BURSTS OF MYS­TERY

The source of mys­te­ri­ous bursts of ra­dio waves has been con­found­ing sci­en­tists for a decade. Now we’re QDOO\ PDNLQJ VRPH SURJUHVV ZULWHV Govert Schilling

Sky at Night Magazine - - CONTENTS -

Decades af­ter their dis­cov­ery, sci­en­tists are fi­nally be­gin­ning to un­der­stand fast ra­dio bursts.

Duncan Lorimer, a ra­dio as­tronomer at West Virginia Univer­sity in the US, had no idea what to ex­pect when his stu­dent David Narke­vic re-an­a­lysed ob­ser­va­tions from July 2001, car­ried out with the 64m Parkes ra­dio tele­scope in Aus­tralia. Lorimer cer­tainly couldn’t imag­ine that Narke­vic’s 2007 anal­y­sis would open up a new field in astro­physics that is still mys­ti­fy­ing sci­en­tists af­ter 10 years.

The up­shot: thou­sands of times each day, an ul­tra-brief, in­tense burst of ra­dio waves is gen­er­ated some­where in the Uni­verse, and as­tronomers have frus­trat­ingly few clues as to their ori­gin. “There’s still an aw­ful lot of work ahead of us,” ob­serves Lorimer.

The ‘Lorimer burst’ of 24 July 2001 was no freak event. Oc­ca­sion­ally, other Parkes ob­ser­va­tions also caught sim­i­lar fast ra­dio bursts (FRBs), last­ing a few mil­lisec­onds at most and oc­cur­ring ran­domly in the sky. Tak­ing the ex­tremely nar­row field of view of the ra­dio tele­scope into ac­count, it was easy to cal­cu­late that these mys­te­ri­ous ex­plo­sions must in fact be very fre­quent. But what are they, and where do they come from?

If you don’t know the lo­ca­tion of an FRB (ei­ther within our own Milky Way or far beyond), you

can’t de­duce the en­er­gies of the out­burst. It could be a rel­a­tively mi­nor ex­plo­sion on the sur­face of a nearby dwarf star, or a ti­tanic event at the edge of the ob­serv­able Uni­verse.

As­tronomers had only one clue: all FRBs dis­play a rel­a­tively strong ‘dis­per­sion’, which means that lower-fre­quency waves lag be­hind higher-fre­quency ones. This ef­fect is caused by the waves pass­ing through a ten­u­ous gas of charged par­ti­cles. High dis­per­sion means lots of in­ter­ven­ing par­ti­cles, cor­re­spond­ing to large dis­tances. But no one could be com­pletely sure.

Clos­ing in on the sig­nal

To com­pli­cate mat­ters, a sin­gle-dish ra­dio tele­scope like Parkes has a rel­a­tively low spa­tial res­o­lu­tion on the sky, so the lo­ca­tions of the bursts weren’t very pre­cisely known. That made it im­pos­si­ble to carry out fol­low-up ob­ser­va­tions with larger tele­scopes.

This only changed in April 2015 when a team led by Evan Keane of the Square Kilo­me­tre Ar­ray Or­gan­i­sa­tion pointed the Aus­tralia Tele­scope Com­pact Ar­ray (ATCA) at a re­gion of sky in the con­stel­la­tion of Ca­nis Ma­jor, where Parkes had dis­cov­ered a fast ra­dio burst just a few hours ear­lier.

On the ba­sis of the Parkes de­tec­tion, the sky po­si­tion of FRB 150814 couldn’t be de­ter­mined to a pre­ci­sion bet­ter than some 15 ar­cmin­utes (half the width of the full Moon). But ATCA is an in­ter­fer­om­e­ter ar­ray with much sharper vi­sion, and in the 0.25º ‘er­ror box’ of FRB 150814, Keane and his col­leagues found a slowly fad­ing ra­dio source, lo­cated in a galaxy at a dis­tance of six bil­lion lightyears. The after­glow of FRB 150418 was rem­i­nis­cent of the after­glow of a short gamma-ray burst, which are the re­sult of neu­tron star col­li­sions in re­mote gal­ax­ies.

