Discovery of coherent pulsations from the Ultraluminous X-ray Source NGC 7793 P13

TL;DR

This paper reports the discovery of coherent pulsations from the ultraluminous X-ray source NGC 7793 P13, confirming it as an accreting neutron star with super-Eddington luminosity and providing insights into its magnetic field and emission mechanisms.

Contribution

First detection of pulsations in NGC 7793 P13, establishing it as a neutron star ULX and analyzing its spin-up and magnetic field properties.

Findings

Pulsations detected at ~0.42s in multiple datasets

Neutron star with luminosity ~1e40 erg/s, above Eddington limit

Persistent spin-up indicating magnetic field of ~1.5e12 G

Abstract

We report the detection of coherent pulsations from the ultraluminous X-ray source NGC 7793 P13. The ~0.42s 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 (>>5 sigma) detection of coherent pulsations demonstrates that the compact object in P13 is a neutron star with an observed peak luminosity of ~1e40 erg/s (assuming isotropy), well above the Eddington limit for a 1.4 M_sun accretor. This makes P13 the second ultraluminous X-ray source 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 ~ 1.5e12 G, typical of many 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.