A new pulsating neutron star in the ultraluminous X-ray source NGC 4559 X7?

Context. Ultraluminous X-ray sources (ULX) are extragalactic objects with observed X-ray luminosities largely above the Eddington limit for a 10 M-circle dot black hole. Currently, it is believed that ULXs host super-Eddington accreting neutron stars or stellar mass black holes. However, the exact proportion of the two populations of compact objects is not yet known. Aims. We investigate the properties of the ULX NGC 4559 X7 (hereafter X7), which shows flux variability up to a factor of five on both long (months to years) and short (hours to days) timescales. A flaring activity was also observed during the highest flux epochs of the source. Flares are unpredictable. They have different durations (but similar rising and decay times) and are all flat topped in flux. The latter suggests that at the flare peaks, there is likely a common switch-off mechanism for the accretion onto the compact object. Methods. We analysed all available XMM-Newton and Swift/XRT observations in order to fully investigate the spectral and temporal evolution of X7, looking for short- and long-term variability. We applied a Lomb-Scargle search to look for long-term periodicities. We also looked for coherent signals through accelerated searches that included orbital corrections. We described the X7 spectral properties with two thermal components plus a cut-off power-law model. Results. We found three well-defined spectral states where the spectral variability is mainly driven by the two harder components, with the thermal one clearly following a correlation between its temperature and luminosity. In addition, a pulsed signal at 2.6 s-2.7 s was detected in two XMM-Newton observations. The significance of these coherent signals is relatively weak, but they are found in two different observations with the same parameter space for the orbital properties. If confirmed, the pulsation would imply a high spin-down of 10(-9) s s(-1), which could be extreme amongst the known pulsating ULXs, and X7 would become a new extragalactic ULX pulsar. Conclusions. We discuss the spectral and temporal results of X7 in the context of super-Eddington accretion onto a stellar-mass compact object. In particular, we suggest that the source might likely host a neutron star.