A transient ultraluminous X-ray source in NGC 55

TL;DR

This paper reports the discovery and analysis of a new transient ultraluminous X-ray source in NGC 55, showing significant luminosity increase, spectral softening, and variability likely due to accretion rate changes or disc precession.

Contribution

The study identifies a new transient ULX in NGC 55 and characterizes its spectral and temporal properties with deep X-ray observations, providing insights into ULX behavior and variability mechanisms.

Findings

ULX-2 reached a luminosity >1.6 x 10^{39} erg/s

The source exhibits a soft X-ray spectrum with two thermal components

X-ray variability occurs on timescales of about a month

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 10$^{38}$ erg s$^{-1}$ in a 2010 XMM-Newton observation. Thanks to new and deeper observations ($\sim$ 130 ks each), we show that the source reaches a luminosity peak $>1.6 \times 10^{39}$ 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.