Where are the pulsars at the centre of the Milky Way?

As we all know, in addition to a black hole with a mass of 4 million times that of the sun, the centre of the Milky Way is also an incredibly dense star-gathering area. The density of stars is about the same as that of globular star clusters. There are an average of 2 stars per cubic light-year here (within a few hundred light-years). At first glance, you may think that this is not much. But do you know the density of stars around the solar system? There are only 0.004 particles per cubic light-year.

Since there are so many stars in the centre of the Milky Way, it stands to reason that the centre of the Milky Way should be very bright, right? However, on the contrary, since the solar system is located on the galactic disk, when we look toward the galactic centre, our line of sight will be blocked by the interstellar dust on the galactic disk, so it is difficult for us to see the light of stars near the visible light band. This is why the Milky Way is always a dark dust lane in starry sky photos.

Studying the galactic centre usually requires the use of shorter wavelength gamma rays, hard X-rays, or longer wavelength micron waves and radio. For example, when we took pictures of the black hole in the centre of the Milky Way, we used a radio telescope. By arranging multiple radio telescopes around the world, we can capture the radio waves released by the accretion disk of the black hole at the centre of the Milky Way, thereby drawing a silhouette of the black hole at the centre of the Milky Way.

We know that accretion disks are not unique to black holes. In a binary star system, when one of the stars collapses into a compact star, it may accrete the other companion star. For example, a neutron star uses its own tidal force to strip off the outer layer of the companion star and then forms an accretion disk around itself. This process will make the neutron star become a pulsar that rotates faster and faster.

Since the density of stars in the Galactic Center is 500 times that around us, it stands to reason that there should be a lot of stellar debris near the Galactic Center. There must be a large number of pulsars gathered there, estimated to be at least several thousand. But the strange thing is that so far we can hardly find any pulsars within more than 80 light-years near the galactic center. Why is this?

Of course, it is not that the black hole in the centre of the galaxy eats them all. After all, the stars that can form neutron stars are not small in mass and their lifespan is not long (maybe only a few hundred million years). Therefore, under normal circumstances, the formation rate of neutron stars in the galactic centre region should be very high. Moreover, although the black hole in the centre of the Milky Way appears to be millions of times the mass of the sun, a simple calculation based on the Schwarzschild radius formula shows that its size is only on the order of tens of millions of kilometres. As for the central region of the Milky Way just mentioned, the range is measured in light years, and the difference between the two is several orders of magnitude.

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So what do astronomers think about this?

One view is that the neutron stars here may be “short-lived ghosts”, such as magnetars with super-strong magnetic fields. Because the magnetar’s magnetic field decays quickly, its high-energy radiation does not last long. Compared with the 10 million to 100 million years of ordinary pulsars, the lifetime of magnetars is only 10,000 years. So, we can’t find pulsars here, maybe because they cooled down a long time ago.

But not all stars will eventually become magnetars, so astronomers raised another possibility: some neutron stars may be “expelled from their homes.”

We know that stars in the universe are often not alone, especially those massive stars, many of which have their own companion stars. So when one of the stars in the binary system goes supernova, its companion star may be kicked out under the powerful impact. In this way, the remaining neutron star sky has a certain ability (tidal force) but has nowhere to use it and can only stay there quietly.

Of course, not all stars can drive their “wife” out of the house. Those binary stars that have each experienced a supernova explosion and have not separated will eventually become a pair of binary neutron stars that “grow old together” and dance their waltz quietly in interstellar space from then on.

Regardless of the above situation, these silent neutron stars are difficult to detect.

However, although these reasons can be explained, they cannot explain much. Then astronomers raised another possibility: these neutron stars might have been killed by black holes! Of course, this black hole does not refer to the galactic centre black hole, but a very mini black hole, usually less than 1 times the mass of the sun, or even only the mass of an asteroid.

Where did such a small black hole come from? Yes, of course, it is the primordial black hole formed in the early universe. Since there is no stellar collapse, the size of the original black hole is not restricted and can be very large or very small. If there are indeed a large number of mini black holes in the universe, it is indeed difficult for us to detect them from an observational perspective, so some scientists also regard small-mass primordial black holes as one of the candidates for dark matter.

Last time, we said that some scientists suggested that neutron stars may heat themselves by capturing dark matter. If the dark matter at this time is the original black hole, it is not as simple as heating itself, but it may be hollowed out from the inside by the black hole.

You might say: “Such a small black hole would have been evaporated by Hawking radiation long ago, right?”

As mentioned before, according to the “Hawking evaporation limit”, a 1 billion-ton black hole is enough to survive to this day, and its size is only as large as an atom. So if such a small black hole really exists, it is not impossible for a neutron star to capture the black hole.

In fact, we have talked about this situation of stars capturing small black holes before. For example, we mentioned last time that there may be black holes inside the sun. The same is true for stellar remnants such as neutron stars. After these neutron stars capture small black holes, the black holes will slowly eat them away from the interior of the neutron star, eventually growing into a quiet stellar black hole. If all the neutron stars near the galactic centre end up like this, it might really explain the lack of neutron stars.

But not long ago, in an article temporarily posted on Arxiv, a research team from Europe re-examined the possibility of such a neutron star being disintegrated from the inside by a primordial black hole. They concluded that the idea that neutron stars captured primordial black holes could not explain the absence of neutron stars.

The research team calculated the probability of the pulsar capturing the primordial black hole and the probability that the neutron star would further collapse into a stellar black hole after capture. The results show that under the specific circumstances of the simplified model, the probability that the pulsar will capture the primordial black hole during its lifetime (~10 million years) is at most 30%. Moreover, for the binary star system where the pulsar is located, the external primordial black hole is a third party with a relatively small mass. In this unstable triangular three-body motion, the third party with a small mass is usually thrown out of the system.

However, the current research on the capture of primordial black holes by neutron stars is still relatively rough, because the density of stars in the centre of the Milky Way is very high, and the reality is usually not a simple three-body relationship, but a more complicated multi-body motion. This problem is indeed difficult to solve at this stage. In the future, as more advanced observation equipment is put into use, the mystery may be solved.

In short, why there are no pulsars in the centre of the Milky Way is still an unsolved mystery in astronomy.

Articles & News Sources:

• [1] Wikipedia: Stellar density

• [2] scopethegalaxy: Pulsars vs Magnetars

• [3] phys.org: Neutron stars could be capturing primordial black holes

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Papers & Abstracts:

• [1] Y. Génolini, P. D. Serpico, and P. Tinyakov. Revisiting primordial black hole capture into neutron stars. PHYSICAL REVIEW D (2020). 102(8)

• [2] Roberto Caiozzo, Gianfranco Bertone, Florian Kühnel. Revisiting Primordial Black Hole Capture by Neutron Stars. arXiv preprint arXiv:2404.08057