Astronomers Discover Long-Hidden Small Black Hole in Unusually Evolved Binary System

Chinese researchers have discovered a promising mass-gap black hole using radial velocity and astrometry methods. The study was published online in Astronomy of nature on September 10, and was conducted by a team led by Dr. Wang Song, an associate researcher at the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC).

Over the past six decades, scientists have discovered two dozen stellar-mass black holes using X-ray methods. The mass distribution of these black holes, mostly between 5 and 25 solar masses, shows a rarity of black holes with masses ranging from 3 to 5 solar masses.

The mass gap may be caused by special mechanisms during supernova explosions that prevent the formation of black holes in this mass range, or it could be due to an observational bias, since binaries with lower-mass black holes are more easily disrupted by natal kicks during supernova explosions and are therefore harder to detect.

Although recent observations of gravitational waves by the Laser Interferometer Gravitational-Wave Observatory have revealed the existence of compact objects within this mass gap, the question of whether low-mass black holes could exist in binaries remains a matter of debate. Such a system was expected to be non-interacting and non-emitting X-rays and could be searched for using radial velocity and astrometric methods.

Using spectroscopy obtained from the Large Sky Area Multi-Object Spectroscopic Telescope (LAMOST) and astrometry data from Gaia, this study conducted a search for binaries containing compact components.

Researchers have discovered a low-mass dark object located in the binary system G3425. The visible star is a red giant with a mass of about 2.7 solar masses, while the mass of the dark object is about 3.6 solar masses, with a range of 3.1 to 4.4 solar masses. There is no luminous contribution from any other component of the system besides the red giant, which proves that the dark companion is a black hole, whose mass falls within the mass range.

In particular, G3425 is a large binary star with an orbital period of about 880 days and an eccentricity close to zero. Researchers have struggled to explain its formation by standard binary evolution processes. Therefore, the formation of this surprisingly large circular orbit challenges current theories of binary evolution and supernova explosions.

The study demonstrates that the combination of radial velocity and astrometry can efficiently detect compact objects at rest in binary systems. This intriguing system strongly suggests the existence of binary systems containing low-mass black holes and can provide new insights into the formation and evolution of binary systems.