GRB 230204B: GIT Discovery of a Fast Fading Afterglow Associated with an Energetic GRB from a Massive-Star Progenitor

TL;DR

GRB 230204B is analyzed as a hyper-energetic long GRB exhibiting a prominent photospheric component in its prompt emission and a rapidly fading optical afterglow. Through time-integrated, episode-wise, and time-resolved spectroscopy, the authors show a strong thermal component coexisting with non-thermal emission, enabling constraints on the jet Lorentz factor, photospheric radius, and base radius. Broadband afterglow modeling favors a wind-type circum-burst medium with a narrow, collimated jet, a Wolf-Rayet progenitor with low metallicity, and a true energy budget exceeding 10^52 erg. The study finds meaningful correlations between prompt and afterglow properties, supporting a unified energy reservoir scenario and highlighting the importance of photospheric emission and wind environments in hyper-energetic GRBs.

Abstract

We present a comprehensive multi-wavelength study of a bright gamma-ray burst GRB 230204B, analyzing both prompt and afterglow emissions. This GRB is highly energetic, with an isotropic equivalent energy emission $E_{\mathrm{iso}} \sim 2.2 \times 10^{54}\ \mathrm{erg}$, released during the prompt emission. The GROWTH-India Telescope discovered a bright afterglow ($m_r = 15.55$) that faded rapidly ($\propto t^{-1.82}$). The prompt emission shows strong thermal photospheric emission, along with a non-thermal high-energy component. We explore the evolution of these components and find them to be consistent with theoretical expectations. Afterglow modeling reveals an energetic jet $E_{tot} \gtrsim 10^{52}\ \mathrm{erg}$ expanding into a wind-type medium viewed nearly on-axis, suggesting a massive star progenitor with strong winds. We also explore correlations between the prompt emission and afterglow that may help to understand the complete picture of GRB progenitors.

Figures (10)

• Figure 1: The optical afterglow of GRB 230204B was discovered by the GROWTH-India Telescope (GIT) in $r'$ filter at UT 2023-02-04T23:22:37.6, as reported by 2023GCN.33269....1S. The source was both bright and rapidly fading, consistent with the expected behavior of a GRB afterglow.
• Figure 2: Multi-panel light curve analysis of GRB 230204B using Fermi-GBM detectors. Top panels: Background-subtracted light curves from NaI detectors 8 and 7 (10--900 keV), and BGO detector 1 (10--38,000 keV), color-coded by episode. Middle panel: Comparison of low-energy (10--120 keV) and mid-energy (120--400 keV) light curves from NaI 8, highlighting the spectral evolution during the burst. The structure reveals four distinct emission episodes, with significant variability in intensity and spectral hardness across them. Bottom panel: Bayesian blocks (dashed gray) and final binned intervals (shaded red) used for time-resolved spectral analysis, labeled with bin numbers. The binning ensures sufficient counts in high-energy channels (e.g., BGO 1) for reliable spectral fitting. Figure is plotted with packages -- GDT-CoreGDT-Fermi and ThreeML2015arXiv150708343V.
• Figure 3: Time-resolved spectral evolution of GRB 230204B using both the Band and PL+BB models. Top three panels: Light curve and spectral evolution from the Band function fits in the 10--38000 keV range. The panels show (i) total flux, (ii) low- and high-energy photon indices ($\alpha$, $\beta$), and (iii) peak energy ($\mathrm{E_{p}}$). Bottom five panels: Temporal evolution from the Power-law + Blackbody (PL+BB) model. Shown are: (iv) total flux along with separate contributions from the PL and BB components in 10--38000 keV, (v) their fluxes in 10--1000 keV, (vi) the ratio of BB to total flux, (vii) blackbody temperature ($kT$), and (viii) the PL photon index ($\Gamma$).
• Figure 4: Correlation plots for GRB 230204B: (a) Amati relation and (b) Yonetoku relation. Both panels show that GRB 230204B belongs to the population of energetic long GRBs and conforms well to established empirical relations.
• Figure 5: Time-resolved analysis of the photospheric emission during Episode 2 of GRB 230204B. From top to bottom, the panels show: (1) total flux (blue) and blackbody (BB) flux (red) in the 10-38000 keV energy range, (2) the fractional BB contribution, (3) BB temperature ($kT$), (4) effective transverse size of the emitting region ($\mathcal{R}$), (5) bulk Lorentz factor ($\Gamma_{j}$), (6) photospheric radius ($r_{\mathrm{ph}}$), (7) saturation radius ($r_{\mathrm{s}}$), and (8) base radius of the flow ($r_{\rm 0}$). The evolution of these parameters reflects the dynamics of the outflow at the photosphere.