Photometric and Spectroscopic Observations of GRB 210104A: Bright Reverse-shock Emission and Dense Circumburst Environment

Early afterglow observations of gamma-ray bursts (GRBs) are valuable for exploring the properties of their jets and ambient medium. We report our photometric and spectroscopic observations of GRB 210104A and discuss its jet properties with multiwavelength data. Our spectroscopic observation reveals several absorption features and a tentative redshift of 0.46 is identified. A bright optical flare that has a peak brightness of R = 13 mag at 112 ± 7 s was observed in the R band during 67 ~ 165 s post the GRB trigger. The flux of the R-band afterglow decays with a slope of α _O = - 0.91 ± 0.03 at t > 650 s. The early X-ray afterglow lightcurve is a smooth bump, and it decays with a slope of α _X = -1.18 ± 0.01 at late epoch. Our joint spectral fit to the optical-X-ray afterglows during (1.1-1.3) × 10⁴ s yields a photon index Γ_O,X = -1.82 ± 0.04. The derived host galaxy extinction is A _R = 0.87. Attributing the early optical flare to the reverse-shock (RS) emission and the late optical-X-ray emission to the forward shock emission, the optical and X-ray lightcurves at t < 3 × 10⁴ s can be well fit adopting a Markov Chain Monte Carlo algorithm. Comparing the properties of GRB 210104A with other GRBs that have detection of bright RS emission, we show that its jet is mildly magnetized (R _B = 28), with high radiation efficiency (77%), is subenergetic (E _k,iso = 4.5 × 10⁵¹ erg), and moderately relativistic (Γ₀ ~ 35) in a density medium (n ₀ ~ 417 cm^-3). It follows the tight L _γ,iso-E _p,z-Γ₀ relation as with typical GRBs.