WHAT I RE­ALLY WANT TO KNOW IS…

Dr Charles Kil­patrick WKH UVW WR VSRW OLJKW IURP D JUDYLWDWLRQDO ZDYH HYHQW VD\V LW ZLOO KHOS WHOO XV KRZ JROG LV FUHDWHG

Sky at Night Magazine - - CONTENTS - IN­TER­VIEWED BY PAUL SUTHER­LAND

In Oc­to­ber, astronomers made the dra­matic an­nounce­ment that a rip­ple in space-time, a grav­i­ta­tional wave, had been matched with an ex­plo­sive event in a dis­tant galaxy. It’s the first time we’ve ever seen light associated with a grav­i­ta­tional wave event.

I was the first per­son to set eyes on this re­mark­able sight, thought to be the merg­ing of two neu­tron stars in a kilo­nova. I had spot­ted it on an im­age of the galaxy which lies 130 mil­lion lightyears away.

I am part of a group called the One-Me­ter Two-Hemi­spheres (1M2H) Col­lab­o­ra­tion. We use two 1m tele­scopes. One is the Swope at Las Cam­panas, Chile, and the other is the Nickel tele­scope, at the Lick Ob­ser­va­tory in Cal­i­for­nia. To­gether, they give us cov­er­age of the en­tire sky.

The grav­i­ta­tional wave event, la­belled SSS17a, was de­tected on Au­gust 17 by two ob­ser­va­to­ries, LIGO in the US and Virgo in Italy. Al­most si­mul­ta­ne­ously, two space tele­scopes, Fermi and In­te­gral, de­tected a gamma-ray burst, dubbed GRB 170817A. Along with other astronomers we re­ceived a rapid email alert, and quickly swung into ac­tion to try to lo­cate it.

LIGO and Virgo pointed us to a re­gion of sky where the wave’s source was thought to be. It was a very large area of around 30 square de­grees, equiv­a­lent to more than 100 full moons, but we had a strat­egy for search­ing for it which in­volved iden­ti­fy­ing likely gal­ax­ies. We would then pho­to­graph those gal­ax­ies and look for some­thing new that was not there in archived im­ages.

On high alert

As soon as we got the alert, one of my col­leagues put to­gether a list of gal­ax­ies at the sort of dis­tance at which the event was thought to have oc­curred. We then sent that list of tar­gets to our ob­server on the Swope tele­scope in Chile. We knew we had to be quick be­cause the patch of sky we needed to check was very close to the Sun and would set a cou­ple of hours af­ter twi­light. Our ob­server went through the gal­ax­ies one by one, and I be­gan re­duc­ing the data in my of­fice at the Univer­sity of Cal­i­for­nia Santa Cruz as each im­age was recorded and sent to me. And in the ninth im­age, when I looked at a galaxy known as NGC 4993, which is in Hy­dra, I saw a source that I did not see in the archived pic­ture. Our im­age of the galaxy was taken al­most ex­actly 11 hours af­ter LIGO de­tected the grav­i­ta­tional wave, and it took me just 20 min­utes to find the vis­i­ble source. At the time, I was re­ally fo­cused on the process, and it was ex­cit­ing to be wrapped up in it. But then our team leader Ryan Fo­ley, also of the Univer­sity of Cal­i­for­nia Santa Cruz, said: ‘Wow. That’s it. That’s the source’. And it very slowly dawned on me what a big deal this was. We had en­tered a new era in as­tron­omy where we can si­mul­ta­ne­ously de­tect grav­i­ta­tional waves and elec­tro­mag­netic emis­sion from one event. The SSS17a event has been called ‘multi-mes­sen­ger as­tron­omy’ be­cause it was de­tected by tele­scopes ob­serv­ing right across the spec­trum, from gam­maray and X-ray tele­scopes in space, through ul­tra­vi­o­let, op­ti­cal and the in­frared, to ra­dio wave­lengths. It is im­por­tant to study be­cause it tells us a lot about where heavy el­e­ments in the Uni­verse come from. Neu­tron stars are in­cred­i­bly dense. Just a tea­spoon of ma­te­rial from one would weigh a bil­lion tonnes on Earth. It is thought that when two neu­tron stars be­gin to col­lide, they throw off neu­trons, which form the build­ing blocks of re­ally heavy el­e­ments such as gold, plat­inum, lead and ura­nium. I’ve heard it sug­gested that gold sev­eral hun­dred times the mass of the Earth was pro­duced by this one merger. As an op­ti­cal as­tronomer, I’m re­ally in­ter­ested in how of­ten neu­tron star merg­ers hap­pen. Hope­fully, when we’ve seen a few more of them, we will be able to fig­ure out the rate at which they oc­cur.

This grav­i­ta­tional event was born of a kilo­nova cre­ated by two col­lid­ing neu­tron stars

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