What is mass gap? Over the past sixty years, scientists have discovered two dozen stellar-mass black holes using the X-ray method. The mass distribution of these black holes, mainly between 5 to 25 solar masses, reveals a scarcity of black holes with masses ranging from 3 to 5 solar masses, contrary to black hole formation theories (Figure 1).

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

Although recent gravitational wave observations by the Laser Interferometer Gravitational-Wave Observatory reveal the existence of compact objects within this mass gap (such as GW190425 and GW230529), the question of whether low-mass black holes can exist in binaries remains a matter of debate. Such a system is expected to be non-interacting and without X-ray emission, and can be searched for using radial-velocity and astrometric methods.

Using spectroscopy obtained from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) and astrometry data from Gaia, we conducted a search for stars exhibiting radial-velocity variation and astrometric solutions, with the aim of identifying binaries with compact components. We discovered a low-mass dark object located in a binary system named G3425 (Figure 2).

The visible star, a red giant, has a mass of about 2.7 solar masses, while the dark object’s mass is about 3.6 solar masses, with a more precise range of 3.1 to 4.4 solar masses. Spectral disentangling confirms that there is no light contribution from any other component in the system besides the red giant, evidencing that the unseen companion is a black hole, with a mass falling within the mass gap. This finding strongly suggests the existence of binary systems containing low-mass black holes.

More notably, G3425 is a wide binary exhibiting an orbital period of approximately 880 days and a near-zero eccentricity (Figure 3). The formation of G3425 challenges our understanding of the processes of binary evolution and supernova explosion.

(1) Similar to the well-known Gaia BH1 and BH2, its orbit appears too wide to have formed through common-envelope evolution. Our numerical simulation shows that a wide orbit similar to G3425 could only be produced by adopting a high ejection efficiency of common envelope.

(2) Different with Gaia BH1 and BH2, G3425 exhibits a surprisingly circular orbit. The low mass of the black hole in G3425 indicates that a lot of material needs to be ejected, making the binary likely to be unbound or have a high eccentricity, which cannot be circularized by tidal torque within a Hubble time. Clearly, G3425 cannot form through a dynamical capture of the giant star by a black hole.

Any other explanation about the formation? G3425 could be initially a triple system, with the observed giant star as an outermost component and an inner binary containing two massive stars. The present black hole formed as a result of a merger of the inner binary after long-term evolution. It is also possible that the central unseen object still contains two less-massive compact objects. In this case, it would be an exciting candidate for the merger of binary neutron stars or a neutron star and a white dwarf, which could be detected in the future by gravitational-wave observations.

What’s next? The study demonstrates that combining radial velocity and astrometry can effectively detect quiescent compact objects in binary systems. G3425’s selection from Gaia DR3 data, which observed 1.46 billion sources, covering only 1/100 of Galactic stars, suggests that hundreds of such systems exist in the Milky Way. Future spectroscopic and astrometric observations, in particular, the upcoming Gaia DR4, may help to unveil a low-mass black-hole binary population with a variety of parameters and provide profound insights into the formation and evolution of binary systems.  

Link: https://www.nature.com/articles/s41550-024-02359-9