  Scattering coefficients can diverge at spectral singularities. In such
situation, the stationary solution becomes a laser solution with outgoing waves
only. We explore a parity-time (PT)-symmetric non-Hermitian two-arm
Aharonov-Bohm interferometer consisting of three coupled resonators enclosing
synthetic magnetic flux. The synthetic magnetic flux does not break the PT
symmetry, which protects the symmetric transmission. The features and
conditions of symmetric, asymmetric, and unidirectional lasing at spectral
singularities are discussed. We elucidate that lasing affected by the
interference is asymmetric; asymmetric lasing is induced by the interplay
between the synthetic magnetic flux and the system's non-Hermiticity. The
product of the left and right transmissions is equal to that of the
reflections. Our findings reveal that the synthetic magnetic flux affects light
propagation, and the results can be applied in the design of lasing devices.

Optics;Quantum Physics