Constraint on 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 T cool ∼ 0.2 keV and the presence of ∼ 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 T hot ∼ 2 keV with a radial dependent profile T(r) ∝ 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 ∼ 6 − 10 M non-rotating black hole or the system hosts a weakly magnetized neutron star with B 2 × 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.