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

DALL'OSSO, SimoneSimoneDALL'OSSOPerna, RosalbaRosalbaPernaPAPITTO, ALESSANDROALESSANDROPAPITTOBozzo, EnricoEnricoBozzoSTELLA, LuigiLuigiSTELLA2020-05-142020-05-1420160035-8711http://hdl.handle.net/20.500.12386/24823The ultraluminous accreting pulsar M82-X2 (NuSTAR J095551+6940.8) offers an unprecedented opportunity to study the disc-magnetosphere interaction in a new regime 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 rm ∼ 108 cm is approximately half the corotation radius rco, and radiation pressure dominates the disc out to rtr ≲ 109 cm. In the radiation-pressure-dominated regime, rm grows very slowly as the mass inflow rate drops: as a result, rm < rco remains valid until dot{M} ≳dot{M}_E, the Eddington accretion rate, allowing a wide range of accretion luminosities to the NS. Once dot{M} < dot{M}_E accretion on to the NS is inhibited because rm > rco, 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.STAMPAenThe accretion regimes of a highly magnetized NS: the unique case of NuSTAR J095551+6940.8Article10.1093/mnras/stw1102-s2.0-84976892782000373583900060https://academic.oup.com/mnras/article/457/3/3076/2588907https://arxiv.org/abs/1512.015322016MNRAS.457.3076DFIS/05 - ASTRONOMIA E ASTROFISICAScienze Fisiche Settori ERC (ERC) di riferimento::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation::PE9_10 High energy and particle astronomy – X-rays, cosmic rays, gamma rays, neutrinos