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

Journal
Date Issued
2020
Author(s)
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Castillo, G. A. Rodríguez
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Esposito, P.
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Belfiore, A.
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Luca, A. De
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Rigoselli, M.
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Earnshaw, H. P.
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Roberts, T. P.
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Walton, D. J.
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Haberl, F.
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Salvaggio, C.
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Tiengo, A.
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Brightman, M.
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D’Agostino, D.
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Dall’Osso, S.
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Fürst, F.
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Harrison, F. A.
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Mapelli, M.
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Middleton, M.
DOI
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 (LX ∼ 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.
Volume
890
Issue
2
Start page
166
Uri
Url
Issn Identifier
0004-637X
Ads BibCode
2020ApJ...890..166P
Rights
open.access
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