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反常X射线脉冲星/软伽马射线重复暴:带壳层的奇异星?

AXPs/SGRs: strange stars with crusts?

  • 摘要: 反常X射线脉冲星(AXP)和软伽马射线重复暴(SGR)的辐射被认为是由其强磁场的耗散驱动的,称为磁星(或译磁陀星、超磁星)。通过结合计时和能量观测结果,可以很容易地理解磁星模型。从计时角度来看,假设磁偶极辐射计算的AXP和SGR的磁场强度非常强。从能量角度来看,AXP和SGR的X射线/软伽马射线光度大于其自转能量损失率。因此,可以合理地假设高能辐射来自磁能耗散,磁星模型已被广泛讨论(或接受)。然而,我们认为:(1)通过假设自转能量损失由磁偶极辐射主导来计算磁场可能存在争议,我们建议也应考虑外流粒子携带的能量;以及(2)X射线光度大于自转能量损失率的事实并不一定意味着辐射能量来自磁场衰减,这需要进一步的观测检验。此外,一些观测事实与磁星模型相冲突,例如对反磁星、高磁场脉冲星的观测,以及对AXP/SGR的射电和X射线观测。因此,我们提出了一种带壳层的奇异星模型作为替代方案,可以解释AXP/SGR的更多观测事实。

     

    Abstract: The emission of anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) is believed to be powered by the dissipation of their strong magnetic fields, which coined the name “magnetar”. By combining timing and energy observational results, the magnetar model can be easily appreciated. From a timing perspective, the magnetic field strengths of AXPs and SGRs, which are calculated under the assumption of dipole radiation, are extremely strong. From an energy perspective, the X-ray/soft gamma-ray luminosities of AXPs and SGRs are larger than their rotational energy loss rates (i.e., L_\rmX>\dot E_\rmrot). It is thus reasonable to assume that the high-energy radiation comes from magnetic energy decay, and the magnetar model has been extensively discussed (or accepted). However, we argue that: (ⅰ) Calculating magnetic fields by assuming that rotational energy loss is dominated by dipole radiation (i.e., \dotE_\rmrot\simeq\dotE_\mu) may be controversial, and we suggest that the energies carried by outflowing particles should also be considered. (ⅱ) The fact that X-ray luminosity is greater than the rotational energy loss rate does not necessarily mean that the emission energy comes from the magnetic field decaying, which requires further observational testing. Furthermore, some observational facts conflict with the “magnetar” model, such as observations of anti-magnetars, high magnetic field pulsars, and radio and X-ray observations of AXPs/SGRs. Therefore, we propose a crusted strange star model as an alternative, which can explain many more observational facts of AXPs/SGRs.

     

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