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 10 39 erg s −1 . The ULX population counts several hundreds objects but only a minor fraction is 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 × 10 39 erg s −1 ), have spectra that can be described by a two-thermal component model as often found in ULXs. ULX-1 shows also one order of magnitude in flux variability on short-term timescales (hundreds to thousand ks). 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 (labelled as ULX-5) with a luminosity of ∼ 10 39 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.

A multimission catalogue of ultraluminous X-ray source candidates

“…This is a key consideration here, since the latter are now being detected in increasing numbers as our X-ray archives continue to grow (e.g. Middleton et al 2012Middleton et al , 2013Soria et al 2012;Pintore et al 2018Pintore et al , 2020Earnshaw et al 2019aEarnshaw et al , 2020van Haaften et al 2019;Brightman et al 2020b;Walton et al 2021), and may be of particular interest in the context of identifying good ULX pulsar candidates (e.g. Tsygankov et al 2016;Earnshaw, Roberts & Sathyaprakash 2018;Song et al 2020).…”

A new transient ultraluminous X-ray source in NGC 7090

“…This is the latest member of the growing population of transient ULXs (e.g. Soria et al 2012;Middleton et al 2012Middleton et al , 2013Pintore et al 2018Pintore et al , 2020Earnshaw et al 2019aEarnshaw et al , 2020Brightman et al 2020). Remarkably, ULX3 is the third such source in the galaxy NGC 7090 alone (e.g.…”

“…Furthermore, as the archive of X-ray data continues to grow, and along with it the number of nearby galaxies with multiple observing epochs, an increasing number of new/transient ULXs are being reported (e.g. Soria et al 2012;Middleton et al 2012Middleton et al , 2013Pintore et al 2018Pintore et al , 2020Earnshaw et al 2019aEarnshaw et al , 2020van Haaften et al 2019;Brightman et al 2020). The high-amplitude long-term variability Although this is distinct behaviour from the known ULX pulsars, which spend extended periods at highly super-Eddington luminosities, these may be interesting objects in terms of connecting ULX pulsars to the sub-Eddington X-ray pulsar population.…”

The slow heartbeats of an ultra-luminous X-ray source in NGC 3621

“…Such marked short time-scale variability is clearly at odds with most of the ULXs, where only a small sample of sources is highly variable on time-scales shorter than a few hours (e.g. Earnshaw & Roberts 2017;Pintore et al 2020). 4X J1118 can be classified as broadeneddisc or hard-ultraluminous source (Sutton et al 2013a), and, by showing both relatively hard spectra and obvious variability, may depart from the common variable- when-softer connection observed in other ULXs (Sutton et al 2013b;Middleton et al 2015b).…”