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

TL;DR

This paper reports the discovery and analysis of ULX-3 in NGC 925, a transient ultraluminous X-ray source exhibiting extreme variability, with implications for understanding accretion processes in ULXs.

Contribution

It presents the first detailed long-term observational study of ULX-3, revealing its transient nature and spectral properties, and discusses possible mechanisms behind its variability.

Findings

ULX-3 shows flux variability factor >26 over years.

Spectral analysis indicates a hard power-law spectrum in high states.

Long-term behavior may involve propeller effect or superorbital periods.

Abstract

We report the discovery of a third ULX in NGC 925 (ULX-3), detected in November 2017 by Chandra at a luminosity of $L_{\rm X} = (7.8\pm0.8)\times10^{39}$ 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 $L_{\rm X} = (3.8\pm0.5)\times10^{38}$ 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 $\Gamma=1.6\pm0.1$, whereas the XMM-Newton detection is slightly softer, with $\Gamma=1.8^{+0.2}_{-0.1}$ 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.