation’s Karl G Jansky Very Large Array (VLA) in New Mexico, USA; Atacama Large Millimetre Array (ALMA) in Chile; and Submillimetre Array (SMA) in Hawaii. This compilation is now one of the most detailed datasets for a GRB afterglow date. Upon combining and analysing the data from all the telescopes, the astronomers were flummoxed as the radio measurements were brighter than expected based on the X-ray and visible light of the afterglow.
Laskar said, “With GRB 221009A being the brightest burst known so far, a real mystery lay in what would come after the initial burst of Gamma-Rays. As the jets slam into the gas surrounding the dying star, a bright ‘afterglow’ of light is produced across the entire spectrum. The afterglows of GRBs fade quite rapidly, which means we had to be quick and nimble in capturing the light before it disappeared, taking its secrets with it.”
The co-author of the study, Kate Alexander, assistant professor, Astronomy, University of Arizona, said, “At first, we were very excited because we thought we might have captured the fleeting signature of a `reverse shock’ aka a shock wave going backward through the jet and lighting it up in the radio.” While such a signature has been predicted to show up at radio frequencies, its presence has been confirmed only in a few cases. Catching one such in the brightest GRB ever could have helped the researchers zero in on the composition of GRB jets, a puzzle that remains poorly understood to this day.
Laskar said, “What we found in the radio looks a bit like a reverse shock in the sense that there is extra emission at radio frequencies. But we also have models for how a reverse shock spectrum should evolve with time, and the radio spectrum of GRB 221009A faded much too slowly. So, either we don’t understand reverse shocks, or we’ve found a completely new emission component.”
Alexander said, “Our GMRT observations were essential for this study because when combined with the rest of the data, they were key in pinning down the peak frequency and peak brightness of this emission component. From that, we were able to calculate that the outflow producing this light must consist of a small amount of mass moving at about 99.4% of the speed of light.”
Laskar concluded, “Without the GMRT, we would still be groping in the dark with many levels of uncertainty in this new emission component. While we may have to go back to the drawing board to thrash out what is going on, thanks to GMRT, we have an excellent starting point from a fantastic set of measurements.”
According to astronomers, long-duration GRBs are the birth cry of a black hole, formed as the core of a massive star collapses under its own weight. The newborn black hole launches powerful jets of plasma at speeds near that of light which pierce through the collapsing star and shine in Gamma-Rays.