A physical scenario for the high and low X-ray luminosity states in the transitional pulsar PSR J1023+0038

The binary system PSR J1023+0038 (J1023) hosts a neutron star and a low-mass companion. J1023 is the best studied transitional pulsar, alternating a faint eclipsing millisecond radio pulsar state to a brighter X-ray active state. At variance with other low-mass X-ray binaries, this active state reaches luminosities of only ~10³⁴ erg s^-1, showing strong, fast variability. In the active state, J1023 displays: I) a high state (L_X ~ 7 × 10³³ erg s^-1, 0.3-80 keV) occurring ~80% of the time and during which X-ray pulsations at the neutron star spin period are detected (pulsed fraction ~ 8%); II) a low state (L_X ~ 10³³ erg s^-1) during which pulsations are not detected (≲ 3%); and III) a flaring state during which sporadic flares occur in excess of ~ 10³⁴ erg s^-1, with no pulsation too. The transition between the high and the low states is very rapid, on a ~10 s timescale. Here we propose a plausible physical interpretation of the high and low states based on the (fast) transition among the propeller state and the radio pulsar state. We modelled the XMM-Newton spectra of the high, low and radio pulsar states, and found a good agreement with this physical picture.