Date: 27.10.2026

Andrzej Zdziarski (CAMK PAN,WAW)

Where are the spins of stellar-mass black holes?

Abstract:
In recent years, we have measured the spins of merging black holes with relatively high accuracy from their gravitational-wave signals. They are generally low, with the estimated average effective spin parameter close to zero. In contrast, many accreting black-hole binaries have been measured to have high spins, some close to the maximum spin parameter of 1 (e.g., greater than 0.9985 at 3 sigma in Cygnus X-1). I will present our recent results regarding this discrepancy. In particular, I will discuss possible systematic effects affecting spin measurements in accreting systems and whether they can be reconciled with those from gravitational-wave observations. See New Astronomy Reviews (2026), 102, 101746 (arXiv:2506.00623).