GRB 210704A: A Luminous Fast Blue Transient in a GRB Afterglow at $z = 2.34$

TL;DR

This paper analyzes GRB 210704A, revealing a luminous, fast blue optical transient linked to a gamma-ray burst with a high Lorentz factor, suggesting a connection between LFBOTs and GRBs.

Contribution

It presents the first detailed multi-wavelength analysis linking LFBOT-like emission to a GRB with a successful jet, expanding understanding of transient phenomena.

Findings

The redshift of GRB 210704A is determined to be z=2.34.

The optical/infrared excess peaks around 7 days after the burst and is extremely luminous.

The excess emission resembles LFBOTs and is modeled as a refreshed shock.

Abstract

We present detailed, multi-wavelength analysis of GRB 210704A: a Fermi Gamma-ray Burst Monitor discovered and Fermi Large Area Telescope (LAT) detected gamma-ray burst (GRB). The burst is dominated by a short ($\approx 2$ 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 $T_0 + 7$ d ($2$ d in the rest frame). The excess is extremely luminous ($M_{r} = -22.0$ 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.