Earliest simultaneous multi-color optical observations of GRB 230328B: from 41 seconds to the host-galaxy identification

Abstract

We present a multi-wavelength analysis of the long-duration gamma-ray burst GRB 230328B. Fermi/GBM observations reveal a typical Type II burst with a duration of about 22 s. Using a photometric redshift of about 1.5 derived from the host galaxy, we find that the burst energetics, with an isotropic-equivalent energy of about 6.4*10^52 erg, are consistent with established empirical correlations for long gamma-ray bursts. The optical, X-ray, and radio afterglow exhibits a complex temporal evolution, featuring an early onset bump followed by a pronounced late-time achromatic rebrightening at about 4000 s. Through MCMC modeling, we find that the afterglow can be explained by forward shock emission with late energy injection. Broadband spectral energy distribution fitting reveals significant line-of-sight dust extinction, corresponding to a visual extinction of about 0.8 magnitudes, consistent with Milky Way or Large Magellanic Cloud dust properties. The burst originated in a relatively young, highly absorbed S0-type host galaxy, whose morphological analysis suggests that it may be part of a system of interacting galaxies. Finally, late-time optical monitoring reveals no signature of an accompanying supernova.