The Ultraluminous X-ray sources population of the galaxy NGC 7456

Pintore, F.; Marelli, M.; Salvaterra, R.; Israel, G.L.; Rodríguez Castillo, G.A.; Esposito, P.; Belfiore, A.; De Luca, A.; Wolter, A.; Mereghetti, S.; Stella, L.; Rigoselli, M.; Earnshaw, H.P.; Pinto, C.; Roberts, T.P.; Walton, D.J.; Bernardini, F.; Haberl, F.; Salvaggio, C.; Tiengo, A.; Zampieri, L.; Bachetti, M.; Brightman, M.; Casella, P.; D'Agostino, D.; Dall'Osso, S.; Fuerst, F.; Harrison, F.A.; Mapelli, M.; Papitto, A.; Middleton, M.

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.

Citation

Pintore, F., Marelli, M., Salvaterra, R., Israel, G., Rodríguez Castillo, G., Esposito, P., Belfiore, A., De Luca, A., Wolter, A., Mereghetti, S., Stella, L., Rigoselli, M., Earnshaw, H., Pinto, C., Roberts, T., Walton, D., Bernardini, F., Haberl, F., Salvaggio, C., Tiengo, A., …Middleton, M. (2020). The Ultraluminous X-ray sources population of the galaxy NGC 7456. Astrophysical Journal, 890(2), Article 166. https://doi.org/10.3847/1538-4357/ab6ffd

• Journal Article Type: Article
• Acceptance Date: Jan 20, 2020
• Online Publication Date: Feb 26, 2020
• Publication Date: Feb 20, 2020
• Deposit Date: Feb 28, 2020
• Publicly Available Date: Mar 15, 2020
• Journal: Astrophysical Journal
• Print ISSN: 0004-637X
• Electronic ISSN: 1538-4357
• Publisher: American Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 890
• Issue: 2
• Article Number: 166
• DOI: https://doi.org/10.3847/1538-4357/ab6ffd
• Public URL: https://durham-repository.worktribe.com/output/1269321
• Related Public URLs: https://arxiv.org/pdf/2001.08752.pdf