A model for the challenging "bi-drifting" phenomenon in PSR J0815+09

A new drifting pulsar, PSR J0815+09, was discovered in the Arecibo drift-scan searches. An intriguing feature of this source is that within the four pulse components in the integrated pulse profile, the sub-pulse drifting direction in the two leading components is opposite from that in the two trailing components. In view that the leading theoretical model for pulsar sub-pulse drifting can only interpret one-direction sub-pulse drifting, the observed "bi-drifting" phenomenon from PSR J0815+09 poses a great challenge to the pulsar theory. The inner annular gap (IAG), a new type of inner particle accelerator, was recently proposed to explain both $\gamma$-ray and radio emission from pulsars. Here we show that the coexistence of the IAG and the conventional inner core gap (ICG) offers a natural interpretation to the bi-drifting phenomenon. In particular, the peculiar drifting behavior in PSR J0815+09 can be reproduced within the inverse Compton scattering (ICS) model for pulsar radio emission.