Constraining axion by polarized prompt emission from gamma ray bursts

A polarized gamma ray emission spread over a sufficiently wide energy band from a strongly magnetized astrophysical object like gamma ray bursts (GRBs) offers an opportunity to test the hypothesis of invisible axion. The axionic induced dichroism of gamma rays at different energies should cause a misalignment of the polarization plane for higher energy events relative to that one for lower energies events resulting in the loss of statistics needed to form a pattern of the polarization signal to be recognized in a detector. According to this, any evidence of polarized gamma rays coming from an object with extended magnetic field could be interpreted as a constraint on the existence of the invisible axion for a certain parameter range. Based on reports of polarized MeV emission detected in several GRBs we derive a constraint on the axion–photon coupling. This constraint g a γ γ ⩽ 2.2 × 10 − 11 GeV − 1 calculated for the axion mass m a = 10 − 3 eV is competitive with the sensitivity of CAST and becomes even stronger for lower masses.