A bright ultraluminous X-ray source in NGC 5907

Sutton, A.D.; Roberts, T.P.; Gladstone, J.C.; Farrell, S.A.; Reilly, E.; Goad, M.R.; Gehrels, N.

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.

Citation

Sutton, A., Roberts, T., Gladstone, J., Farrell, S., Reilly, E., Goad, M., & Gehrels, N. (2013). A bright ultraluminous X-ray source in NGC 5907. Monthly Notices of the Royal Astronomical Society, 434(2), 1702-1712. https://doi.org/10.1093/mnras/stt1133

• Journal Article Type: Article
• Publication Date: Sep 11, 2013
• Deposit Date: Sep 11, 2013
• Publicly Available Date: Jan 27, 2014
• Journal: Monthly Notices of the Royal Astronomical Society
• Print ISSN: 0035-8711
• Electronic ISSN: 1365-2966
• Publisher: Royal Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 434
• Issue: 2
• Pages: 1702-1712
• DOI: https://doi.org/10.1093/mnras/stt1133
• Keywords: Accretion discs, Black hole physics, X rays, Binaries, Galaxies.