Constraint on the accretion of NGC 6946 X-1 using broadband X-ray data

TL;DR

This study uses broadband X-ray data to analyze NGC 6946 X-1, revealing evidence of super-critical accretion, a dual thermal disk structure, and constraining the nature of the compact object as either a black hole or neutron star.

Contribution

It provides detailed broadband spectral analysis of NGC 6946 X-1, supporting super-Eddington accretion and constraining the compact object's mass and magnetic field.

Findings

Evidence of optically thick wind indicating super-critical accretion

Presence of two thermal disk components with distinct temperatures

Estimated compact object mass consistent with a stellar-mass black hole or neutron star

Abstract

We analyze broadband X-ray data of NGC 6946 X-1 and probe plausible accretion scenarios in this ULX. NGC 6946 X-1 is a persistent soft source with broadband continuum spectra described by two thermal disk components. The cool accretion disk temperature $\rm T_{cool} \sim 0.2$ keV and the presence of $\sim 0.9$ keV emission/absorption broad feature suggests the evidence of optically thick wind due to super-critical accretion. The hot geometrically modified accretion disk has an inner temperature of $\rm T_{hot} \sim 2$ keV with a radial dependent profile $\rm T(r) \propto r^{-0.5}$, expected in a slim disk scenario. Further, the measurement based on a realistic inclination angle of the disk indicates that the mass of the host compact object is comparable to $\rm \sim 6-10 ~M_{\odot}$ non-rotating black hole or the system hosts a moderately magnetized neutron star with $\rm B \lesssim 2 \times 10^{11}$ G magnetic field. Overall, the detected spectral curvature, high luminosity, flux contribution from two thermal disk components, and estimated accretion rate imprint the super-Eddington accretion scenario.