A Spectroscopic Angle on Central Engines in Accreting Neutron Stars

High-resolution spectroscopic studies of absorbing disk winds and atmospheres in low-mass X-ray binaries (LMXBs) have been key in advancing our understanding of these systems and the accretion process, in general. These analyses, however, do not account for an important geometric effect expected to be present and detectable in the data: the contribution of a small component of the absorber’s orbital motion to the velocity width of absorption lines due to the relative size of the central emitting region (hereafter, central engine). We developed a new method to constrain the size of the central engine in accreting compact objects by quantifying the magnitude of this effect. We applied this method on the Chandra/HETG spectra of three ultra-compact and short period neutron star X-ray binaries in which evidence of gravitationally redshifted absorption, owing to an inner-disk atmosphere, has recently been reported. For our most sensitive spectrum (XTE J1710-281), we obtain a 1-sigma upper bound of 310 km/s on the magnitude of this geometric effect and a central engine of size < 60 GM/c². These initial constraints compare favorably to those obtained via microlensing in quasars and approach the sensitivity of constraints via relativistic reflection in neutron stars. This sensitivity will increase with further exposures, as well as the launch of future microcalorimeter and grating missions.