GRB X-ray plateaus as evidence that the afterglow begins before the prompt gamma-ray emission

TL;DR

This study tests the prior activity model for GRB X-ray plateaus by modeling rest-frame afterglows with a reference time $T_0$ before the prompt emission and a luminosity scale $L_0$ at trigger. Using a large Swift sample with measured redshifts, they fit $L_x(t)=L_0(1+t/T_0)^{-p}$ via Bayesian MCMC to obtain $L_0$, $T_0$, and $p$ (or equivalently $ abla$) for about 300 GRBs. They report a robust anti-correlation between $L_0$ and $T_0$ with ~0.7 dex scatter and a three-parameter $L_0$–$T_0$–$E_gamma_iso$ relation that spans nine decades in $L_0$, with a four-parameter version including the decay index $ abla$ reducing the scatter to ~0.37 dex. The distribution of $T_0$ is log-normal with mean around $10^3$ s, implying the afterglow often begins before the prompt emission, consistent with persistent engine activity and with Einstein Probe findings of early X-ray emission.

Abstract

Most GRB X-ray afterglow light curves are characterised by a plateau, followed by a normal power-law decay interpreted as afterglow emission. Despite the numerous alternative interpretations, the origin of the plateau remains unclear. In the early years of Swift, it was suggested that the plateau might be afterglow radiation, that started before the prompt gamma-ray emission, and its time profile would be an artefact of assuming the start time of the prompt gamma-ray emission as zero time (the so-called "prior activity model"). We aim to test this scenario by leveraging the current Swift sample of early X-ray afterglows of GRBs with measured redshifts. We modelled the GRB rest-frame X-ray afterglow luminosities assuming a simple power-law with the true reference time preceding the prompt gamma-ray emission trigger time by T_0 and the X-ray luminosity L_0 at the trigger time as free parameters. For 90% GRBs of our sample, the model provided a successful description. In 10 cases the afterglow peak is identified and modelled appropriately. Using the 300 GRBs with accurate parameters' estimates, we confirm the anti-correlation between L_0 and T_0 with 0.7 dex scatter. In addition, selecting the subsample of 180 from the literature with reliable estimates of isotropic-equivalent energy E_gamma,iso, peak luminosity L_gamma,iso, and intrinsic peak energy E_p,i of the nuFnu spectrum of the prompt gamma-ray emission, we find a correlation between L_0, T_0, and E_gamma,iso (0.4 dex scatter) over nine decades in L_0 and common to all kinds of GRBs. The afterglow likely begins in most cases before the start of the detected prompt gamma-ray emission. As also suggested by the recent discoveries of Einstein Probe of X-ray emission starting long before the prompt gamma-rays, our results suggest that the occurrence of prior activity could be much more frequent than what has tacitly been assumed so far.

Figures (4)

• Figure 1: Top: example of LC successfully modelled with Eq. \ref{['eq:mod']}. Bottom: example of LC which exhibits a peak following the steep decay and which was modelled with Eq. \ref{['eq:AGmod']} as the afterglow rise caused by the deceleration of the relativistic ejecta. This example is one of the four out of nine GRBs whose rise was succesfully modelled as RS emission.
• Figure 2: Top: $L_0$ vs. $T_0$ for two separate groups: (i) L-GRBs (red circles, excluding the AG-rise cases), whose best-fit PL and 1$\sigma$ uncertainty region are shown by the red solid line and red-shaded area, respectively; (ii) S+SEE-GRBs, blue squares and cyan pentagons, respectively, whose best-fit PL is shown with the blue solid line within the blue shaded area of the 1$\sigma$ region. The L-GRBs of the AG-rise sample (green stars) were not used for the modelling. Also shown are the corresponding marginalised distributions. Bottom: $L_0$-$T_0$-$E_{\gamma,{\rm iso}}$ relation for the 180 GRBs with accurate measurements available, along with the best-fit model and 2$\sigma$ uncertainty region.
• Figure 3: Top: observed XRT LC of 241213A in the observer frame, which was also detected by Einstein Probe/WXT 105 s earlier (see Table \ref{['tab:EP']}). A successful modelling with a different zero time yields $T_0=250_{-50}^{+70}$ s. Bottom: same LC as in the top panel, but with the reference time set to 250 s prior to the Swift trigger time. The LC is modelled with a SPL. The observed delay between soft X- and gamma-rays is comparable with the value inferred from the prior activity model.
• Figure 4: Top: all the LCs displayed by shifting the rest-frame zero time backward by $T_0$. Red, blue, cyan, and green lines correspond to L-GRBs, S-GRBs, SEE-GRBs, and AG-rise cases, respectively. The grey portions were ignored by the modelling, being interpreted as internal activity. Bottom: same data as in the top panel, except that the reference time coincides with the BAT trigger time.