Ultraluminous X-ray sources (ULXs) are extragalactic accreting compact objects, characterized by isotropic X-ray luminosities well above the Eddington limit for a 10 Msun black hole. Over the past decade, an unexpected population of pulsating ULXs (PULXs) has emerged, providing unique evidence for the existence of extremely super-Eddington accreting neutron stars and challenging our current understanding of accretion processes. Discovering new PULXs is crucial for unraveling the physical mechanisms at play under super-Eddington conditions and perform population studies. Pulsations in PULXs are transient, hence other neutron stars in ULXs might still be undiscovered. One approach to identify new PULX candidates is to look for ULXs with spectral and temporal properties resembling those of known PULXs. Most ULXs exhibit significant flux variations over time. However, a small subset — including PULXs — shows extreme variability (by orders of magnitude) on both short and long timescales, with their origin still under debate.
The galaxies NGC 4559 and NGC 7456 host a broad population of ULXs, with large flux variations in both short and long timescales. Long-term Swift/XRT monitoring and XMM-Newton observations caught their highly variable nature. A ULX in NGC 4559, spectrally very hard, showed also a hint of pulsation in two, well separated in time, datasets.
We will discuss the properties of this ULX sample in the context of super-Eddington accretion, aiming to distinguish whether the compact objects in these sources are neutron stars or black holes.
The chaos in highly variable ultraluminous X-ray sources