Speaker
Description
Ultraluminous X-ray sources (ULXs) are among the most luminous non-nuclear X-ray emitters in nearby galaxies, with luminosities exceeding the Eddington limit expected for stellar-mass black holes. They were historically considered promising candidates for hosting intermediate-mass black holes (IMBHs). However, the discovery of coherent pulsations in several ULXs, together with detailed broadband X-ray observations, has demonstrated that many of these systems are powered by neutron stars undergoing super-Eddington accretion. These findings have significantly reshaped our understanding of extreme accretion and radiation processes around compact objects.
Hyperluminous X-ray sources (HLXs), characterized by luminosities above 1e41 erg/sec, have long been regarded as the strongest observational candidates for IMBHs. Nevertheless, the possibility that neutron stars may also power some HLXs challenges this interpretation and motivates renewed investigation into the physical mechanisms driving these sources.
Here, we examine the spectral properties of ULXs and HLXs using a physically motivated model that considers neutron stars as the central accretors. In this framework, the observed emission is described by a combination of soft thermal radiation and high-energy synchrotron emission originating within a strongly magnetized neutron-star magnetosphere. The resulting spectral features are consistent with an accreting neutron-star scenario, while the particle acceleration processes responsible for potential non-thermal emission remain an active area of research.
Future missions such as NewAthena will play a transformative role in this field. Its large collecting area and high-resolution spectroscopy will enable detailed studies of faint spectral features, improved characterization of outflows and accretion environments, and the detection of fainter ULX populations, providing crucial insights into the nature of these extreme sources.
