Reviewed by Danielle Ellis, B.Sc.Sep 11 2024
Using radial velocity and astrometry techniques, Chinese scientists identified a potential mass-gap black hole. A team led by Dr. WANG Song, an associate researcher from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC), performed the study, which was published online in Nature Astronomy on September 10th, 2024.
An artistic image of G3425 binary system, including a visible red giant and an invisible low-mass black hole. Image Credit: WANG Song
Over the last six decades, scientists have used X-ray technologies to discover two dozen stellar-mass black holes. The mass distribution of these black holes, which is mostly between five and 25 solar masses, indicates a shortage of black holes with masses ranging from three to five solar masses.
The mass gap could 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 observational bias, as binaries containing lower-mass black holes are more easily disrupted by natal kicks during supernova explosions and thus harder to detect.
Although recent gravitational wave measurements by the Laser Interferometer Gravitational-Wave Observatory have shown the existence of compact objects inside this mass gap, the topic of whether low-mass black holes could exist in binaries is still debated. Such a system is predicted to be non-interacting and devoid of X-ray emission, and it could be identified using radial-velocity and astrometric approaches.
This study used spectroscopy from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) and astrometry data from Gaia to hunt for binaries with compact components.
Researchers have detected a low-mass dark object in a binary system called G3425. The visible star is a red giant with a mass of around 2.7 solar masses, whereas the dark object has a mass of around 3.6 solar masses, with a more exact range of 3.1 to 4.4. There is no light contribution from any other component in the system than the red giant, indicating that the dark companion is a black hole with a mass that is inside the mass gap.
More notably, G3425 is a broad binary with an orbital period of around 880 days and a near-zero eccentricity. The researchers found it impossible to explain its formation using typical binary evolutionary mechanisms. As a result, the genesis of this remarkably large circular orbit calls into question conventional ideas of binary evolution and supernova explosion.
The results show that combining radial velocity and astrometry can efficiently discover quiescent compact objects in binary systems. This interesting system strongly implies the presence of binary systems containing low-mass black holes, and it has the potential to provide new insights into binary system formation and development.
Journal Reference:
Wang, S., et. al. (2024) A potential mass-gap black hole in a wide binary with a circular orbit. Nature Astronomy. doi.org/10.1038/s41550-024-02359-9