Abstract
ABSTRACT We present a multiwavelength study of the long-duration gamma-ray burst GRB 230328B, combining prompt $\gamma$-ray observations with exceptionally early simultaneous optical photometry and extensive X-ray, radio, and host-galaxy follow-up. NUTTELA-TAO began simultaneous $g^{\prime }r^{\prime }i^{\prime }$ observations only 41 s after the Swift/BAT trigger, corresponding to approximately 16 s in the rest frame for the adopted host-galaxy photometric redshift of $z_{\rm phot}=1.54\pm 0.06$. Unlike sequential multiband measurements, these observations provide an instantaneous optical spectral slope without temporal-interpolation uncertainties. The early optical continuum remains statistically consistent with a constant slope from approximately $10^{2}$ to $7\times 10^{3}$ s, providing no evidence for rapid dust destruction. The absence of significant colour evolution across the pronounced optical rebrightening at approximately $4\times 10^{3}$ s demonstrates that it is achromatic and favours a dynamical rather than spectral origin. Broad-band modelling shows that the optical, X-ray, and radio evolution can be broadly explained by forward-shock emission with late energy injection, although alternative scenarios cannot be excluded. The red optical continuum is consistent with substantial line-of-sight extinction in a massive, dusty host galaxy. Late-time observations are insufficient to constrain a typical GRB-associated supernova at the adopted redshift. GRB 230328B therefore provides a benchmark for connecting the earliest simultaneous optical colours with afterglow dynamics and the host environment of a representative long GRB.