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 proposed to host a stellar-mass black hole of less than 15 M<SUB>☉</SUB> in a binary system with orbital period of 64 d and a 18-23 M<SUB>☉</SUB> B9Ia companion. Within the EXTraS (Exploring the X-ray Transient and variable Sky) 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<SUB>X</SUB> ∼ 2.1 × 10<SUP>39</SUP> and 5 × 10<SUP>39</SUP> erg s<SUP>-1</SUP> (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 × 10<SUP>13</SUP> G close to the base of the accretion column appears to be in agreement with the properties of the system.