When two neutron stars collide, it creates a kilonova. The event causes both gravitational waves and emissions of electromagnetic energy. In 2017 the LIGO-Virgo gravitational-wave observatories detected a merger of two neutron stars about 130 million light-years away in the galaxy NGC 4993. The merger is called GW170817, and it remains the only cosmic event observed in both gravitational waves and electromagnetic radiation.
Astronomers have watched the expanding debris cloud from the kilonova for years. A clearer picture of what happens in the aftermath is emerging.
A team of researchers have pieced together the story of GW170917 in a paper titled “The emergence of a new source of X-rays from the binary neutron star merger GW170817.” The lead author is Aprajita Hajela, a graduate student in the Department of Physics & Astronomy at Northwestern University. The paper is published in The Astrophysical Journal Letters.
Over the years, astronomers have trained a whole suite of scientific eyes on the expanding cloud, uncovering more and more detail about these cosmic calamities. GW170817 is an unprecedented opportunity to study the kilonova phenomena because astronomers observed gravitational waves and electromagnetic radiation from the merger. The gravitational waves (GW) tell researchers about pre-merger activity, and the electromagnetic observations tell them about the post-merger physical properties.
When two neutron stars merge, it produces a cloud of debris and a burst of light called a kilonova. A pair of astronomers introduced the model for neutron star mergers in 1998. They said that the mergers synthesized radioactive nuclei that provide a long-term heat source for the expanding debris envelope. The optical and infrared light in the kilonova comes from the decay of elements like platinum and gold created during the merger. When LIGO and Virgo detected GWs from GW170817, other telescopes detected the optical and infrared light hours later.
This is the first optical image ever to show an event initially detected as a Gravitational Wave (GW), designated GW170817, pictured left. Afterglow, designated as SSS17a, is left over from the explosion of two neutron stars that collided in galaxy NGC 4993 (shown centre). Only 10.9 hours after triggering the largest astronomical search in history, the Swope 1-m telescope at the Las Campanas Observatory in Chile discovered GW170817’s afterglow. Four days later, the image on the right shows afterglow dimming in brightness and changing from blue to red. CREDIT: Las Campanas Observatory, Carnegie Institution of Washington (Swope + Magellan)
The Chandra X-ray Observatory was also watching. Chandra saw nothing at first, which was unusual. Scientists expect kilonovae to produce x-rays in jets of high-energy particles. Now scientists think there was a jet, but it wasn’t pointed at Earth. Chandra eventually detected x-rays when the jets impacted the surrounding gas and dust, causing them to widen and slow down. Then later in 2018, the x-ray emissions declined again.
The x-rays have remained stable since the end of 2020. The inset x-ray image in the top picture is from Chandra data from December 2020 and January 2021. The x-rays come from both GW170917 and the host galaxy.
Scientists think there could be two explanations for the steadying of x-ray emissions.
On 17 August 2017, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo Interferometer detected gravitational waves from the collision between two neutron stars. Within 12 hours, observatories identified the event’s source within the lenticular galaxy NGC 4993, shown in this image gathered with the NASA/ESA Hubble Space Telescope. The associated stellar flare, a kilonova, is visible in the Hubble observations. This is the first time astronomers have observed the optical counterpart of a gravitational wave event. Hubble observed the kilonova gradually fading over six days, as shown in these observations taken between 22 and 28 August (insets). Image Credit: Hubble/NASA/ESA
The first explanation is that a kind of shock is involved, akin to a sonic boom. When the cloud of debris from the kilonova slams into gas around GW170817, the material is heated. The temperature is enough to produce x-rays and can account for the steady kilonova afterglow Chandra detected.
The artist’s illustration depicts this. The blue in the image is the debris responsible for the glow. The orange and red show the shock. The jets have faded over time, and the blue arcs in the image show where the jets struck surrounding material.
The second explanation is entirely different. It says that the neutron star merger collapsed into a remnant black hole. In this scenario, material falling into the black hole is heated enough to emit x-rays, a known phenomenon around black holes.
The team of researchers behind the new paper says that only one of the two explanations can spell out what’s happening. It would be an improbable coincidence for both sources to be producing x-rays simultaneously in the same place. They also point out that scientists have never observed a kilonova afterglow nor accretion-powered emissions like this before.
Further observations should determine the cause of the x-ray afterglow. Astronomers will continue to observe GW170817 in both x-rays and radio waves. If the glow comes from the kilonova, the radio emissions should brighten in the coming months and years. But if the glow comes from material falling into a black hole, then the x-rays should stay steady or decline rapidly, but there’ll be no radio emissions over time.
“Measuring the time of the peak of the kilonova afterglow, which probed the ejecta dynamics independent of shock microphysics, would offer a unique opportunity to do calorimetry of the kilonova’s fastest ejecta,” they write. This is important because it relates to whether or not the merger left a remnant black hole. If there’s a high-velocity tail in the ejecta, it can create excessive x-ray emissions that “… argues against the prompt collapse of the merger remnant into a black hole.”
Astrophysicists know that black holes emit electromagnetic radiation in x-ray wavelengths. The Chandra X-ray Observatory has imaged many of them. This Chandra image shows Centaurus A, which is not part of this study but is the site of a supermassive black hole, shining brightly. Credit: X-ray: NASA/CXC/U.Birmingham/M.Burke et al.
On the other hand, that same ejecta might emit “… a constant (or declining) source of X-ray emission in the next thousands of days that is not accompanied by bright radio emission.” If that’s the case, the authors say, it shows that the merger collapsed into a black hole. That represents another scientific opportunity because it “… will unveil how accretion processes work on a compact-object remnant of a BNS merger a few years after its birth.”
When LIGO detected the first gravitational waves in 2016, they opened a new window into the Universe. One hundred years before their detection, Einstein predicted them in his general theory of relativity. Three researchers who played a central role in detecting GWs received the 2017 Nobel Prize in physics.
Since the first detection, LIGO and Virgo have detected many more black holes and neutron star mergers. The combination of GW detections and quick and enduring follow-up electromagnetic observations have confirmed some theoretical work, including discovering that kilonovae produce heavy elements.
This paper, along with other articles published on the kilonova, has confirmed theoretical predictions about these events. Astrophysicists predicted that kilonovae are a significant source of heavy elements in the Universe. The type of emissions and their flux both support that. “The spectrum and flux evolution of the kilonova emission from GW170817 was in agreement with theoretical predictions, demonstrating that mergers of neutron stars are one of the major sources of heavy elements in our Universe,” the paper says.
In 2019 a team of European researchers, using data from the X-shooter instrument on ESO’s Very Large Telescope, found signatures of strontium formed in the GW170817 neutron-star merger. This artist’s impression shows two tiny but dense neutron stars when they merge and explode as a kilonova. In the foreground, we see a representation of freshly created strontium. Image Credit: ESO/L. Calçada/M. Kornmesser
Scientists have learned a lot about neutron star mergers and kilonovae since the 1998 paper outlining how they work. We know they can create either a single massive neutron star or collapse into a black hole. We know the merger can create an extraordinarily powerful magnetic field that’s trillions of times more potent than Earth’s puny magnetic field and that they can make that field in milliseconds. Astrophysicists know they can produce gamma-ray bursts and that kilonovae can synthesize heavy elements like strontium.
But scientists are excited about the future. “Observations of GW170817 are mapping an uncharted territory of the BNS (binary neutron star) merger phenomenology and have far-reaching theoretical implications,” the authors write in their paper.
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.
