低磁场磁星SGR 0418 + 5729的爆发衰变
The outburst decay of the low magnetic field magnetar SGR 0418+5729
摘要：
我们报告了低磁场磁星SGR 0418 + 5729爆发衰变的长期X射线监测，使用所有可用的X射线数据与RXTE，SWIFT，钱德拉，和XMM-牛顿观测，从2009年6月发现的源，到2012年8月。时序分析使我们能够获得SGR 0418 + 5729周期导数的第一次测量：\dot{P} = 4（1）x10 ^{-15} s/s，在~3.5 sigma置信水平下显著。这导致B_dip~6 x 10 ^{12} G的表面偶极磁场。该测量确认SGR 0418 + 5729为最低磁场磁星。在从爆发开始到~1200天的通量和光谱演化之后，我们观察到负责X射线发射的微小热点从~0.9 keV到0.3 keV的温度逐渐冷却。与此同时，X射线通量减少了大约3个数量级：从大约1.4 × 10 ^{-11}到1.2 × 10 ^{-14}尔格/秒/厘米^2。深层无线电、毫米波、光学和伽马射线观测都没有探测到对应的源，这意味着它的多波段发射受到严格的限制，以及化石盘的存在受到限制。通过模拟SGR 0418 + 5729的磁热长期演化，我们推断其实际年龄约为55万年，诞生时的偶极磁场约为10 ^{14} G。爆发的特点表明，存在一个薄的扭曲束与一个小的热点在其基础上。在爆发后的最初几个月里，星系束解旋，而热点的温度和大小都在下降。我们估计低磁场磁星的爆发率约为每个星系每年一个，我们简要讨论了这样的结果在其他几个天体物理背景下的后果。
英文摘要：
We report on the long term X-ray monitoring of the outburst decay of the low magnetic field magnetar SGR 0418+5729, using all the available X-ray data obtained with RXTE, SWIFT, Chandra, and XMM-Newton observations, from the discovery of the source in June 2009, up to August 2012. The timing analysis allowed us to obtain the first measurement of the period derivative of SGR 0418+5729: \dot{P}=4(1)x10^{-15} s/s, significant at ~3.5 sigma confidence level. This leads to a surface dipolar magnetic field of B_dip ~6x 10^{12} G. This measurement confirms SGR 0418+5729 as the lowest magnetic field magnetar. Following the flux and spectral evolution from the beginning of the outburst up to ~1200 days, we observe a gradual cooling of the tiny hot spot responsible for the X-ray emission, from a temperature of ~0.9 to 0.3 keV. Simultaneously, the X-ray flux decreased by about 3 orders of magnitude: from about 1.4x10^{-11} to 1.2x10^{-14} erg/s/cm^2 . Deep radio, millimeter, optical and gamma-ray observations did not detect the source counterpart, implying stringent limits on its multi-band emission, as well as constraints on the presence of a fossil disk. By modeling the magneto-thermal secular evolution of SGR 0418+5729, we infer a realistic age of ~550 kyr, and a dipolar magnetic field at birth of ~10^{14} G. The outburst characteristics suggest the presence of a thin twisted bundle with a small heated spot at its base. The bundle untwisted in the first few months following the outburst, while the hot spot decreases in temperature and size. We estimate the outburst rate of low magnetic field magnetars to be about one per year per galaxy, and we briefly discuss the consequences of such result in several other astrophysical contexts.
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