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.

中文翻译：

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NGC 7793 中的一个新候选脉冲 ULX

我们在此报告 NGC 7793 ULX-4 的发现，它是 NGC 7793 中一个新的瞬态超高光 X 射线源 (ULX)，该旋涡星系因拥有多个 ULX 而闻名。这个新源在 2012 年爆发，当时它被 XMM-Newton 和 Swift X 射线望远镜探测到。爆发达到了 3.4 × 1039 erg s-1 的峰值光度并持续了大约 8 个月，之后源的光度低于 1037 erg s-1；以前的钱德拉观测将低态光度限制在 ~2 × 1036 erg s-1 以下，这意味着变异性至少为 1000 倍。我们提出了四种可能的光学对应物，这些对应物在星系的档案 HST 观测中发现。在 XMM-Newton 信号中发现了 2.52 Hz 的脉动，其意义为 $\sim 3.4\、\sigma$，并且相关的自旋上升为 $\dot{f} = 3.5\times 10^{-8 }$ Hz s−1。