Arecibo detects Fast Radio Burst

The 305-m Arecibo Radio Telescope in Puerto Rico has detected a Fast Radio Burst (FRB), proving that these mysterious bursts of radiation are indeed real!  FRBs are very intense, short-lived (generally a few milliseconds) single event bursts of radio radiation. They appear to originate at extragalactic distances but are of unknown origin. The first FRB was discovered in 2007 by Duncan Lorimer and collaborators, after analysing archival data from the 64-m Parkes radio telescope. This extremely intense burst of radio was subsequently dubbed the ‘Lorimer Burst’. In 2012 another FRB candidate was detected in Parkes archival data, but this time potentially from within our own Galaxy. Then in 2013, four more FRBs were found, again from Parkes data and again  at apparently extragalactic distances. Their origin remains a mystery, and the fact that all 6 known FRBs were detected with Parkes has cast some doubt about their astrophysical origin.

The 305-m Arecibo radio telescope in Puerto Rico. (Credit: NAIC)

The Arecibo FRB is the first detected independently of Parkes, confirming that these strong radio tranients are not due to terrestrial sources local to Parkes (in New South Wales, Australia) or instrumental errors in the Parkes telescope. The Arecibo FRB, known as FRB 121102, was detected 2 November 2012, with the radio burst lasting just 3 milliseconds. It has properties very similar to the other 6 FRBs, which means that there is something happening out there in the universe producing these strong radio bursts, though astronomers are still unsure about their origin or even their distance from us.

How do astronomers determine distances to radio sources?  This can be estimated from the dispersion measure (DM), which is a “smearing” of the pulse of radio radiation as it travels through space.  Like all forms of electromagnetic radiation, radio waves travel at the speed of light through a vacuum. However, space is not a true vacuum. The interstellar medium of our Galaxy, for example, is filled with gas and dust and charged particles. As radio photons travel through space, electrostatic interaction between the photons and the charged particles – especially electrons – cause delays in the propagation of the light.  More energetic (higher frequency) photons tend to push past the free electrons with little effect on their propagation speed, while lower energy (lower frequency) photons are more significantly delayed.  The amount of smearing of the radio signal (or the amount of delay between the arrival time of the lower frequency compared with the higher frequency radio photons) provides an estimate of how far away the signal originated, as well as the amount of material between the source as the Earth.

While the signal from FRB 121102 came from the direction of the plane of our Galaxy, it appears to have originated at an extragalactic distant.  Its dispersion measure is about three times greater that DM expected from our Galaxy, indicated it came from outside our Galaxy.

Discovery plots of the Fast Radio Burst FRB 121102 made with the Arecibo telescope. The top panel shows the dispersion measure, the bottom left shows that strong signal lasting only a few milliseconds, and the bottom right panel shows the pulse versus non-pulse. (Credit: Spitler et al. 2014)

Discovery plots of the Fast Radio Burst FRB 121102 made with the Arecibo telescope. The top panel shows the dispersion measure profile; the bottom left shows that strong signal lasting only a few milliseconds; and the bottom right panel shows the pulse versus non-pulse. (Credit: Spitler et al. 2014)

The nature of the origin of this burst – and the other FRBs – remains a mystery. Theories include coalescing neutron stars, the collapse of supermassive stars, or even evaporating black holes.  To learn more about the origin of FRBs and to study them in detail, astronomers need huge telescopes that can survey large areas of the sky in a short period of time. This is just what the Square Kilometre Array and the Molonglo telescope will do. Until then, astronomers will do doubt come up with a wide range of theories to explain the origins of FRBs!

For more information, see

[Sarah Maddison & Themiya Nanayakkara]

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