典型风吸积X射线脉冲星Vela X-1的IXPE观测
IXPE observations of the quintessential wind-accreting X-ray pulsar Vela X-1
AI总结：
利用IXPE首次观测典型风吸积X射线脉冲星Vela X-1，测得相位平均低偏振度并揭示其随能量和脉冲相位的显著变化。
摘要：
吸积X射线脉冲星的辐射曾被预期具有高偏振度，一些估计认为偏振度可高达80%。然而，为了检验不同模型并阐明发射过程及发射区几何结构，需要对X射线脉冲星进行相位分辨和能量分辨偏振测量。在此，我们展示使用Imaging X-ray Polarimetry Explorer（IXPE，成像X射线偏振探测器）对吸积X射线脉冲星Vela X-1进行观测所获得的首批结果。Vela X-1被认为是风吸积大质量X射线双星系统的典型范例，该系统由一颗高磁化中子星组成，它从其超巨星伴星吸积物质。对Vela X-1相位平均数据的光谱偏振分析揭示，在偏振角（PA）为-47.3±5.4度时，偏振度（PD）为2.3±0.4%。较低的PD与在其他X射线脉冲星上获得的结果一致，并且可能与中子星大气的逆温度结构有关。能量分辨分析显示，5 keV以上的PD达到6–10%，并且与2–3 keV能段的数据相比，PA存在90度的差异。相位分辨光谱偏振分析发现PD处于0–9%范围内，PA在-80至40度之间变化。
英文摘要：
The radiation from accreting X-ray pulsars was expected to be highly polarized, with some estimates for the polarization degree of up to 80%. However, phase-resolved and energy-resolved polarimetry of X-ray pulsars is required in order to test different models and to shed light on the emission processes and the geometry of the emission region. Here we present the first results of the observations of the accreting X-ray pulsar Vela X-1 performed with the Imaging X-ray Polarimetry Explorer (IXPE). Vela X-1 is considered to be the archetypal example of a wind-accreting high-mass X-ray binary system, consisting of a highly magnetized neutron star accreting matter from its supergiant stellar companion. The spectro-polarimetric analysis of the phase-averaged data for Vela X-1 reveals a polarization degree (PD) of 2.3$\pm$0.4% at the polarization angle (PA) of -47.3$\pm$5.4 deg. A low PD is consistent with the results obtained for other X-ray pulsars and is likely related to the inverse temperature structure of the neutron star atmosphere. The energy-resolved analysis shows the PD above 5 keV reaching 6-10%, and a 90 deg difference in the PA compared to the data in the 2-3 keV range. The phase-resolved spectro-polarimetric analysis finds a PD in the range 0-9% with the PA varying between -80 and 40 deg.
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