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

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 ${\mathrm{T}}_{\mathrm{cool}}\sim 0.2$ 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 ${\mathrm{T}}_{\mathrm{hot}}\sim 2$ keV with a radial dependent profile $\mathrm{T}(\mathrm{r})\propto {\mathrm{r}}^{-0.5}$, 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 $\sim 6-10{\mathrm{M}}_{\odot }$ non-rotating black hole or the system hosts a moderately magnetized neutron star with $\mathrm{B}\lesssim 2\times 10^{11}$ 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.

10 pages, 4 figures, accepted for publication in ApJ

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