NGC 1850 BH1

Lore & Background

The binary system NGC 1850 BH1 was discovered by a team led by Sara Saracino in 2022. The team identified a visible star of approximately 5 solar masses at the main-sequence turnoff, which was beginning to fill its Roche lobe. Using radial velocity and light curve modeling, they determined the mass of the unseen companion to be 11.1 solar masses, which would firmly classify it as a black hole. However, no X-ray emission from the source was detected by the Chandra observatory, casting doubt on the black hole interpretation.

A 2022 study by Kareem El-Badry and Kevin B. Burdge argued that the mass of the visible star was likely overestimated, finding that its observed density is too low to exceed about 1.5 solar masses. They concluded the visible star is most likely a ~1.1 solar mass stripped star with a large radius of 5.7 solar radii, which would be consistent with a ~3.5 solar mass main-sequence companion rather than a black hole. A study by Heloise Stevance, Steven Parsons, and Jan J. Eldridge reached a similar conclusion, finding that a black-hole binary system with the observed mass parameters is extremely unlikely and that a stripped star system is a better explanation.

A follow-up study by Sara Saracino's team in 2023 refuted these alternative interpretations, stating that the possibility of a black hole should not be ruled out. They argued that even in the stripped-star scenario, the unseen object, if a star, should still be bright enough to be detectable in the binary's spectrum, and would have to be obscured by an optically thick accretion disk. They also noted that even if the mass of the unseen star was lower than originally calculated, it would still not rule out a black hole, since the object would still be more massive than the TOV limit.

Reader's Guide

NGC 1850 BH1 holds significance as a contested candidate for a stellar-mass black hole in a binary system outside the Milky Way. Its discovery by Sara Saracino's team in 2022 represented a potential breakthrough in finding black holes in the Large Magellanic Cloud. The subsequent debate over its nature highlights the challenges in identifying black holes via indirect methods, particularly when X-ray emission is absent. The alternative interpretation—that the system contains a stripped star and a main-sequence companion—illustrates how uncertainties in stellar mass and evolutionary state can lead to radically different conclusions. The 2023 rebuttal by the original discovery team underscores the ongoing nature of the dispute, with arguments based on detectability of the companion and the TOV limit. The system thus serves as a case study in the complexities of binary system analysis and the importance of multi-wavelength observations in resolving such ambiguities.

Did You Know?

• NGC 1850 BH1 is located in the Large Magellanic Cloud.
• A 2023 study by Saracino's team argued that the black hole possibility should not be ruled out.