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 2017 November by Chandra at a luminosity of L_X = (7.8 ± 0.8) × 10³⁹ 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 2017 January at a much lower luminosity of L_X = (3.8 ± 0.5) × 10³⁸ erg s⁻¹. With an additional Chandra nondetection 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 Γ = 1.6 ± 0.1, whereas the XMM-Newton detection is slightly softer, with Γ = 1.8_(-0.1)^(+0.2), and is also well-fitted with a broadened disk 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.