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 \sim 10 ^ { -3 } Hz , from low frequency spectral index \alpha = 0.1 to high frequency spectral index \alpha = 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 \times 10 ^ { 4 } M _ { \odot } , 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 \mu Jy/beam .
Using the X-ray/radio fundamental plane , we calculate a black hole mass upper limit of 1.95 \times 10 ^ { 5 } M _ { \odot } .
Therefore , this ULX is consistent with being an IMBH accreting in the low/hard state .