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

¹ European Space Astronomy Centre (ESAC), Science Operations Departement, 28692 Villanueva de la Cañada, Madrid, Spain
e-mail: felix.fuerst@sciops.esa.int
² Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
³ Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA
⁴ Université de Toulouse; UPS-OMP; IRAP, Toulouse, France
⁵ CNRS; IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse cedex 4, France
⁶ Department of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK
⁷ Raman Research Institute, C. V. Raman Avenue, Sadashivanagar, Bangalore 560080, India

Received: 24 April 2018
Accepted: 17 May 2018

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 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 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.