  We study the fermionic vacuum energy of vacua with and without being applied
an external magnetic field. The energetic difference of two vacua leads to the
vacuum decaying and the vacuum-energy releasing. In the context of quantum
field theories, we discuss why and how the vacuum energy can be released by
spontaneous photon emissions and/or paramagnetically screening the external
magnetic field. In addition, we quantitatively compute the vacuum energy
released, the paramagnetic screening effect and the rate and spectrum of
spontaneous photon emissions. The possibilities of experimentally detecting
such an effect of vacuum-energy releasing and this effect accounting for the
anormalous X-ray pulsar are discussed.
