Variable polarisation and Doppler tomography of PSR J1023+0038 - Evidence for the magnetic propeller during flaring?

Abstract

Transitional millisecond pulsars are systems that alternate between an accreting low-mass X-ray binary (LMXB) state, and a non-accreting radio pulsar state. When at the LMXB state, their X-ray and optical light curves show rapid flares and dips, origin of which is not well understood. We present results from our optical and NIR observing campaign of PSR J1023+0038, a transitional millisecond pulsar observed in an accretion state. Our wide band optical photopolarimetry indicates that the system shows intrinsic linear polarisation, the degree of which is anticorrelated with optical emission, i.e. the polarisation could be diluted during the flares. However, the change in position angle during the flares suggests an additional emerging polarised component during the flares. We also find, based on our H$_\alpha$ spectroscopy and Doppler tomography, that there is indication for change in the accretion disc structure/emission during the flares, possibly due to a change in accretion flow. This, together with changing polarisation during the flares, could mark the existence of magnetic propeller mass ejection process in the system. Furthermore, our analysis of flare profiles in both optical and NIR shows that NIR flares are at least as powerful as the optical ones and both can exhibit transition time scales less than 3 sec. The optical/NIR flares therefore seem to originate from a separate, polarised transient component, which might be due to Thomson scattering from propeller ejected matter.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical Society, Accepted 2017 November 8. Received 2017 November 8; in original form 2017 August 1