Discovery of a 0.42-s pulsar in the ultraluminous X-ray source NGC 7793 P13

TL;DR

This paper reports the discovery of a 0.42-second pulsar in the ultraluminous X-ray source NGC 7793 P13, confirming the neutron star nature of the compact object and revealing super-Eddington accretion.

Contribution

It presents the first detection of pulsations in NGC 7793 P13, establishing it as a neutron star ULX with a multipolar magnetic field, advancing understanding of super-Eddington accretion.

Findings

Pulsations at ~0.42 s detected in XMM-Newton data.

Neutron star confirmed as the compact object.

Super-Eddington accretion rates inferred.

Abstract

NGC 7793 P13 is a variable (luminosity range ~100) ultraluminous X-ray source (ULX) proposed to host a stellar-mass black hole of less than 15 M$_{\odot}$ in a binary system with orbital period of 64 d and a 18-23 M$_{\odot}$ B9Ia companion. Within the EXTraS project we discovered pulsations at a period of ~0.42 s in two XMM-Newton observations of NGC 7793 P13, during which the source was detected at $L_{\mathrm{X}}\sim2.1\times10^{39}$ and $5\times10^{39}$ erg s$^{-1}$ (0.3-10 keV band). These findings unambiguously demonstrate that the compact object in NGC 7793 P13 is a neutron star accreting at super-Eddington rates. While standard accretion models face difficulties accounting for the pulsar X-ray luminosity, the presence of a multipolar magnetic field with $B$ ~ few $\times$ 10$^{13}$ G close to the base of the accretion column appears to be in agreement with the properties of the system.