Unveiling NGC 470 HLX-1: A Neutron Star in an HLX?

Ultraluminous X-ray sources (ULXs) are X-ray binaries with luminosities exceeding the Eddington limit of stellar-mass compact objects. Hyperluminous X-ray sources (HLXs), a rare subclass of ULXs with luminosities above 10^41 erg/s, are often considered prime candidates for hosting intermediate-mass black holes (IMBHs). However, recent discoveries of neutron stars in ULXs have heightened interest in uncovering the true nature of these sources. Identifying neutron star (NS) candidates in HLX systems is particularly crucial to revising our understanding of these enigmatic objects. In this context, we present a detailed broadband analysis of NGC 470 HLX-1 using simultaneous XMM-Newton and NuSTAR observations. Our findings challenge the traditional IMBH interpretation for HLXs, instead suggesting that this source harbors a stellar-mass compact object accreting at an exceptionally high Eddington ratio. Spectral modeling reveals parameters consistent with a neutron star accretor, positioning NGC 470 HLX-1 as a unique member of the HLX class. Additionally, we explore high-energy X-ray emission mechanisms, including synchrotron processes, and find the derived magnetic field strength and particle density to align with those of neutron stars in Galactic X-ray binaries. These findings redefine HLXs by demonstrating that they can host stellar-mass compact objects, thereby questioning the prevailing IMBH-centric paradigm. Furthermore, it offers valuable insights into the physics of extreme accretion and the diverse nature of compact objects. These results underscore the pivotal role of broadband X-ray observations in revealing the intrinsic properties of HLXs like NGC 470 HLX-1, paving the way for more comprehensive studies of similar systems.