The two Ultraluminous X-ray sources in the galaxy NGC 925

TL;DR

This study analyzes two ultraluminous X-ray sources in galaxy NGC 925 using high-quality XMM-Newton and NuSTAR data, revealing ULX-1 as a super-Eddington stellar black hole and questioning the IMBH nature of ULX-2.

Contribution

First simultaneous high-quality X-ray analysis of NGC 925 ULXs, providing new spectral classifications and physical interpretations of their accretion states.

Findings

ULX-1 is a broadened disc ultraluminous X-ray source.

ULX-1 likely viewed at small inclination angles through a wind cone.

ULX-2's properties do not match typical IMBH or Galactic black hole states.

Abstract

NGC 925 ULX-1 and ULX-2 are two ultraluminous X-ray sources in the galaxy NGC 925, at a distance of 8.5 Mpc. For the first time, we analyzed high quality, simultaneous XMM-Newton and NuSTAR data of both sources. Although at a first glance ULX-1 resembles an intermediate mass black hole candidate (IMBH) because of its high X-ray luminosity ($(2$$-$$4)\times10^{40}$ erg s$^{-1}$) and its spectral/temporal features, a closer inspection shows that its properties are more similar to those of a typical super-Eddington accreting stellar black hole and we classify it as a `broadened disc' ultraluminous X-ray source. Based on the physical interpretation of this spectral state, we suggest that ULX-1 is seen at small inclination angles, possibly through the evacuated cone of a powerful wind originating in the accretion disc. The spectral classification of ULX-2 is less certain, but we disfavour an IMBH accreting at sub-Eddington rates as none of its spectral/temporal properties can be associated to either the soft or hard state of Galactic accreting black hole binaries.