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2016MNRAS.456.3840E - Mon. Not. R. Astron. Soc., 456, 3840-3854 (2016/March-2)

A variable ULX and possible IMBH candidate in M51a.

EARNSHAW H.M., ROBERTS T.P., HEIL L.M., MEZCUA M., WALTON D.J., DONE C., HARRISON F.A., LANSBURY G.B., MIDDLETON M.J. and SUTTON A.D.

Abstract (from CDS):

Ultraluminous X-ray source (ULX)-7, in the northern spiral arm of M51, demonstrates unusual behaviour for an ULX, with a hard X-ray spectrum but very high short-term variability. This suggests that it is not in a typical ultraluminous state. We analyse the source using archival data from XMM-Newton, Chandra and NuSTAR, and by examining optical and radio data from HST and Very Large Array. Our X-ray spectral analysis shows that the source has a hard power-law spectral shape with a photon index Γ ∼ 1.5, which persists despite the source's X-ray luminosity varying by over an order of magnitude. The power spectrum of the source features a break at 6.5^+0.5_-1.1x10–3 Hz, from a low-frequency spectral index of α_1=-0.1^+0.5_-0.2 to a high-frequency spectral index of α_2=6.5^+0.05_-0.14, making it analogous to the low-frequency break found in the power spectra of low/hard state black holes (BHs). We can take a lower frequency limit for a corresponding high-frequency break to calculate a BH mass upper limit of 1.6x103 M. Using the X-ray/radio Fundamental Plane, we calculate another upper limit to the BH mass of 3.5x104 M for a BH in the low/hard state. The hard spectrum, high rms variability and mass limits are consistent with ULX-7 being an intermediate-mass BH; however we cannot exclude other interpretations of this source's interesting behaviour, most notably a neutron star with an extreme accretion rate.