Speaker
Description
Ultraluminous X-ray sources (ULXs) are an intriguing class of X-ray binaries, most of them observed in nearby galaxies, with luminosity > 10^39 erg/s. Observational evidences favor their interpretation as super-Eddington accretors. Swift/XRT has played a fundamental role in the characterization of ULXs long-term evolution, being the only active X-ray telescope, which allows regular and repeated visits of a source for a long period (e.g. a year). Thanks to Swift/XRT monitorings, it has been possible to discover and study a multiplicity of long-term variability behaviors in ULXs, such as super-orbital periodicities, flaring activity, bimodal flux distribution (e.g. Pintore et al. 2021, Salvaggio et al. 2022) and to discover new transient sources (e.g. Pintore et al. 2018, Robba et al. 2022). I will recap the long-term properties of ULXs, considering both persistent and transient sources, fundamental to understanding these elusive class of objects, focusing on the uniqueness of Swift/XRT for these kind of studies. I will show our results, among which the variability and spectral analysis of the ULXs in NGC925, containing one of the most luminous ULXs known (i.e. L > 10^40 erg/s) and a transient source with a long-term periodicity (Salvaggio et al. 2022).
