Modelling of many gamma-ray burst prompt emission spectra sometimes requires a (quasi) thermal spectral component in addition to the Band function that sometimes leads to a double-hump spectrum, the origin of which remains unclear. In photospheric emission models, a prominent thermal component broadened by sub-photospheric dissipation is expected to be released at the photospheric radius, r_(ph) ∼10¹² cm. We consider an ultra-relativistic strongly magnetized steady outflow with a striped-wind magnetic-field structure undergoing gradual and continuous magnetic energy dissipation at r < r_s that heats and accelerates the flow to a bulk Lorentz factor Γ(r) = Γ_∞min [1, (r/r_s)^(1/3)], where typically r_(ph) < r_s. Similar dynamics and energy dissipation rates are also expected in highly variable magnetized outflows without stripes/field-reversals. Two modes of particle energy injection are considered: (a) power-law electrons, e.g. accelerated by magnetic reconnection, and (b) distributed heating of all electrons (and e±-pairs), e.g. due to magnetohydrodynamic instabilities. Steady-state spectra are obtained using a numerical code that evolves coupled kinetic equations for a photon-electron-positron plasma. We find that (i) the thermal component consistently peaks at (1+z)E_(pk) ∼ 0.2−1 MeV, for a source at redshift z, and becomes sub-dominant if the total injected energy density exceeds the thermal one, (ii) power-law electrons cool mainly by synchrotron emission whereas mildly relativistic and almost monoenergetic electrons in the distributed heating scenario cool by Comptonization on thermal peak photons, (iii) both scenarios can yield a low-energy break, and (iv) the ∼0.5(1+z)⁻¹ keV X-ray emission is suppressed in scenario (a), whereas it is expected in scenario (b). Energy-dependent linear polarization can differentiate between the two particle heating scenarios.

中文翻译：

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磁化流出中逐渐耗散的 GRB 谱

许多伽马射线暴瞬发发射光谱的建模有时需要（准）热光谱分量，以及有时会导致双峰光谱的带函数，其起源尚不清楚。在光球发射模型中，预计会在光球半径 r_(ph) ∼10¹² cm 处释放由亚光球耗散加宽的显着热分量。我们考虑具有条纹风磁场结构的超相对论强磁化稳定流出，在 r < r_s 处经历逐渐和连续的磁能耗散，加热并加速流动到体积洛伦兹因子 Γ(r) = Γ_∞min [ 1, (r/r_s)^(1/3)]，其中通常 r_(ph) < r_s。在没有条纹/场反转的高度可变的磁化流出中也预期有类似的动力学和能量耗散率。粒子能量注入的两种模式被考虑：（a）幂律电子，例如通过磁重联加速，以及（b）所有电子（和e±-对）的分布式加热，例如由于磁流体动力学不稳定性。稳态光谱是使用数字代码获得的，该代码为光子-电子-正电子等离子体演化出耦合动力学方程。我们发现 (i) 对于红移 z 处的源，热分量始终在 (1+z)E_(pk) ∼ 0.2−1 MeV 处达到峰值，如果总注入能量密度超过热能密度，则热分量变得次要，(ii) 幂律电子主要通过同步加速器发射冷却，而分布式加热场景中的温和相对论和几乎单能电子通过热峰值光子的 Comptonization 冷却，(iii) 两种情况都可以产生低能量中断，以及 (iv) ∼0.5(1+z)⁻¹ keV X 射线发射在场景 (a) 中被抑制，而在场景 (b) 中是预期的。依赖于能量的线性极化可以区分两种粒子加热方案。