A variable ULX and possible IMBH candidate in M51a

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)_(−1.1) × 10^(−3) Hz, from a low-frequency spectral index of ɑ_1 = 0.1^(+0.5)_(-0.2) to a high-frequency spectral index of ɑ_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 1.6x10^3 M_☉. Using the X-ray/radio fundamental plane we calculate another upper limit to the BH mass of 3.5x10^4 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.