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Black hole (BH) high mass X-ray binary (HMXB) systems are likely theprogenitors of the BH-BH mergers detected in gravitational waves byLIGO/Virgo/KAGRA (LVK). Yet BHs in the gravitational wave sources reach highermasses (100Msun) than BHs in HMXBs (20Msun), and have typically lower spins(a<0.25 with tail extending to larger values) than the published values for BHsin HMXBs (a>0.9). This could suggest that these two classes of systems belongto different populations (apples and oranges) but we show here that it does nothave to be the case. The difference in masses is easily explained as the knownHMXB-BHs are in galaxies with relatively high-metallicity, so their progenitorstars are subject to strong mass loss from winds which leads to relatively lowmass BH at core collapse. Conversely, LVK allows for detection of BHs fromlow-metallicity galaxies that naturally produce more massive stellar-originBHs. The explanation of the difference in BH spins is not as straightforward,but we show that in HMXBs the BH spin value derivation depends strongly on howthe HMXB accretion disk emission is modeled. We demonstrate this explicitly forthe case of Cyg X-1, showing excellent spectral fits with low spin (a=0.1) whentaking into account the Comptonization of the disk photosphere. Similar resultshave recently been shown for LMC X-1 (allowing for a=0.1). We suggest that suchlow BH spins are generic for all three HMXB BHs. Hence we conclude that currentobservations are consistent with LVK BHs and HMXB BHs belonging to the samepopulation (all apples).

Common origin of black holes in high mass X-ray binaries and in gravitation-wave sources