Fürst, F.Walton, D. J.Harrison, F. A.Stern, D.Barret, D.Brightman, M.Fabian, A. C.Grefenstette, B.Madsen, K. K.Middleton, M. J.Miller, J. M.Pottschmidt, KatjaPtak, A.Rana, V.Webb, N.2023-08-312023-08-312016-11-03Fürst, F., D. J. Walton, F. A. Harrison, D. Stern, D. Barret, M. Brightman, A. C. Fabian, et al. “Discovery of Coherent Pulsations from the Ultraluminous X-Ray Source NGC 7793 P13.” The Astrophysical Journal Letters 831, no. 2 (November 2016): L14. https://doi.org/10.3847/2041-8205/831/2/L14.https://doi.org/10.3847/2041-8205/831/2/L14http://hdl.handle.net/11603/29491We report the detection of coherent pulsations from the ultraluminous X-ray source (ULX) NGC 7793 P13. The ≈0.42 s nearly sinusoidal pulsations were initially discovered in broadband X-ray observations using XMM-Newton and NuSTAR taken in 2016. We subsequently also found pulsations in archival XMM-Newton data taken in 2013 and 2014. The significant (≫5σ) detection of coherent pulsations demonstrates that the compact object in P13 is a neutron star, and given the observed peak luminosity of ≈10⁴⁰ erg s⁻¹ (assuming isotropy), it is well above the Eddington limit for a 1.4 M⊙ accretor. This makes P13 the second ULX known to be powered by an accreting neutron star. The pulse period varies between epochs, with a slow but persistent spin-up over the 2013–2016 period. This spin-up indicates a magnetic field of B ≈ 1.5 × 10¹² G, typical of many Galactic accreting pulsars. The most likely explanation for the extreme luminosity is a high degree of beaming; however, this is difficult to reconcile with the sinusoidal pulse profile.6 pagesen-USThis work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.Public Domain Mark 1.0http://creativecommons.org/publicdomain/mark/1.0/Text