Neutron Stars Could be Capturing Primordial Black Holes

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, along with professors Gianfranco Bertone and Florian Kühnel) examined 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 matnetar 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 percent 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 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 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.”

Magnetars form as normal neutron star/pulsar objects, but a magnetar’s interior dynamo converts heat and rotational energy into magnetic energy. That action pumps up the magnetic field strength to create the magnetar. Only about a tenth of supernova explosions result in a magnetar rather than a neutron star or pulsar.

Pulsar disruption happens in binary star systems where one star is more massive than the other. It explodes as a supernova, creating a neutron star that is detectable by its radio emissions. Eventually, it slows down as it loses energy. The other star may or may not explode, and it could eventually swell up and begin feeding its material to the neutron star. When that happens, the neutron star spins up again, and it becomes a disrupted recycled neutron star. However, this process still doesn’t explain the lack of pulsars in the Milky Way’s core region.

Using Primordial Black Hole Capture to Explain Missing Pulsars

Caizzo’s team decided to use a model to understand what’s happening. “Ourmodel is for a general Neutron star,” he pointed out. “We consider the millisecond pulsar scenario as a possible avenue for constraints, since these objects are much older than regular pulsars and hence are much more likely to interact with PBHs over their lifetime.”

Millisecond pulsars—that is, pulsars spinning extremely fast—are a good avenue 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.

For More Information

Revisiting Primordial Black Hole Capture by Neutron Stars
Searching for Pulsars in the Galactic Centre at 3 and 2 mm