Months after

The Washington Post - - FRONT PAGE - BY SARAH KA­PLAN

a daz­zling su­per­nova was spot­ted in the Hawai­ian sky, sci­en­tists puz­zle over what caused it.

When a bril­liant, be­wil­der­ing ce­les­tial body flashed into be­ing in a nearby galaxy last sum­mer, it trans­fixed the as­tron­omy com­mu­nity.

No one had ever wit­nessed any­thing like it. Over the course of just a few days, the cos­mic flare grew un­til it was al­most 100 bil­lion times brighter than the sun. Soon after the sight­ing was re­ported, sci­en­tists were ob­serv­ing it in ev­ery wave­length of the elec­tro­mag­netic spec­trum.

Even after hun­dreds of hours of ef­fort, the sig­nal de­fies easy ex­pla­na­tion — pos­ing the tan­ta­liz­ing pos­si­bil­ity that astronomers have de­tected some­thing en­tirely new.

In pub­lished stud­ies and pre­sen­ta­tions Thurs­day at the win­ter meet­ing of the Amer­i­can As­tro­nom­i­cal So­ci­ety in Seat­tle, sci­en­tists de­bated what as­tro­phys­i­cal phe­nom­e­non could be be­hind this puz­zling flare.

Sev­eral re­search teams ar­gue that it may come from the birth of a black hole or a col­lapsed star. Only an in­cred­i­bly dense ob­ject shin­ing through a cloud of stel­lar de­bris could power such a fast and bril­liant sig­nal, these sci­en­tists say.

Oth­ers ar­gue the flare was emit­ted by a star in its death throes as it was de­voured by a black hole.

Events like this “are new fron­tiers for dis­cov­ery,” said Raf­faella Margutti, a North­west­ern Univer­sity as­tro­physi­cist who led re­search sug­gest­ing the flare comes from a new­born black hole. What­ever the ori­gins of this sig­nal, study­ing it — and find­ing more — could al­low sci­en­tists to probe the uni­verse’s most ex­otic lo­cales, where the laws of physics are pushed to their ex­tremes.

The June 16, 2018, de­tec­tion came from Hawaii’s AT­LAS tele­scope, which sur­veys the night sky for su­per­novas and other pow­er­ful, short-lived phe­nom­ena.

These events, known as “tran­sients,” typ­i­cally take a few weeks to reach full bright­ness. But this flare un­folded faster. By day three, it was about 10 times as bright as a stan­dard ex­plod­ing star, and it sent a shock wave zoom­ing out­ward at 20,000 miles a sec­ond — 10 per­cent of the speed of light.

An ur­gent mes­sage was posted on the As­tronomer’s Tele­gram, an on­line ser­vice for astronomers to rapidly re­port in­ter­est­ing ob­ser­va­tions. Thanks to the site’s ran­dom­ized three-let­ter nam­ing sys­tem, the ob­ject was dubbed AT2018­cow, or “the Cow” for short.

Within a week, the AT­LAS ob­ser­va­tion was fol­lowed up by more than two dozen teams of astronomers us­ing tele­scopes based on at least four con­ti­nents and in space.

“There’s a lot of in­ter­est,” Robert Rut­ledge, ed­i­tor in chief of the As­tronomer’s Tele­gram and an as­tro­physi­cist at McGill Univer­sity in Canada, said at the time. “I think it’s the most no­tices for any in­di­vid­ual ob­ject in such a short pe­riod of time.”

Sev­eral stud­ies sug­gested the ob­ject was a mere 200 mil­lion light-years away — prac­ti­cally in our back­yard, by cos­mic stan­dards.

Yet the Cow seemed to strain ev­ery model de­vised to ex­plain it, said Dan Per­ley, an as­tronomer at Liver­pool John Moores Univer­sity in Bri­tain. It car­ried light sig­na­tures of hy­dro­gen and he­lium — typ­i­cal el­e­ments for a star like our own sun — rather than the car­bon, oxy­gen and ni­tro­gen usu­ally seen in a su­per­nova.

Anna Ho, a Cal­tech re­searcher, led an in­ves­ti­ga­tion into the event us­ing mil­lime­ter wave­lengths, a part of the elec­tro­mag­netic spec­trum that is slightly higher in en­ergy than ra­dio waves but which is rarely used to study su­per­nova. To the re­searchers’ sur­prise, the Cow kept get­ting brighter in mil­lime­ter wave­lengths — some­thing astronomers have never seen be­fore.

