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

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 $L_{\mathrm{X}}\sim 2.1\times 10^{39}$ and $5\times 10^{39}$ 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.

5 pages, 5 figures, 2 tables; Version accepted for publication in MNRAS Letters