The mass of Cygnus X-1's black hole challenges stellar evolution models

February 18, 2021

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.

American Association for the Advancement of Science

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