Into the Abyss: Understanding Black Hole Formation by Birth Shock and Neutrino Emission

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An artist’s impression of VFTS 243 in the Tarantula Nebula. Credit: ESO/L. Calcada.

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An artist’s impression of VFTS 243 in the Tarantula Nebula. Credit: ESO/L. Calcada.

A new study in Physical Assessment Letters investigates the conditions of black hole formation by dying stars, particularly the role of neutrino-induced birth shocks in the formation process.

Black holes are among the most mysterious objects in the universe, with gravitational forces so strong that not even light can escape them. Based on the evidence we have so far, black holes are stellar corpses, meaning they are born when stars die.

However, the exact mechanisms of their formation are still a mystery. The new study tackles some of these mysteries by examining processes such as stellar mass ejection and neutrino emission, which play a crucial role in the formation of black holes. spoke with first author Dr. Alejandro Vigna-Gómez, a postdoctoral researcher at the Max Planck Institute for Astrophysics in Germany.

When asked about his motivation for studying black hole formation, he said, “For the past decade, my work has focused on the intersection of the physics of binary stars and supernovae.”

“My interest has grown in the wake of recent breakthroughs in black hole astronomy. In recent years I realized that massive black holes could provide important insights into the stellar collapse processes that result in their formation.”

Natal kicks and neutron stars

When a star larger than our Sun reaches the end of its life, it triggers an extremely bright and violent explosion called a supernova explosion. These explosions are so bright that they can briefly exceed the brightness of an entire galaxy and release a large number of neutrinos, leaving behind a neutron star.

The stellar mass ejected during the explosion has a speed of thousands of kilometers per second, but is not always evenly distributed. This asymmetry leads to large-scale asymmetries in the explosion remnants observed in neutron stars.

This asymmetrical ejected mass causes a kickback to the neutron star, a so-called birth kick, causing it to move through the galaxy at high speed. Natal kicks have previously been observed in neutron stars, but not in black holes.

Black holes form when, instead of an explosion, a dying star collapses in on itself. So we come to the researchers’ question: can birth shocks also play a role in the formation of black holes?

Black hole binaries

‘In recent years, several binary stars with black holes have been discovered in our Milky Way and its surroundings. They are usually detected via X-rays, but only a few have been detected via single-line spectroscopy. [a different method] as X-ray silent double stars,” said Dr. Vigna-Gómez.

These binary star systems do not emit significant amounts of X-rays, which can be indicative of the evolutionary stages of the stars in the binary star system.

The researchers chose the galaxy VFTS 243 for their research because it houses one of the most massive black holes of these binary stars.

The binary star system consists of a black hole and a massive star. The researchers wanted to study the conditions under which the black hole formed, such as the lost stellar mass and the birth shocks that accompanied its formation.

The researchers built on recent observations of disappearing stars, stars that died and became black holes without an explosion. Furthermore, these black hole mass (this is the official term) binary stars are inert, meaning there is little interaction between the star and the black hole after the black hole has formed.

Restrictions on the birth kick

The researchers used a semi-analytical approach to calculate the probability that a birth kick during the formation of the black hole would lead to the observed configuration of the system.

To analyze the formation of the system, the researchers used several constraints, such as the orbital period, the eccentricity and the systemic radial velocity of the system. They additionally performed estimates for long-term neutrino asymmetries during the formation of the black hole (assuming this was due to a complete collapse and not an explosion).

Dr. Vigna-Gómez summarized the findings by saying, “We find that VFTS 243’s black hole formed without an explosion and had a low neutrino natal kick, if any. This suggests that neutrinos were emitted almost evenly in all directions when the massive neutrinos were emitted. progenitor fell into a black hole.”

For VFTS 243, the researchers limited the birth speed to less than or equal to about 10 kilometers per second. They found that the most likely scenario is that about 0.3 solar masses were ejected, presumably in neutrinos, and that the black hole received a birth shock of about 4 kilometers per second.

Future work

These findings have implications for the formation of other black holes, suggesting that some may form via complete collapse, without an explosion.

Furthermore, the long-term neutrino emission is preferentially spherically symmetric (equal in all directions), which explains the lack of a strong birth kick for the binary system.

Dr. Vigna-Gómez added: “It seems that the theoretical intuition we built on the fact that black holes reduced birth shocks relative to neutron stars was correct.”

‘This analysis shows that VFTS 243 can be used as a benchmark system for the simulation of supernovae collapsing into the core, i.e. simulations of stars collapsing into black holes of around ten solar masses should match the small neutrino asymmetries and birth shocks that cause them. we concluded for VFTS 243.”

Building frameworks for a population of black holes would be the next step for the researchers in their attempt to understand the evolution of massive stars.

More information:
Alejandro Vigna-Gómez et al, Constraints on Neutrino Natal Kicks from Black-Hole Binary VFTS 243, Physical Assessment Letters (2024). DOI: 10.1103/PhysRevLett.132.191403.

Magazine information:
Physical Assessment Letters

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