AI 中文总结
本研究针对软X射线暂现源EP241113a的特殊能谱与余辉特征，提出其低峰值能量源于低洛伦兹因子脏火球的耗散光球层辐射，并推测喷流由中微子湮灭驱动，为大质量坍缩恒星喷流研究提供新视角。
AI 中文摘要
EP241113a是由爱因斯坦探针（Einstein Probe, EP）探测到的一个软X射线暂现源，其各向同性等效能量（$E_{γ,\rm{iso}}\boldsymbol{\sim} 10^{51}{\\, \rm erg}$）与经典伽马射线暴（gamma-ray bursts, GRBs）相当，但峰值能量（$E_{\rm{p}} \lesssim 1 \\, {\rm keV}$）异常低，使其以>3σ的置信度偏离了典型的$E_{\rm{p}}$--$E_{γ,\rm{iso}}$（Amati）关系。EP241113a的余辉也很不寻常，其平台期光度极低，且截至最新观测尚未发现喷流拐折。这些特性被解释为源于一个高能“脏火球”，即一个能量与经典GRBs相当但重子负载高得多的相对论性喷流，其整体洛伦兹因子为$Γ\boldsymbol{\sim} 20$。在这篇《快报》（Letter）中，我们提出低$E_{\rm{p}}$源于这种低$Γ$喷流中的耗散光球层辐射。随着$Γ$降低，光球层向外移动，而内激波半径向内移动，导致耗散发生在光球层深处，使瞬时辐射以光球层主导。同时，热化半径也向外移动，降低了形成光谱峰值的共动辐射温度。结合较弱的洛伦兹增亮效应，这些作用自然地将观测到的$E_{\rm{p}}$移至软X射线波段。我们进一步指出，这种低$Γ$喷流可能由大质量恒星核心坍缩过程中形成的超吸积盘内的中微子-反中微子（$ν\barν$）湮灭提供能量。与Blandford-Znajek过程相比，中微子湮灭可通过中微子驱动风的重子夹带产生更“脏”的喷流。因此，类似EP241113a的事件为研究大质量坍缩恒星喷流的耗散物理和发射机制提供了新的见解。
英文摘要
EP241113a, a soft X-ray transient detected by the Einstein Probe (EP), has an isotropic-equivalent energy ($E_{γ,\rm{iso}}\sim 10^{51}{\, \rm erg}$) comparable to classical gamma-ray bursts (GRBs) but an exceptionally low peak energy ($E_{\rm{p}} \lesssim 1 \, {\rm keV}$), placing it off the canonical $E_{\rm{p}}$--$E_{γ,\rm{iso}}$ (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 $Γ\sim 20$. In this {\it Letter}, we propose that low $E_{\rm{p}}$ 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 $E_{\rm{p}}$ into the soft X-ray band. We further suggest that such a low-$Γ$ jet may be powered by neutrino--antineutrino ($ν\barν$) 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.
Comments12 pages, 5 figures; published in ApJL
Journal refApJL 1006, L62 (2026)
