A new candidate pulsating ULX in NGC 7793

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 $\mathit{\text{XMM-Newton}}$ and the $\mathit{\text{Swift}}$ 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 $\mathit{\text{Chandra}}$ 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 $\mathit{\text{XMM-Newton}}$ signal was found at 2.52 Hz, with a significance of $\sim 3.4\sigma$, and an associated spin-up of $\dot{f}=3.5\times {10}^{-8}$ Hz.s${}^{-1}$. NGC 7793 is therefore the first galaxy to host more than one pulsating ULX.