The accretion regimes of a highly magnetized NS: The unique case of NuSTAR J095551+6940.8

The ultraluminous accreting pulsar M82-X2 (NuSTAR J095551+6940.8) offers an unprecedented opportunity to study the disc-magnetosphere interaction in a newregime of supercritical accretion. The source X-ray emission has been highly variable during the last 15 yrs. It ranged from a maximum of ~2 × 10⁴⁰ erg s^-1 through intermediate values ~ a few × 10³⁹ erg s^-1, and down to a minimum below 2 × 10³⁸ erg s^-1 that we have determined here, by analysing archival Chandra HRC observations of the source at an epoch at which it was undetected. We interpret the source variability via a magnetically threaded disc model: when at peak luminosity, the neutron star (NS) is close to spin equilibrium, its inner disc edge r_m ~ 10⁸ cm is approximately half the corotation radius r_co, and radiation pressure dominates the disc out to r_tr ≲ 10⁹ cm. In the radiation-pressure-dominated regime, r_m grows very slowly as the mass inflow rate drops: as a result, r_m < r_co remains valid until M˙ ≳ M˙_E, the Eddington accretion rate, allowing a wide range of accretion luminosities to the NS. Once M˙ < M˙_E accretion on to the NS is inhibited because r_m > r_co, and the source luminosity is expected to drop by a large factor. We conclude that a magnetically threaded accretion disc surrounding a highly magnetized NS (B ≲ 10¹³ G), and transitioning between the radiation-pressure and gas-pressure dominated regimes, offers the best interpretation for all the currently observed properties of NuSTAR J095551+6940.8.