Over the past six decades, scientists have discovered two dozen stellar-mass black holes using X-ray methods. These black holes have a mass distribution mainly between 25 solar mas, which shows a scarcity of black holes with masses ranging from 3 to 5 solar masses.
The mass gap might happen because unique processes during supernova explosions stop black holes in this mass range from forming. Or, it could be that we can’t easily see lower-mass black holes because they are often disrupted by the explosion and harder to detect.
Recently, Chinese scientists used radial velocity and astrometry methods to discover a promising mass-gap black hole in the binary system G3425.
Recent gravitational wave detections by LIGO have found compact objects in a mass range where we previously thought no black holes existed. It is still debated whether low-mass black holes can exist in binary systems. These black holes would be hard to detect because they don’t emit X-rays. Instead, they can be searched for using radial-velocity and astrometric methods.
This study used spectroscopy from the LAMOST telescope and astrometry data from Gaia to look for binary star systems that might contain these compact objects.
The bright star is a red giant and weighs about 2.7 times as much as the Sun. The dark object has a mass of around 3.6 times the Sun, which could range from 3.1 to 4.4 times the Sun. Since no light comes from anything else in the system, this dark companion must be a black hole, with its mass in the “mass gap” range.
G3425 is a wide binary system with an orbit of about 880 days and nearly no tilt. Its usual wide and circular orbit is complicated to explain with typical theories of binary star evolution and supernova explosions.
The study shows that radial velocity and astrometry can detect compact objects in binary systems. This finding suggests that binary systems may have low-mass black holes and offers new insights into how these systems form and evolve.
Journal Reference:
- Song Wang et al., A potential mass-gap black hole in a wide binary with a circular orbit, Nature Astronomy (2024). DOI: 10.1038/s41550-024-02359-9
