Magnetic field transport in accretion disk: Black holes vs neutron stars

A variety of astrophysical objects can launch jets: X-ray binaries, active galactic nuclei, gamma-ray bursts, and dwarf novae. Jetted astrophysical systems typically possess an accretion disk, with the jet-launching compact object at the center. The radiative powers emitted by the jets and their accretion disks are highly correlated. This correlation implies that accretion and ejection processes are dynamically linked. The magnetic field could provide this link because it is essential for both accretion and ejection. Indeed, the magnetic field drives accretion through the magneto rotational instability (MRI), launches, and accelerates the jets. However, the structure and origin of the magnetic fields are not well understood. Namely, It is a puzzle how the accretion disk manages to transport the magnetic fields from large distances to the compact object. Surprisingly this transport appears to be robust and effective across a wide range of central compact object types, e.g., black holes and neutron stars. Using first-principles 3D general relativistic magnetohydrodynamic (GRMHD) simulations, I investigate the nature of magnetic field transport in both neutron star and black hole accretion systems. I find that the direction of the magnetic transport changes sign between these two qualitatively different systems. I will first detail how the accretion disk advects the magnetic flux inwards towards the black hole. I will show how the magnetic flux transport emerges from the balance between advection and diffusion. I will report, on the surprising result, that while near a black hole the advection brings the magnetic flux inward, and diffusion expels it outward, the situation is the opposite near neutron stars! More specifically, near a neutron star, the diffusion is responsible for bringing the magnetic flux in, and the advection pushes it outwards. These results provide important insights into the robustness of the jet launching mechanism.