A wide star-black-hole binary system from radial-velocity measurements

Abstract: All stellar mass black holes have hitherto been identified by X-rays emitted by gas that is accreting onto the black hole from a companion star. These systems are all binaries with black holes below 30 M${\odot}$$^{1-4}$. Theory predicts, however, that X-ray emitting systems form a minority of the total population of star-black hole binaries$^{5,6}$. When the black hole is not accreting gas, it can be found through radial velocity measurements of the motion of the companion star. Here we report radial velocity measurements of a Galactic star, LB-1, which is a B-type star, taken over two years. We find that the motion of the B-star and an accompanying H$\alpha$ emission line require the presence of a dark companion with a mass of $68^{+11}{-13}$ M$_{\odot}$, which can only be a black hole. The long orbital period of 78.9 days shows that this is a wide binary system. The gravitational wave experiments have detected similarly massive black holes$^{7,8}$, but forming such massive ones in a high-metallicity environment would be extremely challenging to current stellar evolution theories$^{9-11}$.

• The paper documents the discovery of a wide LB-1 system featuring a B-type star and a 68 M⊙ black hole, identified through detailed radial velocity measurements.
• The study employs precise spectroscopic data from LAMOST, GTC/OSIRIS, and Keck/HIRES to determine the system's 78.9-day orbital period and mass characteristics.
• The findings challenge standard black hole formation theories in metal-rich environments, prompting further exploration of undetected massive stellar remnants.

Analysis and Implications of a Wide Star-Black-Hole Binary System

The paper documents the identification of a wide binary system consisting of a B-type star, LB-1, and a non-accreting black hole (BH), challenging longstanding assumptions that such systems can only be detected through X-ray emissions from accretion processes. This discovery has been substantiated through extensive radial velocity measurements, coupled with spectroscopic observations to confirm the presence of a dark companion with a significant mass.

Key Findings

1. Binary System Characteristics: The LB-1 system is comprised of a B-type star and a dark companion, indicated at 68$^+11_{-13}$ M$_\odot$. The orbit of this system displays a period of 78.9 days, suggesting a wide separation between the star and its companion. This challenges existing theories which posit that substantial mass loss and dynamic interactions would prohibit such wide pairings.
2. Mass Measurements and Implications: The mass determination of the companion as 68 M$_\odot$ indicates it is a black hole. Such measurements significantly exceed what is predicted for black holes forming in high-metallicity environments, suggesting the need to revise current stellar evolution models.
3. Spectral Analysis and Radial Velocities: Important to the discovery was the analysis of radial velocity data from the Large Aperture Multi-Object Spectroscopic Telescope (LAMOST), GTC/OSIRIS, and Keck/HIRES observations, which solidified the presence of a highly massive and virtually invisible companion.
4. Non-Conforming to Traditional Black Hole Models: The reported black hole mass in LB-1 contradicts theoretical models that predict a maximum mass of approximately 25 M$_\odot$ at solar metallicity, posing questions about the processes and environments necessary for such large black hole formation.
5. Potential Triple System Evolution: One hypothesis proposed in the study is the initial configuration of LB-1's precursor as a triple system. The massive companion could have formed through a merging process between two heavy stars, followed by subsequent accretion onto the resultant black hole, challenging traditional binary formation theories.

Implications and Future Directions

The implications of this discovery are multifold, prompting a reevaluation of black hole formation theories and their evolution, particularly in metal-rich environments. This system's characteristics hint at more diverse evolutionary paths than recognized before, including potential mergers and collapses in isolated or complex multi-star systems. Future work could focus on examining other long-orbital period, low-luminosity systems for hidden massive companions, employing similar radial velocity and spectral techniques as those used in this study.

The findings also hold promise for advancing gravitational wave astronomy. The discovery could point to undetected populations of massive stellar remnants, which may contribute to significant gravitational wave signatures when involved in further cosmic events.

Conclusions

The potential for undiscovered populations of non-accreting black holes widens the investigation scope beyond traditional X-ray binary searches. The findings related to LB-1 suggest a more intricate picture of black hole distribution and formation than previously thought, necessitating greater focus on radial velocity measurements and non-conventionally understood astronomical signatures.

This work represents a paradigm shift in black hole astronomy, emphasizing the importance of exploring the details of radial velocity changes, spectral emissions, and the pursuit of additional means for detecting these elusive systems. The bridge between theoretical prediction and observational data continues to stimulate robust debate and innovation in the astronomical community.