— The mys­tery of fast ra­dio bursts

KATIE MACK is a the­o­ret­i­cal as­tro­physi­cist who fo­cuses on find­ing new ways to learn about the early uni­verse and fun­da­men­tal physics.

Cosmos - - Contents -

Solv­ing the mys­tery of fast ra­dio bursts.

THE BEST SCIENCE STO­RIES are mys­tery sto­ries. Some­thing un­ex­plained oc­curs, the de­tec­tives gather their clues, the­o­ries are pro­posed and shot down. In the end, if all goes well, the mys­tery is solved – at least un­til the next time some­thing goes bump in the night.

One of the most per­plex­ing mys­ter­ies in as­tron­omy to­day is the fast ra­dio burst, or FRB. Al­most 10 years ago, as­tronomer Dun­can Lorimer at West Vir­ginia Univer­sity no­ticed a shock­ingly bright, in­cred­i­bly quick sig­nal in data col­lected by the Parkes ra­dio te­le­scope ob­ser­va­tory in New South Wales a few years be­fore. Only a few mil­lisec­onds long, the burst was as bril­liant as some of the bright­est gal­ax­ies ra­dio astronomers had ever ob­served.

In­trigu­ingly, the sig­nal swept across ra­dio fre­quen­cies, mim­ick­ing the be­hav­iour of bright flashes of ra­di­a­tion from very dis­tant pul­sars – ul­tra-dense stars that emit reg­u­lar pulses of light. A sig­nal that spreads across fre­quen­cies usu­ally in­di­cates that cos­mic mat­ter is dis­pers­ing the light, in the same way a prism spreads white light into a rain­bow.

But while the burst looked a lot like a pulsar blip, it didn’t re­peat the way pulsar sig­nals do, and no other te­le­scope de­tected it. Dubbed the “Lorimer Burst”, it stood for years as a one-off event.

Given its unique­ness, some sug­gested it must have been some kind of Earth­based in­ter­fer­ence, or per­haps sim­ply a glitch in the Parkes te­le­scope.

To­day, fast ra­dio bursts are no longer anom­alies. With a hint of what to look for – very short, bright events – astronomers have scoured data from the Parkes te­le­scope and other ra­dio tele­scopes around the world. FRBS are now so nu­mer­ous it’s hard to keep up with their dis­cov­ery.

Yet FRBS are a study in con­tra­dic­tions. So far, only one source re­peats, but at such ir­reg­u­lar in­ter­vals that astronomers have not been able to de­ter­mine a pat­tern. Only two bursts have co­in­cided with emis­sions in vis­i­ble or any other kind of light, which is nec­es­sary to pin­point the source of the FRB since the ra­dio tele­scopes can’t give an ex­act lo­ca­tion.

How­ever, one of those two bursts now ap­pears more likely to be a chance align­ment than a true cor­re­la­tion, and the other paints the pic­ture of an ex­plo­sion with such odd char­ac­ter­is­tics it is hard to rec­on­cile with any known model.

Care­ful anal­y­sis of dif­fer­ent FRB sig­nals has sug­gested ex­plo­sions of young stars, or old stars, or even col­li­sions be­tween stars, but none of those fit with an FRB that re­peats.

One of the big­gest open ques­tions is ex­actly how far away FRBS are. Ev­ery at­tempt to work out their dis­tance has been in­con­clu­sive. Even the pat­tern of their lo­ca­tions in the sky is odd. If they’re all far be­yond our own galaxy, we would ex­pect them to ap­pear at ran­dom places in the sky. If they’re all in our galaxy, we should see them mostly along the plane of the Milky Way, where most of the stars are. In ac­tu­al­ity, we’ve found them to lie some­what more of­ten above or be­low the plane of the galaxy, not ran­domly like dis­tant sources, and not in the plane like close ones. But with only 20 or so seen so far, it is hard to draw a con­clu­sion.

Thanks to FRBS, we are now look­ing at the uni­verse in a new way, re­design­ing our ob­ser­va­tion strate­gies and scour­ing the data for su­per-short-du­ra­tion events. Just as ev­ery new ob­serv­ing wave­length we try or in­stru­men­tal tech­nique we de­velop opens a new win­dow on to the uni­verse, this new fron­tier may al­low us to see an en­tire zoo of cos­mic events that were hap­pen­ing all along, un­seen. It wouldn’t be sur­pris­ing to find that FRBS rep­re­sent a di­verse fam­ily of cos­mic ex­plo­sions rather than one kind of thing.

The key to solv­ing this mys­tery will be to catch an FRB in the act and, at the same time, see its fin­ger­prints on a sig­nal de­tected with an­other kind of light, thus al­low­ing us to see the galaxy it came from.

Astronomers are al­ready de­sign­ing sur­veys that watch for FRBS with ra­dio tele­scopes and scour the sky with op­ti­cal, in­frared, or gamma ray tele­scopes around the world si­mul­ta­ne­ously. Once we have a hand­ful of real-time FRBS along with their host gal­ax­ies, we will start to close this case and, more likely than not, open sev­eral ex­cit­ing new ones.


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