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Starts With A Bang

How close is the nearest black hole to Earth?

The ESA's Gaia mission just broke the record for closest black hole by over 1,000 light-years. Is there an even closer one out there?
This view of part of the Milky Way showcases three zoom levels. At left, the individual star system known as Gaia DR3 4373465352415301632 is shown, which contains a binary companion of ~10 solar masses and an orbital period of 185.6 days (center). At right, an illustration of how the star might appear due to the lensing effect of the black hole is also shown.
(Credit: T. Müller (MPIA), PanSTARRS DR1 (K. C. Chambers et al. 2016), ESA/Gaia/DPAC (CC BY-SA 3.0 IGO))
Key Takeaways
  • Ever since the first black hole was discovered, the X-ray binary Cygnus X-1, scientists have wondered just how close the nearest black hole to us truly is.
  • With techniques such as X-ray binary measurements and gravitational wave observations, we've discovered many candidates and confirmed black holes, but all are thousands (or more) of light-years away.
  • Using a novel technique and data set to find detached black hole-star binaries, a new record holder, Gaia BH1, is just 1560 light-years away. It holds the current record; likely not for long.

All across the Universe, massive stars collapse and die.

The anatomy of a very massive star throughout its life, culminating in a Type II Supernova when the core runs out of nuclear fuel. The final stage of fusion is typically silicon-burning, producing iron and iron-like elements in the core for only a brief while before a supernova ensues. If the core of this star is massive enough, it will produce a black hole when the core collapses.
(Credit: Nicolle Rager Fuller/NSF)

From core-collapse supernovae, neutron stars and black holes form.

The visible/near-IR photos from Hubble show a massive star, about 25 times the mass of the Sun, that has winked out of existence, with no supernova or other explanation. Direct collapse is the only reasonable candidate explanation, and is one known way, in addition to supernovae or neutron star mergers, to form a black hole for the first time.
(Credit: NASA/ESA/C. Kochanek (OSU))

Stars and gas directly collapse, forming black holes.

supermassive black hole
This snippet from a supercomputer simulation shows just over 1 million years of cosmic evolution between two converging cold streams of gas. In this short interval, just a little over 100 million years after the Big Bang, clumps of matter grow to possess individual stars containing tens of thousands of solar masses each in the densest regions. This could provide the needed seeds for the Universe’s earliest, most massive black holes, as well as the earliest seeds for the growth of galactic structures.
(Credit: M.A. Latif et al., Nature, 2022)

Finally, neutron star mergers create black holes, too.

When two neutron stars collide, if their total mass is great enough, they won’t just result in a kilonova explosion and the ubiquitous creation of heavy elements, but will lead to the formation of a novel black hole from the post-merger remnant. Gravitational waves and gamma-rays from the merger appear to travel at indistinguishable speeds: the speed of all massless particles.
(Credit: Robin Dienel/Carnegie Institution for Science)

These black holes roam the Universe, devouring whatever matter contacts their event horizons.

sagittarius A*
On September 14, 2013, astronomers caught the largest X-ray flare ever detected from the supermassive black hole at the center of the Milky Way, known as Sagittarius A*. In X-rays, no event horizon is visible at these resolutions; the “light” is purely disk-like. However, we can be certain that only matter remaining outside the event horizon generates light; matter passing within it gets added to the black hole’s mass, inevitably infalling into the black hole’s central singularity.
(Credit: NASA/CXC/Amherst College/D.Haggard et al.)

Inspiraling, merging objects emit gravitational waves, allowing black hole detections terrestrially.

black hole merger
A mathematical simulation of the warped space-time near two merging black holes. The colored bands are gravitational-wave peaks and troughs, with the colors getting brighter as the wave amplitude increases. The strongest waves, carrying the greatest amount of energy, come just before and during the merger event itself. From inspiraling neutron stars to ultramassive black holes, the signals that we should expect the Universe to generate ought to span more than 9 orders of magnitude in frequency and can reach peak power outputs of ~10^23 Suns.
(Credit: SXS Collaboration)

We also detect the X-rays emitted by black holes feeding off of binary companions.

