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A variable ULX and possible IMBH candidate in M51a.

Earnshaw, Hannah M. and Roberts, Timothy P. and Heil, Lucy M. and Mezcua, Mar and Walton, Dominic J. and Done, Chris and Harrison, Fiona A. and Lansbury, George B. and Middleton, Matthew J. and Sutton, Andrew D. (2016) 'A variable ULX and possible IMBH candidate in M51a.', Monthly notices of the Royal Astronomical Society., 456 (4). pp. 3840-3854.


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.

Item Type:Article
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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Publisher 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.
Date accepted:12 December 2015
Date deposited:25 January 2016
Date of first online publication:14 January 2016
Date first made open access:25 January 2016

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