TAPIR Seminar

Disk accretion onto weakly magnetized astrophysical objects possessing a surface (such as planets, stars, white dwarfs, neutron stars) necessarily proceeds via a boundary layer that forms at the inner edge of the disk. In this layer the highly supersonic angular velocity of the disk material must drop from the Keplerian value in the disk to the rotation rate of the accreting object. Up to a half of all accretional energy is released in this layer, strongly affecting the observational appearance of such objects. How this happens and what physical processes mediate the angular momentum, mass, and energy transport inside the boundary layer has remained a mystery for decades.

In this talk I will describe our discovery of a robust physical mechanism that naturally provides transport in the highly supersonic boundary layers of accretion disks, thus paving a way to understanding the properties of these disk regions. This mechanism relies on the excitation of large scale acoustic modes at the accretor-disk interface that propagate both into the star and in the disk. I will show that this transport mechanism is fully global in nature, leads to observable quasi-periodic variability of emission, and may affect not only the disk emission, but also the thermodynamical state of the accreting star. The implications of this finding for our understanding of the high- and low-energy accreting objects will be discussed.