  In experiments with the photosensitive Belousov-Zhabotinsky reaction (PBZR)
we found a stable three-dimensional organizing center that periodically emits
trigger waves of chemical concentration. The experiments are performed in a
parameter regime with negative line tension using an open gel reactor to
maintain stationary non-equilibrium conditions. The observed periodic wave
source is formed by a scroll ring stabilized due to its interaction with a
no-flux boundary. Sufficiently far from the boundary, the scroll ring expands
and undergoes the negative line tension instability before it finally develops
into scroll wave turbulence. Our experimental results are reproduced by
numerical integration of the modified Oregonator model for the PBZR. Stationary
and breathing self-organized pacemakers have been found in these numerical
simulations. In the latter case, both the radius of the scroll ring and the
distance of its filament plane to the no-flux boundary after some transient
undergo undamped stable limit cycle oscillations. So far, in contrary to their
stationary counterpart, the numerically predicted breathing autonomous
pacemaker has not been observed in the chemical experiment.
