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Abstract  We present the current results of an investigation into M51 ULX‐7, using archival data from XMM‐ Newton ,  Chandra  and NuSTAR, and optical and radio data from HST and VLA. The source has a consistently hard power‐law X‐ray spectrum and high short‐term variability. This is unusual variability behaviour for a ULX, as we would expect highly variable ULXs to have soft energy spectra. The power spectrum features a break at ∼ 10 –3  Hz, from low frequency spectral index α = 0.1 to high frequency spectral index α = 0.8, analogous to the low frequency break found in power spectra of black holes accreting in the low/hard state. We do not observe a corresponding high frequency break, however taking the white noise level as a frequency lower limit of the break, we can calculate a black hole mass upper limit of 9.12×10 4  M, assuming that the ULX is in the low/hard state. While there is no radio detection, we find a flux density upper limit of 87 µJy/beam. Using the X‐ray/radio fundamental plane, we calculate a black hole mass upper limit of 1.95×10 5  M®. Therefore, this ULX is consistent with being an IMBH accreting in the low/hard state. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

An intermediate‐mass black hole candidate in M51?