Ending the prompt phase in photospheric models of gamma-ray bursts

¹ Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris (IAP), 98 bis boulevard Arago, 75014 Paris, France
e-mail: filip.alamaa@iap.fr
² Department of Physics, KTH Royal Institute of Technology, and The Oskar Klein Centre, 10691 Stockholm, Sweden
³ Institut Universitaire de France, Ministère de l’Enseignement Supérieur et de la Recherche, 1 rue Descartes, 75231 Paris Cedex 05, France

Received: 17 October 2023
Accepted: 2 January 2024

Abstract

The early steep decay, a rapid decrease in X-ray flux as a function of time following the prompt emission, is a robust feature seen in almost all gamma-ray bursts with early enough X-ray observations. This peculiar phenomenon has often been explained as emission from high latitudes of the last flashing shell. However, in photospheric models of gamma-ray bursts, the timescale of high-latitude emission is generally short compared to the duration of the steep decay phase, and hence an alternative explanation is needed. In this paper we show that the early steep decay can directly result from the final activity of the dying central engine. We find that the corresponding photospheric emission can reproduce both the temporal and spectral evolution observed. This requires a late-time behaviour that should be common to all gamma-ray burst central engines, and we estimate the necessary evolution of the kinetic power and the Lorentz factor. If this interpretation is correct, observation of the early steep decay can give us insights into the last stages of central activity, and provide new constraints on the late evolution of the Lorentz factor and photospheric radius.