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Saturday, 2 November 2019

For the first time, astrophysicists could have detected a "mini black hole"

The black holes are certainly part of the most mysterious objects in the cosmic catalog. Planned from the beginning of the 20th century as part of general relativity, current models describe several types: stellar black holes, intermediate black holes and supermassive black holes. Each of these types has been detected in recent years. But the models also provide other hypothetical black holes, such as primordial black holes, black micro holes or even mini black holes. The latter would be black holes whose mass would be between that of a neutron star and that of a stellar black hole . And for the first time, cosmologists could have detected one.

Astronomers generally look for black holes in our galaxy by observing the X-rays that are emitted when black holes absorb material from nearby stars. The thermal friction within the accretion disk generates, in fact, the emission of a powerful electromagnetic radiation. In more distant galaxies , this search may involve the detection of gravitational waves produced by the fusion of two black holes.

But a group of researchers wondered if there might be relatively low mass black holes that do not emit the tell-tale X-ray signals. Such hypothetical black holes would probably exist in a binary system with another star, although they orbit sufficiently far from this star not to absorb too large quantities.

Mini black holes: they would betray themselves by the variations of brightness of their companion star

The researchers speculated that these small black holes would not emit detectable X-rays and would therefore remain invisible to astronomers, according to Todd Thompson, a professor of astronomy at Ohio State University. " We are pretty sure there must be many, many of these black holes in the binary systems with stars in the galaxies, but we have not detected them because they are hard to find ."

Thompson and his colleagues looked for evidence of these black holes in the stellar companions of the proposed objects. They screened the data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE), containing information on the light spectrum of more than 100,000 stars in our galaxy.

The information in this study revealed changing spectra for each of these stars. Such a change could mean that a particular star is gravitating around an invisible companion. After conducting this analysis, the researchers examined changes in brightness of a subset of stars that could gravitate around black holes, using data from another mission called ASA-SN (All-Sky Automated Survey). for Supernovae).

An object too massive to be a neutron star ... but not massive enough to be a stellar black hole

This is how the researchers discovered a massive black object, caught in a gravitational embrace with a giant star in rapid rotation, located about 10'000 light-years from Earth, on the edge of our galaxy (near the constellation of the Coachman).

They estimated that the mass of this object was about 3.3 times that of our Sun, too massive to be a neutron star and not massive enough compared to that of known black holes. The results were published in the journal Science.

Graphic presenting the observational constraint posed on the mass of the object. The most accurate result indicates about 3.3 solar masses. Credits: Todd A. Thompson et al. 2019

The most massive neutron stars that astrophysicists know are 2.1 solar masses, while the least massive black hole known is about five to six times the mass of our Sun, according to Thompson. However, the lower mass limit of the new object found corresponds to 2.6 times the mass of the Sun, which, according to astronomers, constitutes the theoretical upper limit of star mass. More massive than that, the neutron star would collapse into a black hole.

The realistic hypothesis of the detection of a black mini-hole

So, this dark and mysterious object " could be the most massive neutron star ever seen. Just at the border after which she can no longer exist. In fact, I would be even more excited if that were true. But it is more than likely a relatively low mass black hole, theorized but never discovered before, "says Thompson.

Mini black holes, black holes whose mass is located between that of neutron stars and stellar black holes, have been theorized for many years. Credits: LIGO-Virgo, Frank Elavsky, Northwestern (Modified by Todd Thompson)

Dejan Stojkovic, a cosmologist and professor of physics at the University of Arts and Sciences, Buffalo, confirms these results. " It's probably a black hole, because it's too massive to be a neutron star, unless it's a kind of unusual star. The discovery seems very reasonable, but not unexpected, because astronomers know that there are black holes of lower mass .

Thompson said he was eagerly awaiting future discoveries, such as information on the inclination of the star's orbit around the dark object that the European Space Agency's Gaia spacecraft could muster during the day. a next mission. This could help researchers to more accurately measure the mass of the dark object.


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