What did the team of tele­scopes re­ally see?

Ob­serv­ing two neu­tron stars col­lid­ing in dif­fer­ent wave­lengths has re­vealed what oc­curred 1.7 bil­lion light years away

All About Space - - The Kilonova Story -

“All neu­tron star merg­ers pro­duce grav­i­ta­tional waves; this is some­thing we’re ex­tremely con­fi­dent about”

Dr Van Eerten

States, and the Univer­sity of Mary­land at Col­lege Park (UMCP) tells All About Space. “Luck­ily the sig­nal was also caught by NASA's Neil Gehrels Swift Observatory. Swift has a sharper view than Fermi and clearly pin­pointed the GRB's po­si­tion in the sky. This al­lowed as­tronomers to use other tele­scopes and ob­serve that re­gion of the sky in other wave­lengths such as ra­dio, op­ti­cal and X-ray.

“The Mag­el­lan tele­scope de­tected the op­ti­cal light 36 hours later, whereas NASA's Chan­dra X-ray Tele­scope saw its X-ray af­ter­glow eight days af­ter the gamma-ray burst was seen for the first time.” This chain of events was also helped by data taken by the Hub­ble Space Tele­scope; the Dis­cov­ery Chan­nel Tele­scope (DCT), a 4.3-me­tre (14.1-foot) tele­scope in Ari­zona, United States; the European Southern Observatory’s Very Large Tele­scope at the Paranal Observatory in Chile and the Gemini South tele­scope also si­t­u­ated in Chile at Cerro Pachón.

What led to this in­ves­ti­ga­tion, though? What made as­tronomers look at the sig­nal and think that this war­ranted ex­tra ob­ser­va­tion time us­ing mul­ti­ple tele­scopes? The an­swer to that is ex­plained by a fel­low re­searcher in the study, Dr Hen­drik Van Eerten of the Univer­sity of Bath, United King­dom. “The X-ray in­stru­ment on board Swift de­tected a sig­nal that re­mained un­usu­ally steady over a long pe­riod of time. This was so un­ex­pected,” ex­plains Van Eerten to All About Space. “A rapid fade-out, or at least any sort of fade­out, is more usual for 'short' GRBs – the type caused by merg­ing neu­tron stars. To get a more in-depth ob­ser­va­tion in the X-rays, with bet­ter spa­tial res­o­lu­tion, the Chan­dra X-ray tele­scope was utilised.”

The gamma-ray burst

ex­hib­ited not only had a no­tably short fade-out, but it was ex­tremely weak as well. In fact, it has one of the low­est en­er­gies ever de­tected by the Swift tele­scope.

Just over a day af­ter the de­tec­tion fol­low-up ob­ser­va­tions were con­ducted in bright op­ti­cal and X-ray wave­lengths. Af­ter such data had been col­lected and the op­tions ruled out there was only one ex­pla­na­tion to this co­nun­drum, and the re­searchers an­nounced that they had just wit­nessed the kilo­nova caused by the merger of two neu­tron stars 1.7 bil­lion light years away from Earth.

Un­like a su­per­nova – a stel­lar ex­plo­sion that sig­nals the end of a star’s life – a kilo­nova has a dif­fer­ent process and emits dif­fer­ent forms of en­ergy. The most com­mon form of su­per­nova, a Type II, oc­curs when the star can­not main­tain nu­clear fu­sion and causes a col­lapse of outer ma­te­rial against its solid core, thus ex­hibit­ing the ex­plo­sion that thrusts large amounts of ma­te­rial into the cos­mos - this type of ex­plo­sion can shine with the bright­ness of 10 bil­lion Suns. What is left be­hind af­ter a su­per­nova can either be­come a black hole or a neu­tron star, which is a star that has

“Op­ti­cal tells us about the kilo­nova; X-rays tell us about the jet and the new­born black hole”

Dr Eleonora Troja

a large mass tightly jammed into a sphere that is about the size of a city.

When two of these neu­tron stars col­lide they emit vastly greater amounts of en­ergy, and the fi­nal bang at the end of its crescendo can re­sult in a gamma-ray burst, which was spot­ted in GRB150101B. These events are the cre­ators of some of the uni­verse’s most ex­otic and heav­i­est el­e­ments. “It [the ejecta] is so hot, dense and neu­tron-rich that un­sta­ble mas­sive nu­clei are formed faster than they can de­cay. Even­tu­ally they do de­cay to more sta­ble iso­topes, just about ev­ery­thing with atomic num­bers from about 30 to about 80,” ex­plains Dr Geoffrey Ryan of the Univer­sity of Mary­land at Col­lege Park (UMCP), United States, to All About Space. “You've heard of gold, I'm sure. But there's also irid­ium, tung­sten, sil­ver, pal­la­dium, all the way down to ru­bid­ium and se­le­nium. Even the lan­thanides, like eu­ropium, are made.”

