Gemini North telescope on Hawai'i reveals first dormant, stellar-mass black hole in our cosmic backyard
From: Association of Universities for Research in
Astronomy (AURA)
November 4, 2022 -- Astronomers
using the International Gemini Observatory, operated by NSF's NOIRLab, have
discovered the closest-known black hole to Earth. This is the first unambiguous
detection of a dormant stellar-mass black hole in the Milky Way. Its close
proximity to Earth, a mere 1600 light-years away, offers an intriguing target
of study to advance our understanding of the evolution of binary systems.
Black holes are the
most extreme objects in the Universe. Supermassive versions of these
unimaginably dense objects likely reside at the centers of all large galaxies.
Stellar-mass black holes -- which weigh approximately five to 100 times the
mass of the Sun -- are much more common, with an estimated 100 million in the
Milky Way alone. Only a handful have been confirmed to date, however, and
nearly all of these are 'active' -- meaning they shine brightly in X-rays as
they consume material from a nearby stellar companion, unlike dormant black
holes which do not.
Astronomers using the
Gemini North telescope on Hawai'i, one of the twin telescopes of the
InternationalGemini Observatory, operated by NSF's NOIRLab, have discovered the
closest black hole to Earth, which the researchers have dubbed Gaia BH1. This
dormant black hole is about 10 times more massive than the Sun and is located
about 1600 light-years away in the constellation Ophiuchus, making it three
times closer to Earth than the previous record holder, an X-ray binary in the
constellation of Monoceros. The new discovery was made possible by making
exquisite observations of the motion of the black hole's companion, a Sun-like
star that orbits the black hole at about the same distance as the Earth orbits
the Sun.
"Take the Solar
System, put a black hole where the Sun is, and the Sun where the Earth is, and
you get this system," explained Kareem El-Badry, an astrophysicist at the
Center for Astrophysics | Harvard & Smithsonianand the Max Planck Institute
for Astronomy, and the lead author of the paper describing this discovery.
"While there have been many claimed detections of systems like this,
almost all these discoveries have subsequently been refuted. This is the first
unambiguous detection of a Sun-like star in a wide orbit around a stellar-mass
black hole in our Galaxy."
Though there are likely
millions of stellar-mass black holes roaming the Milky Way Galaxy, those few
that have been detected were uncovered by their energetic interactions with a
companion star. As material from a nearby star spirals in toward the black
hole, it becomes superheated and generates powerful X-rays and jets of
material. If a black hole is not actively feeding (i.e., it is dormant) it
simply blends in with its surroundings.
"I've been
searching for dormant black holes for the last four years using a wide range of
datasets and methods," said El-Badry. "My previous attempts -- as
well as those of others -- turned up a menagerie of binary systems that masquerade
as black holes, but this is the first time the search has borne fruit."
The team originally
identified the system as potentially hosting a black hole by analyzing data
from the European Space Agency's Gaia spacecraft. Gaia captured the minute irregularities
in the star's motion caused by the gravity of an unseen massive object. To
explore the system in more detail, El-Badry and his team turned to the Gemini
Multi-Object Spectrograph instrument on Gemini North, which measured the
velocity of the companion star as it orbited the black hole and provided
precise measurement of its orbital period. The Gemini follow-up observations
were crucial to constraining the orbital motion and hence masses of the two
components in the binary system, allowing the team to identify the central body
as a black hole roughly 10 times as massive as our Sun.
"Our Gemini
follow-up observations confirmed beyond reasonable doubt that the binary
contains a normal star and at least one dormant black hole," elaborated
El-Badry. "We could find no plausible astrophysical scenario that can
explain the observed orbit of the system that doesn't involve at least one
black hole."
The team relied not
only on Gemini North's superb observational capabilities but also on Gemini's
ability to provide data on a tight deadline, as the team had only a short
window in which to perform their follow-up observations.
"When we had the
first indications that the system contained a black hole, we only had one week
before the two objects were at the closest separation in their orbits.
Measurements at this point are essential to make accurate mass estimates in a
binary system," said El-Badry. "Gemini's ability to provide
observations on a short timescale was critical to the project's success. If
we'd missed that narrow window, we would have had to wait another year."
Astronomers' current
models of the evolution of binary systems are hard-pressed to explain how the
peculiar configuration of Gaia BH1 system could have arisen. Specifically, the
progenitor star that later turned into the newly detected black hole would have
been at least 20 times as massive as our Sun. This means it would have lived
only a few million years. If both stars formed at the same time, this massive
star would have quickly turned into a supergiant, puffing up and engulfing the
other star before it had time to become a proper, hydrogen-burning, main-sequence
star like our Sun.
It is not at all clear
how the solar-mass star could have survived that episode, ending up as an
apparently normal star, as the observations of the black hole binary indicate.
Theoretical models that do allow for survival all predict that the solar-mass
star should have ended up on a much tighter orbit than what is actually
observed.
This could indicate
that there are important gaps in our understanding of how black holes form and
evolve in binary systems, and also suggests the existence of an
as-yet-unexplored population of dormant black holes in binaries.
"It is interesting
that this system is not easily accommodated by standard binary evolution
models," concluded El-Badry. "It poses many questions about how this
binary system was formed, as well as how many of these dormant black holes there
are out there."
"As part of a
network of space- and ground-based observatories, Gemini North has not only
provided strong evidence for the nearest black hole to date but also the first
pristine black hole system, uncluttered by the usual hot gas interacting with
the black hole," said NSF Gemini Program Officer Martin Still. "While
this potentially augurs future discoveries of the predicted dormant black hole
population in our Galaxy, the observations also leave a mystery to be solved --
despite a shared history with its exotic neighbor, why is the companion star in
this binary system so normal?"
Gemini North
observations were made as part of a director's discretionary time program
(program id: GN-2022B-DD-202).
The International
Gemini Observatory is operated by a partnership of six countries, including the
United States through the National Science Foundation, Canada through the
National Research Council of Canada, Chile through the Agencia Nacional de
Investigación y Desarrollo, Brazil through the Ministério da Ciência, Tecnologia
e Inovações, Argentina through the Ministerio de Ciencia, Tecnología e
Innovación, and Korea through the Korea Astronomy and Space Science Institute.
These Participants and the University of Hawaii, which has regular access to
Gemini, each maintain a "National Gemini Office" to support their
local users.
https://www.sciencedaily.com/releases/2022/11/221104113504.htm
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