Searching the Surroundings of a Fast Radio Burst


Fast radio bursts are mysterious astronomical phenomena — for now. To understand how they form, we need to take a closer look at where they live. A new study does just that, with the help of some very sensitive astronomical instruments.

The Gran Telescopio Canarias during the day. [University of Florida]

The Fascination of Fast Radio Bursts

Fast radio bursts (FRBs) are exactly what they say they are: short, bright radio signals that last milliseconds at most. Their energy levels make them especially intriguing, since there aren’t many processes that can produce such large amounts of energy so quickly. Another constraint is that FRBs have been detected in all kinds of galaxies, meaning that whatever produces FRBs can’t be overly unique.

Radio telescopes today have the capability to precisely isolate FRBs in their host galaxies, meaning that we can probe the environments that produce FRB sources. The closest known FRB we’ve confidently isolated is called FRB 20180916B (though see this post for a new discovery that may be closer!), which is nearly 500 million light-years away. High-resolution observations have shown that FRB 20180916B is located in a distinct star-forming region, but what can we see if we look even closer?

In a recent study, a group of researchers led by Shriharsh P. Tendulkar (Tata Institute of Fundamental Research, India) studied the surroundings of FRB 20180916B in the highest detail yet, getting down to a scale of hundreds of light-years.

The velocity of gas in the host galaxy of FRB 20180916B, with the location of the FRB shown by a red cross. The contours come from Hubble images of the galaxy and depend on the flux detected in the image. [Adapted from Tendulkar et al. 2021]

Searching Through Gas and Stars

For their study, Tendulkar and collaborators used the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope and the MEGARA spectrograph on the Gran Telescopio Canarias. Taken together, the observations span mainly optical wavelengths, which are sensitive to gas and stars.

The gas serves two important functions: it can be used to determine how much star formation is happening in a region, and it can also be used to measure motion. Tendulkar and collaborators used the latter property to determine that FRB 20180916B’s home region is likely rotating with the large galaxy in its vicinity. This rules out the possibility that the FRB source is actually hosted in a smaller, less distinct satellite galaxy.

The star-forming region closest to FRB 20180916B as seen by Hubble, with its V-shape highlighted. The FRB’s location is shown by the green ellipse with a green arrow pointing towards it. [Adapted from Tendulkar et al. 2021]

Running Away from Home

Tendulkar and collaborators also found that the star formation happening around FRB 20180916B is at an interesting stage: it’s not extremely active, but it hasn’t gone placid either, suggesting that the region is still rather young.

FRB 20180916B is also a significant distance from the nearest group of stars. So, if the FRB source was born in that group, it had to have traveled between 800,000 to 7 million years to get to where it is now. This puts constraints on what the source of FRB 20180916B is, since not many astronomical objects can remain as energetic as FRB sources as they age.

So what’s behind FRB 20180916B? After considering possible scenarios, Tendulkar and collaborators zero in on X-ray or gamma-ray binaries, which consist of a neutron star and a massive companion star. However, to be certain that these sorts of objects are FRB sources, we’d need large samples of well-studied binaries — which is certainly doable with the radio telescopes we have now!


“The 60 pc Environment of FRB 20180916B,” Shriharsh P. Tendulkar et al 2021 ApJL 908 L12. doi:10.3847/2041-8213/abdb38