Weighing in at roughly 21 solar masses, the black hole in the X-ray binary system Cygnus X-1 is so massive that it challenges current stellar evolution models, a new study reveals. Ultimately, the mass of a black hole is determined by its parent star’s properties and is generally constrained by the mass lost to stellar winds throughout its lifetime. If a black hole interacts with a binary companion star, the system emits X-rays and can sometimes form radio jets, which make the systems visible to electromagnetic observations as an X-ray binary. Measurements from known x-ray binaries have shown that black holes in these systems all have masses below 20 solar masses (M?), with the largest being 15-17 M?. However, gravitational wave detections of black hole merger events have found more massive black holes, reaching upwards of 50 M?, revealing a discrepancy that challenges current theories on black hole formation from massive stars. Here, James Miller-Jones and colleagues present new observations of Cygnus X-1 – a well-studied stellar-mass black hole located in our Milky Way Galaxy – using the Very Long Baseline Array (VLBA). Between May 29 and June 3, 2016, they performed six observations (one per day) of Cygnus X-1 with the VLBA. Using the new data and archival observations, Miller-Jones et al. refined the distance to the X-ray binary and found it to be farther away than previously estimated, thus raising the inferred mass of the system’s black hole to 21 M?. The new measurements establish Cygnus X-1 as the most massive electromagnetically detected stellar-mass black hole currently known. According to the authors, for a black hole this massive to exist in the Milky Way, the mass lost through stellar winds during the progenitor star’s evolution must have been lower than what current models predict.
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