In this artist’s representation, the merger of two neutron stars to form a black hole (hidden within bright bulge at center of image) generated opposing, high-energy jets of particles (blue) that heated up material around the stars, making it emit X-rays (reddish clouds). The Chandra X-ray Observatory is still detecting X-rays from the event today. They could be produced by a shock wave in the material around the black hole, or by material falling violently into the black hole (yellowish disk around central bulge). Credit: X-ray data from NASA, CXC and Northwestern Univ./A. Hajela; visual by NASA/CXC/M. Weiss
When two neutron stars spiral into one another and merge to form a black hole—an event recorded in 2017 by gravitational wave detectors and telescopes worldwide—does it immediately become a black hole? Or does it take a while to spin down before gravitationally collapsing past the event horizon into a black hole?
Ongoing observations of that 2017 merger by the Chandra X-ray Observatory, an orbiting telescope, suggests the latter: that the merged object stuck around, likely for a mere second, before undergoing ultimate collapse.
The evidence is in the form of an X-ray afterglow from the merger, dubbed GW170817, that would not be expected if the merged neutron stars collapsed immediately to a black hole. The afterglow can be explained as a rebound of material off the merged neutronstars, which plowed through and heated the material around the binary neutron stars. This hot material has now kept the remnant glowing steadily more than four years after the merger threw material outward in what’s referred to as a kilonova. X-ray emissions from a jet of material that was detected by Chandra shortly after the merger would otherwise be dimming by now.
While the excess X-ray emissions observed by Chandra could come from debris in an accretion disk swirling around and eventually falling into the black hole, astrophysicist Raffaella Margutti of the University of California, Berkeley, favors the delayed collapse hypothesis, which is predicted theoretically.
“If the merged neutron stars were to collapse directly to a black hole with no intermediate stage, it would be very hard to explain this X-ray excess that we see right now, because there would be no hard surface for stuff to bounce off and fly out at high velocities to create this afterglow,” said Margutti, UC Berkeley associate professor of astronomy and of physics. “It would just fall in. Done. The true reason why I’m excited scientifically is the possibility that we are seeing something more than the jet. We might finally get some information about the new compact object.”
Margutti and her colleagues, including first author Aprajita Hajela, who was Margutti’s graduate student when she was at Northwestern University before moving to UC Berkeley, report their analysis of the X-ray afterglow in a paper recently accepted for publication in The Astrophysical Journal Letters.
X-ray sources captured by Chandra, including, at top, the black hole that formed from the merger of two neutron stars and was first observed in 2017. Credit: NASA, CXC and Northwestern Univ./A. Hajela
The radioactive glow of a kilonova
Gravitational waves from the merger were first detected on Aug. 17, 2017, by the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) and the Virgo collaboration. Satellite- and ground-based telescopes quickly followed up to record a burst of gamma rays and visible and infrared emissions that together confirmed the theory that many heavy elements are produced in the aftermath of such mergers inside hot ejecta that produces a bright kilonova. The kilonova glows because of light emitted during the decay of radioactive elements, like platinum and gold, that are produced in the merger debris.
Chandra, too, pivoted to observe GW170817, but saw no X-rays until nine days later, suggesting that the merger also produced a narrow jet of material that, upon colliding with the material around the neutron stars, emitted a cone of X-rays that initially missed Earth. Only later did the head of the jet expand and begin emitting X-rays in a broader jet visible from Earth.
The X-ray emissions from the jet increased for 160 days after the merger, after which they steadily grew fainter as the jet slowed down and expanded. But Hajela and her team noticed that from March 2020—about 900 days after the merger—until the end of 2020, the decline stopped, and the X-ray emissions remained approximately constant in brightness.
“The fact that the X-rays stopped fading quickly was our best evidence yet that something in addition to a jet is being detected in X-rays in this source,” Margutti said. “A completely different source of X-rays appears to be needed to explain what we’re seeing.”
The researchers suggest that the excess X-rays are produced by a shock wave distinct from the jets produced by the merger. This shock was a result of the delayed collapse of the merged neutron stars, likely because its rapid spin very briefly counteracted the gravitational collapse. By sticking around for an extra second, the material around the neutron stars got an extra bounce that produced a very fast tail of kilonova ejecta that created the shock.
