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arXiv · 1805.05336

A tale of two periods: determination of the orbital ephemeris of the super-Eddington pulsar NGC 7793 P13

F. Fuerst·D. J. Walton·M. Heida·F. A. Harrison·D. Barret·M. Brightman·A. C. Fabian·M. J. Middleton·C. Pinto·V. Rana·F. Tramper·N. Webb·P. Kretschmar
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Abstract

We present a timing analysis of multiple XMM-Newton and NuSTAR observations of the ultra-luminous pulsar NGC 7793 P13 spread over its 65d variability period. We use the measured pulse periods to determine the orbital ephemeris, confirm a long orbital period with P_orb = 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 analyse 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.

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F. Fuerst, D. J. Walton, M. Heida, F. A. Harrison, D. Barret, M. Brightman, A. C. Fabian, M. J. Middleton, C. Pinto, V. Rana, F. Tramper, N. Webb, P. Kretschmar. 2018-05-14. A tale of two periods: determination of the orbital ephemeris of the super-Eddington pulsar NGC 7793 P13. https://doi.org/10.1051/0004-6361%2F201833292

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