For almost a decade, astronomers have observed intense bursts of radio waves from the distant cosmos whose origins were unknown. The source of one such burst has now been identified, but this has only deepened the mystery. See Letter p.58

Suppose your eyes could see radio-frequency light — what would the sky look like? For a long time, we thought we knew exactly: you would see an impressively bright Milky Way galaxy, smoke rings from exploded stars, plumes of gas escaping from black holes and blinking radio emissions from cosmic lighthouses called pulsars. In 2007, we learnt that there is something else1: fast radio bursts (FRBs) that seem to appear randomly in the sky. FRBs are radio signals that are so short and elusive that astronomers have been unable to tell exactly where they come from. On page 58, Chatterjee et al.2 report the localization of an FRB with impressively high precision. However, what they saw there is not what anyone had expected.

Detecting FRBs is not easy. After all, our eyes are not sensitive to radio waves, and although the largest telescopes of the 100-metre class are extremely sensitive, they observe only a relatively small region of the sky. It therefore came as a surprise when, almost a decade ago, a seemingly isolated radio flash was discovered that lasted for only a few milliseconds1. Later, it turned out that such flashes are not rare at all3. In fact, if we had radio-sensitive eyes, we would see the sky light up roughly twice a minute4 — as if there were a constant radio meteor shower.

The radio sky is therefore not static, and the study of transient astronomical phenomena is booming as never before. Some ultrashort radio flashes are associated with lightning or with interactions of cosmic particles with Earth5. However, whereas these signals are created within our own atmosphere, FRBs are spectacularly different because they seem to come from the distant Universe.

A key signature of FRBs is that they change 'radio colour' — when FRBs light up, they quickly switch from high to low frequency. This effect is well known and is typically caused by dispersion of the radio waves as they pass through the tenuous interstellar gas in the Milky Way. However, in the case of FRBs, we observe unusually large dispersions, implying that the radio waves travel through a considerable fraction of the cosmos before their detection. Consequently, FRBs are currently the most distant (and, therefore, the brightest) radio signals in the known Universe.

What could produce so much radio-frequency light in such a short time? Initially, it seemed as if these bursts were one-off occurrences, perhaps caused by some cataclysmic event, such as the collapse or merging of neutron stars into a black hole. However, this idea took a hit when an FRB, detected using the William E. Gordon Telescope at the Arecibo Observatory in Puerto Rico, was seen to repeat6. Was this object an ultra-bright pulsar in a distant galaxy? Unfortunately, no pulsation period was found and the radio spectrum was very different from that of typical pulsars.

At the same time as the repeating FRB was reported, another study7 claimed to have found a radio source associated with an FRB that seemed to be fading on a timescale of a few days. Could this source be the afterglow of a cosmic explosion? And, if so, are there two classes of FRB: those that repeat and those that result from explosions? The claim of this study was quickly questioned8, however, because the source did not continue to fade as predicted — it behaved more like the faint and erratically variable radio emission from an underfed supermassive black hole9.

Chatterjee et al. targeted the repeating FRB using an interferometer — a network of radio telescopes that provides a much better spatial resolution than a single telescope. When the repeating FRB lit up again, Chatterjee and colleagues detected the burst using the Karl G. Jansky Very Large Array in New Mexico, discovering a faint, persistent source that emitted both radio and optical light (Fig. 1). Figure 1: Homing in on a fast radio burst (FRB). Chatterjee et al.2 report the localization of an FRB — a bright emission of radio waves that lasts for only a few milliseconds1. a, The authors use observations from a network of radio telescopes called the Very Large Array to produce a radio image of the FRB source (indicated by the square). The colours represent the flux density of radio waves from low (red) to high (white). b, The authors then use observations from the Gemini optical telescope to find a source of visible light (indicated by the circle) that is coincident with the radio source. Full size image

The FRB had finally left a trace. So, what is this object? Using telescopes of the European VLBI Network and of the US Very Long Baseline Array, Chatterjee et al. produced images of the persistent radio source at even higher resolution. As was found in the previous study7, the authors' source seems to resemble the radio emission generated by a supermassive black hole. Is this simply a coincidence, or was the claim of the previous study correct, after all? Indeed, to me, the radio spectrum and variability of the authors' source look a lot like the low-power black holes that I have studied over the past 25 years.

But if this were true, most astronomers would have expected a bright galaxy to be present, because large black holes are typically found only in large galaxies. It is therefore utterly confusing that the faint optical source coincident with the persistent radio source can at most be a small 'dwarf' galaxy, if it is a galaxy at all.

Perhaps the authors' optical source is a dwarf galaxy that contains a supermassive black hole, or is the nucleus of a disrupted galaxy or even just an isolated black hole. Maybe the persistent source is something completely different — for example, an exploding star 'disguised' to look like a black hole. And are these bursts made by the black hole itself, or by something else in orbit around it? After all, supermassive black holes are typically surrounded by dense star clusters. Chatterjee and colleagues, and the rest of the astrophysics community, are left scratching their heads.

Still, even without a clear answer, the authors' finding is a real game-changer, and the hunt for FRBs is afoot. Undoubtedly, the work will lead to many new claims in the coming months, but will the case be closed quickly? As good detectives, we should avoid adopting newly emerging dogmas too soon, even when we think we have caught the suspect red-handed. FRBs are nimble fugitives, and are not necessarily all alike. I am sure that there is still much more to be discovered in the radio sky.Footnote 1

Notes

References 1 Lorimer, D. R., Bailes, M., McLaughlin, M. A., Narkevic, D. J. & Crawford, F. Science 318, 777–780 (2007). 2 Chatterjee, S. et al. Nature 541, 58–61 (2017). 3 Thornton, D. et al. Science 341, 53–56 (2013). 4 Champion, D. J. et al. Mon. Not. R. Astron. Soc. 460, L30–L34 (2016). 5 Buitink, S. et al. Nature 531, 70–73 (2016). 6 Spitler, L. G. et al. Nature 531, 202–205 (2016). 7 Keane, E. F. et al. Nature 530, 453–456 (2016). 8 Williams, P. K. G. & Berger, E. Astrophys. J. Lett. 821, L22 (2016). 9 Bassa, C. G. et al. Mon. Not. R. Astron. Soc. 463, L36–L40 (2016). Download references

Author information Affiliations Heino Falcke is in the Department of Astronomy, Institute for Mathematics, Astrophysics and Particle Physics, Radboud University Nijmegen, 6500 Nijmegen, the Netherlands. Heino Falcke Authors Heino Falcke View author publications You can also search for this author in PubMed Google Scholar Corresponding author Correspondence to Heino Falcke.

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