A new candidate pulsating ULX in NGC 7793

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 XMM-Newton and the Swift X-ray telescope. The outburst reached a peak luminosity of 3.4 × 1039 erg s-1 and lasted for about eight months, after which the source went below a luminosity of 1037 erg s-1; previous 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 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.