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 Tcool ∼ 0.2 keV and the presence of a ∼0.9 keV emission/absorption broad feature suggest evidence of an optically thick wind due to supercritical accretion. The hot geometrically modified accretion disk has an inner temperature of Thot ∼ 2 keV with a radially 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 a ∼6–10 M⊙ nonrotating black hole or the system hosts a moderately magnetized neutron star with a B ≲ 2 × 1011 G magnetic field. Overall, the detected spectral curvature, high luminosity, flux contribution from two thermal disk components, and estimated accretion rate support the super-Eddington accretion scenario.