ULX-7 , in the northern spiral arm of M51 , demonstrates unusual behaviour for an ultraluminous X-ray source , 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 VLA .
Our X-ray spectral analysis shows that the source has a hard power-law spectral shape with a photon index \Gamma \sim 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 } \times 10 ^ { -3 } Hz , from a low-frequency spectral index of \alpha _ { 1 } = -0.1 ^ { +0.5 } _ { -0.2 } to a high-frequency spectral index of \alpha _ { 2 } = 0.65 ^ { +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 1.6 \times 10 ^ { 3 } M _ { \odot } .
Using the X-ray/radio fundamental plane we calculate another upper limit to the BH mass of 3.5 \times 10 ^ { 4 } M _ { \odot } 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 can not exclude other interpretations of this source ’ s interesting behaviour , most notably a neutron star with an extreme accretion rate .