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Cosmic transients, such as gamma-ray bursts and fast radio bursts, have beenused to constrain the Einstein's Equivalence Principle (EEP) through theparametrized-post-Newtonian (PPN) formalism. In this approach, the time delayof photons with different energies from these cosmic transients are used toobtain upper bounds on the difference of the PPN $\gamma$ parameter. In thiswork we assume that an important contribution to the time delay is due to thedark matter halo of the Milky Way and consider the dark matter massdistribution given by the Navarro--Frenk--White profile. We obtain the upperlimit on the difference of the PPN parameter $\gamma$ for the polarizedgamma-ray emission of GRB 110721A, $\Delta \gamma < 1.06 \times 10^{-28}$, themost stringent limit to date on the EEP. In addition, we show that a verysimilar upper bound is obtained if, instead of having a dark matter component,a visible matter density profile and a non-minimal gravitational couplingbetween curvature and matter are present.

Cosmic Transients, Einstein's Equivalence Principle and Dark Matter Halos