測量小尺度星際磁場:基於脈衝星閃爍的新方法

Abstract

星際介質(ISM)具有高度非均勻性，當脈衝星發出的電磁波穿越這些不規則結 構時，會產生干涉現象，進而形成脈衝星閃爍(scintillation)，這類似於星星在 大氣中閃爍的現象。傳統上，要在天文單位(AU)或亞天文單位(sub-AU)尺度上 測量磁場極為困難，因為缺乏足夠解析度的觀測工具。然而，脈衝星閃爍提供了 一個獨特的機會，使我們能夠探索這些微小尺度的結構。 本研究針對一個在 PSR B0834+06 歷史資料中發現的小尺度結構進行探討， 該結構被稱為「1毫秒特徵(1-ms feature)」，在閃爍模型中被視為一個次級透 鏡。我們結合脈衝星閃爍分析技術、一種新的相位重建方法，以及偏振資料，進 行旋轉量(RM)的測量，並推導對應的磁場。 我們在七個頻率波段上測量了 1毫秒特徵的旋轉量。儘管信號中含有不少雜 訊，但我們仍能對該亞天文單位尺度上的磁場強度設置上限。此方法為未來系統 性地測量類似「1毫秒特徵」的微小結構中的磁場提供了可能，並有望推進我們對 脈衝星計時陣列(PTAs)與星際介質中的磁流體動力學(MHD)紊流的理解。 此方法論也在旋轉量測量有突破性地發展，它克服了電離層的分量，並在旋轉量 測量上有兩個數量級的精確度提升。

The interstellar medium (ISM) is highly inhomogeneous, and as electromagnetic waves from pulsars travel through these irregularities, they experience interference effects—manifesting as pulsar scintillation, analogous to the atmospheric twinkling of stars. Traditionally, measuring magnetic fields on astronomical unit (AU) or sub- AU scales has been challenging due to the lack of suitable probes. However, pulsar scintillation offers a unique window into these small-scale structures. In this study, we investigate a compact structure in the ISM—referred to as the “1-ms feature”—identified in archival data of PSR B0834+06. This feature is modeled as a secondary lens within the scintillation framework. By leveraging pulsar scintillation techniques, a novel phase-retrieval method, and polarized data, we measure the Faraday rotation through the rotation measure (RM) and infer the corresponding magnetic field. We extract RM values for the 1-ms feature across seven frequency bands. Al- though the signal is noisy, our analysis allows us to calculate a magnetic field, and place an upper limit on the magnetic field strength at sub-AU scales. This methodol- ogy establishes a framework for systematic magnetic field measurements in similarly compact features, potentially advancing our understanding of pulsar timing arrays (PTAs) and magnetohydrodynamic (MHD) turbulence in the ISM. This method also bypasses the traditional ionospheric contamination in RM and gives 2 order of magnitude improvement on the measurement of RM.