An intermediate-mass black hole candidate in M51?
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 9.12×104 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.
- Publication:
-
Astronomische Nachrichten
- Pub Date:
- May 2016
- DOI:
- 10.1002/asna.201612328
- arXiv:
- arXiv:1510.02277
- Bibcode:
- 2016AN....337..448E
- Keywords:
-
- accretion;
- accretion disks;
- black hole physics;
- galaxies: individual (M51);
- X-rays: binaries;
- X-rays: individuals (M51 ULX-7);
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 6 pages, 4 figures, conference proceedings from the 2015 XMM-Newton Science Workshop, to be published in Astronomical Notes (Astronomische Nachrichten)