FASTTHERADIO BURSTS: BAFFLING LIGHTHOUSES OF THE SKY Anirudh Kulkarni examines what fast radio bursts are and how they occur.
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ast radio bursts have had their causes shrouded in mystery ever since their discovery, and astrophysicists are racing towards figuring out what triggers them. Once thought of as a rare phenomenon, they are now known to be ubiquitous with about an estimated 1,000 coming towards us each day from all over the sky.
light which is on the order of hundreds of nanometres (a tenth of the thickness of a hair). Radio frequencies also range from MHz (like your favourite FM station) to GHz. However, the radio that you listen to has been transmitted by human sources, whereas the radio waves detected by telescopes come from outer space.
Metaphorically, they seem to be like lighthouses distributed across the universe beaming away radio waves into space.
In 2007, the Parkes telescope in Australia received a strong burst of radio signal that had never been detected before. Now known as the Lorimer burst, this discovery was a special case of Fast Radio Bursts (FRBs), which have fuelled scientific curiosity and public imagination ever since. Metaphorically, they seem to be like lighthouses distributed across the universe beaming away radio waves into space.
Radio waves are the same waves you use when you tune into your cherished evening radio show. They sit on the electromagnetic spectrum just like visible light – the colours that we see. However, radio waves have very long wavelengths: measuring from about tens of centimetres to metres, unlike visible
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Radio telescopes have antennas which detect these radio waves. The Lorimer burst, named after the first author of the paper reporting the event, lasted 5ms and had a frequency of 1.2–1.5 GHz. To know how far the signal travelled, scientists looked at the dispersion of the signal through intergalactic space before arriving at the telescope. Dispersion is the hallmark of refraction, a feature of light. When visible white light passes through a prism, it gets separated into different frequencies, which we see as colours. Rainbows are caused when light gets dispersed by rain drops. Furthermore, these different frequencies travel at different speeds in the refractive medium. Intergalactic space is not empty; rather it is
filled with free electrons which disperse light waves passing through it. As a result, the higher frequencies of radio waves reach us first and lower frequencies reach us later. By studying the amount of dispersion, one can estimate the distance travelled by the radio signal. The FRB detected in 2007 seemed to originate from a very distant place in our universe. Pinpointing the precise source of the FRB, however, was not easy. Radio telescopes have a good time resolution (in the order of milliseconds) but do not have a good angular resolution. The Parkes telescope can only pinpoint the source to an area of 1000 galaxies but cannot be any more precise. We needed more events like the Lorimer burst to find out the location. Unfortunately, some of the subsequently detected FRBs turned out to be anomalies caused by the activity of nearby microwave ovens or lightning rods which bogged some research down. Dubbed as “perytons”, after the mythical winged stag that casts a human shadow, these anomalies did not, however, deter scientists from pursuing their study. In 2012, an FRB that repeated every 6 seconds was discovered. This was the key to trying to find out more about the source. To improve the localisation, scientists used an interferometer, a combination of many radio telescopes that are spread apart. In
