The Double-Burst Nature and Early Afterglow Evolution of Long GRB 110801A

TL;DR

GRB 110801A exhibits a distinct double-burst prompt structure with an optical rise preceding the second high-energy episode, suggesting two separate emission components. The authors perform a comprehensive multi-wavelength analysis, including joint broadband spectral fitting (optical to $\gamma$-rays) and afterglow modeling with reverse and forward shocks, to disentangle prompt and afterglow contributions. They find that a two-component spectral model, comprising a power-law and a Band function, best describes the second prompt episode, with the power-law component linked to the first burst's afterglow and the Band component to the second burst's prompt emission; a synchrotron interpretation for the high-energy emission is also viable. The afterglow modeling yields $Γ_0 \sim 60$, $θ_j \sim 0.09$ and $E_{k,iso} \sim 10^{54.8}$ erg, consistent with a strong, collimated jet and providing constraints on the microphysical parameters, enabling a coherent picture of the double-burst GRB and its early afterglow dynamics.

Abstract

We present a comprehensive temporal and spectral analysis of the long-duration gamma-ray burst GRB 110801A, utilizing multi-band data from the Neil Gehrels Swift Observatory and ground-based telescopes. The $γ$-ray emission exhibits a distinct two-episode (``double-burst'') structure. Rapid follow-up observations in the optical and X-ray bands provide full coverage of the second burst. The optical light curve begins to rise approximately 135 s after the trigger, significantly preceding the second emission episode observed in X-rays and $γ$-rays at $\sim 320$ s. This chromatic behavior suggests different physical origins for the optical and high-energy emissions. Joint broadband spectral fitting (optical to $γ$-rays) during the second episode reveals that a two-component model, consisting of a power-law plus a Band function, provides a superior fit compared to single-component models. We interpret the power-law component as the afterglow of the first burst (dominating the optical band), while the Band component is attributed to the prompt emission of the second burst (dominating the high-energy bands). A physical synchrotron model is also found to be a viable candidate to explain the high-energy emission. Regarding the afterglow, the early optical light curve displays a sharp transition from a rise of $\sim t^{2.5}$ to $\sim t^{6.5}$, which is well-explained by a scenario involving both reverse shock (RS) and forward shock (FS) components. We constrain the key physical parameters of the burst, deriving an initial Lorentz factor $Γ_0 \sim 60$, a jet half-opening angle $θ_j \sim 0.09$, and an isotropic kinetic energy $E_{\rm k,iso} \sim 10^{54.8}$ erg.

Figures (5)

• Figure 1: Multi-band emission light curves of GRB 110801A. Data are collected by Swift BAT (green), XRT (red), and UVOT. Note that the WHITE-band data are in orange and U-band data are in cyan. The vertical dashed lines represent the start and end times of the intervals selected for the spectral analysis.
• Figure 2: The SEDs of GRB 110801A prompt emission. (a)(b) PL only: spectral fitting of the 319--379 s and 389--449 s interval (PL only model). (c)(d) PL with Band: spectral fitting of the 319--379 s and 389--449 s interval (PL with Band model). (e)(f) PL with BB: spectral fitting of the 319--379 s and 389--449 s interval (PL with BB model). (g)(h) PL with SYN model: spectral fitting of the 319--379 s and 389--449 s interval (PL with SYN model). Dotted lines represent the unabsorbed PL component, dash-dot lines represent the unabsorbed Band or BB or SYN component, and the solid gray line represents the predictions generated by the model.
• Figure 3: The broad band SED of GRB 110801A afterglow from $T_0+4500$ to $T_0+58500$ s with the equivalent photon arrival time of $\sim13400$ s after the BAT trigger. Blue points are observed data of XRT from 0.3 keV to 10.0 keV. Colorful points are the observed data in different UV/optical bands. Gray solid lines represent the predicted absorbed model. Gray and black dash-dot lines represent Lyman-$\alpha$ and Lyman-limit lines. Dashed line is the unabsorbed power-law model with a spectral index of $2.00$.
• Figure 4: GRB 110801A light curves from XRT, UVOT and OAO data. In this scenario, we considers the optical peak as dominated by RS. The open squares represent XRT data excluded from afterglow fitting. Dashed line represents the RS contribution while solid line represents the sum contribution of all components.
• Figure 5: Constrained parameters of the afterglow model for GRB 110801A. Here $\Gamma_0$ is the initial Lorentz factor, $\epsilon_e$ is fraction of energy of relativistic electrons,$\epsilon_b$ is fraction of the energy of magnetic field, $\theta_j$ is the half-opening angle in radians, $E_{\rm k,iso}$ is the isotropic kinetic energy, $p$ is the electron energy distribution index, $n_0$ is the medium number density. And the additional subscripts $f$ and $r$ represent the FS and RS, respectively. $f_{sys,wh}$, $f_{sys,w1}$ and $f_{sys,u}$ are the free parameters to the magnitudes.