A transient ultraluminous X-ray source in NGC 55
Date Issued
2022
Author(s)
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Kosec, P
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Middleton, M
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Roberts, T
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Walton, D
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.
Volume
515
Issue
4
Start page
4669
Issn Identifier
0035-8711
Ads BibCode
2022MNRAS.515.4669R
Rights
open.access
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