Fast Radio Burst, an open question
Less than ten years ago, a new class of celestial phenomena were discovered, Fast Radio Bursts (FRBs). They are transient radio impulses lasting a few milliseconds, so rapid that it is not possible to understand from which part of the sky they come from. They are sometimes detected by radiotelescopes, and were observed for the first time in real time in 2014 by Parkes radiotelescope in Australia.
Being the events so rapid, it is almost impossible to measure their exact position in the sky accurately and to understand if these phenomena occur inside or outside our galaxy. Since each FRB takes along a “sign” of the amount of gas it has crossed, it is crucial to understand how far away an FRB is in order to measure cosmological parameters, e.g average universe density, but also to better understand the objects, e.g. neutron stars and black holes, which can originate radio bursts.
In February 2016, a team of astronomers detected the afterglow of a Fast Radio Burst, and identified the position of the radio burst which was named FRB150418.
The observation results, published in Nature journal, have been questioned for a long time and a study of the Harvard-Smithsonian Centre for Astrophysics, conducted by Williams and Berger, proposed WISE J0716-19 galaxy, which is positioned approximately in the same area of the sky where the radio burst originated, as the host of the radio emission.
However, in order to confirm that it was actually an afterglow, the brightness from the galaxy should have completely faded away over time, while researchers from the Smithsonian Centre have found a persistent radio source, the intensity of which varies randomly, reaching levels that could match the signal identified: the hypothesis seemed to be put aside due to the signal persistence.
A recent study carried out by an international collaboration under the leadership of Marcello Giroletti of the INAF Institute of Radioastronomy in Bologna, and with support from the EVN European Radiotelescope Network, investigated more in detail the anonymous and remote WISE J0716-19 elliptic galaxy, suggesting again that this could be the host of FRB150418. The paper has been submitted to the Astronomy & Astrophysics journal for publication.
To better understand the role of WISE J0716 -19 galaxy, identified by Williams and Berger, the team led by Giroletti hypothesised that the most effective way was to obtain a high-resolution image of the galaxy which enables researchers to verify if it hosts a compact and active nucleus.
In order to get the best resolution, researchers used the VLBI (Very Long Baseline Interferometry) technique: they used the European EVN (European VLBI Network) to have a number of different antennas to work together, also including INAF radiotelescopes in Medicina, near Bologna, and Noto, in Sicily, as if they were a large single radiotelescope.
«In March 2016, we made the first observation», said Giroletti, «which actually revealed the presence of a compact radio source inside the galaxy nucleus. This was a very important achievement, difficult to obtain, since the source is very weak and it was necessary to obtain a very sensitive image to reveal it».
Thank to this successful early observation, the team has continued the campaign to confirm galaxy variability, a characteristic that would have closed the “file”, by confirming that WISE J0716-19 galaxy is a “normal” active galaxy, almost certainly not associated with the radio burst.
«Surprisingly, the following observations», said Giroletti, «despite revealing the source, did not show any variability, this is way the question remains open».