The Ultraluminous X-Ray Sources Population of the Galaxy NGC 7456

Abstract

Ultraluminous X-ray sources (ULXs) are a class of accreting compact objects with X-ray luminosities above 10³⁹ erg s⁻¹. The ULX population counts several hundred objects but only a fraction are well studied. Here we present a detailed analysis of all ULXs hosted in the galaxy NGC 7456. It was observed in X-rays only once in the past (in 2005) by XMM-Newton. but the observation was short and strongly affected by high background. In 2018, we obtained a new, deeper (~90 ks) XMM-Newton observation that allowed us to perform a detailed characterization of the ULXs hosted in the galaxy. ULX-1 and ULX-2, the two brightest objects (L_X ~ 6−10 × 10³⁹ erg s⁻¹), have spectra that can be described by a model with two thermal components, as often found in ULXs. ULX-1 also shows one order of magnitude in flux variability on short-term timescales (hundreds to thousands of kiloseconds). The other sources (ULX-3 and ULX-4) show flux changes of at least an order of magnitude, and these objects may be candidate transient ULXs, although longer X-ray monitoring or further studies are required to ascribe them to the ULX population. In addition, we found a previously undetected source that might be a new candidate ULX (labeled as ULX-5), with a luminosity of ~10³⁹ erg s⁻¹ and hard power-law spectral shape, whose nature is still unclear and for which a background active galactic nucleus cannot be excluded. We discuss the properties of all the ULXs in NGC 7456 within the framework of super-Eddington accretion onto stellar-mass compact objects. Although no pulsations were detected, we cannot exclude that the sources host neutron stars.