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This magnetar is a highly magnetized neutron star. This artist’s illustration shows an outburst from a magnetar. Neutron stars that spin rapidly and give out radiation are called pulsars, and specific pulsars are rare in the core of the Milky Way. Credit: NASA/JPL-CalTech
The Milky Way has a missing pulsar problem in its core. Astronomers have tried to explain this for years. One of the more interesting ideas comes from a team of astronomers in Europe and invokes dark matter, neutron stars, and primordial black holes (PBHs).
Astronomer Roberto Caiozzo, of the International School for Advanced Studies in Trieste, Italy, led a group examining the missing pulsar problem. “We do not observe pulsars of any kind in this inner region (except for the magnetar PSR J1745-2900),” he wrote in an email.
“This was thought to be due to technical limitations, but the observation of the magnetar seems to suggest otherwise.” That magnetar orbits Sagittarius A*, the black hole at the core of the Milky Way.
The team examined other possible reasons why pulsars don’t appear in the core and looked closely at magnetar formation as well as disruptions of neutron stars. One intriguing idea they examined was the cannibalization of primordial black holes by neutron stars.
The team explored the missing-pulsar problem by asking the question: could neutron star-primordial black hole cannibalism explain the lack of detected millisecond pulsars in the core of the Milky Way? Let’s look at the main players in this mystery to understand if this could happen.
Neutron stars, pulsars, and little black holes, oh my
Theory suggests that primordial black holes were created in the first seconds after the Big Bang. “PBHs are not known to exist,” Caiozzo points out, “but they seem to explain some important astrophysical phenomena.” He pointed at the idea that supermassive black holes seemed to exist at very early times in the universe and suggested that they could have been the seeds for these monsters.
If there are PHBs out there, the upcoming Nancy Grace Roman Telescope could help find them. Astronomers predict they could exist in a range of masses, ranging from the mass of a pin to around 100,000 the mass of the sun. There could be an intermediate range of them in the middle, the so-called “asteroid-mass” PBHs. Astronomers suggest these last ones as dark matter candidates.
Dark matter makes up about 27% of the universe, but beyond suggesting that PBH could be part of the dark matter content, astronomers still don’t know exactly what it is. There does seem to be a large amount of it in the core of our galaxy. However, it hasn’t been directly observed, so its presence is inferred. Is it bound up in those midrange PBHs? No one knows.
The third player in this missing pulsar mystery is neutron stars. They’re huge, quivering balls of neutrons left over after the death of a supergiant star of between 10 and 25 solar masses. Neutron stars start out very hot (in the range of 10 million K) and cool down over time.
They start out spinning very fast and they do generate magnetic fields. Some emit beams of radiation (usually in radio frequencies) and as they spin, those beams appear as “pulses” of emission. That earned them the nickname “pulsar.” Neutron stars with extremely powerful magnetic fields are termed “magnetars.”
The missing pulsar problem
Astronomers have searched the core of the Milky Way for pulsars without much success. Survey after survey detected no radio pulsars within the inner 25 parsecs of the galaxy’s core. Why is that? Caizzo and his co-authors suggested in their paper, posted to the arXiv preprint server, that magnetar formation and other disruptions of neutron stars that affect pulsar formation don’t exactly explain the absence of these objects in the galactic core.
“Efficient magnetar formation could explain this (due to their shorter lifetime),” he said, “But there is no theoretical reason to expect this. Another possibility is that the pulsars are somehow disrupted in other ways.”
Usually, disruption happens in binary star systems where one star is more massive than the other and it explodes as a supernova. The other star may or may not explode. Something may kick it out of the system altogether. The surviving neutron star becomes a “disrupted” pulsar. They aren’t as easily observed, which could explain the lack of radio detections.
If the companion isn’t kicked out and later swells up, its matter gets sucked away by the neutron star. That spins up the neutron star and affects the magnetic field. If the second star remains in the system, it later explodes and becomes a neutron star. The result is a binary neutron star. This disruption may help explain why the galactic core seems to be devoid of pulsars.
Using primordial black hole capture to explain missing pulsars
Caizzo’s team decided to use two-dimensional models of millisecond pulsars—that is, pulsars spinning extremely fast—as a way to investigate the possibility of primordial black hole capture in the galactic core.
The process works like this: a millisecond pulsar interacts in some way with a primordial black hole that has less than one stellar mass. Eventually, the neutron star (which has a strong enough gravitational pull to attract the PBH) captures the black hole. Once that happens, the PBH sinks to the core of the neutron star. Inside the core, the black hole begins to accrete matter from the neutron star.
Eventually, all that’s left is a black hole with about the same mass as the original neutron star. If this occurs, that could help explain the lack of pulsars in the inner parsecs of the Milky Way.
Could this happen? The team investigated the possible rates of capture of PBHs by neutron stars. They also calculated the likelihood that a given neutron star would collapse and assessed the disruption rate of pulsars in the galactic core. If not all the disrupted pulsars are or were part of binary systems, then that leaves neutron star capture of PBHs as another way to explain the lack of pulsars in the core. But, does it happen in reality?
Missing pulsar tension continues
It turns out that such cannibalism cannot explain the missing pulsar problem, according to Caizzo. “We found that in our current model PBHs are not able to disrupt these objects but this is only considering our simplified model of 2 body interactions,” he said. It doesn’t rule out the existence of PHBs, only that in specific instances, such capture isn’t happening.
So, what’s left to examine? If there are PHBs in the cores and they’re merging, no one’s seen them yet. But, the center of the galaxy is a busy place. A lot of bodies crowd the central parsecs. You have to calculate the effects of all those objects interacting in such a small space. That “many-body dynamics” problem has to account for other interactions, as well as the dynamics and capture of PBHs.
Astronomers looking to use PBH-neutron star mergers to explain the lack of pulsar observations in the core of the galaxy will need to better understand both the proposed observations and the larger populations of pulsars.
The team suggests that future observations of old neutron stars close to Sgr A* could be very useful. They’d help set stronger limits on the number of PBHs in the core. In addition, it would be useful to get an idea of the masses of these PBHs, since those on the lower end (asteroid-mass types) could interact very differently.
More information:
Roberto Caiozzo et al, Revisiting Primordial Black Hole Capture by Neutron Stars, arXiv (2024). DOI: 10.48550/arxiv.2404.08057
Citation:
Neutron stars could be capturing primordial black holes (2024, April 29)
retrieved 29 April 2024
from https://phys.org/news/2024-04-neutron-stars-capturing-primordial-black.html
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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.