In late Fe­bru­ary 2016, Keane’s team wrote in Na­ture that FRBs are most likely one-off

“If FRB 121102 is re­peat­ing, you can just keep an eye on the sus­pect part of the sky to lo­calise a new out­burst in real time”

cat­a­strophic events, even though they couldn’t be 100 per cent sure about the as­so­ci­a­tion be­tween the FRB and the ATCA ra­dio source. But just a week later, Na­ture pub­lished an­other pa­per by Cana­dian, Amer­i­can and Dutch ra­dio as­tronomers that came to a very dif­fer­ent con­clu­sion.

Us­ing the 305m ra­dio tele­scope at Arecibo Ob­ser­va­tory in Puerto Rico, Paul Scholz of McGill Univer­sity in Mon­treal and his col­leagues had dis­cov­ered a re­peat­ing fast ra­dio burst: FRB 121102 in Auriga also dis­played out­bursts in May and June 2015. So what­ever was caus­ing these bursts didn’t de­stroy its source. As Lorimer says: “It’s ba­si­cally im­pos­si­ble for a cat­a­clysmic event to pro­duce re­peat­ing bursts.”

Now, the hunt was re­ally on. If FRB 121102 is re­peat­ing ev­ery now and then, you can just keep an eye on the sus­pect part of the sky with a large in­ter­fer­om­e­ter to lo­calise a new out­burst in real time. Pa­tience paid off, even­tu­ally. In Septem­ber last year, both the Very Large Ar­ray in New Mex­ico and the Euro­pean VLBI Net­work suc­ceeded in trac­ing down the source of the bursts to a small, in­con­spic­u­ous dwarf galaxy at a dis­tance of some 2.5 bil­lion lightyears. “It’s an ob­ser­va­tional break­through,” said NASA as­tro­physi­cist and gamma-ray burst ex­pert Neil Gehrels when he learned about the re­sults, just be­fore his un­timely death in Fe­bru­ary 2017. From the known dis­tance, as­tronomers could now cal­cu­late the burst’s en­ergy – about as much in one mil­lisec­ond as the Sun pours out in 24 hours.

Cana­dian-Dutch ra­dio as­tronomer Jason Hes­sels now be­lieves FRBs are

oc­ca­sional ex­plo­sions from ex­tremely rapidly spin­ning, highly mag­ne­tised neu­tron stars. Others think that the re­peat­ing bursts may oc­cur in the ac­cre­tion disc sur­round­ing the black hole that prob­a­bly lurks in the dwarf galaxy’s core. Mean­while, Lorimer is not so sure that there’s just one type of fast ra­dio burst – af­ter all, FRB 121102 is the only one known to re­peat so far. “My guess is that there are mul­ti­ple classes,” he says.

So yes, there’s still a lot of work to do. Astro­physi­cists look for­ward to the in­au­gu­ra­tion this year of the CHIME ra­dio tele­scope in Canada, which may de­tect a few dozen FRBs per day. Mean­while, Dutch and South African as­tronomers are about to de­ploy the 65cm op­ti­cal MeerLICHT tele­scope at the South African As­tro­nom­i­cal Ob­ser­va­tory in Suther­land, a very promis­ing in­stru­ment in the search for the true na­ture of FRBs.

MeerLICHT will au­to­mat­i­cally and con­tin­u­ously scan the same re­gion of sky as the South African ra­dio in­ter­fer­om­e­ter MeerKAT, some 250km to the north. As soon as MeerKAT hap­pens to catch a fast ra­dio burst, a pos­si­ble op­ti­cal after­glow will be cap­tured by MeerLICHT, en­abling de­tailed fol­low-up ob­ser­va­tions. “No one has ever tried this ap­proach be­fore,” says project man­ager Steven Bloe­men of Rad­boud Univer­sity in Ni­jmegen in the Nether­lands. “It may rev­o­lu­tionise the field.”

Fast ra­dio bursts have been some­thing of an enigma, last­ing only for a few mil­lisec­onds and seem­ingly oc­cur­ring at ran­dom

The FRB after­glow is sim­i­lar to that left be­hind by a short gamma-ray burst, which are cre­ated when bi­nary neu­tron stars col­lapse into one an­other

The ar­rival of the ‘dis­persed’ Lorimer burst; in­set: the same burst once dedis­per­sion has been ap­plied

Some be­lieve that FRBs may be caused by in­ter­mit­tent ex­plo­sions on rapidly spin­ning neu­tron stars, but there is no con­sen­sus so far

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