Dur­ing a kilo­nova a fast-mov­ing jet of en­ergy is also emit­ted from the source and is nor­mally fir­ing from both ends of a newly formed black hole, per­pen­dic­u­lar to its plane of ro­ta­tion. “We can study these two, very dif­fer­ent events [the kilo­nova and the jet] be­cause we have the abil­ity to see the sky through dif­fer­ent lenses: op­ti­cal tells us about the kilo­nova and the pro­duc­tion of heavy el­e­ments; X-rays tell us about the jet and the new­born black hole,” says Troja.

On 17 Au­gust 2017 a world­wide fleet of tele­scopes all turned their heads to the con­stel­la­tion of Hy­dra to fol­low up on a grav­i­ta­tional-wave de­tec­tion made by LIGO and Virgo. It was then that as­tronomers found the first-ever vis­i­ble coun­ter­part to a grav­i­ta­tional-wave de­tec­tion. GW170817 was lo­cated 130 mil­lion light years from Earth. As­tronomers were able to de­duce that this event was also the re­sult of two neu­tron stars merg­ing.

The as­tronomers in­volved in the anal­y­sis of GRB150101B have claimed that it is a cos­mic ‘rel­a­tive’ of GW170817. Troja, Ryan and Van Eerten all agree that these two events are re­mark­ably sim­i­lar and could well be re­lated. The main sim­i­lar­i­ties are the un­usu­ally faint and short-lived GRB, but they also both emit­ted bright, blue op­ti­cal light which lasted over a pe­riod of days – the X-ray emis­sion last­ing even longer – and their lo­cal en­vi­ron­ment also strikes a re­sem­blance.

“Both GRB150101B [and GW170817] are dim for gamma-ray bursts, we be­lieve be­cause we are view­ing them off-axis, away from the core of the jet. Both are also op­ti­cally bright,” says Ryan. “In the case of GRB150101B, in op­ti­cal wave­lengths it ap­pears brighter than you would ex­pect for the af­ter­glow alone, and the dif­fer­ence is about the bright­ness ex­pected for a kilo­nova. Both events also oc­cur in the out­skirts of the same type of galaxy: old, lu­mi­nous, and el­lip­ti­cal.”

What ap­pears to be miss­ing though is the de­tec­tion of the far more valu­able grav­i­ta­tional waves. These waves were pre­dicted by Ein­stein’s gen­eral the­ory of rel­a­tiv­ity over a cen­tury ago which states that the most in­tense events in the uni­verse, such as black holes or neu­tron stars merg­ing, will cre­ate rip­ples in time and space. The LIGO mis­sion was the first to prove this true with its first de­tec­tion in Septem­ber 2015. The Ad­vanced LIGO ex­per­i­ment wasn’t fully func­tional un­til the month be­fore the first grav­i­ta­tional-wave de­tec­tion, how­ever, mean­ing that the de­tec­tors missed the chance to view GRB150101B by eight months. But

even if LIGO was work­ing and star­ing at that patch of sky at the right time, there is no cer­tainty it would have de­tected any­thing due to the colos­sal dis­tance be­tween Earth and GRB150101B, which is over 1,000-times fur­ther away than GW170817! How­ever, the re­searchers are still ex­tremely con­fi­dent that grav­i­ta­tional waves were still cre­ated, even if they weren’t able to de­tect them.

“All neu­tron star merg­ers pro­duce grav­i­ta­tional waves; this is some­thing the as­tro­nom­i­cal com­mu­nity is ex­tremely con­fi­dent about. The rea­son for this con­fi­dence is that, when it comes to pre­dic­tions of grav­i­ta­tional waves, merg­ers of neu­tron stars and of black holes are ex­tremely straight­for­ward ex­er­cises in gen­eral rel­a­tiv­ity,” says Van Eerten. "What mat­ters for us is whether these merg­ers oc­cur suf­fi­ciently close enough to de­tect with grav­i­ta­tional-wave de­tec­tors.”

