Mysterious gamma radiation from the centers of galaxies gives rise to neutron stars

(ORDO NEWS) — Excess gamma radiation from the center of the Milky Way succumbed to explanation. The best candidate for the role of its source turned out to be an unaccounted for population of millisecond pulsars, which is formed according to a “non-standard” mechanism.

Dark matter is the most significant and mysterious “material” component of our Universe. It makes up to 85% of the entire gravitating mass of the Universe, but still defies attempts to detect it directly.

Dark matter was present in the early epochs of the universe, and its clusters contributed to the formation of galaxies, and now it “fastens” their contents through gravity – that’s practically all that is reliably known about it.

One of the candidates for the role of dark matter is the so-called WIMPs (weakly interacting massive particles), or weakly interacting massive particles.

They could have formed immediately after the Big Bang, when the energy of the particles exceeded everything that can be achieved in colliders, and after cooling, form stable clusters the size of a galaxy.

According to one of the assumptions, dark matter particles can be Majorana fermions and annihilate with each other – then they could be detected by scattered gamma radiation emanating from areas of their high concentration (that is, the centers of galaxies).

Similar radiation was detected when analyzing data from the Fermi Space Telescope , which surveyed the sky in gamma rays.

An excess of radiation with an energy of about two gigaelectronvolts comes from a few central kiloparsecs of the Milky Way and is concentrated towards its center. It was not possible to completely explain this excess with the help of known phenomena, and scientists have repeatedly tried to connect it with the annihilation of dark matter.

Here astronomers had a hard time. We do not know the mass of dark matter particles, and the energy of radiation from their annihilation can be anything – in the range from several kiloelectronvolts to hundreds of gigaelectronvolts (the energy of visible light is two or three electronvolts, and radiation in a medical x-ray machine is tens of kiloelectronvolts).

Identifying their signal among many other processes is akin to looking for a needle in a haystack, especially in such a turbulent and unexplored region as the center of the Galaxy.

Dark matter eludes again mysterious gamma radiation from the centers of galaxies gives rise to neutron stars 2
Center of the Milky Way in infrared and X-rays

A team of scientists led by Anuj Gautam from the Australian National University has studied possible mechanisms for the formation of pulsars , which can create an excess of gamma radiation. It turned out that it can emit a previously unknown population of neutron stars that exist in the center of the Milky Way.

Neutron stars , some of the most extreme objects in space, have many ways to emit high-energy radiation, and they are found in abundance in the galactic center.

Usually they are produced in supernova explosions, and pulsars with a short rotation period are able to generate exactly the radiation that was discovered in the analysis of the Fermi data. However, previous attempts to get them involved have met with difficulties.

Pulsars are rarely seen directly from this distance, but massive stars often exist in multiple systems. Their population, sufficient to form the required number of pulsars, would also give rise to too many X-ray binaries – and they would be clearly visible in the center of the Galaxy.

In addition, when they are born in a supernova explosion, pulsars often receive a powerful return and fly away in a random direction. Such pulsars simply could not “crowd” in the center of the galaxy.

Dark matter eludes again mysterious gamma radiation from the centers of galaxies gives rise to neutron stars 3
The “thorn” at the bottom left is the trace of a pulsar that has received a return and leaves the supernova remnan

And yet, the supposed spectrum of the pulsar emission matches the observed spectrum too well to deviate from this explanation. Scientists have tested whether other mechanisms are capable of producing enough pulsars, and one of them really came up – the collapse of heavy white dwarfs that make up binary stars.

Usually, the absorption of the matter of a companion star by a white dwarf leads to outbursts of new ones . When a white dwarf overeats, the hydrogen on its surface explodes and dissipates into space, after which accretion resumes.

But some of the reaction products remain on the surface of the white dwarf, so this process may not repeat forever, but only until the Chandrasekhar limit is reached . Approaching it is accompanied by such a strong compression and heating of the white dwarf matter that it triggers a thermonuclear reaction in its entire mass.

What happens next depends on the composition of the white dwarf: the lighter elements it contains, the more energy is released during the explosion. Carbon-oxygen white dwarfs explode as a whole, becoming Type I supernovae , while oxygen-neon dwarfs, whose explosion energy is not enough to completely disperse their matter, collapse into pulsars.

It turned out that enough of these pulsars could accumulate in the center of the Milky Way to eliminate the discrepancy between the predicted gamma radiation and the observed one, and at the same time explain the diffuse microwave radiation emanating from the center of the Galaxy.

Thus, dark matter has slipped away again, and all our knowledge of it is still limited to circumstantial evidence. The search for a needle in a haystack continues, and leads to the emergence of more and more new knowledge about the haystack itself.

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