An Intermediate-Mass Black Hole Candidate in M51?

TL;DR

This study investigates M51 ULX-7, suggesting it may host an intermediate-mass black hole based on X-ray spectral analysis, variability, and mass estimates from timing and radio data.

Contribution

It provides evidence supporting the presence of an intermediate-mass black hole in M51 ULX-7 through multi-wavelength analysis and timing properties.

Findings

Black hole mass upper limit of ~9.12 x 10^4 solar masses from power spectrum analysis.

Black hole mass upper limit of ~1.95 x 10^5 solar masses from X-ray/radio fundamental plane.

Source exhibits unusual variability for a ULX with a hard spectrum.

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 ~1e-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 9.12e4 solar masses, 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 micro-Jy/beam. Using the X-ray/radio fundamental plane, we calculate a black hole mass upper limit of 1.95e5 solar masses. Therefore, this ULX is consistent with being an IMBH accreting in the low/hard state.