A NuSTAR study of the 55 ks hard X-ray pulse-phase modulation in the magnetar 4U 0142+61
A NuSTAR study of the 55 ks hard X-ray pulse-phase modulation in the magnetar 4U 0142+61
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DOI：
10.1093/pasj/psy129
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发表时间：
2018-10
影响因子：
2.3
通讯作者：
K. Makishima;Hiroaki Murakami;T. Enoto;K. Nakazawa
中科院分区：
文献类型：
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作者：
K. Makishima;Hiroaki Murakami;T. Enoto;K. Nakazawa
对磁星4U0142+61于2014年3月获取的总时间跨度为258 k的档案数据进行了分析。这是为了再次确认该源的硬x射线脉冲相位中的55 ks调制，该调制是在2009年的Suzaku观测中发现的（Makishima et al. 2014）。事实上，在8.68917秒用核之星探测到的10-70 keV x射线脉冲，也被发现在相同的55ks周期（或一半的值）被相位调制（98%的置信度或更高）。此外，对2013年获得的另一个Suzaku数据集4U0142+61的简要分析再次证实了15-40 keV脉冲中相同的55 ks相位调制。因此，用Suzaku（2009年和2013年）和NuSTAR（2014年）在一个一致的时期检测到硬x射线脉冲相位调制。但调制幅度变化明显；与Suzaku（2009）的A=0.17 s，与Suzaku（2013）的A=1.2 s，与NuSTAR的A=0.17 s。此外，在观测的前1/3和后2/3之间，用NuSTAR检测到的相位调制特性差异很大。在能量低于10 keV时，Suzaku和NuSTAR均未检测到脉冲相位调制。这些结果强化了Makishima et al.（2014）的观点；4U0142+61中的中子星保持自由进动，但轴向有轻微的变形，这可能是由于1e16 g的超强环形磁场造成的。进动的摆动角度应该保持不变，但脉冲相位调制幅度在几个月到几年的时间尺度上发生变化，这可能是由于恒星轴周围硬x射线发射模式的不对称性发生了变化。
Archival NuSTAR data of the magnetar 4U0142+61, acquired in 2014 March for a total time span of 258 ks, were analyzed. This is to reconfirm the 55 ks modulation in the hard X-ray pulse phases of this source, found with a Suzaku observation in 2009 (Makishima et al. 2014). Indeed, the 10-70 keV X-ray pulsation, detected with NuSTAR at 8.68917s, was found to be also phase-modulated (at 98 percent confidence or higher) at the same 55 ks period, or half that value. Furthermore, a brief analysis of another Suzaku data set of 4U0142+61, acquired in 2013, reconfirmed the same 55 ks phase modulation in the 15-40 keV pulses. Thus, the hard X-ray pulse-phase modulation was detected with Suzaku (in 2009 and 2013) and NuSTAR (in 2014) at a consistent period. However, the modulation amplitude varied significantly; A=0.17 s with Suzaku (2009), A=1.2 s with Suzaku (2013), and A=0.17 s with NuSTAR. In addition, the phase modulation properties detected with NuSTAR differed considerably between the first 1/3 and the latter 2/3 of the observation. In energies below 10 keV, the pulse-phase modulation was not detected with either Suzaku or NuSTAR. These results reinforce the view of Makishima et al. (2014); the neutron star in 4U0142+61 keeps free precession, under a slight axial deformation due probably to ultra-strong toroidal magnetic fields of 1e16 G. The wobbling angle of precession should remain constant, but the pulse-phase modulation amplitude varies on time scales of months to years, presumably as asymmetry of the hard X-ray emission pattern around the star's axis changes.