Gamma-ray burst X-ray plateaus as evidence of pre-prompt afterglow

¹ Department of Physics and Earth Science, University of Ferrara, Via Saragat 1, I-44122 Ferrara, Italy
² INFN – Sezione di Ferrara, Via Saragat 1, 44122 Ferrara, Italy
³ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Piero Gobetti 101, 40129 Bologna, Italy
⁴ Astrophysics Research Institute, Liverpool John Moores University, Liverpool Science Park IC2, 146 Brownlow Hill, Liverpool L3 5RF, UK
⁵ INAF, Osservatorio Astronomico d’Abruzzo, Via Mentore Maggini snc, 64100 Teramo, Italy

^⋆ Corresponding author: guidorzi@fe.infn.it

Received: 30 July 2025
Accepted: 22 September 2025

Abstract

Context. Most gamma-ray burst (GRB) X-ray afterglow light curves are characterised by a plateau, followed by a normal power-law decay, interpreted as afterglow emission – radiation emitted by the shocked interstellar medium swept up by the blast wave. Despite numerous alternative interpretations, the origin of the plateau remains unclear. In the early years of the Neil Gehrels Swift Observatory, it was suggested that the plateau might be afterglow radiation that began before the prompt gamma-ray emission, with its time profile appearing as an artefact of assuming the prompt gamma-ray emission start time as zero (the so-called ‘prior activity model’).

Aims. We aim to test the plausibility of the prior activity model by leveraging the current Swift sample of early X-ray afterglows of GRBs with measured redshifts, which is more than eight times larger than the one originally used (463 vs. 56).

Methods. 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 T₀ and the X-ray luminosity L₀ at the trigger time as free parameters. We tested each case applying both χ² and runs tests.

Results. For 90% of our sample, the model provides a successful description. In ten cases, the afterglow peak is identified and modelled appropriately. Using the 300 GRBs with accurate parameter estimates, we confirm the anti-correlation between L₀ and T₀ with 0.7 dex scatter. In addition, selecting the sub-sample of 180 from the literature with reliable estimates of the isotropic-equivalent released energy E_γ, iso, peak luminosity L_γ, iso, and intrinsic peak energy E_p, i of the ν F_ν spectrum of the prompt gamma-ray emission, we find a correlation between L₀, T₀, and E_γ, iso (0.4 dex scatter) over nine decades in L₀ and common to all kinds of GRBs.

Conclusions. The afterglow likely begins in most cases before the start of the detected prompt gamma-ray emission by a lognormally-distributed rest-frame delay with a mean of 10³ s and 0.8 dex dispersion. As also suggested by the recent discoveries from the Einstein Probe of X-ray emission beginning long before the prompt gamma rays, our results suggest that prior activity may be much more frequent than what has tacitly been assumed.