A Repeating FRB: Guest Post by Paul Scholz

This is a guest post by Paul Scholz, currently a graduate student working with Vicky Kaspi at McGill University. Paul specializes in radio and X-ray observations of pulsars. He was second author on this Nature Paper describing the discovery of a repeating(!!) fast radio burst, and was the first person to notice the repeated bursts it in the data from the follow-up observations. -dave

There's been a few posts already on this blog about FRBs  but a new result in a Nature paper last week, on which I'm co-author, adds a big twist to the story. As a reminder (or in case you haven't read Dave's and Dusty's posts) Fast Radio Bursts (FRBs) are short (~millisecond) bursts detected at radio frequencies, that originate from cosmological distances, or at least that's what's implied from their dispersion measures. The twist is that we've discovered repeating bursts from the same source as a previously discovered FRB. First I'll give a bit of background and then I'll get to why the result is exciting.

FRB 121102 was detected in the PALFA survey in 2013 by Laura Spitler and was the first FRB seen at a telescope other than Parkes. This was an important step in out understanding of this new phenomenon as it put to rest fears that FRBs were caused be some terrestrial or atmospheric phenomena local to Parkes. (since then the Green Bank Telescope has also found an FRB) 

The Parkes FRBs have so far never been seen to repeat despite quite a bit of telescope time dedicated to determining whether or not they do. This, along with their extreme distances, has led to many cataclysmic explanations for FRBs such as the collapse of a rapidly rotating neutron star into a black hole, and the merger of neutron stars or white dwarfs. 

For the PALFA FRB we set out to put a robust limit on the repeatability of bursts from the source of FRB121102. So, we performed a campaign of observations using Arecibo in May and June 2015. When we searched those observations for bursts we found some very bright signals. In total we found 10 new bursts that had the same dispersion measure as the original FRB 121102.

11 bursts from FRB 121102 including the original burst. They're corrected for the $\nu^2$ dispersion sweep and the shape of the instrumental bandpass.

It was immediately obvious that this was a big deal in the study of FRBs. Our discovery of repeating bursts from FRB 121102 shows that the source of these bursts cannot be cataclysmic, that is the source must survive the event. Also, six of the bursts were found within 10 minutes of each other, that says that it can't be caused by an event that occurs on longer timescales than that, such as giant flares from magnetars.

A model for FRBs that does fit with these bursts that can repeat on relatively short timescales is supergiant pulses from an extragalactic pulsar or magnetar. This also fits with the unusual and highly variable burst spectra that we see (see the right panels in the above plot), since similar spectral variability is seen in the Crab pulsar, the prime example of a young, energetic pulsar in a supernova remnant in our Galaxy. Another that might fit is radio counterparts to short (millisecond to second) X-ray bursts from a magnetar.

At any rate, this result shows that FRBs can repeat. But, it most definitely does not imply that all FRBs repeat and that some of them cannot be of the cosmological, cataclysmic flavor. The Keane et al. Nature paper that came out a week before ours suggested the merging of two neutron stars into a black hole (i.e. a cataclysmic event) as the origin of their bursts. But the host-galaxy association that that interpretation was based on has since been questioned (see Dave's post). Still, keep in mind that the Parkes-discovered FRBs have been followed up for many hours and no repeat bursts have been found. Perhaps they are all repeating and Parkes is missing the fainter bursts that we can see from FRB 121102 since we're using Arecibo. But this seems like especially bad luck, surely Parkes would have seen at least a second burst from one of the 15 FRBs discovered there by now.

This leads to the exciting prospect that there are two or more physical origins for FRBs. Time will tell: we'll keep characterizing FRB 121102, other FRBs will continue to be monitored for repeats, and in the next few years large field-of-view radio telescopes, such as the CHIME telescope in Canada, should give us a flood of new FRBs that will give us a much better picture of the FRB population.


EDIT - For posterity, here is a repeating Furby. -dave