Welcome to the new version of CaltechAUTHORS. Login is currently restricted to library staff. If you notice any issues, please email coda@library.caltech.edu
Published March 11, 2016 | Published + Submitted
Journal Article Open

A variable ULX and possible IMBH candidate in M51a


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)_(−1.1) × 10^(−3) Hz, from a low-frequency spectral index of ɑ_1 = 0.1^(+0.5)_(-0.2) to a high-frequency spectral index of ɑ_2 = 6.5^(+0.05)_(−0.14), 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 1.6x10^3 M_☉. Using the X-ray/radio fundamental plane we calculate another upper limit to the BH mass of 3.5x10^4 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.

Additional Information

© 2016 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2015 December 12. Received 2015 November 17. First published online January 14, 2016. We gratefully acknowledge support from the Science and Technology Facilities Council (HE through grant ST/K501979/1 and TR as part of consolidated grant ST/L00075X/1) and from NASA (MM through Chandra Grant G05-16099X). AS is supported by an appointment to the NASA Postdoctoral Program at Marshall Space Flight Center, administered by Oak Ridge Associated Universities through a contract with NASA. The scientific results reported in this paper are based on data obtained from the Chandra Data Archive, and on archival observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. This research has also made use of data obtained with NuSTAR, a project led by Caltech, funded by NASA and managed by NASA/JPL, and has utilized the NUSTARDAS software package, jointly developed by the ASDC (Italy) and Caltech (USA). Further results are based on observations made with the NASA/ESA Hubble Space Telescope, and obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA) and the Canadian Astronomy Data Centre (CADC/NRC/CSA); and the National Radio Astronomy Observatory, which is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

Attached Files

Submitted - 1512.04825v1.pdf

Published - MNRAS-2016-Earnshaw-3840-54.pdf


Files (8.9 MB)
Name Size Download all
2.7 MB Preview Download
6.2 MB Preview Download

Additional details

August 20, 2023
August 20, 2023