标题： 费米大面积望远镜对耀变体爆发中光谱硬化现象的系统搜索 显示英文标题

标题： A Systematic Search for Spectral Hardening in Blazar Flares with the Fermi-Large Area Telescope

摘要： 耀变体是活动星系核（AGN）的一个子类，通过相对论喷流发射非热辐射，其特征是通量和偏振变化迅速。极端高同步辐射峰耀变体（EHSPs），其同步辐射峰超过10$^{17}$ Hz，对于理解耀变体现象的全部范围和测试喷流物理模型至关重要。然而，已知的极端耀变体数量很少，因此这类天体仍研究不足。本工作旨在利用费米伽马射线空间望远镜上的大面积望远镜（LAT）数据，系统地识别和表征最极端的$\gamma$-射耀变体。重点在于光谱硬化，即在较高能量下，$\gamma$-射线光谱变得更硬，尤其是在爆发期间。这是首次对EHSP群体的光谱硬化进行专门分析，因为以前的研究仅在个别源中探索过这一现象。我们分析了从4FGL-DR2目录中选出的138个耀变体，这些耀变体具有高的同步辐射峰值频率和充分采样的光变曲线。爆发期被自动识别，每个爆发都被分析，并通过基于两种光谱模型似然比的检验统计量来评估光谱硬化的显著性。我们识别出两个有光谱硬化迹象的爆发事件，在4FGL J0238.4$-$3116和PKS 2155$-$304中，后者被两种方法独立检测到，但指的是同一时期。这些事件与统计波动的预期一致，表明光谱硬化是一种罕见现象（<0.1%）。这些结果限制了其频率，并支持了一种平滑变化的幂律耀变体辐射模型，促使未来的多波段研究以澄清这些罕见爆发是否反映了耀变体喷流中的不同物理过程。 显示英文摘要

摘要： Blazars are a subclass of active galactic nuclei (AGN) that emit non-thermal radiation through relativistic jets, characterized by rapid flux and polarization variability. Extreme high synchrotron-peaked blazars (EHSPs), with synchrotron peaks exceeding 10$^{17}$ Hz, are essential for understanding the full range of blazar phenomena and testing jet physics models. However, the number of known extreme blazars is small, so this class of objects remains poorly studied. This work aims to systematically identify and characterize the most extreme $\gamma$-ray blazars using data from the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. The focus is on spectral hardening, where the $\gamma$-ray spectrum becomes harder at higher energies, particularly during flaring episodes. This represents the first dedicated analysis of spectral hardening across a population of EHSPs, as previous studies explored it only in individual sources. We analyze 138 blazars selected from the 4FGL-DR2 catalog with high synchrotron peak frequencies and well-sampled light curves. Flaring periods are automatically identified, and each flare is analyzed, with the significance of spectral hardening assessed through a test statistic based on the likelihood ratio of two spectral models. We identify two flaring episodes with indications of spectral hardening, in 4FGL J0238.4$-$3116 and PKS 2155$-$304, the latter detected independently by both methods but referring to the same period. These events are consistent with expectations from statistical fluctuations, suggesting that spectral hardening is a rare occurrence (< 0.1 %). These results constrain its frequency and support a smoothly varying power-law blazar emission model, motivating future multi-wavelength studies to clarify whether these rare flares reflect distinct physical processes within blazar jets.