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 3.4×1039 erg\ s−1 and lasted for about 8 months, after which the source went below a luminosity of 1037 erg\ s−1; previous \textit{Chandra} observations constrain the low-state luminosity below ∼ 2×1036 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 ∼3.4σ, and an associated spin-up of f˙=3.5×10−8 Hz.s−1. NGC 7793 is therefore the first galaxy to host more than one pulsating ULX.
@article{arxiv.2103.11650,
title = {A new candidate pulsating ULX in NGC 7793},
author = {Erwan Quintin and Natalie Webb and Andrés Gúrpide and Matteo Bachetti and Felix Fürst},
journal= {arXiv preprint arXiv:2103.11650},
year = {2021}
}