“We think the kilonova afterglow emission is produced by shocked material in the circumbinary medium,” Margutti said. “It is material that was in the environment of the two neutron stars that was shocked and heated up by the fastest edge of the kilonova ejecta, which is driving the shock wave.”
The radiation is reaching us only now because it took time for the heavy kilonova ejecta to be decelerated in the low-density environment and for the kinetic energy of the ejecta to be converted into heat by shocks, she said. This is the same process that produces radio and X-rays for the jet, but because the jet is much, much lighter, it is immediately decelerated by the environment and shines in the X-ray and radio from the very earliest times.
The merger of two neutron stars produced a black hole (center, white) and a burst of gamma-rays generated by a narrow jet or beam of high-energy particles, depicted in red. Initially the jet was narrow and undetectable by Chandra, but as time passed the material in the jet slowed down and widened (blue) as it slammed into surrounding material, causing the X-ray emission to rise as the jet came into direct view by Chandra. This jet and its oppositely directed counterpart were likely generated by material falling onto the black hole after it formed. Credit: NASA/CXC/K. DiVona
An alternative explanation, the researchers note, is that the X-rays come from material falling towards the black hole that formed after the neutron stars merged.
“This would either be the first time we’ve seen a kilonova afterglow or the first time we’ve seen material falling onto a black hole after a neutron star merger,” said co-author Joe Bright, a UC Berkeley postdoctoral researcher. “Either outcome would be extremely exciting.”
Chandra is now the only observatory still able to detect light from this cosmic collision. Follow-up observations by Chandra and radio telescopes could distinguish between the alternative explanations, however. If it is a kilonova afterglow, radio emission is expected to be detected again in the next few months or years. If the X-rays are being produced by matter falling onto a newly formed black hole, then the X-ray output should stay steady or decline rapidly, and no radio emission will be detected over time.
Margutti hopes that LIGO, Virgo and other telescopes will capture gravitational waves and electromagnetic waves from more neutron star mergers so that the series of events preceding and following the merger can be pinned down more precisely and help reveal the physics of black hole formation. Until then, GW170817 is the only example available for study.
“Further study of GW170817 could have far-reaching implications,” said co-author Kate Alexander, a postdoctoral researcher who also is from Northwestern University. “The detection of a kilonova afterglow would imply that the merger did not immediately produce a black hole. Alternatively, this object may offer astronomers a chance to study how matter falls onto a black hole a few years after its birth.”
Margutti and her team recently announced that the Chandra telescope had detected X-rays in observations of GW170817 performed in December 2021. Analysis of that data is ongoing. No radio detection associated with the X-rays has been reported.
More information:
The emergence of a new source of X-rays from the binary neutron star merger GW170817, arXiv:2104.02070 [astro-ph.HE] arxiv.org/abs/2104.02070
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Did rapid spin delay 2017 collapse of neutron stars into black hole? (2022, March 5)
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NAIROBI, Kenya (AP) — The body of Ugandan Olympic athlete Rebecca Cheptegei — who died after being set on fire by her partner in Kenya — was received Friday by family and anti-femicide crusaders, ahead of her burial a day later.
Cheptegei’s family met with dozens of activists Friday who had marched to the Moi Teaching and Referral Hospital’s morgue in the western city of Eldoret while chanting anti-femicide slogans.
She is the fourth female athlete to have been killed by her partner in Kenya in yet another case of gender-based violence in recent years.
Viola Cheptoo, the founder of Tirop Angels – an organization that was formed in honor of athlete Agnes Tirop, who was stabbed to death in 2021, said stakeholders need to ensure this is the last death of an athlete due to gender-based violence.
“We are here to say that enough is enough, we are tired of burying our sisters due to GBV,” she said.
It was a somber mood at the morgue as athletes and family members viewed Cheptegei’s body which sustained 80% of burns after she was doused with gasoline by her partner Dickson Ndiema. Ndiema sustained 30% burns on his body and later succumbed.
Ndiema and Cheptegei were said to have quarreled over a piece of land that the athlete bought in Kenya, according to a report filed by the local chief.
Cheptegei competed in the women’s marathon at the Paris Olympics less than a month before the attack. She finished in 44th place.
