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

TL;DR

This study presents detailed photometric and spectroscopic observations of GRB 210104A, revealing bright reverse shock emission, a dense circumburst environment, and insights into jet properties, with implications for understanding GRB physics.

Contribution

First comprehensive analysis of GRB 210104A combining multiwavelength data, identifying reverse shock emission, and characterizing its jet and environment properties.

Findings

Bright optical flare attributed to reverse shock emission

Dense circumburst medium with density ~417 cm^-3

Jet is mildly magnetized with high efficiency and moderate Lorentz factor

Abstract

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 $112\pm 7$~s was observed in the $R$ band during $67\sim 165$ seconds post the GRB trigger. The flux of the $R$-band afterglow decays with a slope of $\alpha_{\rm O}={-0.91\pm 0.03}$ at $t>650$~s. The early X-ray afterglow lightcurve is a smooth bump, and it decays with a slope of $\alpha_{\rm X}=-1.18\pm 0.01$ at late epoch. Our joint spectral fit to the optical-X-ray afterglows during $(1.1-1.3)\times 10^{4}$~s yields a photon index $\Gamma_{\rm O,X}=-1.82\pm 0.04$. 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 $t<3\times 10^4$~s can be well fit adopting an 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 ($R_{\rm B}=28$), with high radiation efficiency ($77\%$), is sub-energetic ($E_{\rm k, iso}=4.5\times 10^{51}$ erg), and moderately relativistic ($\Gamma_0\sim 35$) in a density medium ($n_{0}\sim 417\;{\rm cm}^{-3}$). It follows the tight $L_{\gamma,\rm iso}-E_{\rm p,z}-\Gamma_{0}$ relation as with typical GRBs.