The ultraluminous X-ray source M81 X-6: a weakly magnetised neutron star with a precessing accretion disc?

Vol. 669
2. Astrophysical processes

The ultraluminous X-ray source M81 X-6: a weakly magnetised neutron star with a precessing accretion disc?

Ultra-luminous X-ray (ULX) sources are X-ray point sources in external galaxies, not coincident with the nucleus, and with a luminosity greater than the Eddington one for a black hole of ~10 solar masses (i.e., L_x>~ 10^39 erg s-1). Under simplified assumptions, no sources should be brighter than this limit, the radiation pressure acting as a damper. The ULX sources in nearby galaxies are brighter and easier to study. In this paper, Amato et al. report on the study of ULX X-6 in the grand-design spiral galaxy M81, at ~3.6 Mpc. Thanks to a 12 yr monitoring with the Swift satellite, the authors show that ULX X-6 oscillates between two main X-ray states: one hard and bright and the other soft and dimmer, with luminosities that vary by a factor of ~2. The authors explain the super-orbital variability connected with these variations as being due to the (Lense-Thirring) precession of the accretion disk, induced by a neutron star with a relatively low magnetic field (B~10^10 G).