Reflection Spectra of Accretion Disks Illuminated by an Off-axis Corona

Abstract Relativistic reflection features in the X-ray spectra of accreting black holes are considered to be generated by the illumination of the accretion disk by the hot corona. In this work, we present a numerical method for the emission line profile and the reflection spectrum produced by an off-axis X-ray source. The X-ray source is considered as a point source, as in the lamppost scenario, except that it is located off-axis and moves at arbitrary velocity. The observed flux for the distant observer is calculated directly without priority evaluation of the emissivity on the accretion disk, which allows our model to be applicable to the point source that deviates from the axis of the black hole spins and moves with a velocity. To study the impact of the off-axis geometry on the measurement of source properties, we simulate observations for a black hole binary with NuSTAR and eXTP. We compare the simulation with the observation of the phase-resolved spectra of the low-frequency quasiperiodic oscillation observed by the Insight Hard X-ray Modulation Telescope. Due to the nonaxisymmetric illumination on the accretion disk, parameters of the model are not reproduced by the lamppost model, including the corona height, radial velocity, and the reflection fraction. On the other hand, all the model parameters are recovered through the off-axis model.