Global kinetic modeling of the intrabinary shock in spider pulsars

Among the most fascinating astrophysical objects are the “spider pulsars,” which are a particular type of very rapidly-rotating, extremely compact neutron stars in a binary system. Spider pulsars, which are classified as either Redbacks (RBs) or Black Widows (BWs), are in tight binary orbits with a low-mass stellar companion. The pulsar emanates an intense relativistic wind. When impacting on the companion star, the wind exacerbates ablation and mass loss of the companion, which is then “devoured” by the pulsar wind --- hence, the evocative name of these systems. A research team at Columbia University will use state-of-the-art computer simulations to better understand spider pulsars, and astrophysical shock waves in general. The lead researcher will also design a new course, “Back-of-the-envelope physics, astronomy, and beyond," to introduce students in the Columbia Bridge to Ph.D. scholars program to the art of looking at reality with quantitative eyes, using personal experience and prior knowledge as a guide to drawing order-of-magnitude conclusions, and thus to being prepared to make decisions that are grounded in quantitative reasoning. The relativistic pulsar wind consists of toroidal stripes of opposite magnetic field polarity, separated by current sheets of hot plasma. The wind terminates in a strong shock, where the ram pressure of the pulsar wind is balanced by the companion wind or its magnetosphere. The intrabinary shock may be an efficient site of particle acceleration, which would then explain why X-ray and gamma-ray lightcurves of spider pulsars are often modulated on the orbital period. Therefore, the scrutiny of BWs and RBs can help uncover the largely unknown physics of pulsar winds in millisecond pulsars, and aid in understanding where the transition occurs from a Poynting flux-dominated to a particle-dominated flow. This project will involve first-principles global kinetic Particle-In-Cell simulations of the intrabinary shock in spider pulsars, so one can simultaneously capture the microscopic physics of magnetic field dissipation, particle acceleration and ensuing emission, as well as the global morphology and curvature of the shock and the resulting macroscopic flow dynamics. While the main focus is on spider pulsars, the question of particle acceleration in magnetically-dominated striped shocks is of profound relevance to the physics of Poynting-flux-dominated relativistic outflows in other systems, like jets of active galactic nuclei and gamma-ray bursts. This award advances the goals of the Windows on the Universe Big Idea. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.