Astronomers may have detected a ‘dark’ free-floating black hole

Scientists estimate there are 200 million black holes scattered across the Milky Way galaxy!

Key Takeaways

Astronomers may have detected a free-floating black hole using gravitational microlensing.
This discovery could mark the first “dark” stellar remnant not paired with another star.

The mysterious object could also be a neutron star if its mass is below 2.2 solar masses.
Two competing studies using the same data propose differing masses for the compact object.
Further Hubble data could resolve debates and refine black hole detection methods.

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A Cosmic Ghost Discovered

Astronomers from UC Berkeley, led by Casey Lam and Jessica Lu, may have detected the first-ever free-floating black hole or neutron star. This stellar remnant, referred to as OB110462, was observed through gravitational microlensing—a phenomenon where a compact object’s immense gravity bends and magnifies light from a more distant star. The team estimates its mass to be between 1.6 and 4.4 solar masses. However, with objects below 2.2 solar masses potentially being neutron stars, its true identity remains uncertain.

This discovery sheds light on “dark” stellar remnants, which are challenging to detect as they lack the typical luminous companions or accretion activity seen in binary systems. Gravitational microlensing offers a new approach to studying these enigmatic objects.

Competing Theories and Observations

While the UC Berkeley-led team suggests the object could be a neutron star or a lower-mass black hole, a separate team from the Space Telescope Science Institute (STScI) in Baltimore analyzed the same microlensing data and proposed a mass of 7.1 solar masses, strongly supporting the black hole hypothesis. Both groups analyzed data from the Hubble Space Telescope, as well as microlensing surveys OGLE and MOA, which provided photometric measurements of the distant star’s brightening.

The teams also debated the object’s velocity. The Berkeley group reported a speed below 30 km/s, suggesting a quiet formation, possibly from a failed supernova. In contrast, the STScI team measured a velocity of 45 km/s, hinting at a supernova origin and a possible “kick” imparted during the event.

a field of orangish stars against a light orange background — Hubble Space Telescope image of a distant star that was brightened and distorted by an invisible but very compact and heavy object between it and Earth. The compact object — estimated by UC Berkeley astronomers to be between 1.6 and 4.4 times the mass of our sun — could be a free-floating black hole, one of perhaps 200 million in the Milky Way galaxy. (Image courtesy of STScI/NASA/ESA)

Future Implications

The findings emphasize the importance of refining microlensing techniques to estimate the mass and distance of compact objects. If confirmed, this discovery could deepen our understanding of black hole formation and the population of stellar remnants in the Milky Way, estimated to include 200 million black holes.

Continued Hubble observations, planned for fall 2022, aim to resolve these uncertainties, offering clearer insights into whether OB110462 is a black hole or neutron star. Regardless, the discovery opens a new window into studying the hidden remnants of massive stars.