When a massive star orbits a stellar corpse, like a neutron star or black hole, the remnant can accrete matter, heating and accelerating it, leading to the emission of X-rays. These X-ray binaries were how all stellar mass black holes, until the advent of gravitational wave astronomy, were discovered, and are still how most of the Milky Way’s known black holes have been found.
(Credit: ESO/L. Calçada/M.Kornmesser)

These X-ray binaries, traditionally, have revealed the closest black holes: several thousands of light-years distant.

mass gap
The most up-to-date plot, as of November, 2021, of all the black holes and neutron stars observed both electromagnetically and through gravitational waves. While these include objects ranging from a little over 1 solar mass, for the lightest neutron stars, up to objects a little over 100 solar masses, for post-merger black holes, gravitational wave astronomy is presently only sensitive to a very narrow set of objects. The closest black holes have all been found as X-ray binaries, until the November 2022 discovery of Gaia BH1.
(Credit: LIGO-Virgo-KAGRA / Aaron Geller / Northwestern)

However, two other methods hold promise: microlensing and black hole-star binaries with detached orbits.

black hole hit earth
If a black hole were on a collision course with Earth, we wouldn’t have any warning from the black hole itself, but it would distort and bend the light from background objects, revealing its presence. The fact that mass bends spacetime, regardless of what types of light it gives off, is a key to finding black holes that may be hiding in the nearby Universe.
(Credit: the Ohio State University)

Microlensing occurs whenever a mass intervenes between a luminous object and ourselves.

When a gravitational microlensing event occurs, the background light from a star gets distorted and magnified as an intervening mass travels across or near the line-of-sight to the star. The effect of the intervening gravity bends the space between the light and our eyes, creating a specific signal that reveals the mass and speed of the intervening object in question. All masses are capable of bending light via gravitational lensing, from low-mass planets to high-mass black holes.
(Credit: Jan Skowron/Astronomical Observatory, University of Warsaw)

The characteristic brightening pattern reveals the interloper’s mass and other properties.

The relativistic, light-bending effects shown here are caused by the strong gravitational lensing effects of a foreground black hole. Both the background of the Milky Way and a lensed star are shown here. This method would reveal both a lensed star in a detached binary orbit with the black hole as well as an interloping black hole that caused a microlensing event.
(Credit: T. Müller (MPIA), PanSTARRS DR1 (K. C. Chambers et al. 2016), ESA/Gaia/DPAC (CC BY-SA 3.0 IGO))

Meanwhile, black holes orbiting normal stars will influence the star’s observed motion and position.

By tracking a star’s redshift-and-blueshift over time, a candidate companion’s mass can be uncovered.

dark matter
The idea of the radial velocity method is that if a star has an unseen, massive companion, whether an exoplanet or a black hole, observing its motion and position over time, if possible, should reveal the companion and its properties. This remains true, even if there’s no detectable light emitted from the companion itself.
(Credit: E. Pécontal)

Observing its changing position over time should match the companion candidate’s predictions, confirming its partner.

Overview of the radial velocities for Gaia-BH1 as obtained by the LAMOST survey and from follow-up observations with the MagE, GMOS, XSHOOTER, ESI, FEROS and HIRES spectrographs. Points with error bars are measurements, gray lines are drawn from the posterior when jointly fitting thse radial velocity spectra and the Gaia astrometric constraints.
(Credit: El-Badry et al., MNRAS, 2022)

The ESA’s Gaia mission leveraged this method, discovering today’s nearest black hole: Gaia BH1.

Just 1560 light-years away, this record is temporary.

Gaia BH1, at ~10 solar masses, with an orbital period of ~180 days, and located just 1560 light-years away, now holds the record (as of 2022) for closest black hole known to our Solar System.
(Credit: El-Badry et al., MNRAS, 2022)

Upcoming missions, like Nancy Roman, should reveal even closer black holes.

dark energy
This illustration compares the relative sizes of the areas of sky covered by two surveys: the upcoming Nancy Roman Telescope’s High Latitude Wide Area Survey, outlined in blue, and the largest mosaic led by Hubble, the Cosmological Evolution Survey (COSMOS), shown in red. In current plans, the Roman survey will be more than 1,000 times broader than Hubble’s, revealing how galaxies cluster across time and space as never before, enabling the tightest constraints on evolving dark energy, and revealing more microlensing events, including possibly extremely close black holes, than ever before.
(Credit: NASA/GSFC)

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