  We show that the electromagnetic moments of unstable particles (resonances)
have an absorptive contribution which quantifies the change of the particle's
lifetime in an external electromagnetic field. To give an example we compute
here the imaginary part of the magnetic moment for the cases of the muon and
the neutron at leading order in the electroweak coupling. We also consider an
analogous effect for the strongly-decaying $\Delta$(1232) resonance. The result
for the muon is Im$ \mu = e G_F^2 m^3/768 \pi^3$, with $e$ the charge and $m$
the mass of the muon, $G_F$ the Fermi constant, which in an external magnetic
field of $B$ Tesla give rise to the relative change in the muon lifetime of
$3\times 10^{-15} B$. For neutron the effect is of a similar magnitude. We
speculate on the observable implications of this effect.
