The (re)appearance of NGC 925 ULX-3, a new transient ULX

Abstract

We report the discovery of a third ULX in NGC 925 (ULX-3), detected in November 2017 by Chandra at a luminosity of $L_{\mathrm{X}}=(7.8\pm 0.8)\times 10^{39}$ erg s${}^{-1}$. Examination of archival data for NGC 925 reveals that ULX-3 was detected by Swift at a similarly high luminosity in 2011, as well as by XMM-Newton in January 2017 at a much lower luminosity of $L_{\mathrm{X}}=(3.8\pm 0.5)\times 10^{38}$ erg s${}^{-1}$. With an additional Chandra non-detection in 2005, this object demonstrates a high dynamic range of flux of factor >26. In its high-luminosity detections, ULX-3 exhibits a hard power-law spectrum with $\Gamma =1.6\pm 0.1$, whereas the XMM-Newton detection is slightly softer, with $\Gamma =1.8_{-0.1}^{+0.2}$ and also well-fitted with a broadened disc model. The long-term light curve is sparsely covered and could be consistent either with the propeller effect or with a large-amplitude superorbital period, both of which are seen in ULXs, in particular those with neutron star accretors. Further systematic monitoring of ULX-3 will allow us to determine the mechanism by which ULX-3 undergoes its extreme variability and to better understand the accretion processes of ULXs.

9 pages, 3 figures, 1 table. Accepted for publication in ApJ; updated to accepted version with minor revisions