XRB continuum fitting with sensitive high-energy X-ray detectors

Abstract

The launch of the Nuclear Spectroscopic Telescope Array (NuSTAR) heralded a new era of sensitive high-energy X-ray spectroscopy for X-ray binaries (XRBs). In this paper we show how multiple physical parameters can be measured from the accretion disc spectrum when the high-energy side of the disc spectrum can be measured precisely using NuSTAR. This immediately makes two exciting developments possible. If the mass and distance of the source are known, the continuum fitting method can be used to calculate the spin and inner disc inclination independently of the iron line fitting method. If the mass and distance are unknown, the two methods can be combined to constrain these values to a narrow region of parameter space. In this paper we perform extensive simulations to establish the reliability of these techniques. We find that with high-quality spectra, spin and inclination can indeed be simultaneously measured using the disc spectrum. These measurements are much more precise at higher spin values, where the relativistic effects are stronger. The inclusion of a soft X-ray snapshot observation alongside the NuSTAR data significantly improves the reliability, particularly for lower temperature discs, as it gives a greatly improved measurement of the disc peak. High signal-to-noise data are not necessary for this, as measuring the peak temperature is relatively easy. We discuss the impact of systematic effects on this technique, and the implications of our results such as robust measurements of accretion disc warps and XRB mass surveys.