Long-term pulse period evolution of the ultra-luminous X-ray pulsar NGC 7793 P13

¹ Quasar Science Resources SL for ESA, European Space Astronomy Centre (ESAC), Science Operations Department, 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 European Southern Observatory, Garching, Germany
⁴ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius, Italy
⁵ INAF – IASF Palermo, Via U. La Malfa 153, 90146 Palermo, Italy
⁶ Department of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK
⁷ Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA
⁸ CNRS, IRAP, 9 Av. Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
⁹ European Space Astronomy Centre (ESAC), Science Operations Department, 28692 Villanueva de la Cañada, Madrid, Spain
¹⁰ CRESST, Department of Physics, and Center for Space Science and Technology, UMBC, Baltimore, MD 21250, USA
¹¹ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹² Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
¹³ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
¹⁴ Dr. Karl-Remeis-Sternwarte and ECAP, Sternwartstr. 7, 96049 Bamberg, Germany

Received: 22 February 2021
Accepted: 28 April 2021

Abstract

Ultra-luminous X-ray pulsars (ULXPs) provide a unique opportunity to study persistent super-Eddington accretion. Here we present the results of a long-term monitoring campaign of ULXP NGC 7793 P13, focusing on the pulse period evolution and the determination of the orbital ephemeris. Over our four year monitoring campaign with Swift, XMM-Newton, and NuSTAR, we measured a continuous spin-up with an average value of Ṗ ≈ −3.8 × 10⁻¹¹ s s⁻¹. We find that the strength of the spin-up is independent of the observed X-ray flux, indicating that despite a drop in observed flux in 2019, accretion onto the source has continued at largely similar rates. The source entered an apparent off-state in early 2020, which might have resulted in a change in the accretion geometry as no pulsations were found in observations in July and August 2020. We used the long-term monitoring to update the orbital ephemeris, as well as the periodicities seen in both the observed optical and UV magnitudes and the X-ray fluxes. We find that the optical and UV period is very stable over the years, with P_UV = 63.75_−0.12^+0.17 d. The best-fit orbital period determined from our X-ray timing results is 64.86 ± 0.19 d, which is almost a day longer than previously implied, and the X-ray flux period is 65.21 ± 0.15 d, which is slightly shorter than previously measured. The physical origin of these different flux periods is currently unknown. We study the hardness ratio of the XMM-Newton and NuSTAR data between 2013−2020 to search for indications of spectral changes. We find that the hardness ratios at high energies are very stable and not directly correlated with the observed flux. At lower energies we observe a small hardening with increased flux, which might indicate increased obscuration through outflows at higher luminosities. Comparing the changes in flux with the observed pulsed fraction, we find that the pulsed fraction is significantly higher at low fluxes. This seems to imply that the accretion geometry already changed before the source entered the deep off-state. We discuss possible scenarios to explain this behavior, which is likely driven by a precessing accretion disk.