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

¹ INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
e-mail: sergio.campana@brera.inaf.it
² Università dell’Insubria, Dipartimento di Scienza e Alta Tecnologia, via Valleggio 11, 22100 Como, Italy
³ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
⁴ Instituto de Ciencias de l’Espacio (ICE, CSIC-IEEC), Campus UAB, Carrer Can Magrans s/n, 08193 Barcelona, Spain
⁵ INAF, Osservatorio Astronomico di Roma, via di Frascati 33, 00044 Monteporzio Catone (Roma), Italy
⁶ ICREA Institució Catalana de Recerca i Estudis Avançats, 08010 Barcelona, Spain

Received: 1 June 2016
Accepted: 21 July 2016

Abstract

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.