Astronomers Peer Deep in the Lair of a Cosmic Radio Burster
An international team of astronomers, among whom Daniele Michili and Jason Hessels of the UvA's Anton Pannekoek Institute, has used two of the world’s largest radio telescopes to show that a mysterious source of radio bursts is in an astonishingly extreme and unusual environment. This discovery suggests that the strange source is in the close vicinity of a massive black hole, or within a nebula of unprecedented power. The team presented their findings at the American Astronomical Society’s winter meeting in Washington, DC. The results appear on the cover of the January 11th edition of Nature.
Fast Radio Bursts (FRBs) are a recently discovered type of astrophysical signal coming from deep in extragalactic space. Their physical origin remains a mystery. Using data from the Arecibo Observatory, Puerto Rico, and the Green Bank Telescope, West Virginia, astronomers have now shown that the radio bursts from the source FRB 121102 have a property known as polarization. The behavior of this polarized light allows them to probe the source’s environment in a new way and to 'peer into the lair’ of the mysterious burster.
Polarized light is likely familiar to anyone who has used polarized sunglasses to cut down on the glare of sunlight reflected off water. If polarized radio waves travel through a region with a magnetic field, the polarization gets “twisted” by an effect known as Faraday rotation: the stronger the magnetic field, the greater the twisting. The amount of twisting observed in FRB 121102’s radio bursts is among the largest ever measured in a radio source, and the researchers conclude that the bursts are passing through an exceptionally strong magnetic field in a dense plasma (a hot, ionized gas).
'The only known sources in the Milky Way that are twisted as much as FRB 121102 are in the galactic center, which is a dynamic region near a massive black hole. Maybe FRB 121102 is in a similar environment in its host galaxy,' says Daniele Michilli, PhD candidate at the University of Amsterdam and ASTRON, the Netherlands Institute for Radio Astronomy. 'However, the twisting of the radio bursts could also be explained if the source is located in a powerful nebula or supernova remnant,' he adds.
Key to the discovery was detecting the bursts at higher radio frequency than ever before. 'We developed a new observing setup at the Arecibo Observatory to do this, and our colleagues from the Breakthrough Listen project at the Green Bank Telescope confirmed the results with observations at even higher radio frequencies,' says Andrew Seymour, USRA astronomer at Arecibo Observatory. 'What’s more, the polarization properties and shapes of these bursts are similar to radio emission from young, energetic neutron stars in our galaxy. This provides support to the models that the radio bursts are produced by a neutron star,' he adds.
A year ago, the research team pinpointed the location of FRB 121102 and reported that it lies in a star-forming region of a dwarf galaxy at a distance of over 3 billion light-years from Earth. At this great distance, an enormous amount of energy is needed to power each burst: roughly as much energy in a single millisecond as the Sun releases in an entire day. FRB 121102 is the only known repeating FRB, and this has also raised the question of whether it has a different origin compared to apparently non-repeating FRBs. 'FRB 121102 was already unique because it repeats; now the huge Faraday rotation we have observed singles it out yet again. We’re curious as to whether these two unique aspects are linked,' says Daniele Michilli.
As a novel way of visualizing the shapes of the bursts, team member Anne Archibald (University of Amsterdam) has made 3D printed models, which show the brightness of each burst as a function of both time and the observed radio frequency. These designs are freely available for download so you can print your own copy: https://www.thingiverse.com/thing:2723399.
'We are continuing to monitor how the properties of the bursts change with time,' says Jason Hessels, Associate Professor at the University of Amsterdam and ASTRON. 'With these observations we hope to distinguish between the two competing hypotheses of a neutron star either near a black hole or embedded in a powerful nebula.'
With a number of wide-field radio telescopes now coming online, more such sources are expected to be discovered in the coming year, and astronomers are poised to answer more fundamental questions about FRBs.
An extreme magneto-ionic environment associated with fast radio burst source FRB121102. By: D. Michilli, A. Seymour, J. W. T. Hessels, L. G. Spitler, V. Gajjar, A. M. Archibald, G. C. Bower, S. Chatterjee, J. M. Cordes, K. Gourdji, G. H. Heald, V. M. Kaspi, C. J. Law, C. Sobey, E. A. K. Adams, C. G. Bassa, S. Bogdanov, C. Brinkman, P. Demorest, F. Fernandez, G. Hellbourg, T. J. W. Lazio, R. S. Lynch, N. Maddox, B. Marcote, M. A. McLaughlin, Z. Paragi, S. M. Ransom, P. Scholz, A. P. V. Siemion, S. P. Tendulkar, P. Van Rooy, R. S. Wharton, D. Whitlow