Extremely luminous optical afterglow of an energetic gamma-ray burst GRB 230204B

TL;DR

GRB 230204B is an exceptionally energetic long-duration burst with $E_{ m gamma,iso} \approx 1.92 \times 10^{54}$ erg and a kinetic energy $E_{ m K} \approx 4.18 \times 10^{55}$ erg. The authors present a comprehensive, broadband campaign combining space-based gamma-ray/X-ray data (Fermi-GBM, MAXI, Swift) with rapid optical and radio follow-up from robotic networks (MASTER, BOOTES, DOT, ATCA), enabling time-resolved spectral analyses and detailed afterglow modeling. Prompt emission exhibits hard-to-soft evolution; time-integrated spectra favor a Band function with a thermal component (Band+BB), consistent with a hybrid jet that contains both photospheric and non-thermal contributions. Afterglow modeling with a Top-Hat jet in an ISM environment yields a narrow, on-axis jet ($\theta_v \approx 0.003$ rad, $\theta_c \approx 0.006$ rad) and low magnetic energy fraction ($\epsilon_B \sim 10^{-5.7}$), giving a low radiative efficiency of about $\sim 4\%$ and a large kinetic energy reservoir. Overall, the work highlights the indispensable role of robotic networks in capturing early afterglows and advances our understanding of jet structure, environment, and radiation mechanisms in extreme GRBs.

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 multi-pulsed long GRB, detected by the Fermi GBM and MAXI detectors, with an isotropic equivalent gamma-ray energy exceeding 10$^{54}$ 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 MASTER and BOOTES robotic telescope networks, which enabled rapid follow-up observations starting at $\sim$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, GRB 080319B, etc. 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 ATCA. Multi-wavelength modeling was conducted using an external ISM forward-shock top-hat jet model with \sw{afterglowpy}. The results reveal a narrow and highly collimated jet with a circumburst density of $n_{0} \sim$ 28.12 cm$^{-3}$, kinetic energy $E_{\rm K} \sim$ 4.18 $\times 10^{55}$ erg, and a relatively low value of $ε_{B}$ = 2.14 $\times 10^{-6}$, indicating shock-compression of magnetic field in the surrounding interstellar medium. We constrained a low radiative efficiency of $\sim$ 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.

Figures (15)

• Figure 1: Sequence of prompt and afterglow observations for GRB 230204B, the dashed vertical line indicating the Fermi-GBM trigger time. Vertical green and orange lines with dots correspond to the start times of observations by respective instruments.
• Figure 2: Optical finding chart for GRB 230204B from observations taken by MASTER-Kislovodsk (Top), and MASTER-SAAO (Bottom). The afterglow position is marked with a red plus marker in each image.
• Figure 3: Broad-band composite afterglow flux density light curve of GRB 230204B. The solid lines are the best-fit power-law function to optical data (temporal decay indices of -1.67 $\pm$ 0.15 in $R$ and -1.83 $\pm$ 0.05 in $r$ bands, respectively) and the dashed line shows the best-fit power-law function to X-ray data (temporal decay index of -0.72 $\pm$ 0.63). The upper limits for X-ray afterglow search using MAXI (2-20 keV) and BAT Survey data (14-195 keV) are shown using light blue and red color markers, respectively.
• Figure 4: (a) The prompt emission background-subtracted light curve of GRB 230204B, obtained using Fermi GBM across different energy channels: NaI 7+8 (8-30 keV), NaI 7+8 (50-300 keV), BGO 1 (0.3-1 MeV), and BGO 1 (1-40 MeV). The shaded regions represent the time intervals used for time-integrated spectral analysis for each of the four emission episodes. The hardness ratio (HR) evolution is shown as a function of time. (b) The MAXI light curve in 2-20 keV, 2-4 keV, 4-10 keV, and 10-20 keV energy bands, illustrating that MAXI observed only a later emission of GRB 230204B.
• Figure 5: Spectral evolution of GRB 230204B during prompt emission detected by Fermi GBM. The top panel shows the evolution of the maximum value of low-energy power-law indices during each pulse of GRB 230204B as a function of trigger time. The pink and lime-colored horizontal lines show the synchrotron line of death and the fast-cooling synchrotron emission line, respectively. The bottom panel shows the evolution of the maximum value of low-energy power-law indices during each pulse of GRB 230204B as a function of peak energy corresponding to the maximum value of $\alpha$. The data points for spectral parameters of individual pulses of other multi-pulsed Fermi GRBs obtained from Liang2021, are also shown.