[Paper Review] Precise Measurements of Self-absorbed Rising Reverse Shock Emission from Gamma-ray Burst 221009A

Joe Bright, Lauren Rhodes+19 more|arXiv (Cornell University)|Mar 23, 2023

Gamma-ray bursts and supernovae4 citations

TL;DR

This study presents the first precise, multi-frequency radio observations of self-absorbed rising reverse shock emission from the exceptionally bright gamma-ray burst GRB 221009A, using rapid follow-up from multiple radio telescopes. It constrains the outflow's Lorentz factor, internal energy, and size, and accurately predicts the peak frequency of the reverse shock within the first few hours post-burst, offering a benchmark for relativistic jet physics.

ABSTRACT

The deaths of massive stars are sometimes accompanied by the launch of highly relativistic and collimated jets. If the jet is pointed towards Earth, we observe a "prompt" gamma-ray burst due to internal shocks or magnetic reconnection events within the jet, followed by a long-lived broadband synchrotron afterglow as the jet interacts with the circum-burst material. While there is solid observational evidence that emission from multiple shocks contributes to the afterglow signature, detailed studies of the reverse shock, which travels back into the explosion ejecta, are hampered by a lack of early-time observations, particularly in the radio band. We present rapid follow-up radio observations of the exceptionally bright gamma-ray burst GRB 221009A which reveal an optically thick rising component from the reverse shock in unprecedented detail both temporally and in frequency space. From this, we are able to constrain the size, Lorentz factor, and internal energy of the outflow while providing accurate predictions for the location of the peak frequency of the reverse shock in the first few hours after the burst.

Motivation & Objective

• To overcome the lack of early-time radio observations of reverse shocks in gamma-ray bursts, particularly in the optically thick regime.
• To precisely measure the self-absorbed rising component of the reverse shock emission in GRB 221009A across multiple radio frequencies.
• To constrain the physical parameters of the relativistic outflow, including Lorentz factor, internal energy, and size, using the observed light curves.
• To predict the peak frequency of the reverse shock emission in the first few hours after the burst based on observed evolution.
• To provide a benchmark for future studies of reverse shock emission in long-duration gamma-ray bursts.

Proposed method

• Conducted rapid multi-band radio follow-up observations of GRB 221009A using the Arcminute Microkelvin Imager Large Array (AMI-LA), Allen Telescope Array (ATA), e-MERLIN, ASKAP, and other facilities.
• Performed smooth, broken power-law fitting to radio light curves across frequencies from 1.5 to 17.69 GHz, with a smoothing parameter s=2 to model the rise and decay phases.
• Used the observed flux density evolution to infer the spectral evolution of the reverse shock, particularly the optically thick self-absorbed component.
• Applied the standard fireball model with synchrotron self-absorption (νsa), cooling break (νc), and minimum injection frequency (νm) to interpret the spectral evolution.
• Combined data from public archives (MeerKAT, Japanese VLBI) with new observations to improve temporal and frequency coverage.
• Accounted for systematic uncertainties (10% for AMI-LA, 10% for ATA, 5% for e-MERLIN and ASKAP) in flux density measurements during fitting and analysis.

Experimental results

Research questions

• RQ1 What are the physical parameters (Lorentz factor, internal energy, size) of the relativistic outflow in GRB 221009A as inferred from the reverse shock light curve?
• RQ2 How does the self-absorbed rising component of the reverse shock evolve in time and frequency, and what does this reveal about the shock dynamics?
• RQ3 At what time and frequency does the peak flux density of the reverse shock emission occur in the first few hours after the burst?
• RQ4 How well can the observed radio light curves be modeled using a smoothly broken power-law fit, and what does this imply about the shock structure?
• RQ5 Can the observed radio emission be uniquely attributed to a reverse shock, and what constraints does this place on the circum-burst medium and jet properties?

Key findings

• The reverse shock emission exhibits a rising phase with a power-law index of F ∝ t^{1.34±0.02}, indicating optically thick self-absorption.
• The decay phase follows F ∝ t^{-0.82±0.04}, consistent with the expected evolution of a self-absorbed synchrotron component.
• The peak flux density occurs at 6.09±0.09 hours post-burst at 17.69 GHz, with a flux density of 57.2±0.6 mJy.
• The observed peak frequency evolution across bands (3–17.69 GHz) allows for accurate prediction of the reverse shock peak frequency within the first few hours.
• The outflow has a Lorentz factor of approximately 100–150, internal energy of ~10^52 erg, and a size of ~10^12–10^13 cm, consistent with relativistic jet models.
• The data show no detectable emission at the GRB position in pre-burst surveys (VLASS and NVSS), with a 3σ upper limit of ~450 μJy/beam, confirming the transient nature of the signal.