Whether these events are fre­quent or not is some­thing that still needs to be de­ter­mined. The evo­lu­tion of tele­scopes and de­tec­tors has helped a lot with this re­search. Hope­fully with the fine ar­ray of ground- and space-based tele­scopes avail­able to mod­ern as­tronomers the de­tec­tions of such events could be­come more fre­quent. When ask­ing Ryan what this dis­cov­ery means for our un­der­stand­ing of sci­ence and the uni­verse, he replies: “Short-term, iden­ti­fy­ing the shared prop­er­ties will help us de­tect sim­i­lar events in the fu­ture. Long-term, with more ob­ser­va­tions we can build up a cohesive pic­ture of these events and bet­ter un­der­stand the physics that drives them.”

Both of these goals cen­tre heav­ily on gath­er­ing as much data as pos­si­ble. Now that as­tronomers know what they have to look out for it’s a mat­ter of keep­ing a keen eye out for sim­i­lar events and build­ing a data­base of neu­tron star merg­ers through­out the uni­verse. With a more com­pre­hen­sive archive as­tronomers can im­prove on find­ing the vis­i­ble coun­ter­part to grav­i­ta­tional-wave sig­nals. If an­other enor­mous event was on the verge of tran­spir­ing as­tronomers would be well-equipped to un­der­stand the true in­tri­ca­cies of the merger.

This in­cludes un­der­stand­ing how the cos­mos’ most ex­otic el­e­ments, which make up ev­ery­day life on Earth, are made.

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1 A neu­tron star’s powerNeu­tron stars are in­cred­i­bly dense and small, with up to 2.1-times the mass of the Sun packed into a star the size of a city. This makes a deep dent in space-time.2Stuck in grav­ity’s danceWhen bi­nary neu­tron stars are in­ter­twined in a grav­ity-in­duced dance, grav­i­ta­tional waves are spread through space-time, such like the rip­ples caused by throw­ing a stone into a pond.3When two neu­tron stars be­come one black holeAs the or­bits shrink and en­ergy is lost, the merger is im­mi­nent. As the stars merge a flurry of the high­est en­ergy is re­leased as a kilo­nova be­gins, and this was spot­ted by NASA’s Fermi satel­lite on 1 Jan­uary 2015.4 The aftermath of en­ergyAf­ter the merger the team of tele­scopes were able to ob­serve X-ray and op­ti­cal emis­sions of the black hole, kilo­nova and its jets. The blue op­ti­cal light and a long-life X-ray emis­sion gave away its iden­tity. 5Fermi Space Tele­scopeFermi was the first to spot the sig­nal. It lasted 12 mil­lisec­onds – enough to catch the at­ten­tion of the as­tronomers. 6Neil Gehrels Swift Observatory Swift was able to pin­point the lo­ca­tion of GRB150101B as it has a sharper view than its com­pan­ion, the Fermi tele­scope. 7Chan­dra X-ray Observatory Chan­dra was cru­cial in re­solv­ing the pres­ence of two nearby sources and char­ac­ter­is­ing their prop­er­ties. 8Hub­ble Space Tele­scopeThe im­ages of the galaxy that ac­com­mo­dated GRB150101B taken by Hub­ble struck a sim­i­lar re­sem­blance to the host galaxy of GW170817. 9Mag­el­lan Tele­scopeBy us­ing Mag­el­lan’s Inamori-Mag­el­lan Areal Cam­era and Spec­tro­graph, or IMACS, op­ti­cal light from the burst was de­tected 36 hours later. 10Dis­cov­ery Chan­nel Tele­scope Pho­to­met­ric and spec­tro­scopic anal­y­sis was made us­ing the Dis­cov­ery Chan­nel Tele­scope, which can ob­serve in near-ul­tra­vi­o­let to far-in­frared.11Gemini South Tele­scopeGemini South showed that the source ex­hib­ited the usual dom­i­nant af­ter­glow com­po­nent af­ter ten days and that the kilo­nova had al­ready faded.12Very Large Tele­scopeThe VLT im­aged the source two days af­ter the GRB de­tec­tion in near-in­frared us­ing its High Acu­ity Wide field K-band Imager (HAWK-I). A kilo­nova pro­duces many ex­otic el­e­ments, such as gold, plat­inum and ura­nium The jet formed from GRB150101B can re­veal a lot about a new­born black hole

A neu­tron star is not much big­ger than Mu­nich, Ger­many, but a tea­spoon of the star can weigh as much as the en­tire hu­man pop­u­la­tionThe LIGO de­tec­tors were not op­er­a­tional dur­ing the time ofthe GRB150101B de­tec­tion

Grav­i­ta­tional waves are a con­se­quence of en­ergy be­ing re­leased by the merger of twoenor­mous cos­mic ob­jects A su­per­nova can be up to 100-times brighter than a kilo­nova

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