A lonely stellar-mass black hole has been discovered for the first time by a team of scientists that involved the critical expertise of researchers at the University of St Andrews.

The black hole's presence was revealed from a tiny shift in the position of an observed background star resulting from the curvature of spacetime by the black hole in accordance with Einstein's theory of Relativity.

This new discovery opens the door to obtaining the demographics of black holes throughout the Milky Way, providing key observational data on the late stages of the evolution of stars. The measurements, at the very limits of the capabilities of the Hubble Space Telescope, were a team effort by academics at the University of St Andrews and the Space Telescope Science Institute (STScI).

Theoretical models suggest there could be 100 million black holes hidden among the hundreds of billions of stars in the Milky Way. These have escaped detection because they neither emit light nor other forms of electromagnetic radiation of their own (with the sole exception of an undetectable tiny amount of Hawking radiation) nor do they reflect the light of other luminous bodies as planets can.

However, a black hole's own gravity does affect the path of light in its vicinity, bending it by an amount proportional to its mass, explained by Albert Einstein's theory of General Relativity as a result of massive bodies curving spacetime.

Dr Martin Dominik, Reader in the School of Physics and Astronomy at the University of St Andrews, said: "Einstein did it again – black holes make themselves invisible, but they cannot hide their gravity. It was amazing to see how two observable signatures of the gravitational bending of light by the black hole matched up – the shift in position and an apparent brightening of the observed background star."

So far, all known black holes within the stellar mass range had been in binary systems, in which two astronomical bodies orbit each other due to gravitational attraction.

The observed gravitational bending of light is the very same effect that caused the change in position of stars near the edge of the Sun measured by Arthur Eddington and colleagues during the Solar eclipse of 29 May 1919, which observations eventually made Einstein famous.

However, for such bending of light to be substantial, an observed star needs to be closely aligned on the sky with an intervening massive body, and such alignments are rare.

With a one in a million chance, dedicated surveys like OGLE (Optical Gravitational Lensing Experiment) or MOA (Microlensing Observations in Astrophysics) use 1- to 2m-class telescopes for monitoring hundreds of millions of stars towards the Milky Way bulge in order to catch ongoing so-called gravitational microlensing events.

These are characterised by a transient brightening of the observed star due to the bending of light changing the amount of total light received in two unresolved images.

One such gravitational microlensing event independently detected by either team in 2011, known alternatively as MOA-2011-BLG-191 or OGLE-2011-BLG-0462, had an unusually long duration, which could either have been due to the intervening body moving slowly on the sky with respect to the observed source star, or alternatively due to the mass of the deflector being large. With stellar-mass black holes being expected in the range between 3 and 20 Solar masses, as compared to a typical stellar-mass gravitational lens of 0.3 Solar masses, and given that the event was furthermore compatible with the lens object being dark, it made a good candidate for microlensing by a black hole.

Consequently, over the following six years a team lead by Dr Kailash Sahu, working at the Space Telescope Science Institute (STScI) in Baltimore and a long-standing collaborator of Dr Dominik, used the Hubble Space Telescope (HST) to track the position of the source star that caused this event. As they had investigated 20 years ago, the positional shift of the centroid of light composed by the two unresolved images is observable for a much longer period of time than the brightening, and these observations would provide the mass of the gravitational lens unambiguously.

These measurements proved challenging: not only was the target of interest close to a much brighter star, from which it is not resolved on the ground-based images but, moreover, the gravitational deflection of light is quite small. Right at the limits of what is possible with HST, the positional measurements have an uncertainty of only 0.2 milli-arcseconds, which is 10,000 times less than the bending angle of 2 arcseconds that Eddington looked at, and approximately the 6 billionth part of the full angle.

The amount of positional shift of the source star relates both to the mass of the gravitational lens and its distance, while the size of the Earth's orbit provides a ruler for the distance, leading to an offset of the relative position angle between source star and intervening lens, which prominently shows in the brightening of the source as a function of time. Together, these effects gave a lens mass of about 7 Solar masses, as well as a distance of 1.6 kpc (or about 5000 light-years).

Such a mass is above the range expected for any other single or binary objects of negligible luminosity. Stellar-mass black holes are the end-product of heavy stars that are at least 20 times more massive than the Sun. These stars explode as supernovae, and the remnant core is crushed by gravity into a black hole. Because the self-detonation of stars is not perfectly symmetric, the remnant black hole may get a kick. The inferred transverse velocity of the gravitational lens turned out to be an outlier amongst stars at similar distance, further supporting that it is indeed a black hole.

There is much scope for further detections of isolated stellar-mass black holes using the same approach with ESA's Gaia satellite, aiming at a precise mapping of stellar positions and velocities throughout the Milky Way and, unless stellar-mass black holes are far less common than we currently think, the capabilities of the Nancy Grace Roman Space Telescope, to be launched within the next five years, will turn these into a routine job.

Dr Dominik added: "We identified gravitational bending of light by a dark object of seven Solar masses in the Milky Way. It's an isolated stellar-mass black hole that failed to hide, and many others are set to fail as well in the near future."

The paper, 'An Isolated Stellar-Mass Black Hole Detected Through Astrometric Microlensing' is published in The Astrophysical Journal and is available online.

Image: An illustration of a close-up look at a black hole drifting through our Milky Way galaxy. The black hole is the crushed remnant of a massive star that exploded as a supernova.
Image credit: FECYT, IAC.