NuSTAR and Swift observations of the very high state in GX 339-4: Weighing the black hole with X-rays

TL;DR

This study uses NuSTAR and Swift X-ray observations to measure the black hole's spin, inclination, mass, and distance in GX 339-4 during its very high state, employing relativistic reflection and continuum fitting methods.

Contribution

It introduces a novel approach by combining reflection and continuum fitting to simultaneously constrain multiple physical parameters of the black hole using only X-ray spectral data.

Findings

Black hole spin measured as a=0.95+0.02-0.08

Inclination of the inner disk is 30±1 degrees

Estimated black hole mass is 9.0+1.6-1.2 solar masses

Abstract

We present results from spectral fitting of the very high state of GX~339-4 with NuSTAR and Swift. We use relativistic reflection modelling to measure the spin of the black hole and inclination of the inner disk, and find a spin of $a=0.95^{+0.02}_{-0.08}$ and inclination of $30${\deg}$\pm1$ (statistical errors). These values agree well with previous results from reflection modelling. With the exceptional sensitivity of NuSTAR at the high-energy side of the disk spectrum, we are able to constrain multiple physical parameters simultaneously using continuum fitting. By using the constraints from reflection as input for the continuum fitting method, we invert the conventional fitting procedure to estimate the mass and distance of GX~339-4 using just the X-ray spectrum, finding a mass of $9.0^{+1.6}_{-1.2}M_\odot$ and distance of $8.4\pm0.9$ kpc (statistical errors).