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

Fürst, F. and Walton, D. J. and Heida, M. and Bachetti, M. and Pinto, C. and Middleton, M. J. and Brightman, M. and Earnshaw, H. P. and Barret, D. and Fabian, A. C. and Kretschmar, P. and Pottschmidt, K. and Ptak, A. and Roberts, T. and Stern, D. and Webb, N. and Wilms, J. (2021) Long-term pulse period evolution of the ultra-luminous X-ray pulsar NGC 7793 P13. Astronomy and Astrophysics, 651 . Art. No. A75. ISSN 0004-6361. doi:10.1051/0004-6361/202140625.

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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.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Fürst, F.0000-0003-0388-0560
Walton, D. J.0000-0001-5819-3552
Heida, M.0000-0002-1082-7496
Bachetti, M.0000-0002-4576-9337
Pinto, C.0000-0003-2532-7379
Middleton, M. J.0000-0002-8183-2970
Brightman, M.0000-0002-8147-2602
Earnshaw, H. P.0000-0001-5857-5622
Barret, D.0000-0002-0393-9190
Fabian, A. C.0000-0002-9378-4072
Kretschmar, P.0000-0001-9840-2048
Pottschmidt, K.0000-0002-4656-6881
Ptak, A.0000-0001-5655-1440
Roberts, T.0000-0001-8252-6337
Stern, D.0000-0003-2686-9241
Wilms, J.0000-0003-2065-5410
Additional Information:© ESO 2021. Article published by EDP Sciences. Received 22 February 2021; Accepted 28 April 2021; Published online 16 July 2021. We would like the thank the referee for the very useful comments that helped to improve the manuscript. DJW and MJM acknowledge support from STFC Ernest Rutherford fellowships. This research has made use of data obtained with NuSTAR, a project led by Caltech, funded by NASA and managed by NASA/JPL, and has utilized the nustardas software package, jointly developed by the ASDC (Italy) and Caltech (USA). This research has also made use of data obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester, and also made use of the XRT Data Analysis Software (XRTDAS) developed under the responsibility of the ASI Science Data Center (ASDC), Italy. This research has made use of a collection of ISIS functions (ISISscripts) provided by ECAP/Remeis observatory and MIT ( The material is based upon work supported by NASA under award number 80GSFC17M0002.
Group:NuSTAR, Space Radiation Laboratory
Funding AgencyGrant Number
Science and Technology Facilities Council (STFC)UNSPECIFIED
Subject Keywords:accretion, accretion disks – X-rays: binaries – stars: neutron – pulsars: individual: NGC7793 P13
Record Number:CaltechAUTHORS:20210520-150032176
Persistent URL:
Official Citation:Long-term pulse period evolution of the ultra-luminous X-ray pulsar NGC 7793 P13. F. Fürst, D. J. Walton, M. Heida, M. Bachetti, C. Pinto, M. J. Middleton, M. Brightman, H. P. Earnshaw, D. Barret, A. C. Fabian, P. Kretschmar, K. Pottschmidt, A. Ptak, T. Roberts, D. Stern, N. Webb and J. Wilms. A&A, 651 (2021) A75; DOI:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:109230
Deposited By: George Porter
Deposited On:21 May 2021 14:17
Last Modified:28 Jul 2021 17:53

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