Extremely Luminous Optical Afterglow of an Energetic Gamma-Ray Burst GRB 230204B

Gupta, Rahul and Racusin, Judith and Lipunov, Vladimir and Hu, Y.-D. and Gulati, Ashna and Castro-Tirado, Alberto J. and Murphy, Tara and Serino, Motoko and Zhirkov, Kirill and Shilling, S. and Oates, Samantha R. and Leung, James K. and Parsotan, T. and Ror, Amit K. and Pandey, Shashi B. and Iyyani, S. and Sharma, V. and Aryan, A. and Bai, Jin-Ming and Balanutsa, Pavel and Buckley, David and Caballero-García, María D. and Carrasco-García, I. M. and Castellón, A. and Castillo, Sebastián and Cui, Chen-Zhou and Fan, Yu-Feng and Fernández-García, Emilio and Segura, Guillermo García- and Gritsevich, Maria and Guziy, Sergiy and Hiriart, David and Lee, William H. and Jeong, Soomin and del Pulgar, Carlos Jesus Pérez and Olivares, Ignacio and Park, Il H. and Pérez-García, Ignacio and Razzaque, S. and Sánchez-Ramírez, Rubén and Tiurina, Nataly and Topolev, Vladislav and Wang, Chuan-Jun and Wu, Si-Yu and Xin, Yu-Xin and Xiong, Ding-Rong and Zhao, Xiao-Hong and Mao, Jirong and Lun, Bao-Li and Kai, Ye (2026) Extremely Luminous Optical Afterglow of an Energetic Gamma-Ray Burst GRB 230204B. The Astrophysical Journal, 997 (2): 246. ISSN 0004-637X

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Abstract

Robotic telescope networks play an important role in capturing early and bright optical afterglows, providing critical insights into the energetics and emission mechanisms of GRBs. In this study, we analyze GRB 230204B, an exceptionally energetic and multipulsed long GRB, detected by the Fermi Gamma-ray Burst Monitor and MAXI detectors, with an isotropic equivalent gamma-ray energy exceeding 1054 erg. Time-resolved spectral analysis reveals a transition in the prompt emission from hard (sub-photospheric-dominated) spectra during early pulses to softer (synchrotron-radiation-dominated) spectra in later pulses, indicative of a hybrid jet composition. We report the discovery and characterization of the optical afterglow using the Mobile Astronomical System of Telescope-Robots (MASTER) and Burst Observer and Optical Transient Exploring System (BOOTES) robotic telescope networks, which enabled rapid follow-up observations starting at ∼1.3 ks post-burst. The optical luminosity at this time was exceptionally high, surpassing that of many other optically bright GRBs, such as GRB 990123 and GRB 080319B. This places the burst among the most luminous optical GRBs observed to date. Long-term radio observations extending to 335 days post-burst were conducted with the Australia Telescope Compact Array. Multiwavelength modeling, incorporating data from MASTER, BOOTES, Devasthal Optical Telescope, Swift/XRT, and radio observations, was conducted using an external interstellar medium (ISM) forward-shock top-hat jet model with afterglowpy. The results reveal a narrow and highly collimated jet with a circumburst density of n0 ∼ 28.12 cm−3, kinetic energy EK ∼ 4.18 × 1055 erg, and a relatively low value of ϵB = 2.14 × 10−6, indicating shock-compression of the magnetic field in the surrounding ISM. We constrained a low radiative efficiency of ∼4.3%. This study highlights the indispensable contribution of robotic networks to early afterglow observations and advances our understanding of GRB 230204B unique characteristics and underlying jet physics.