2016•Maryland Shared Open Access Repository (USMAI Consortium)Open access
DISCOVERY OF COHERENT PULSATIONS FROM THE ULTRALUMINOUS X-RAY SOURCE NGC 7793 P13
Felix S. Fürst, Dominic J. Walton, Fiona A. Harrison, Daniel Stern, D. Barret, Murray Brightman, A. C. Fabian, Brian W. Grefenstette, Kristin Kruse Madsen, Matthew J. Middleton, Jon M. Miller, K. Pottschmidt, Andrew F. Ptak, Vikram Rana, Natalie A. Webb
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Abstract
We 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.
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