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dc.contributor.authorWang, S.
dc.contributor.authorSoria, Roberto
dc.contributor.authorUrquhart, Ryan
dc.contributor.authorLiu, J.
dc.identifier.citationWang, S. and Soria, R. and Urquhart, R. and Liu, J. 2018. Discovery of two eclipsing X-ray binaries in M51. Monthly Notices of the Royal Astronomical Society. 477 (3): pp. 3623-3645.

We discovered eclipses and dips in two luminous (and highly variable) X-ray sources in M 51. One (CXOM51 J132943.3+471135) is an ultraluminous supersoft source, with a thermal spectrum at a temperature of about 0.1 keV and characteristic blackbody radius of about 104 km. The other (CXOM51 J132946.1+471042) has a two-component spectrum with additional thermal-plasma emission; it approached an X-ray luminosity of 1039 erg s−1 during outbursts in 2005 and 2012. From the timing of three eclipses in a series of Chandra observations, we determine the binary period (52.75 ± 0.63 h) and eclipse fraction (22±0.1 per cent ) of CXOM51 J132946.1+471042. We also identify a blue optical counterpart in archival Hubble Space Telescope images, consistent with a massive donor star (mass of ∼20–35 M⊙). By combining the X-ray light-curve parameters with the optical constraints on the donor star, we show that the mass ratio in the system must be M2 /M1 >~ 18 and therefore the compact object is most likely a neutron star (exceeding its Eddington limit in outburst). The general significance of our result is that we illustrate one method (applicable to high-inclination sources) of identifying luminous neutron star X-ray binaries, in the absence of X-ray pulsations or phase-resolved optical spectroscopy. Finally, we discuss the different X-ray spectral appearance expected from super-Eddington neutron stars and black holes at high viewing angles.

dc.publisherOxford University Press
dc.titleDiscovery of two eclipsing X-ray binaries in M51
dc.typeJournal Article
dcterms.source.titleMonthly Notices of the Royal Astronomical Society

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

curtin.departmentCurtin Institute of Radio Astronomy (Physics)
curtin.accessStatusOpen access

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