[Paper Review] Insight-HXMT and GECAM-C observations of the brightest-of-all-time GRB 221009A

Zhenghua An, S. Antier+176 more|arXiv (Cornell University)|Mar 2, 2023

Gamma-ray bursts and supernovae33 citations

TL;DR

The paper presents joint observations from Insight-HXMT and GECAM-C of GRB 221009A, delivering an unprecedented measurement of its prompt emission and afterglow, including a record E_iso ~1.5×10^55 erg and a jet opening angle of ~0.7 degrees.

ABSTRACT

GRB 221009A is the brightest gamma-ray burst ever detected since the discovery of this kind of energetic explosions. However, an accurate measurement of the prompt emission properties of this burst is very challenging due to its exceptional brightness. With joint observations of extit{Insight}-HXMT and GECAM-C, we made an unprecedentedly accurate measurement of the emission during the first $\sim$1800 s of GRB 221009A, including its precursor, main emission (ME, which dominates the burst in flux), flaring emission and early afterglow, in the hard X-ray to soft gamma-ray band from $\sim$ 10 keV to $\sim$ 6 MeV. Based on the GECAM-C unsaturated data of the ME, we measure a record-breaking isotropic equivalent energy ($E_{ m iso}$) of $\bf \sim 1.5 imes 10^{55}$ erg, which is about eight times the total rest-mass energy of the Sun. The early afterglow data require a significant jet break between 650 s and 1100 s, most likely at $\sim950$ s from the afterglow starting time $T_{AG}$, which corresponds to a jet opening angle of $\sim {0.7^\circ} \ (η_γn)^{1/8}$, where $n$ is the ambient medium density in units of $ m cm^{-3}$ and $η_γ$ is the ratio between $γ$-ray energy and afterglow kinetic energy. The beaming-corrected total $γ$-ray energy $E_γ$ is $\sim 1.15 imes10^{51} \ (η_γn)^{1/4}$ erg, which is typical for long GRBs. These results suggest that this GRB may have a special central engine, which could launch and collimate a very narrowly beamed jet with an ordinary energy budget, leading to exceptionally luminous gamma-ray radiation per unit solid angle. Alternatively, more GRBs might have such a narrow and bright beam, which are missed by an unfavorable viewing angle or have been detected without distance measurement.

Motivation & Objective

• Characterize the prompt emission of GRB 221009A from ~10 keV to ~6 MeV across precursor, main emission, flare, and early afterglow.
• obtain an accurate bolometric energy release and spectral evolution using unsaturated detector data and robust background modeling.
• constrain the jet geometry and beaming correction to derive the true gamma-ray energy and discuss implications for central engine models.
• assess the afterglow evolution and identify jet break timing to infer jet opening angle and energetics.
• evaluate how viewing geometry and jet structure may explain the exceptional brightness of GRB 221009A.

Proposed method

• Joint time-resolved spectral analysis using empirical models (Band, cutoff power-law, power-law, blackbody).
• Use GECAM-C GRD01 LG channel data for absolute normalization and HG channel data to constrain spectral shape with a normalization factor to account for HG undersampling.
• Apply Insight-HXMT HE/CsI background modeling to achieve a net burst light curve across precursor, ME, flare, and afterglow.
• Estimate 20–200 keV flux by combining 0.6–3 MeV HE/CsI data for precursor and early afterglow, enabling complete light curves.
• Fit afterglow with power-law and identify a jet break timing between ~T_AG+650 s and ~T_AG+1100 s, yielding a refined jet opening angle.

Experimental results

Research questions

• RQ1 What is the isotropic-equivalent energy (E_iso) and bolometric energy budget of GRB 221009A based on unsaturated data?
• RQ2 What is the jet opening angle inferred from the afterglow jet break and how does it affect the beaming-corrected gamma-ray energy (E_gamma)?
• RQ3 How does the spectral evolution of the main emission and flare behave, and does it conform to standard GRB correlations (e.g., Amati relation)?
• RQ4 What do the observations imply about the central engine and jet collimation for an event of this luminosity?
• RQ5 How does the afterglow decay slope change across the pre- and post-jet-break phases?

Key findings

• E_iso is measured to be ~1.5×10^55 erg for GRB 221009A, with an average bolometric fluence of (2.24±0.02)×10^−1 erg cm^−2 for the full burst.
• The afterglow shows a jet break around 950 s (with 650–1100 s range), implying a jet opening angle of ~0.7° (dependent on n and η_gamma).
• The beaming-corrected energy E_gamma is ~1.15×10^51 erg (assuming (η_gamma n)^(1/4) scaling), consistent with the standard energy reservoir for long GRBs.
• The peak luminosity reaches ~1.14–1.74×10^54 erg s^−1 (1-s and 50-ms peaks, respectively) within the ME, with an E_peak of 1247.4±91.2 keV for the full burst.
• Spectral evolution shows the low-energy index α below the synchrotron limit, with E_peak exhibiting flux-tracking behavior; BB is not required.
• Both GECAM-C and Insight-HXMT data enable a robust, saturation-free measurement of the prompt emission and early afterglow across 10 keV–6 MeV.