GRB 250706B/C: Insight-HXMT Discovery of a High-Luminosity Burst as a Candidate for Fallback-Regulated Accretion in the Prompt Emission

Abstract

Fallback accretion in collapsar models is often associated with underluminous gamma-ray bursts (GRBs), leading to the widespread view that fallback-fed engines may be intrinsically inefficient at producing high-luminosity events. In this Letter, we present GRB 250706B/C, a luminous long GRB observed by \textit{Insight}-HXMT that exhibits an unusual combination of extreme short-timescale variability and coherent large-scale temporal evolution. The prompt emission contains at least 79 resolved pulses and a minimum variability timescale of $\sim 11$ ms. The pulse widths are nearly independent of photon energy and span a broad distribution with a median FWHM of $\sim 0.30$ s, while the waiting times between adjacent pulses have a median of $\sim 0.38$ s. The prompt-emission envelope exhibits a prolonged rise described by $F(t)\propto (t-t_0{)}^{0.47\pm 0.01}$ followed by a rapid decline. Despite substantial pulse-to-pulse fluctuations, neither the pulse widths nor the waiting times show significant secular evolution during the main emission episode. These features indicate the coexistence of two distinct temporal components, including a slow evolving rising luminosity envelope and rapid stochastic variability. Such behavior is consistent with scenarios in which a time-dependent engine-feeding history regulates the large-scale emission while internal dissipation within the relativistic outflow produces the pulse structure. Within this context, GRB~250706B/C may represent a fallback-fed collapsar operating on a high-luminosity branch, suggesting that fallback itself does not necessarily limit the luminosity scale of GRBs.

Accepted for publication in The Astrophysical Journal Letters