An intermediate‐mass black hole candidate in M51?

Abstract

AbstractWe 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 9.12×104 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 1.95×105 M®. Therefore, this ULX is consistent with being an IMBH accreting in the low/hard state. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)