A bright ultraluminous X-ray source in NGC 5907

Abstract

We present a multimission X-ray analysis of a bright (peak observed 0.3–10 keV luminosity of ∼6 × 1040 erg s−1), but relatively highly absorbed ultraluminous X-ray source (ULX) in the edge-on spiral galaxy NGC 5907. The ULX is spectrally hard in X-rays (Γ ∼ 1.2–1.7, when fitted with an absorbed power law), and has a previously reported hard spectral break consistent with it being in the ultraluminous accretion state. It is also relatively highly absorbed for a ULX, with a column of ∼0.4–0.9 × 1022 atom cm−2 in addition to the line-of-sight column in our Galaxy. Although its X-ray spectra are well represented by accretion disc models, its variability characteristics argue against this interpretation. The ULX spectra instead appear dominated by a cool, optically thick Comptonizing corona. We discuss how the measured 9 per cent rms variability and a hardening of the spectrum as its flux diminishes might be reconciled with the effects of a very massive, radiatively driven wind and subtle changes in the corona, respectively. We speculate that the cool disc-like spectral component thought to be produced by the wind in other ULXs may be missing from the observed spectrum due to a combination of a low temperature (∼0.1 keV), and the high column to the ULX. We find no evidence, other than its extreme X-ray luminosity, for the presence of an intermediate mass black hole (MsBHs, ∼ 102−104 M⊙) in this object. Rather, the observations can be consistently explained by a massive (≳ 20 M⊙) stellar remnant black hole in a super-Eddington accretion state.