UHECRs and GRBs discussed in the previous chapters are some of the most mysterious discoveries of the last century. Their astronomical origin has only recently been constrained by the PAO and various satellite missions since the discovery of X-ray afterglows by Beppo-SAX.

Black holes are natural candidates for powering these emissions. Frame dragging around rotating black holes acts universally on particles and fields alike, which opens a broad range of channels in non-thermal emissions. Furthermore, black holes are scale free, with no intrinsic reference to a particular mass, in sharp contrast to degenerate compact objects, i.e., neutron stars and white dwarfs (see Chapter 1).

Extracting evidence for black holes as inner engines powering these emissions requires detailed analysis of all their radiation channels, taking into account a large diversity in phenomenology as expressed by supermassive and stellar mass black holes in view of their scale-free behavior. Scale-free behavior may also be expressed in ensembles of specific types of sources, provided the ensembles are sufficiently large in number.

Alfvén waves in transient capillary jets

When the magnetosphere around rotating black holes is intermittent, e.g., due to instabilities in the disk or the inner torus magnetosphere [599], magnetic outflows produce terminal Alfvén fronts propagating along their spin axis out to large distances. The approximation of ideal MHD discussed in Chapter 6 assumes negligible dissipation of the electromagnetic field in the fluid, corresponding to an infinite magnetic Reynolds number, which applies to extragalactic radio jets [203].