Caught in the act: a new transient ULX in NGC 6946

The nearby spiral galaxy NGC 6946 was recently observed twice with NuSTAR. The two observations were taken ten days apart, with the second observation simultaneous with an observation with XMM-Newton. In the second, simultaenous observation, we detect a new ultraluminous X-ray source (ULX) not detected in any previous observation of NGC 6946. Over the course of the XMM-Newton observation, its luminosity increases from 1.6 × 10³⁸ to 2.8 × 10³⁹ erg s^-1, undergoing flaring behaviour as it does so. A Chandra observation taken ten days after that fails to detect ULX-4, meaning that its flux decreased in that time by at least a factor of ~25 and the entire transient event lasted a maximum of 20 days (and more likely ~10 days). The spectrum of ULX-4 is very hard, and is equally well described as a cut-off power-law with Γ = 0.7 ± 0.1 and E_cut = 11⁺⁹_-4 keV or as a hot multicolour disc blackbody with T = 4.3^+0.6_-0.5 keV. Its hard spectrum and sudden changes in flux is similar to those exhibited by known ULX pulsars, which can be explained by the 'propeller mechanism'. However, since we only detect ULX-4 once and do not detect any pulsations which would unambiguously identify it as a neutron star, we also consider the possibility of a one-off transient event such as a micro-tidal disruption event of a low-mass star onto a stellar- or intermediate-mass black hole.