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Title: Demonstrating the likely neutron star nature of five M31 globular cluster sources with Swift-NuSTAR spectroscopy
Authors: Maccarone, T. J.
Yukita, M.
Hornschemeier, A.
Lehmer, B. D.
Antoniou, V.
Ptak, A.
Wik, D. R.
Zezas, A.
Boyd, P.
Kennea, J.
Page, Kim
Eracleous, M.
Williams, B. F.
Boggs, S. E.
Christensen, F. E.
Craig, W. W.
Hailey, C. J.
Harrison, F.
Stern, D.
Zhang, W. W.
First Published: 7-Mar-2016
Publisher: Oxford University Press (OUP)
Citation: Monthly Notices of the Royal Astronomical Society, 2016, 458 (4): 3633-3643.
Abstract: We present the results of a joint Swift-NuSTAR spectroscopy campaign on M31. We focus on the five brightest globular cluster X-ray sources in our fields. Two of these had previously been argued to be black hole candidates on the basis of apparent hard-state spectra at luminosities above those for which neutron stars are in hard states. We show that these two sources are likely to be Z-sources (i.e. low magnetic field neutron stars accreting near their Eddington limits), or perhaps bright atoll sources (low magnetic field neutron stars which are just a bit fainter than this level) on the basis of simultaneous Swift and NuSTAR spectra which cover a broader range of energies. These new observations reveal spectral curvature above 6-8 keV that would be hard to detect without the broader energy coverage the NuSTAR data provide relative to Chandra and XMM-Newton. We show that the other three sources are also likely to be bright neutron star X-ray binaries, rather than black hole X-ray binaries. We discuss why it should already have been realized that it was unlikely that these objects were black holes on the basis of their being persistent sources, and we re-examine past work which suggested that tidal capture products would be persistently bright X-ray emitters. We discuss how this problem is likely due to neglecting disk winds in older work that predict which systems will be persistent and which will be transient.
DOI Link: 10.1093/mnras/stw530
ISSN: 0035-8711
eISSN: 1365-2966
Version: Publisher Version
Status: Peer-reviewed
Type: Journal Article
Rights: Copyright © 2016, The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. The file associated with this record is distributed under the Creative Commons “Attribution Non-Commercial No Derivatives” licence, further details of which can be found via the following link:
Description: 12 pages, 3 figures
Appears in Collections:Published Articles, Dept. of Physics and Astronomy

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