Investigating the nature of the ultraluminous X-ray sources in the galaxy NGC 925

TL;DR

This study monitors ultraluminous X-ray sources in galaxy NGC 925, revealing flux variability, spectral states, and a potential 126-day periodicity, enhancing understanding of their accretion regimes and nature.

Contribution

It provides the first detailed variability and spectral analysis of ULXs in NGC 925, identifying a new periodicity and classifying their accretion states.

Findings

ULX-1 is highly luminous with weak spectral variability, in a super-Eddington regime.

ULX-2 and ULX-3 show flux variability and intermediate spectral states.

ULX-3 exhibits a ~126-day periodicity, possibly orbital or super-orbital.

Abstract

Variability is a powerful tool to investigate properties of X-ray binaries (XRB), in particular for Ultraluminous X-ray sources (ULXs) that are mainly detected in the X-ray band. For most ULXs the nature of the accretor is unknown, although a few ULXs have been confirmed to be accreting at super-Eddington rates onto a neutron star (NS). Monitoring these sources is particularly useful both to detect transients and to derive periodicities, linked to orbital and super-orbital modulations. Here we present the results of our monitoring campaign of the galaxy NGC 925, performed with the Neil Gehrels Swift Observatory. We also include archival and literature data obtained with Chandra, XMM-Newton and NuSTAR. We have studied spectra, light-curves and variability properties on days to months time-scales. All the three ULXs detected in this galaxy show flux variability. ULX-1 is one of the most luminous ULXs known, since only 10% of the ULXs exceed a luminosity of $\sim$5$\times$10$^{40}$ erg s$^{-1}$, but despite its high flux variability we found only weak spectral variability. We classify it as in a hard ultraluminous regime of super-Eddington accretion. ULX-2 and ULX-3 are less luminous but also variable in flux and possibly also in spectral shape. We classify them as in between the hard and the soft ultraluminous regimes. ULX-3 is a transient source: by applying a Lomb-Scargle algorithm we derive a periodicity of $\sim$ 126 d, which could be associated with an orbital or super-orbital origin.