Surprise kilonova upends established understanding of long gamma-ray bursts

For practically 20 years, astrophysicists have believed that lengthy gamma-ray bursts (GRBs) resulted solely from the collapse of large stars. Now, a brand new research upends that long-established and long-accepted perception.

Led by Northwestern College, a workforce of astrophysicists has uncovered new proof that no less than some lengthy GRBs may end up from neutron star mergers, which had been beforehand believed to provide solely brief GRBs.

After detecting a 50-second-long GRB in December 2021, the workforce started looking for the lengthy GRB’s afterglow, an extremely luminous and fast-fading burst of sunshine that always precedes a supernova. However, as a substitute, they uncovered proof of a kilonova, a uncommon occasion that solely happens after the merger of a neutron star with one other compact object (both one other neutron star or a black hole).

Along with difficult long-established beliefs about how lengthy GRBs are shaped, the brand new discovery additionally results in new insights into the mysterious formation of the heaviest parts within the universe.

The analysis might be revealed on Dec. 7 within the journal Nature.

“This occasion appears not like anything we now have seen earlier than from an extended gamma-ray burst,” mentioned Northwestern’s Jillian Rastinejad, who led the research. “Its gamma rays resemble these of bursts produced by the collapse of large stars. Given that every one different confirmed neutron star mergers we now have noticed have been accompanied by bursts lasting lower than two seconds, we had each purpose to count on this 50-second GRB was created by the collapse of an enormous star. This occasion represents an thrilling paradigm shift for gamma-ray burst astronomy.”

“After we adopted this lengthy gamma-ray burst, we anticipated it might result in proof of an enormous star collapse,” mentioned Northwestern’s Wen-fai Fong, a senior writer on the research. “As an alternative, what we discovered was very totally different. Once I entered the sector 15 years in the past, it was set in stone that lengthy gamma-ray bursts come from large star collapses. This sudden discovering not solely represents a serious shift in our understanding, but in addition excitingly opens up a brand new window for discovery.”

Fong is an assistant professor of physics and astronomy in Northwestern’s Weinberg Faculty of Arts and Sciences and a key member of the Heart for Interdisciplinary Exploration and Analysis in Astrophysics(CIERA). Rastinejad, a Ph.D. scholar in CIERA and member of Fong’s analysis group, is the paper’s first writer.

Lengthy division

The brightest and most energetic explosions for the reason that Massive Bang, GRBs are divided into two courses. GRBs with durations lower than two seconds are thought-about brief GRBs. If a GRB is longer than two seconds, then it is thought-about an extended GRB. Researchers beforehand believed that GRBs on both facet of the dividing line will need to have totally different origins.

In December 2021, the Neil Gehrels Swift Observatory’s Burst Alert Telescope and the Fermi Gamma-ray House Telescope noticed a brilliant burst of gamma-ray mild, named

GRB211211A. At simply over 50 seconds lengthy, GRB211211A initially did not look like something particular. However situated about 1.1 billion light-years away—which, imagine it or not, is comparatively near Earth—the astrophysicists determined to check this “close by” occasion intimately, utilizing a mess of telescopes that might observe throughout the electromagnetic spectrum.

To picture the occasion with near-infrared wavelengths, the workforce shortly initiated imaging with the Gemini Observatory in Hawaii. After two days of observing with Gemini, Rastinejad frightened that she can be unable to acquire a transparent view.

“The climate was worsening in Hawaii, and we had been so disillusioned as a result of we had been beginning to uncover hints that this burst was not like something we had seen earlier than,” she mentioned. “Fortunately, Northwestern supplies us with distant entry to the MMT Observatory in Arizona, and a perfect instrument was being placed on that telescope the subsequent day. It was cloudy there, however the telescope operators knew how essential this burst was and located a niche between the clouds to take our photographs. It was hectic however so thrilling to get these photographs in actual time.”

‘Telltale signal of a kilonova’

After analyzing the near-infrared photographs, the workforce noticed an extremely faint object that shortly light. Supernovae do not fade as shortly and are a lot brighter, so the workforce realized it discovered one thing sudden that was beforehand believed unimaginable.

“There are a number of objects in our night time sky that fade shortly,” Fong mentioned. “We picture a supply in several filters to acquire shade data, which helps us decide the supply’s id. On this case, crimson shade prevailed, and bluer colours light extra shortly. This shade evolution is a telltale signature of a kilonova, and kilonovae can solely come from neutron star mergers.”

As a result of neutron stars are clear, compact objects, researchers beforehand believed neutron stars didn’t comprise sufficient materials to energy a long-duration GRB. Huge stars, then again, might be tens to a whole bunch of occasions the mass of our sun. Because the dying star collapses, its materials falls inward to feed a newly shaped black hole. However, because of the black hole’s magnetic fields, among the inward-falling materials launches outward at velocities near the velocity of sunshine—powering a GRB.

“Whenever you put two neutron stars collectively, there’s probably not a lot mass there,” Fong defined. “Somewhat little bit of mass accretes after which powers a really short-duration burst. Within the case of large star collapses, which historically energy longer gamma-ray bursts, there’s a longer feeding time.”

Altering the search

The occasion wasn’t the one unusual a part of the research. The GRB’s host galaxy additionally is kind of curious. Named SDSS J140910.47+275320.8, the host galaxy is younger and star-forming, nearly precisely reverse of the one different identified native universe host of a neutron star merger occasion: GW170817’s host galaxy NGC4993. To research the host galaxy, the workforce used knowledge from the W.M. Keck Observatory.

“After the detection of GW170817 and its affiliation with an enormous, red-and-dead host galaxy, many astronomers assumed that hosts of neutron star mergers within the close to universe would look just like NGC4993,” mentioned Anya Nugent, a Northwestern graduate scholar and research co-author. “However this galaxy is pretty younger, actively star forming and never truly that large. Actually, it appears extra just like brief GRB hosts seen deeper within the universe. I feel it adjustments our view of the kinds of galaxies we must always watch after we’re looking for close by kilonovae.”

It additionally adjustments how astrophysicists would possibly strategy the seek for heavy parts, reminiscent of platinum and gold. Though researchers have been capable of research the astronomical factories that produce lighter parts, reminiscent of helium, silicon and carbon, astrophysicists posit that supernova explosions and neutron star mergers produce the heaviest parts. Clear signatures of their creation, nonetheless, are hardly ever noticed.

“Kilonovae are powered by the radioactive decay of among the heaviest parts within the universe,” Rastinejad mentioned. “However kilonovae are very exhausting to look at and fade in a short time. Now, we all know we are able to additionally use some lengthy gamma-ray bursts to search for extra kilonovae.”

Now that the James Webb House Telescope (JWST) is operating, astrophysicists will have the ability to search for extra clues inside kilonovae. As a result of the JWST is able to capturing photographs and spectra of astronomical objects, it might detect particular parts emitted from the item. Utilizing the Webb, astrophysicists lastly would possibly acquire direct observational proof of heavy elements‘ formation.

“Sadly, even the very best ground-based telescopes usually are not delicate sufficient to carry out spectroscopy,” Rastinejad mentioned. “With the JWST, we might have obtained a spectrum of the kilonova. These spectral traces present direct proof that you’ve detected the heaviest parts.”

The research paper is titled “A kilonova following a long-duration gamma-ray burst at 350 Mpc.”