  We are concerned with an inverse problem associated with the fractional
Helmholtz system that arises from the study of viscoacoustics in geophysics and
thermoviscous modelling of lossy media. We are particularly interested in the
case that both the medium parameter and the internal source of the wave
equation are unknown. Moreover, we consider a general class of source functions
which can be frequency-dependent. We establish several general uniqueness
results in simultaneously recovering both the medium parameter and the internal
source by the corresponding exterior measurements. In sharp contrast, these
unique determination results are unknown in the local case, which would be of
significant importance in thermo- and photo-acoustic tomography.
