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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.

A variable ULX and possible IMBH candidate in M51a Thumbnail


Hannah M. Earnshaw

Lucy M. Heil

Mar Mezcua

Dominic J. Walton

Fiona A. Harrison

George B. Lansbury

Matthew J. Middleton

Andrew D. Sutton


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.


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.

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
Keywords Accretion, Accretion discs, Stars: black holes, Black hole physics, Galaxies: individual: M51, X-rays: binaries, X-rays: individual: M51 ULX-7.
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Copyright Statement
This article has been accepted for publication in Monthly notices of the Royal Astronomical Society ©: 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

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