Multi-wavelength analysis of the progenitor of GRB 230307A via Bayesian model comparison

TL;DR

GRB 230307A is analyzed with a Bayesian, multi-wavelength framework to discriminate among compact binary and tidal disruption progenitors. The study combines afterglow modeling with kilonova scenarios and uses nested sampling to compute Bayes factors and predictive scores, identifying a CBC magnetar spin-down kilonova with Ni decay as the preferred model, compatible with either a BNS or NS-WD progenitor. The inferred ejecta properties include $M_{ej}\approx0.06$ and $M_{Ni}\approx4\times10^{-4}$ in solar masses, and the binary's tidal deformability is constrained, yielding insight into the neutron-star equation of state. The large observed offset can be explained by a natal kick with $v'_k$ in the 100–270 km s$^{-1}$ range and initial separation $a_0$ of ~2–3 R⊙, underscoring the value of joint EM observations for probing compact binary evolution and informing future multi-messenger astrophysics.

Abstract

GRB 230307A is one of the brightest long-duration gamma-ray bursts (GRBs) ever detected, yet its progenitor remains uncertain due to the variety of plausible astrophysical scenarios. In this work, we investigate four possible progenitors for GRB 230307A: a binary neutron star (BNS), a neutron star--white dwarf (NS--WD) system, a neutron star--black hole (NS--BH) merger, and a tidal disruption event (TDE) involving a white dwarf and a supermassive black hole. Additionally, we explore three distinct central engine models powering the kilonova associated with the BNS: radioactive decay of $r$-process nuclei in a two-component ejecta model, a magnetar-driven model including magnetic dipole spin-down, and a combined model of magnetar spin-down with ${}^{56}$Ni radioactive decay. We perform Bayesian multi-wavelength light-curve analyses using physically motivated models and priors, and evaluate model performance through Bayes factors and leave-one-out cross-validation (LOO) scores. Our results show a statistical preference for a BNS or NS--WD progenitor producing a kilonova powered by a magnetar and ${}^{56}$Ni decay, characterized by a ${}^{56}$Ni mass of $\sim4\times10^{-4}\,M_{\odot}$ and an ejecta mass of $0.06\,M_{\odot}$. Furthermore, under the assumption of a BNS origin within this model, we infer binary component masses of $m_{1} = 1.81^{+0.46}_{-0.61}\,M_{\odot}$ and $m_{2} = 1.61^{+0.65}_{-0.41}\,M_{\odot}$, with a dimensionless tidal deformability of $\tildeΛ = 471^{+318}_{-395}$. From the component mass posteriors, we infer that the observed offset can be explained by a natal kick as long as the systemic velocity is nearly aligned with the pre-kick orbital motion. In this case, the required kick velocity (co-moving frame) and binary separation range within $v'_{\mathrm{k}}\sim100$--$150~\mathrm{km\,s^{-1}}$, and $a_0\sim2$--$3~R_{\odot}$, respectively.

Figures (5)

• Figure 1: GRB 230307A multi-wavelength light curves. Solid lines show the best-fitting model curves obtained from joint Bayesian inference for different progenitor scenarios (shown in different colors; see App.\ref{['app:inference_resu']} for details). Black dots represent observational data points with uncertainties, and triangles denote upper limits. In some cases, the error bars are not visible because their size is smaller than the plotting scale.
• Figure 2: Multi-wavelength light curves of GRB 230307A from the afterglow-only emission (green) and with the addition of a kilonova, modeled using the BNS two-component model (blue) and the CBC magnetar spin-down model (orange), for the X-ray, optical, and NIR bands. Black dots represent observational data with uncertainties, and triangles indicate upper limits. For certain data points, the error bars are not shown because they are smaller than the scale of the plot.
• Figure 3: Posterior comparison of GRB afterglow model parameters obtained from joint analyses with different possible astrophysical scenarios. The subplots show the residuals relative to the CBC magnetar spin-down model, defined as $2\times\left(\rm{PDF_{KN\, model}}-\rm{PDF_{ref.}}\right)/\left(\rm{PDF_{KN\, model}}+\rm{PDF_{ref.}}\right)$. The shaded gray region denotes the 1$\sigma$ credible interval of the reference model's posterior distributions.
• Figure 4: Corner plot of the binary component masses ($m_1, m_2$) and dimensionless tidal deformability ($\Lambda$) from inference results of the magnetar spin‑down model (orange) and BNS two-component model (blue). The top panel shows the final inference for the magnetar spin‑down model.
• Figure 5: Scatter plots for parameter combinations of $v'_\mathrm{k}-a_0$ (upper) and $\Delta t_\mathrm{P}-a_0$ (lower), consistent with the observed merger offset $\ell_\mathrm{obs} = 38.9$ kpc, as derived from equations (\ref{['integralAp']}-\ref{['vk']}). Colors indicate different assumed values of $\theta_{\mathrm{k}}$.