A Broadband Look at the Old and New ULXs of NGC 6946

Abstract

Two recent observations of the nearby galaxy NGC 6946 with NuSTAR, one simultaneous with an XMM-Newton observation, provide an opportunity to examine its population of bright accreting sources from a broadband perspective. We study the three known ultraluminous X-ray sources (ULXs) in the galaxy, and find that ULX-1 and ULX-2 have very steep power-law spectra with {{Γ }}={3.6}_-0.3^+0.4 in both cases. Their properties are consistent with being super-Eddington accreting sources with the majority of their hard emission obscured and down-scattered. ULX-3 (NGC 6946 X-1) is significantly detected by both XMM-Newton and NuSTAR at L _X = (6.5 ± 0.1) × 10³⁹ erg s^-1, and has a power-law spectrum with Γ = 2.51 ± 0.05. We are unable to identify a high-energy break in its spectrum like that found in other ULXs, but the soft spectrum likely hinders our ability to detect one. We also characterize the new source, ULX-4, which is only detected in the joint XMM-Newton and NuSTAR observation, at L _X = (2.27 ± 0.07) × 10³⁹ erg s^-1, and is absent in a Chandra observation 10 days later. It has a very hard cutoff power-law spectrum with Γ = 0.7 ± 0.1 and {E}_cut}={11}_-4⁺⁹ keV. We do not detect pulsations from ULX-4, but its transient nature can be explained either as a neutron star ULX briefly leaving the propeller regime or as a micro-tidal disruption event induced by a stellar-mass compact object.