The old and new ULXs of NGC 6946

Two recent observations of the nearby galaxy NGC 6946 with NuSTAR, the second 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, as well as a new appearance in the second, simultaneous observation which we call ULX-4. ULX-1 and ULX-2 have very steep power-law spectra with Γ ~ 3.6 in both cases and luminosities LX ∼ 5 × 1038 erg s-1. While not technically ULXs in this observation, their properties are consistent with being ultraluminous supersoft sources - super-Eddington accreting sources with the majority of their hard emission obscured and down-scattered. ULX-3 (also known as NGC 6946 X-1) is significantly detected by both XMM-Newton and NuSTAR, and has a power-law spectrum with Γ = 2.46 ± 0.07. Using the NuSTAR data we are tentatively able to prefer a ULX spectral model that turns over around ~10 keV to a straight power-law for this source. We also characterise the new source ULX-4, which is detected for the first time in the joint XMM-Newton and NuSTAR observation and is absent in a Chandra observation ten days later. It has a very hard spectrum, equally well described as a cut-off power-law with Γ = 0.7 ± 0.2 and E = 12+14-4 keV, or a hot multicolour disc blackbody with T = 4.5 ± 0.5 keV. We do not detect any pulsations from ULX-4, however its unusually hard spectrum and 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.