Speaker
Description
While many ULXs are now known to host neutron star accretors, distinguishing between different accretor types remains challenging without the direct detection of pulsations. Here we present the first broadband (0.3-25 keV) spectroscopic analysis of IC5052 ULX using simultaneous XMM-Newton and NuSTAR observations. Our analysis reveals a remarkably hard spectrum compared to other ULXs studied in the broadband, with a mild high-energy rollover occurring above 10 keV rather than the typical ~8 keV seen in other sources. Single-component models fail to adequately describe the spectral shape. Although a two-component thermal model (diskbb + diskpbb) provides a statistically good fit, the resulting inner disc temperature (~6 keV) is physically problematic. We find that the high-energy excess can be modeled either with a cut-off powerlaw component that could represent emission from an accretion column (suggesting a neutron star accretor), or alternatively with a Comptonizing corona (potentially indicating a black hole accretor), with both scenarios providing statistically equivalent fits. The lack of detected pulsations places an upper limit of ~10% to the pulsed fraction of emission (at 0.3-10 keV), consistent with both interpretations. These findings place IC5052 ULX in a significant position within the ULX population due to its unusually hard spectral properties. We discuss implications for the accretion flow geometry under both neutron star and black hole scenarios, and how future observations might distinguish between these possibilities.
