No signature of the orbital motion of a putative 70 solar mass black hole in LB-1

Abstract: Liu et al. (2019) recently reported the detection of a 68 [+11/-13] solar mass (Msun) black hole (BH) paired with an 8.2 [+0.9/-1.2] Msun B-type sub-giant star in the 78.9-day spectroscopic binary system LB-1. Such a black hole is over twice as massive as any other known stellar-mass black hole with non-compact companions2 and its mass approaches those that result from BH-BH coalescences that are detected by gravitational wave interferometers. Its presence in a solar-like metallicity environment challenges conventional theories of massive binary evolution, stellar winds and core-collapse supernovae, so that more exotic scenarios seem to be needed to explain the existence and properties of LB-1. Here, we show that the observational diagnostics used to derive the BH mass results from the orbital motion of the B-type star, not that of the BH. As a consequence, no evidence for a massive BH remains in the data, therefore solving the existing tension with formation models of such a massive BH at solar metallicity and with theories of massive star evolution in general.

• The paper refutes the massive black hole claim by demonstrating that the supposed radial velocity shifts are artifacts of B-star spectral absorption.
• It employs high-resolution HERMES spectroscopy and atmospheric modeling to reassess the companion’s mass, estimating it around 4 solar masses.
• The findings advocate for improved spectral techniques and revised indirect detection methods in binary systems to accurately interpret stellar dynamics.

An Examination of the Alleged Massive Black Hole in LB-1

The paper authored by Michael Abdul-Masih et al. critically examines the claims made by Liu et al. regarding the existence of an extraordinarily massive black hole in the LB-1 binary system. Initially reported to weigh approximately 68 solar masses, this black hole's existence challenged prevailing models of stellar evolution, particularly in environments with solar-like metallicity. The study aims to clarify these claims through a series of methodological refinements and new observations.

The earlier conclusion of a large black hole mass was predicated on two pivotal arguments: firstly, the characterization of the B-type sub-giant companion and secondly, the purported reflex motion attributed to the black hole. Liu et al. inferred this through the semi-amplitude of 6.4 km/s in the radial velocity (RV) curve of the candidate black hole measured via supposed accretion disc emissions, specifically the H-alpha line. However, this paper unveils that such RV observations primarily result from the superimposition of the absorption lines from the B-type star on a static H-alpha emission, contrary to their attribution to the black hole's reflex motion.

Through high-spectral resolution observations obtained via the HERMES spectrograph and meticulous atmospheric modeling, the researchers establish that the previously observed RV shifts are due to contamination by the B-star's spectral features, rather than any substantive evidence of the black hole's orbital dynamics. Using both empirical data and synthetic simulations, this study robustly reproduces the RV measurements, highlighting that the variations are synchronized anti-phase to the B-star's motion rather than indicating a massive hidden orbiting object.

The implications of this research are significant. Primarily, it addresses the contentious mass estimate of the black hole, noting the lack of evidence for such a massive entity within the system. The analysis limits the companion's minimum mass to that of about 4 solar masses, effectively excluding the massive black hole hypothesis.

The discussion further postulates alternative explanations for the LB-1 system's dynamics. One hypothesis suggests a rapidly rotating main-sequence star or a stripped He star as potential companions. However, such scenarios present observational constraints that align less convincingly with the data, leaving open the possibility of a lower-mass black hole or another ordinary stellar entity.

Conclusively, this study alleviates tensions between previous mass estimates and current stellar evolution theories under solar metallicity. It calls for a reevaluation of indirect methodologies in identifying massive stellar remnants in binary systems. Future investigation could benefit from enhancing spectral resolution and expanding observational strategies to definitively resolve the nature of LB-1 and similar astrophysical objects. This research illustrates the complexity involved in reliably discerning the constituents of distant binary systems, underpinning the importance of meticulous data interpretation and the continued refinement of analytical techniques.