2026/08/04 by Cui-Yuan Dai, Xiang-Yu Wang, Bing Zhang
Physics and Astronomy · #astro-ph.HE
paper · pdf · doi:10.3847/2041-8213/ae8ba8
published as ApJL 1006, L62 (2026) · 12 pages, 5 figures; published in ApJL
arxiv created 2026/08/04 · arxiv updated 2026/08/05
EP241113a, a soft X-ray transient detected by the Einstein Probe (EP), has an isotropic-equivalent energy (E_γ,\rmiso∼ 1051 \rm erg) comparable to classical gamma-ray bursts (GRBs) but an exceptionally low peak energy (E_\rmp \lesssim 1 \rm keV), placing it off the canonical E_\rmp--E_γ,\rmiso (Amati) relation at >3σ confidence. The afterglow of EP241113a is also unusual, with an extremely low plateau luminosity and no jet break up to the latest observation. Such properties have been interpreted as arising from an energetic ``dirty fireball,'' i.e., a relativistic jet with energy comparable to classical GRBs but much higher baryon loading, leading to a bulk Lorentz factor of Γ∼ 20. In this \it Letter, we propose that low E_\rmp arises from dissipative photospheric emission in such a low-Γ jet. As Γ decreases, the photosphere shifts outward while the internal shock radius moves inward, causing dissipation to occur well below the photosphere and making the prompt emission photosphere-dominated. Meanwhile, the thermalization radius also moves outward, reducing the comoving radiation temperature where the spectral peak is established. Combined with weaker Lorentz boosting, these effects naturally shift the observed E_\rmp into the soft X-ray band. We further suggest that such a low-Γ jet may be powered by neutrino--antineutrino (νν) annihilation in a hyperaccreting disk formed during the core collapse of a massive star. Compared with the Blandford--Znajek process, neutrino annihilation can produce a dirtier jet through baryon entrainment from a neutrino-driven wind. EP241113a-like events therefore offer new insight into the dissipation physics and launching mechanisms of jets from collapsing massive stars.