We find that charged unstable particles as well as neutral unstable particles with non-zero magnetic moment which live sufficiently long may emit electromagnetic radiation. This new mechanism is connected with the properties of unstable particles at the post exponential time region. Analyzing the transition times region between exponential and non-exponential form of the survival amplitude it is found that the instantaneous energy of the unstable particle can take very large values, much larger than the energy of this state for times from the exponential time region. Based on the results obtained for the model considered, it is shown that this purely quantum mechanical effect may be responsible for causing unstable particles to emit electromagnetic-, X-or γ-rays at some time intervals from the transition time regions.

We consider a model of a unstable state defined by the truncated Breit-Wigner energy density distribution function. An analytical form of the survival amplitude a(t) of the state considered is found. Our attention is focused on the late time properties of a(t) and on effects generated by the non-exponential behavior of this amplitude in the late time region: In 1957 Khalfin proved that this amplitude tends to zero as t goes to the infinity more slowly than any exponential function of t. This effect can be described using a time-dependent decay rate γ(t) and then the Khalfin result means that this γ(t) is not a constant but at late times it tends to zero as t goes to the infinity. It appears that the energy E(t) of the unstable state behaves similarly: It tends to the minimal energy E min of the system as t → ∞. Within the model considered we find two first leading time dependent elements of late time asymptotic expansions of E(t) and γ(t). We discuss also possible implications of such a late time asymptotic properties of E(t) and γ(t) and cases where these properties may manifest themselves.

The false (unstable) vacuum is discussed from the point of view of the quantum theory of unstable states. Properties of the energy of the system in the unstable vacuum state are studied. Within the model considered, it is shown that at very late times, the energy of the system in the false vacuum state tends to the energy of the system in the true vacuum state as 1/t2.