A black hole wandering at high speed through the Milky Way

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Using the gravitational microlensing technique, two groups of astronomers have found a very compact stellar object in one of the spiral arms of our galaxy and have determined its properties from Einstein's general relativity.

microlenses

are very difficult to study observationally

.

Their gravitational field is so extreme that light is trapped by themselves and they do not emit radiation that can be detected directly by our telescopes.

Black holes are usually studied for the behavior of the surrounding gas or for the apparent effects they have on their companion star when they are part of a binary star system.

Particularly difficult is studying the

.

They are the ones that result when a massive star reaches the end of its days by exploding as a supernova.

And yet, such black holes are thought to be very abundant in all galaxies, including the

.

This is because massive stars live very quickly and, during the life of our galaxy, many have already died and others continue to die.

As a result, it is estimated that there may be

.

Finding one of them has been a real challenge for the last few decades, but now, finally, one may have been located.

Explanatory diagram of a gravitational microlensing.NASA/ESA

To search for these black holes, astronomers have developed an observational method known as the

.

It is based on an effect predicted by Einstein's theory of general

that has also been used to detect many of the most distant known galaxies.

The method consists

with our telescope: an object in the foreground and another, behind, much further away.

The gravitational field of the foreground star deflects the light from the farther star and amplifies it like an optical lens.

When this technique is applied to stellar objects, it is called

.

The way the light is deflected by the foreground object tells a lot about it, even if it were a black hole that you couldn't see directly.

Two teams

Thus, astronomers have been

for decades , patiently waiting for alignments that may occur by chance, creating

.

When one of these alignments occurs, a background star suddenly brightens and, using Einstein's general relativity equations, the mass of the star in the line of sight can be determined.

Using ground-based telescopes, some

, and thanks to this, all kinds of stars (including brown dwarfs) and even exoplanets have been studied.

Two teams of astronomers, one coordinated by

Institute

in

and the other by

, have now found a very compact object that could be a black hole.

To do this, they used data collected by the Hubble Space Telescope for six years (between 2011 and 2017).

The compact stellar object is located in the spiral arm of the Milky Way as seen

, about

.

The researchers have concluded that this very compact object is not an ordinary star for two reasons.

The first is that the duration of the distant star's brightening event

.

However, when microlensing is caused by two ordinary stars, the duration of the event is usually a few days.

during brightening

.

If the foreground object had been an ordinary star, the colors of the aligned stars should have mixed during microlensing.

Both teams also made very high-precision (astrometric) measurements to calculate the spatial deviation of the background star's light.

This deviation turned out to be tiny, just

(the apparent size of a person on the Moon as seen from Earth), but enough to conclude that the foreground object has a mass between 1.6 and 4, 4 solar masses (according to Lam) or even 7 solar masses (according to Sahu).

The arrow marks the star brightened by microlensing.NASA/ESA/K.

SAHU/ J. DEPAQUALE

Neutron star?

Lam's study shows that

, because if its mass were in the lower range we would find ourselves before a neutron star, also a stellar corpse, but with less mass than black holes.

However, Sahu's team, by favoring larger masses, leaves no doubt that

.

This latest research group also calculated that the speed at which the black hole is moving through the Milky Way reaches

, suggesting that the supernova event in which it was created triggered it out of the sky. very violent.

Until now, these low-mass black holes have only been studied through the X-ray radiation that is produced in

and, to a much lesser extent, thanks to the gravitational waves emitted in the merger of two black holes.

Both methods favor the detection of extreme objects.

These new microlensing observations, dealing with the most common objects (be it a black hole or a neutron star), represent

as it opens the door to new detections of these very elusive objects.

It will therefore be necessary to continue monitoring thousands of stars, to detect those brightenings that signal the passage of the numerous black holes that must swarm throughout the Galaxy.

The article by Sahu and collaborators has been published in

Rafael Bachiller is the director of

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