GRB 210704A: A Luminous Fast Blue Transient in a GRB afterglow at <i>z</i> = 2.34

Abstract We present detailed, multi-wavelength analysis of GRB 210704A: a Fermi/GBM discovered and Fermi/LAT detected gamma-ray burst (GRB). The burst is dominated by a short (≈2 s) pulse followed by weaker, softer emission. We line stack our afterglow spectrum and determine the most likely redshift to be z = 2.34. This is corroborated by the photometric redshift of the extended source underlying the GRB. The spectral energy distribution fit parameters, late-time imaging, as well as the GRB’s energetics, spectral lag, and location point to a collapsar nature. Follow-up observations reveal excess optical/infrared emission with respect to a standard afterglow, peaking around T0 + 7 d (2 d in the rest frame). The excess is extremely luminous (Mr = −22.0 mag) and rapidly evolving. Strikingly, it resembles the emission seen in recently discovered Einstein Probe fast X-ray transients EP241021a and EP240414a, as well as the population of luminous fast blue optical transients (LFBOTs). This provides a link between these sources and GRBs. Fermi/LAT observations imply a high Lorentz factor, making this a case where LFBOT-like emission is also associated with a powerful successfully launched jet. We model the excess as likely coming from an energetic refreshed shock.