Revisiting the Multiwavelength Data of GRB 240825A: Implications for a Two-Stage Central Engine and a Dusty Environment

Abstract

Reanalyzing the multi-wavelength data of GRB 240825A, we identify two distinct prompt-emission episodes, i.e. an early, spectrally soft plateau and a subsequent bright, spectrally hard gamma-ray pulse. We show that this signature is consistent with a two-stage central engine model, in which the early soft plateau is powered by the dipolar spin-down radiation of a transient proto-magnetar, while the later hard pulse arises from a relativistic jet launched by fallback accretion onto the nascent black hole via the Blandford-Znajek mechanism. Adopting simulation-motivated initial black hole (BH) masses ($M_{\mathrm{BH},0}$) and spins ($a_0$), our fits to the second episode yield total masses supplied to the post-collapse disk of $M_{\mathrm{fb}}=0.45$--$0.96M_{\odot }$, fallback radii of $r_{\mathrm{fb}}=16.9$--$17.6r_g$, and characteristic fallback timescales of $t_{\mathrm{fb}}=0.13$--$0.25$ s. The corresponding maximum inner accretion rates range from $0.18$ to $0.70M_{\odot }{\mathrm{s}}^{-1}$, which is sensitive to $a_0$. Joint spectral fits with optical-X-ray afterglow data reveal a coupled temporal evolution of dust extinction and equivalent hydrogen column density. Incorporating this extinction evolution, an external forward-reverse shock model in a wind-like medium can account for the radio-to-GeV band afterglow data. Although ambient dust and the bright host galaxy may obscure any associated supernova, late-time optical reddening remains compatible with a supernova contribution. Applying the empirical relation between the prompt spectral peak energy and the SN peak bolometric magnitude gives an illustrative estimate of $M_{\mathrm{bol},\mathrm{peak}}=-19.46\pm 0.35$ mag for a putative associated SN. These results support a two-stage evolution of the central engine and variable dusty environment of GRB 240825A, which provides insights into the nature of its progenitor.

arXiv comment16 pages, 5 figures, 3 tables. Accepted for publication in ApJ