Physical origin of pre-prompt afterglow onset in gamma-ray-burst X-ray plateaus: a central-engine and ejecta model for the $L_{0}$–$T_{0}$–$E_{\gamma,\mathrm{iso}}$ scalings, or a quantitative refutation of the prior-activity interpretation

Guidorzi et al. (2025) showed that the X-ray afterglows of ~90% of 463 Swift gamma-ray bursts (GRBs) are well described by a single power law whose true zero time precedes the prompt gamma-ray trigger by a delay $T_{0}$, and discovered tight two- and three-parameter scalings linking the trigger-time luminosity $L_{0}$, the delay $T_{0}$, and the isotropic-equivalent prompt energy $E_{\gamma,\mathrm{iso}}$ across all GRB classes. If taken at face value, these results imply that the afterglow typically begins before the detected prompt emission, pointing to central engines that launch relativistic outflows up to ~$10^{4}$ s earlier than usually assumed. No physical model explains these scalings, and the finding remains contested. The problem is to construct a physical account — engine scenario, ejecta structure, and prompt-trigger mechanism — that reproduces the observed delay distribution and scaling coefficients, scatters, and class-universality, or to quantitatively refute the prior-activity interpretation with an alternative model that reproduces the same population statistics without prior activity.

Background

A gamma-ray burst (GRB) has two characteristic phases: the prompt emission, a brief (milliseconds to hundreds of seconds) flash of gamma rays powered by internal dissipation inside an ultra-relativistic jet launched by a newly formed compact object, and the afterglow, longer-lived multiwavelength emission produced when the relativistic ejecta are decelerated by the surrounding medium through an external shock. Since 2005, the X-Ray Telescope (XRT) aboard the Neil Gehrels Swift Observatory revealed that a large majority of early X-ray afterglow light curves are not simple power-law decays: they begin with a shallow-decaying plateau lasting $10^{3}$–$10^{4}$ s before steepening into the normal decay (Nousek et al. 2006; Zhang et al. 2006). The physical origin of this plateau has remained one of the longest-standing puzzles of GRB afterglow physics. Proposed explanations include prolonged energy injection from a long-lived central engine (e.g., a magnetar, a rapidly rotating strongly magnetized newborn neutron star, or fallback accretion, the slow infall of bound stellar debris onto the newborn black hole), time-dependent shock microphysics, and off-axis jet geometry.

An alternative proposed already in the Swift era (Ioka et al. 2006, in the context of the apparent radiative-efficiency crisis; Yamazaki 2009) is that the plateau is a timing artefact: if the afterglow actually started before the detected prompt emission, then measuring time from the prompt trigger makes an intrinsically ordinary power-law decay appear shallow. Liang et al. (2009) systematically investigated this prior-activity (zero-time) hypothesis on the canonical afterglows among the first ~400 Swift GRBs, comparing them with the 19 events that exhibited a single power law from early to late times: the canonical decay indices, once modelled with the estimated zero time, became comparable to those of the single-power-law sample, lending support to the interpretation proposed by Yamazaki (2009), though the study was limited to the 56-GRB redshift sample then available. Related empirical plateau scalings between the plateau-end luminosity and its duration — the Dainotti relation — were established under the energy-injection reading (Dainotti et al. 2010, 2016), and some plateaus, notably in GRB 070110 and the short GRB 090515, are attributed to internal engine emission rather than the external shock (Troja et al. 2007; Rowlinson et al. 2010).

Guidorzi et al. (2025) revisited the prior-activity scenario with the full Swift sample of 463 GRBs with measured redshifts. They modeled the rest-frame 1–10 keV afterglow luminosity — computed from Leicester-repository 0.3–10 keV observed fluxes with a k-correction — as $L_{x}(t)=L_{0}(1+t/T_{0})^{-\alpha}$, where the true reference (zero) time precedes the prompt trigger by $T_{0}$ and $L_{0}$ is the luminosity at the trigger; the model described ~90% of the sample. Using the 300 bursts with accurate parameters they confirmed an approximately inverse $L_{0}$–$T_{0}$ correlation with ~0.7 dex scatter, with the same slope for long and short GRBs but a higher long-GRB normalization at fixed $T_{0}$: the fitted intercepts differ by $0.86$ dex (a factor $\approx 7$), which the paper's prose rounds to almost ten times. For the 180 bursts with reliable prompt quantities they found the three-parameter relation $\log L_{0}=47.68\pm0.04+(0.73\pm0.04)\log E_{\gamma,\mathrm{iso},52}-(0.87\pm0.04)\log T_{0,3}$ (with $E_{\gamma,\mathrm{iso},52}$ in units of $10^{52}$ erg and $T_{0,3}$ in units of $10^{3}$ s), with 0.4 dex intrinsic dispersion common to all GRB classes and spanning nine decades in $L_{0}$. The inferred delays form an approximately lognormal distribution with mean ~$10^{3}$ s (rest frame) and 0.8 dex dispersion, reaching ~$10^{4}$ s. This suggests that the afterglow typically begins before the detected prompt gamma-rays, with prior activity far more frequent than tacitly assumed — a conclusion reinforced by discoveries with the Einstein Probe (EP), a wide-field soft X-ray monitoring mission, of soft X-ray emission starting tens of seconds to a day before the prompt gamma-rays in several events (e.g., EP240315a, EP241026a), and by the day-long X-ray transient GRB 250702B (Levan et al. 2025).

