Reappraisal of the Constraints on Heavy Axion-like Particles from Gamma-Ray Bursts
We reassess existing limits and derive new constraints on heavy axion-like particle (ALP) coupling to photons using gamma-ray bursts (GRBs). ALPs can be produced in the hot dense fireball plasma during the initial stage of GRB outflow, thus potentially disrupting the primary fireball and altering the GRB luminosity. We consider the ALP production rate for various GRB parameters in two different energy injection scenarios of GRB fireball formation, and point out that ALP production is less efficient than previously assumed unless a GRB event is exceptionally energetic. We update the existing energy loss bounds using more realistic GRB parameters. We also point out that in the region of parameter space previously constrained by GRB luminosity criterion, ALP production turns out to be still efficient enough to form a secondary fireball via ALP decay to two photons and their subsequent annihilation to electron-positron pair. This secondary fireball reprocesses the gamma-rays from heavy ALP decay into $X$-rays, emitted isotropically from its surface, thus allowing us to probe $\mathcal{O}(100~\mathrm{MeV})$-scale ALPs indirectly using $X$-ray (or future MeV gamma-ray) telescopes, not necessarily directed toward the GRB jet itself. We show that the future point-source sensitivity of $X$-ray and MeV gamma-ray telescopes may allow us to constrain new ALP parameter space.