A new candidate pulsating ULX in NGC 7793

TL;DR

This paper reports the discovery of a new transient ultraluminous X-ray source in NGC 7793, which exhibits pulsations indicating a neutron star, and discusses its properties, optical counterparts, and significance in galaxy ULX populations.

Contribution

The paper presents the first detection of a pulsating ULX in NGC 7793, with detailed analysis of its outburst, luminosity variability, and potential optical counterparts, expanding understanding of ULX diversity.

Findings

Detected a pulsation at 2.52 Hz with 3.4σ significance.

Observed luminosity variability factor of over 1000.

Identified multiple potential optical counterparts in archival data.

Abstract

We report here the discovery of NGC 7793 ULX-4, a new transient ultraluminous X-ray source (ULX) in NGC 7793, a spiral galaxy already well known for harbouring several ULXs. This new source underwent an outburst in 2012, when it was detected by \textit{XMM-Newton} and the \textit{Swift} X-ray telescope. The outburst reached a peak luminosity of 3.4$\times 10^{39}$ erg\ s$^{-1}$ and lasted for about 8 months, after which the source went below a luminosity of $10^{37}$ erg\ s$^{-1}$; previous \textit{Chandra} observations constrain the low-state luminosity below $\sim$ 2$\times 10^{36}$ erg\ s$^{-1}$, implying a variability of at least a factor 1000. We propose four possible optical counterparts, found in archival HST observations of the galaxy. A pulsation in the \textit{XMM-Newton} signal was found at 2.52 Hz, with a significance of $\sim3.4\,\sigma$, and an associated spin-up of $\dot{f} = 3.5\times10^{-8}$ Hz.s$^{-1}$. NGC 7793 is therefore the first galaxy to host more than one pulsating ULX.