Title:	A tale of two periods: determination of the orbital ephemeris of the super-Eddington pulsar NGC 7793 P13
Authors:	Furst, F Walton, D.J. Heida, M Harrison, F.A. Barret, D Brightman, M Fabian, A C Middleton, M J Pinto, C Rana, V Tramper, F Webb, N Kretschmar, P
Keywords:	stars neutron / X-rays binaries / techniques radial velocities / accretion / accretion disks / pulsars individual NGC 7793 P13
Issue Date:	Aug-2018
Publisher:	EDP Sciences for European Southern Observatory
Citation:	Astronomy & Astrophysics, 2018, Vol.616, p A186
Abstract:	We present a timing analysis of multiple XMM-Newton and NuSTAR observations of the ultra-luminous pulsar NGC 7793 P13 spread over its 65 d variability period. We use the measured pulse periods to determine the orbital ephemeris, confirm a long orbital period with Porb = 63.9+0.5−0.6 d, and find an eccentricity of e ≤ 0.15. The orbital signature is imprinted on top of a secular spin-up, which seems to get faster as the source becomes brighter. We also analyze data from dense monitoring of the source with Swift and find an optical photometric period of 63.9 ± 0.5 d and an X-ray flux period of 66.8 ± 0.4 d. The optical period is consistent with the orbital period, while the X-ray flux period is significantly longer. We discuss possible reasons for this discrepancy, which could be due to a super-orbital period caused by a precessing accretion disk or an orbital resonance. We put the orbital period of P13 into context with the orbital periods implied for two other ultra-luminous pulsars, M82 X-2 and NGC 5907 ULX, and discuss possible implications for the system parameters.
Description:	Restricted Access. An open-access version is available at arXiv.org (one of the alternative locations)
URI:	http://hdl.handle.net/2289/6999
ISSN:	0004-6361 1432-0746 (Online)
Alternative Location:	http://adsabs.harvard.edu/abs/2018A%26A...616A.186F https://arxiv.org/abs/1805.05336 https://doi.org/10.1051/0004-6361/201833292
Copyright:	2018 European Southern Observatory
Appears in Collections:	Research Papers (A&A)

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