A variable ULX and possible IMBH candidate in M51a

Earnshaw, Hannah M.; Roberts, Timothy P.; Heil, Lucy M.; Mezcua, Mar; Walton, Dominic J.; Done, Chris; Harrison, Fiona A.; Lansbury, George B.; Middleton, Matthew J.; Sutton, Andrew D.

Abstract

Ultraluminous X-ray source (ULX)-7, in the northern spiral arm of M51, demonstrates unusual behaviour for an ULX, 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 Very Large Array. Our X-ray spectral analysis shows that the source has a hard power-law spectral shape with a photon index Γ ∼ 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+0.5−1.16.5−1.1+0.5 × 10−3 Hz, from a low-frequency spectral index of α1=−0.1+0.5−0.2α1=−0.1−0.2+0.5 to a high-frequency spectral index of α2=6.5+0.05−0.14α2=6.5−0.14+0.05, 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 × 103 M⊙. Using the X-ray/radio Fundamental Plane, we calculate another upper limit to the BH mass of 3.5 × 104 M⊙ 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 cannot exclude other interpretations of this source's interesting behaviour, most notably a neutron star with an extreme accretion rate.

Citation

Earnshaw, H. M., Roberts, T. P., Heil, L. M., Mezcua, M., Walton, D. J., Done, C., …Sutton, A. D. (2016). A variable ULX and possible IMBH candidate in M51a. Monthly Notices of the Royal Astronomical Society, 456(4), 3840-3854. https://doi.org/10.1093/mnras/stv2945

• Journal Article Type: Article
• Acceptance Date: Dec 12, 2015
• Online Publication Date: Jan 14, 2016
• Publication Date: Mar 11, 2016
• Deposit Date: Jan 15, 2016
• Publicly Available Date: Jan 25, 2016
• Journal: Monthly Notices of the Royal Astronomical Society
• Print ISSN: 0035-8711
• Electronic ISSN: 1365-2966
• Publisher: Royal Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 456
• Issue: 4
• Pages: 3840-3854
• DOI: https://doi.org/10.1093/mnras/stv2945
• Keywords: Accretion, Accretion discs, Stars: black holes, Black hole physics, Galaxies: individual: M51, X-rays: binaries, X-rays: individual: M51 ULX-7.
• Public URL: https://durham-repository.worktribe.com/output/1394687
• Related Public URLs: http://arxiv.org/abs/1512.04825