Fermi-GBM Observations of GRB 230307A: An Exceptionally Bright Long-Duration Gamma-ray Burst with an Associated Kilonova

TL;DR

Fermi-GBM observed an exceptionally bright, long-duration gamma-ray burst with a kilonova, revealing complex spectral evolution, high Lorentz factors, and insights into a potential new class of merger-related GRBs.

Contribution

This study presents detailed spectral and temporal analysis of GRB 230307A, highlighting its unique brightness, spectral features, and association with a kilonova, suggesting a new long merger class of GRBs.

Findings

Second highest fluence GRB ever observed

Detection of associated kilonova

High inferred bulk Lorentz factor of 1600

Abstract

On March 7th, 2023 the \textit{Fermi} Gamma-ray Burst Monitor observed the second highest fluence gamma-ray burst (GRB) ever, GRB~230307A. With a duration beyond 100~s, GRB~230307A contains a multitude of rapidly-varying peaks, and was so bright it caused instrumental effects in the GBM detectors. The high fluence of this burst, (6.02 $\pm$ 0.02)$\times$10$^{-3}$ erg cm$^{-2}$, prompted rapid follow-up across the electro magnetic spectrum including the discovery of an associated kilonova. GRB~230307A is one of a few long GRBs with an associated compact merger origin. Three main temporal regions of interest are identified for fine time-resolution spectral analysis: triggering pulse, main emission, and late emission, and the parameter evolution is traced across these regions. The high flux of the burst allowed for the statistical preference of a more complex, physically-motivated model, the Double Smoothly Broken Power Law, over typical spectral fitting functions for GRBs. From this model the evolution of the parameters was found to be in accordance with those expected for synchrotron radiation in the fast-cooling regime. Additionally, it was found that the flux experiences a steep decline in late time intervals, a feature which is often attributed to high-latitude emission, which follows the dissipation episodes. Furthermore, GRB~230307A was found to have one of the highest inferred bulk Lorentz factors of $\Gamma = 1600$. GRB~230307A is a noteworthy burst in terms of flux alone, but additionally provides a unique insight into the possible temporal and spectral characteristics of a new long merger class of GRBs.