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

Abstract We present a multiwavelength study of the long-duration gamma-ray burst GRB 230328B, combining prompt γ-ray observations with exceptionally early simultaneous optical photometry and extensive X-ray, radio, and host-galaxy follow-up. NUTTELA-TAO began simultaneous g′r′i′ 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 zphot = 1.54 ± 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 102 to 7 × 103 s, providing no evidence for rapid dust destruction. The absence of significant color evolution across the pronounced optical rebrightening at approximately 4 × 103 s demonstrates that it is achromatic and favours a dynamical rather than spectral origin. Broadband 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 colors with afterglow dynamics and the host environment of a representative long GRB.

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