[Paper Review] JWST Observations of the Extraordinary GRB 221009A Reveal an Ordinary Supernova Without Signs of $r$-Process Enrichment in a Low-Metallicity Galaxy

P. K. Blanchard, V. Ashley Villar+19 more|arXiv (Cornell University)|Aug 27, 2023

Gamma-ray bursts and supernovae4 citations

TL;DR

This study uses late-time JWST observations of the exceptionally bright GRB 221009A to analyze its supernova and afterglow components. Despite expectations of r-process enrichment from a collapsar origin, the data reveal a typical SN Ic-BL with no evidence of rare-earth element emission, a nickel mass of ≈0.09 M⊙, and a low-metallicity host (≈0.12 Z⊙), suggesting environmental factors—not intrinsic SN properties—may explain the burst's extreme luminosity.

ABSTRACT

Identifying the astrophysical sites of the $r$-process, one of the primary mechanisms by which heavy elements are formed, is a key goal of modern astrophysics. The discovery of the brightest gamma-ray burst of all time, GRB 221009A, at a relatively nearby redshift, presented the first opportunity to spectroscopically test the idea that $r$-process elements are produced following the collapse of rapidly rotating massive stars. Here we present spectroscopic and photometric $ extit{James Webb Space Telescope}$ (JWST) observations of GRB 221009A obtained $+168$ and $+170$ rest-frame days after the initial gamma-ray trigger, and demonstrate they are well-described by a supernova (SN) and power-law afterglow, with no evidence for an additional component from $r$-process emission, and that the SN component strongly resembles the near-infrared spectra of previous SNe, including SN 1998bw. We further find that the SN associated with GRB 221009A is slightly fainter than the expected brightness of SN 1998bw at this phase, concluding that the SN is therefore not an unusual GRB-SN. We infer a nickel mass of $\approx0.09$ M$_{\odot}$, consistent with the lack of an obvious SN detection in the early-time data. We find that the host galaxy of GRB 221009A has a very low metallicity of $\approx0.12$ Z$_{\odot}$ and our resolved host spectrum shows that GRB 221009A occurred in a unique environment in its host characterized by strong H$_2$ emission lines consistent with recent star formation, which may hint at environmental factors being responsible for its extreme energetics.

Motivation & Objective

• To test whether the exceptionally bright GRB 221009A, the brightest GRB ever observed, hosts r-process elements from a collapsar origin.
• To determine whether the supernova associated with GRB 221009A exhibits signatures of r-process nucleosynthesis, as predicted by models of neutron-rich outflows in black hole-forming massive stars.
• To characterize the host galaxy’s metallicity and environment to assess whether extreme conditions there could explain the burst’s luminosity.
• To disentangle the supernova and afterglow components using high-resolution JWST spectroscopy and photometry at rest-frame +168 and +170 days.
• To compare the SN component to known SNe Ic-BL like SN 1998bw to assess its normality and constrain nickel mass.

Proposed method

• Acquired deep near-infrared (NIRSpec) and mid-infrared (MIRI) spectroscopy with the James Webb Space Telescope (JWST) at rest-frame +168 and +170 days post-trigger.
• Fitted the observed spectra using a joint model of a supernova (SN) and power-law afterglow, with extinction laws from [30] and [31] to correct for dust absorption.
• Employed Prospector photometry and spectral fitting to derive host galaxy properties, including gas-phase and stellar metallicities.
• Used H2 emission line ratios to probe the physical conditions in the host’s star-forming region, comparing to fluorescence and shock models.
• Subtracted afterglow contributions using power-law fits (Fν ∝ ν^−0.76±0.07) derived from ALMA and Swift/XRT data to isolate the SN component.
• Compared the extinction-corrected SN spectrum to ground-based late-time spectra of SN 1998bw and SN 2013ge to assess similarity and detect r-process features.

Experimental results

Research questions

• RQ1 Does the supernova associated with GRB 221009A show spectroscopic evidence of r-process elements, as expected from a collapsar origin?
• RQ2 How does the SN component of GRB 221009A compare in luminosity and spectral morphology to known broad-lined Type Ic supernovae like SN 1998bw?
• RQ3 What is the metallicity of the host galaxy and the local environment around GRB 221009A, and does it support the idea of extreme conditions enabling high-energy emission?
• RQ4 Is the afterglow contribution accurately modeled at late times, and does residual flux after subtraction indicate an anomalous SN component?
• RQ5 Do the H2 emission line ratios in the host galaxy point to recent star formation or shock excitation, and what do they imply about the burst's environment?

Key findings

• The supernova component of GRB 221009A is consistent with a typical SN Ic-BL, showing no signs of r-process enrichment, as confirmed by the absence of rare-earth element features in the NIR spectrum.
• The SN is slightly fainter than SN 1998bw at the same phase, with a derived nickel mass of ≈0.09 M⊙, indicating it is not an unusually energetic or broad-lined event.
• The host galaxy has a low gas-phase metallicity of ≈0.12 Z⊙, and the GRB site exhibits a stellar metallicity consistent with this, indicating a metal-poor environment.
• H2 emission lines in the host show strong fluorescence-like ratios, suggesting recent star formation, with a shock model providing a poor fit, indicating a young, active star-forming region.
• Afterglow subtraction using a power-law model (Fν ∝ ν^−0.76±0.07) leaves residual flux in the red, inconsistent with SN 1998bw or SN 2013ge, suggesting the afterglow model may be incomplete at these wavelengths.
• Color-color diagrams (J−H and J−K) of the SN component show no deviation from normal SNe, and no match to r-process enriched SN models, further ruling out significant r-process contribution.