The interpretation faces serious physical obstacles that the 2025 paper explicitly leaves open. First, no model explains why prompt dissipation and early-ejecta energetics produce the observed $L_{0}$–$T_{0}$ and $L_{0}$–$T_{0}$–$E_{\gamma,\mathrm{iso}}$ dependences, or why the same relations hold for long GRBs (produced by collapsars, the collapse of massive stars) and short GRBs (produced by compact-object mergers). Second, delays up to ~$10^{4}$ s require engines that launch jets, or at least substantial relativistic ejecta, far earlier and longer than standard theory assumes. Third, for short GRBs, delays of tens to $10^{4}$ s are in tension with compact-object-merger timing: the binary-neutron-star merger GW170817 was followed by the weak, off-axis GRB 170817A after only 1.7 s (Abbott et al. 2017). Fourth, ~10% of the sample is not fitted by the model — including internal plateaus powered by the central engine and exceptionally complex events like GRB 221009A, the brightest gamma-ray burst yet detected — and systematic multiwavelength corroboration across the population, together with selection effects that bias the observed $T_{0}$ distribution against small delays, remains incomplete. Competing interpretations of the plateau, such as long-lived mildly relativistic reverse shocks in stratified ejecta (Sadeh et al. 2026), have begun to reproduce some plateau phenomenology without prolonged engine activity, but none has addressed the new population scalings.

Problem Statement

Provide a physical account of the pre-prompt onset of the GRB X-ray afterglow — a self-consistent central-engine scenario, ejecta structure and energy budget, and a mechanism that triggers the prompt gamma-ray emission relative to the outflow launch — that predicts afterglow onset preceding the detected prompt trigger by a delay $T_{0}$ distributed as observed (approximately lognormal with mean ~$10^{3}$ s rest frame and 0.8 dex dispersion, extending to ~$10^{4}$ s), and that reproduces the empirical scalings of Guidorzi et al. (2025): the approximately inverse $L_{0}$–$T_{0}$ relation with ~0.7 dex scatter (same slope for long and short GRBs, long-GRB normalization higher at given $T_{0}$ by the fitted intercept offset $0.86$ dex, i.e. a factor $\approx 7$, described in the paper's prose as almost ten times), and the three-parameter relation $\log L_{0}=47.68+0.73\log E_{\gamma,\mathrm{iso},52}-0.87\log T_{0,3}$ with ~0.4 dex intrinsic dispersion common to all GRB classes, with each model parameter fixed a priori on independent physical grounds and its independent source stated explicitly, so that the quoted coefficients are predicted rather than fitted — a reverse-fitted kinematic ansatz is not a physical account. Alternatively, quantitatively refute the prior-activity interpretation by an alternative physical model that reproduces, without prior activity, both the successful single-power-law fits with early reference times for ~90% of the 463-GRB sample and the observed population scalings, at matched statistical treatment (the same sample selection and fitting procedure as the prior-activity analysis of Guidorzi et al. 2025). A complete answer must state how the model accounts for each of the following — as physical realities under the prior-activity branch, or as observational appearances explained without prior activity under the alternative branch — with no single item below sufficing on its own to resolve or refute the interpretation: (i) the class-universality of the relations across long (collapsar) and short (merger) GRBs; (ii) engine longevity constraints implied by delays up to $10^{4}$ s; (iii) compact-object-merger timing for short GRBs, given the 1.7 s delay between GW170817 and GRB 170817A; (iv) the ~10% of GRBs not fitted by the prior-activity model, including internal plateaus (e.g., GRB 070110, GRB 090515) and complex events such as GRB 221009A; and (v) the selection effects and multiwavelength (optical, very-high-energy) constraints that shape and corroborate the observed $T_{0}$ distribution.

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