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A transient ultraluminous X-ray source in NGC 55

Robba, A; Pinto, C; Pintore, F; Rodriguez, G; Ambrosi, E; Barra, F; Cusumano, G; D’Aì, A; Del Santo, M; Kosec, P; Marino, A; Middleton, M; Roberts, T; Salvaggio, C; Soria, R; Wolter, A; Walton, D

A transient ultraluminous X-ray source in NGC 55 Thumbnail


Authors

A Robba

C Pinto

F Pintore

G Rodriguez

E Ambrosi

F Barra

G Cusumano

A D’Aì

M Del Santo

P Kosec

A Marino

M Middleton

C Salvaggio

R Soria

A Wolter

D Walton



Abstract

Ultraluminous X-ray sources (ULXs) are a class of accreting compact objects with X-ray luminosities above 1039 erg s−1. The average number of ULXs per galaxy is still not well constrained, especially given the uncertainty on the fraction of ULX transients. Here, we report the identification of a new transient ULX in the galaxy NGC 55 (which we label as ULX-2), thanks to recent XMM-Newton and the Neil Gehrels Swift Observatory observations. This object was previously classified as a transient X-ray source with a luminosity around a few 1038 erg s−1 in a 2010 XMM-Newton observation. Thanks to new and deeper observations (∼ 130 ks each), we show that the source reaches a luminosity peak >1.6 × 1039 erg s−1. The X-ray spectrum of ULX-2 is much softer than in previous observations and fits in the class of soft ULXs. It can be well described using a model with two thermal components, as often found in ULXs. The time scales of the X-ray variability are of the order of a month and are likely driven by small changes in the accretion rate or due to super-orbital modulations, attributed to precession of the accretion disc, which is similar to other ULXs.

Citation

Robba, A., Pinto, C., Pintore, F., Rodriguez, G., Ambrosi, E., Barra, F., …Walton, D. (2022). A transient ultraluminous X-ray source in NGC 55. Monthly Notices of the Royal Astronomical Society, 515(4), 4669-4674. https://doi.org/10.1093/mnras/stac2018

Journal Article Type Article
Acceptance Date Jul 13, 2022
Online Publication Date Jul 22, 2022
Publication Date 2022-10
Deposit Date Aug 1, 2022
Publicly Available Date Aug 1, 2022
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 515
Issue 4
Pages 4669-4674
DOI https://doi.org/10.1093/mnras/stac2018
Related Public URLs https://doi.org/10.48550/arXiv.2207.09447

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Copyright Statement
This is a pre-copyedited, author-produced PDF of an article accepted for publication in Monthly Notices of the Royal Astronomical Society following peer review. The version of record [Robba, A, Pinto, C, Pintore, F, Rodriguez, G, Ambrosi, E, Barra, F, Cusumano, G, D’Aì, A, Del Santo, M, Kosec, P, Marino, A, Middleton, M, Roberts, T, Salvaggio, C, Soria, R, Wolter, A & Walton, D (2022). A transient ultraluminous X-ray source in NGC 55. Monthly Notices of the Royal Astronomical Society 515(4): 4669–4674.] is available online at: https://doi.org/10.1093/mnras/stac2018






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