[论文解读] Predicting the X-ray flux of evolved pulsar wind nebulae based on VHE gamma-ray observations

M. Mayer, J. Brucker+4 more|arXiv (Cornell University)|Feb 7, 2012

Pulsars and Gravitational Waves Research被引用 7

一句话总结

本文提出了一种时变轻子模型，仅基于甚高能（VHE）伽马射线观测即可预测演化脉冲星风云（PWNe）的X射线流量。通过将模型拟合至VHE数据，并考虑轻子年龄、冷却及空间投影效应，该模型成功预测了已识别PWNe的X射线流量，误差在约2倍以内，从而可估算未关联VHE源的可探测X射线信号。

ABSTRACT

Energetic pulsars power winds of relativistic leptons which produce photon nebulae (so- called pulsar wind nebulae, PWNe). Their spectral energy distribution has a double-humped structure: the first hump lies in the X-ray regime, the second in the gamma-ray range. The X-ray emission is generally understood as synchrotron radiation by highly energetic leptons, the gamma-ray emission as Inverse Compton scattering of energetic leptons with ambient photon fields. The spectral evolution is influenced by the time-dependent spin-down of the pulsar and the decrease of the magnetic field strength with time. Thus, the present spectral appearance of a PWN depends on the age of the pulsar: while young PWNe are bright in X-rays and gamma-rays, the X-ray emission of evolved PWNe is suppressed. Hence, evolved pulsar wind nebulae may offer an explanation of the nature of some of the unidentified VHE (very high-energy, E > 100GeV) gamma-ray sources not yet associated with a counterpart at other wavelengths. The purpose of this work is to develop a model which allows to calculate the expected X-ray fluxes of unidentified VHE gamma-ray sources considered to be PWN candidates. Such an estimate may help to evaluate the prospects of detecting the X-ray signal in deep observations with current X-ray observatories in future studies. We present a time-dependent leptonic model which predicts the broad-band emission of a PWN according to the characteristics of its pulsar. For a sample of representative PWNe, the resulting model predictions in the X-ray and gamma-ray range are compared to observations. The comparison shows that the energy flux of the X-ray emission of identified PWNe from different states of evolution can be roughly predicted by the model. This implies the possibility of an estimate of the non-thermal X-ray emission of unidentified VHE gamma-ray sources in case of an evolved PWN scenario.

研究动机与目标

• 基于仅VHE伽马射线观测，开发一种预测演化脉冲星风云（PWNe）X射线流量的预测模型。
• 评估在演化PWN情景下，未关联VHE伽马射线源的X射线辐射可探测性。
• 在建模宽波段能谱能量分布（SED）时，考虑时间依赖的轻子注入、冷却及空间投影效应。
• 通过调整VHE与X射线观测之间分析区域大小和轻子年龄的差异，改进X射线流量预测。
• 通过估算预期的流量水平，使未来深度X射线观测能够更自信地针对未关联的VHE源进行探测。

提出的方法

• 构建一种时变轻子模型，以描述PWNe中非热辐射的演化，结合脉冲星自转能耗损，表达式为 $ \dot{E}(t) = \dot{E}_0 \left(1 + \frac{t}{\tau_0}\right)^{-(n+1)/(n-1)} $。
• 通过拟合至四组具有不同演化阶段的代表性PWNe的VHE伽马射线谱，约束模型的自由参数。
• 模型同时考虑同一相对论性轻子群体产生的X射线同步辐射和VHE伽马射线逆康普顿散射。
• 基于X射线分析区域大小，重新计算轻子年龄和绝热冷却 timescales，其中 $ \tau_{\text{ad}} = \frac{3}{2} t $。
• 通过使用三维几何模型沿视线方向积分发射，校正投影效应，假设轻子喷流速度为 $ c/3 $。
• 最终将模型预测与X射线、伽马射线及VHE伽马射线数据进行比较，并通过调整使X射线流量预测更贴近观测值。

实验结果

研究问题

• RQ1 仅基于VHE伽马射线观测，能否可靠预测演化PWNe的X射线流量？
• RQ2 与观测值相比，轻子年龄、冷却及空间投影效应对预测X射线流量的影响如何？
• RQ3 当考虑VHE与X射线观测之间分析区域大小差异时，该模型在多大程度上提升了X射线流量预测的准确性？
• RQ4 该模型能否用于估算在PWN情景下，未关联VHE伽马射线源的X射线辐射可探测性？
• RQ5 年轻与年老轻子群体在塑造演化PWNe当前X射线与VHE伽马射线SED中分别起何种作用？

主要发现

• 在修正轻子年龄和空间投影效应后，模型对已识别PWNe的X射线能量通量预测值与实测值相差约2倍以内。
• MSH 15-52的X射线通量预测值为 $ 13.6 \times 10^{-12} \, \text{erg} \, \text{cm}^{-2} \, \text{s}^{-1} $，与实测值 $ 12.0 \pm 0.3 \times 10^{-12} \, \text{erg} \, \text{cm}^{-2} \, \text{s}^{-1} $ 接近。
• 对于HESS J1837-069，模型预测值为 $ 1.55 \times 10^{-12} \, \text{erg} \, \text{cm}^{-2} \, \text{s}^{-1} $，而实测值为 $ 1.0 \times 10^{-12} \, \text{erg} \, \text{cm}^{-2} \, \text{s}^{-1} $，不确定性为 $ \pm 0.4 $。
• 模型成功重现了VHE与伽马射线的SED，但最初因分析区域和轻子年龄假设不匹配而高估了X射线通量。
• 在修正轻子年龄和投影效应后，模型的X射线预测与观测值显著一致，证实了其预测能力。
• 研究表明，演化后的PWNe可在VHE伽马波段保持明亮，而其X射线辐射则变得微弱，支持其作为未关联VHE源候选者的角色。