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Abstract Data derived from general relativistic magnetohydrodynamic simulations of accretion onto black holes can be used as input to a postprocessing scheme that predicts the radiated spectrum. Combining a relativistic Compton scattering radiation transfer solution in the corona with detailed local atmosphere solutions incorporating local ionization and thermal balance within the disk photosphere, it is possible to study both spectral formation and intrinsic spectral variability in the radiation from relativistic accretion disks. With this method, we find that radiatively efficient systems with black holes of 10M_⊙accreting at ≈0.01 in Eddington units produce spectra very similar to those observed in the hard states of X-ray binaries. The spectral shape above 10 keV is well described by a power law with an exponential cutoff. Intrinsic turbulent variations lead to order-unity changes in bolometric luminosity, variations in the logarithmic spectral slope ∼0.1, and factor of 2 alterations in the cutoff energy on timescales ∼50 (M_BH/10M_⊙) ms. Within the corona, the range of gas temperature spans more than 1 order of magnitude. The wide distribution of temperatures is central to defining the spectrum: the logarithmic spectral slope is harder by ∼0.3 and the cutoff energy larger by a factor ∼10–30 than if the coronal temperature everywhere were its mass-weighted mean. 

Abstract The detection of GW170817/AT2017gfo inaugurated an era of multimessenger astrophysics, in which gravitational-wave and multiwavelength photon observations complement one another to provide unique insight into astrophysical systems. A broad theoretical consensus exists, in which the photon phenomenology of neutron star mergers largely rests upon the evolution of the small amount of matter left on bound orbits around the black hole or massive neutron star remaining after the merger. Because this accretion disk is far from inflow equilibrium, its subsequent evolution depends very strongly on its initial state, yet very little is known about how this state is determined. Using both snapshot and tracer particle data from a numerical relativity/MHD simulation of an equal-mass neutron star merger that collapses to a black hole, we show how gravitational forces arising in a nonaxisymmetric, dynamical spacetime supplement hydrodynamical effects in shaping the initial structure of the bound debris disk. The work done by hydrodynamical forces is ∼10 times greater than that due to time-dependent gravity. Although gravitational torques prior to remnant relaxation are an order of magnitude larger than hydrodynamical torques, their intrinsic sign symmetry leads to strong cancellation; as a result, hydrodynamical and gravitational torques have a comparable effect. We also show that the debris disk’s initial specific angular momentum distribution is sharply peaked at roughly the specific angular momentum of the merged neutron star’s outer layers, a fewr_gc, and identify the regulating mechanism. 

Abstract We present a survey of how the spectral features of black hole X-ray binary systems depend on spin, accretion rate, viewing angle, and Fe abundance when predicted on the basis of first-principles physical calculations. The power-law component hardens with increasing spin. The thermal component strengthens with increasing accretion rate. The Compton bump is enhanced by higher accretion rate and lower spin. The Fe K α equivalent width grows sublinearly with Fe abundance. Strikingly, the K α profile is more sensitive to accretion rate than to spin because its radial surface brightness profile is relatively flat, and higher accretion rate extends the production region to smaller radii. The overall radiative efficiency is at least 30%–100% greater than as predicted by the Novikov–Thorne model.