Title:On the thermal line emission from the outflows in ultraluminous X-ray sources

Abstract:The atomic features in the X-ray spectra of the ultraluminous X-ray source (ULX) may be associated with the outflow (Middleton et al. 2015), which may provide a way to explore the physics of the ULXs. We construct a conical outflow model, and calculate the thermal X-ray Fe emission lines from the outflows. Our results show that thermal line luminosity decreases with increasing outflow velocity or/and opening angle of the outflow for a fixed kinetic power of the outflows. Assuming the kinetic power of the outflows to be comparable with the accretion power in the ULXs, we find that the equivalent width can be several eV for the thermal X-ray Fe emission line from the outflows in the ULXs with stellar mass black holes. The thermal line luminosity is proportional to 1/M (M is the black hole mass of the ULX). The equivalent width decreases with the black hole mass, which implies that the Fe line emission from the outflows can hardly be detected if the ULXs contain intermediate mass black holes. Our results suggest that the thermal X-ray Fe line emission should be preferentially be detected in the ULXs with high kinetic power slowly moving outflows from the accretion disks surrounding stellar mass black holes/neutron stars. The recent observed X-ray atomic features of the outflows in the ULX may imply that it contains a stellar mass black hole/neutron star (Pinto et al. 2016).