“There wasn’t just a sin­gle re­lease of en­ergy that hap­pened and then it was over. There had to be en­ergy that was still be­ing pro­duced,” Ho said. “There had to be an ac­tive en­gine.”

Margutti was among the scores of sci­en­tists who sprang into ac­tion to seek that “en­gine.” Like Cap­tain Planet unit­ing the Plan­e­teers, she called on col­leagues around the globe to study the Cow over the next 100 days. Each team mem­ber spe­cial­ized in a dif­fer­ent part of the elec­tro­mag­netic spec­trum and could in­ter­pret what­ever in­sights about the sig­nal were con­veyed in that wave­length of light.

“It was a great ad­ven­ture,” she re­called. “A lot of the time in re­search you sort of know what you’re look­ing at. But our team was fac­ing a sit­u­a­tion where we had tons and tons of data . . . and no clue how to ex­plain it.”

The most in­trigu­ing clue came from a “bump” in the high-en­ergy X-rays. This bump is an un­usual sight around a su­per­nova, but it’s a char­ac­ter­is­tic fea­ture of an ac­cre­tion disk — the halo of su­per­heated ma­te­rial that swirls around an ex­tremely dense ob­ject.

This sig­nal looked as if it could be com­ing from a black hole or a neu­tron star — the dark, dense nugget left be­hind when a mid­size star col­lapses.

Though the­o­ries sug­gest these kinds of com­pact bod­ies are born in the af­ter­math of su­per­novas, sci­en­tists are never able to wit­ness the ini­tial stages of the for­ma­tion process. The clouds of de­bris left be­hind after these cat­a­clysms can last for 1,000 years — and by the time the dust clears, al­low­ing light from the ac­cre­tion disk to pen­e­trate, those im­por­tant early stages are long over.

Some­thing must have been strange about this par­tic­u­lar stel­lar ex­plo­sion.

“We think we must be deal­ing with a very lit­tle amount of ejecta,” or ejected mat­ter, Margutti said. “That’s why we can ac­tu­ally see the in­te­rior right away.”

This ex­pla­na­tion would ac­count for an­other strange fea­ture of the Cow: It doesn’t carry the light sig­na­ture of large amounts of nickel, the el­e­ment that usu­ally lends bright­ness to a su­per­nova.

In this case, Margutti said, the Cow must be con­tin­u­ously pow­ered by the ob­ject at its cen­ter — the black hole or neu­tron star shin­ing through.

An al­ter­na­tive ex­pla­na­tion sug­gests that the Cow came from a tidal dis­rup­tion event — the daz­zling de­struc­tion of a white dwarf star that passed too close to an in­cred­i­bly mas­sive black hole. The black hole’s grav­ity would break the star apart into a swirling stream of gas, gen­er­at­ing an ini­tial burst of light fol­lowed by months of glow in var­i­ous wave­lengths.

But the Cow does not come from a spot in its galaxy where such large black holes are usu­ally found. So the mys­tery per­sists.

More clues may be out there, wait­ing to be found. Pre­vi­ous stud­ies have pin­pointed sim­i­lar ob­jects known as “fast lu­mi­nous blue tran­sients.” But sci­en­tists as­sumed that they were just a vari­a­tion on the stan­dard su­per­nova, so none of those events was in­ves­ti­gated as thor­oughly as the Cow.

Astronomers will not be mak­ing that mis­take again, Per­ley said.

“We should be able to find many more of these in the fu­ture, if we know what to look for,” he said. “And we think that we do now.”

“A lot of the time in re­search you sort of know what you’re look­ing at. But our team was fac­ing a sit­u­a­tion where we had tons and tons of data . . . and no clue how to ex­plain it.” Raf­faella Margutti, North­west­ern Univer­sity as­tro­physi­cist

BILL SAX­TON/NA­TIONAL RA­DIO AS­TRON­OMY OB­SER­VA­TORY/AUI/NSF

An artist’s con­cep­tion de­picts the sort of mys­te­ri­ous cos­mic blast that could be caus­ing the un­prece­dented flare known as AT2018­cow.

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