Cheptegei’s father, Joseph, said that the body will make a brief stop at their home in the Endebess area before proceeding to Bukwo in eastern Uganda for a night vigil and burial on Saturday.
“We are in the final part of giving my daughter the last respect,” a visibly distraught Joseph said.
He told reporters last week that Ndiema was stalking and threatening Cheptegei and the family had informed police.
Four in 10 women or an estimated 41% of dating or married Kenyan women have experienced physical or sexual violence perpetrated by their current or most recent partner, according to the Kenya Demographic and Health Survey 2022.
TEL AVIV, Israel (AP) — A rare Bronze-Era jar accidentally smashed by a 4-year-old visiting a museum was back on display Wednesday after restoration experts were able to carefully piece the artifact back together.
Last month, a family from northern Israel was visiting the museum when their youngest son tipped over the jar, which smashed into pieces.
Alex Geller, the boy’s father, said his son — the youngest of three — is exceptionally curious, and that the moment he heard the crash, “please let that not be my child” was the first thought that raced through his head.
The jar has been on display at the Hecht Museum in Haifa for 35 years. It was one of the only containers of its size and from that period still complete when it was discovered.
The Bronze Age jar is one of many artifacts exhibited out in the open, part of the Hecht Museum’s vision of letting visitors explore history without glass barriers, said Inbal Rivlin, the director of the museum, which is associated with Haifa University in northern Israel.
It was likely used to hold wine or oil, and dates back to between 2200 and 1500 B.C.
Rivlin and the museum decided to turn the moment, which captured international attention, into a teaching moment, inviting the Geller family back for a special visit and hands-on activity to illustrate the restoration process.
Rivlin added that the incident provided a welcome distraction from the ongoing war in Gaza. “Well, he’s just a kid. So I think that somehow it touches the heart of the people in Israel and around the world,“ said Rivlin.
Roee Shafir, a restoration expert at the museum, said the repairs would be fairly simple, as the pieces were from a single, complete jar. Archaeologists often face the more daunting task of sifting through piles of shards from multiple objects and trying to piece them together.
Experts used 3D technology, hi-resolution videos, and special glue to painstakingly reconstruct the large jar.
Less than two weeks after it broke, the jar went back on display at the museum. The gluing process left small hairline cracks, and a few pieces are missing, but the jar’s impressive size remains.
The only noticeable difference in the exhibit was a new sign reading “please don’t touch.”
VICTORIA – The British Columbia government is partnering with a bear welfare group to reduce the number of bears being euthanized in the province.
Nicholas Scapillati, executive director of Grizzly Bear Foundation, said Monday that it comes after months-long discussions with the province on how to protect bears, with the goal to give the animals a “better and second chance at life in the wild.”
Scapillati said what’s exciting about the project is that the government is open to working with outside experts and the public.
“So, they’ll be working through Indigenous knowledge and scientific understanding, bringing in the latest techniques and training expertise from leading experts,” he said in an interview.
B.C. government data show conservation officers destroyed 603 black bears and 23 grizzly bears in 2023, while 154 black bears were killed by officers in the first six months of this year.
Scapillati said the group will publish a report with recommendations by next spring, while an independent oversight committee will be set up to review all bear encounters with conservation officers to provide advice to the government.
Environment Minister George Heyman said in a statement that they are looking for new ways to ensure conservation officers “have the trust of the communities they serve,” and the panel will make recommendations to enhance officer training and improve policies.
Lesley Fox, with the wildlife protection group The Fur-Bearers, said they’ve been calling for such a committee for decades.
“This move demonstrates the government is listening,” said Fox. “I suspect, because of the impending election, their listening skills are potentially a little sharper than they normally are.”
Fox said the partnership came from “a place of long frustration” as provincial conservation officers kill more than 500 black bears every year on average, and the public is “no longer tolerating this kind of approach.”
“I think that the conservation officer service and the B.C. government are aware they need to change, and certainly the public has been asking for it,” said Fox.
Fox said there’s a lot of optimism about the new partnership, but, as with any government, there will likely be a lot of red tape to get through.
“I think speed is going to be important, whether or not the committee has the ability to make change and make change relatively quickly without having to study an issue to death, ” said Fox.
This report by The Canadian Press was first published Sept. 9, 2024.
