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

## TL;DR

This paper demonstrates how high-energy X-ray data from NuSTAR enables precise measurement of black hole spin and disk inclination in X-ray binaries, even when key parameters are unknown, through extensive simulations and combined spectral analysis.

## Contribution

It introduces a method to measure spin and inclination from disk spectra using NuSTAR data, applicable with or without known source parameters, validated by extensive simulations.

## Key findings

- High-quality spectra allow simultaneous measurement of spin and inclination.
- Including soft X-ray data improves measurement reliability, especially for lower temperature disks.
- Systematic effects impact measurements, but the method remains robust for various applications.

## 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 disk spectrum when the high-energy side of the disk 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 disk 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 disk 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 disks, as it gives a greatly improved measurement of the disk 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 disk warps and XRB mass surveys.