2024MNRAS.531..805N

Stellar mass black holes in X-ray binaries (XRBs) are known to display different states characterized by different spectral and timing properties, understood in the framework of a hot corona coexisting with a thin accretion disc whose inner edge is truncated. There are several open questions related to the nature and properties of the corona, the thin disc, and dynamics behind the hard state. This motivated us to perform 2D hydrodynamical simulations of accretion flows onto a $10 \, \mathrm{M}_\odot$ black hole. We consider a two-temperature plasma, incorporate radiative cooling with bremmstrahlung, synchrotron, and Comptonization losses and approximate the Schwarzschild space-time via a pseudo-Newtonian potential. We varied the mass accretion rate in the range $0.02 \le \dot{M}/\dot{M}_{\rm Edd} \le 0.35$. Our simulations show the natural emergence of a colder truncated thin disc embedded in a hot corona, as required to explain the hard state of XRBs. We found that as $\dot{M}$ increases, the corona contracts and the inner edge of the thin disc gets closer to the event horizon. At a critical accretion rate $0.02 \le \dot{M}_{\text{crit }}/\dot{M}_{\rm Edd} \le 0.06$, the thin disc disappears entirely. We discuss how our simulations compare with XRB observations in the hard state.