[论文解读] A different nature between the radio AXPs in comparison to the others SGRs/AXPs

Jaziel G. Coelho, M. Malheiro|arXiv (Cornell University)|Jul 30, 2013

Pulsars and Gravitational Waves Research参考文献 4被引用 3

一句话总结

本文提出，射电波瓣型AXPs（异常X射线脉冲星）是磁場與高B脈衝星相似的自轉驅動中子星，與其他SGRs/AXPs在本質上不同，後者可能為磁化白矮星。研究顯示射電AXPs的X射線光度與自轉能量損失之間存在線性對數關係，支持其為中子星性質；而非射電SGRs/AXPs的X射線光度則與自轉損失功率無關，呈現近似恆定，顯示其起源不同。

ABSTRACT

SGRs/AXPs are considered a subclass of pulsars powered by magnetic energy and not by rotation, as normal radio pulsars. They are understood as strongly magnetized neutron star, with large periods of rotation $P\sim(2-12)$ s, and large spin-down, with typical $\dot{P}\sim(10^{-13}-10^{-10})$ s/s in contrast to $\dot{P}\sim10^{-15}$ s for ordinary pulsars. Their persistent X-ray luminosity, as well as the bursts and flares typical of these sources, are instead believed to be powered by the decay of their ultrastrong magnetic field. SGRs/AXPs typically have a larger X-ray luminosity that can not be explained by their spin-down luminosity ($L_X>\dot{E}_{ m rot}$), unlike rotation-powered pulsars. However, the recent discovery of radio-pulsed emission in four of this class of sources, where the spin-down rotational energy lost $\dot{E}_{ m rot}$ is larger than the X-ray luminosity $L_X$ during the quiescent state - as in normal pulsars - opens the question of the nature of these radio sources in comparison to the others of this class. In this contribution, we show that the radio SGRs/AXPs obey a linear log-log relation between $L_X$ and $\dot{E}_{ m rot}$, very similar to the one satisfied by X-ray and gamma-ray neutron star pulsars, suggesting their neutron star nature. Furthermore, we show that almost all the high-B pulsars are also near the line found for the radio AXPs. In contrast, for almost all the others SGRs/AXPs, $\log L_X$ does not vary too much as function of $\log \dot{E}_{ m rot}$, a phenomenology not shared by X-ray neutron star pulsars, suggesting a different nature for these sources.

研究动机与目标

• 探討射電波瓣型AXPs與其他SGRs/AXPs之間的根本性差異。
• 確定射電AXPs的X射線光度是否由自轉能量損失驅動，如一般脈衝星一般。
• 測試非射電SGRs/AXPs並非磁星，而是具有恆定X射線發射、與自轉損失無關的磁化白矮星的假說。
• 根據其射電活動與自轉損失特性，釐清SGRs/AXPs中X射線光度的物理來源。

提出的方法

• 作者分析來自觀測數據的SGRs/AXPs的X射線光度（$L_X$）與自轉能量損失（$\dot{E}_{\rm rot}$）。
• 對射電波瓣型AXPs的$ L_X $與$ \dot{E}_{\rm rot} $進行對數-對數線性擬合，得出$\log L_X = \log(6.2265 \times 10^7) + 0.7239 \log \dot{E}_{\rm rot}$，與中子星脈衝星一致。
• 將此關係與磁星基本平面及已知高B脈衝星比較，顯示射電AXPs與自轉驅動中子星一致。
• 與非射電SGRs/AXPs的行為對比，後者在不同$ \dot{E}_{\rm rot} $下表現出近乎恆定的$ L_X $，暗示其能量來源不同。
• 評估兩類群體的X射線效率$ \eta_X = L_X / \dot{E}_{\rm rot} $，發現射電AXPs的值為$10^{-2} - 10^{-1}$，與中子星一致，但對白矮星而言過高。
• 利用磁星基本平面與觀測約束，主張非射電SGRs/AXPs更適用於磁化白矮星模型，其磁偶極矩為$10^{34} - 10^{36}$ emu。

实验结果

研究问题

• RQ1 射電波瓣型AXPs是否遵循與一般中子星脈衝星相同的X射線光度-自轉損失功率關係？
• RQ2 為何非射電SGRs/AXPs即使自轉能量損失變化，其X射線光度仍近乎恆定？
• RQ3 射電AXPs的X射線輻射是否由自轉能量驅動（如一般脈衝星），或由磁能衰減驅動（如磁星）？
• RQ4 射電AXPs的觀測X射線效率能否由中子星模型解釋，還是需要其他致密天體？
• RQ5 非射電SGRs/AXPs是否具有不同物理起源，例如為磁化白矮星而非中子星？

主要发现

• 射電波瓣型AXPs在X射線光度與自轉能量損失之間呈現線性對數關係：$\log L_X = \log(6.2265 \times 10^7) + 0.7239 \log \dot{E}_{\rm rot}$，與自轉驅動中子星一致。
• 射電AXPs的X射線效率$ \eta_X = L_X / \dot{E}_{\rm rot} $為$10^{-2} - 10^{-1}$，雖高於一般脈衝星，但仍與中子星模型相符。
• 非射電SGRs/AXPs在廣泛的$ \dot{E}_{\rm rot} $範圍內表現出近乎恆定的X射線光度，顯示其能量來源非來自自轉動能。
• 非射電SGRs/AXPs的觀測X射線光度更適用於磁化白矮星模型，其磁偶極矩為$10^{34} - 10^{36}$ emu。
• 射電AXPs在傳統意義上並非磁星，因其X射線輻射由自轉能量驅動，而非磁能衰減。
• 射電AXPs與高B脈衝星在$ L_X $–$ \dot{E}_{\rm rot} $平面上的對齊，支持其分類為自轉驅動中子星。