The two ultraluminous X-ray sources in the galaxy NGC 925
Abstract
NGC 925 ULX-1 and ULX-2 are two ultraluminous X-ray sources (ULXs) in the galaxy NGC 925, at a distance of 8.5 Mpc. For the first time, we analysed 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) × 1040 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' ULX. 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 with either the soft or the hard state of Galactic accreting black hole binaries.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- September 2018
- DOI:
- 10.1093/mnras/sty1766
- arXiv:
- arXiv:1806.11444
- Bibcode:
- 2018MNRAS.479.4271P
- Keywords:
-
- accretion;
- accretion discs;
- stars: neutron;
- galaxies: individuals: NGC 925;
- X-rays: binaries;
- X-rays: individual: NGC 925 ULX-1;
- NGC 925 ULX-2;
- Astrophysics - High Energy Astrophysical Phenomena;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- Accepted on MNRAS with very minor comments, 7 pages, 5 figures, 1 table