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Relativistic Fe K emission lines from accretion discs and from disc winds encode key information about black holes, and their accretion and feedback mechanisms. We show that these two processes can in principle produce indistinguishable line profiles, such that they cannot be disentangled spectrally. We argue that it is likely that in many cases both processes contribute to the net line profile, and their relative contributions cannot be constrained purely by Fe K spectroscopy. In almost all studies of Fe K emission to date, a single process (either disc reflection or wind Compton scattering) is assumed to dominate the total line profile. We demonstrate that fitting a single process emission model (pure reflection or pure wind) to a hybrid line profile results in large systematic biases in the estimates of key parameters, such as mass outflow rate and spin. We discuss various strategies to mitigate this effect, such as including high-energy data covering the Compton hump, and the implications for future X-ray missions. [ABSTRACT FROM AUTHOR]

We develop a model describing the dynamical and observed properties of disc-dominated tidal disruption events (TDEs) around black holes with the lowest masses (M ≲ few × 106M⊙). TDEs around black holes with the lowest masses are most likely to reach super-Eddington luminosities at early times in their evolution. By assuming that the amount of stellar debris that can form into a compact accretion disc is set dynamically by the Eddington luminosity, we make a number of interesting and testable predictions about the observed properties of bright soft-state X-ray TDEs and optically bright, X-ray dim TDEs. We argue that TDEs around black holes of the lowest masses will expel the vast majority of their gravitationally bound debris into a radiatively driven outflow. A large-mass outflow will obscure the innermost X-ray producing regions, leading to a population of low black hole mass TDEs that are only observed at optical and UV energies. TDE discs evolving with bolometric luminosities comparable to their Eddington luminosity will have near constant (i.e. black hole mass independent) X-ray luminosities, of order L X, max ≡ LM ∼ 1043 − 1044 erg s−1. The range of luminosity values stems primarily from the range of allowed black hole spins. A similar X-ray luminosity limit exists for X-ray TDEs in the hard (Compton scattering dominated) state, and we therefore predict that the X-ray luminosity of the brightest X-ray TDEs will be at the scale LM (a) ∼ 1043 − 1044 erg s−1, independent of black hole mass and accretion state. These predictions are in strong agreement with the properties of the existing population (∼40 sources) of observed TDEs. [ABSTRACT FROM AUTHOR]

We report on a NuSTAR and Swift / XRT observation of the newly discovered X-ray transient MAXI J1820+070. Swift / XRT and NuSTAR have concurrently observed the newly detected source on 2018 March 14. We have simultaneously fitted the broad-band spectra obtained from Swift / XRT and NuSTAR. The observed joint spectra in the energy range 0.6–78.0 keV are well modeled with a weak disc black-body emission, dominant thermal Comptonization, and relativistic reflection fraction. We have detected a fluorescent Iron-Kα line relativistically broadened and a Compton hump at ∼30 keV. We constrain the inner disc radius as well as the disc inclination angle, and their values are found to be 4.1 |$^{+0.8}_{-0.6}$| R ISCO (where R ISCO ≡ radius of the innermost stable circular orbit) or 5.1 |$^{+1.0}_{-0.7}$| r g (where rg ≡ gravitational radius) and 29.8 |$^{+3.0}_{-2.7}$| °, respectively. The best-fitting broad-band spectra suggest that the source was in the hard state and evolving. The source emission is best described by weak thermal emission along with strong thermal Comptonization from a relatively cold, optically thick, geometrically thin and ionized accretion disc. X-ray spectral modeling helps us to understand the accretion and ejection properties in the vicinity of the compact object. [ABSTRACT FROM AUTHOR]