Ultraluminous X-ray sources (ULXs) are a class of accreting compact objects with X-ray luminosities above 10^39 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 × 10^39 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.

A Robba, C Pinto, F Pintore, G Rodriguez, E Ambrosi, F Barra, et al. (2022). A transient ultraluminous X-ray source in NGC 55. MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 515(4), 4669-4674 [10.1093/mnras/stac2018].

A transient ultraluminous X-ray source in NGC 55

A Robba
;
F Barra;
2022-07-22

Abstract

Ultraluminous X-ray sources (ULXs) are a class of accreting compact objects with X-ray luminosities above 10^39 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 × 10^39 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.
Settore FIS/05 - Astronomia E Astrofisica
https://academic.oup.com/mnras/article/515/4/4669/6648842?login=true
A Robba, C Pinto, F Pintore, G Rodriguez, E Ambrosi, F Barra, et al. (2022). A transient ultraluminous X-ray source in NGC 55. MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 515(4), 4669-4674 [10.1093/mnras/stac2018].
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/10447/574326
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