The Swift-BAT monitoring reveals a long-term decay of the cyclotron line energy in Vela X-1
The Swift-BAT monitoring reveals a long-term decay of the cyclotron line energy in Vela X-1
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DOI：
10.1093/mnras/stw1915
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发表时间：
2016-08
影响因子：
4.8
通讯作者：
V. Parola;G. Cusumano;A. Segreto;A. D’Aì
中科院分区：
文献类型：
--
作者：
V. Parola;G. Cusumano;A. Segreto;A. D’Aì
我们研究的回旋共振散射功能（CRSF）的高质量X射线双星船帆座X-1的行为，使用长期的硬X射线监测进行的突发警报望远镜（BAT）的斯威夫特。沿沿着11年BAT调查建立了高统计、强度选择谱。虽然基本线没有显示，但CRSF的二次谐波可以在所有光谱中清楚地检测到，其能量在$\sim 53$ keV和$\sim 58$ keV之间变化，与光度直接相关。通过研究沿沿着调查的沿着四个33个月时间间隔建立的强度选择光谱，我们进一步研究了CRSF在时间上的演变。我们第一次在这个源中发现CRSF能量的长期变化：与源的光度无关，CRSF二次谐波能量在第一和第三时间间隔之间减少了$\sim 0.36$ keV/年，对应于$\sim 3\times 10^{10}$ G/年的磁场明显衰减。强度回旋能量模式是一致的第三和最后的时间间隔之间。这种衰变的一个可能的解释可能是一个吸积丘的沉降，它产生极帽上的极向磁场的扭曲或线形成区域的几何位移。船帆座X-1和海尔座X-1的每单位吸积质量的谱线移动速率与每单位面积的极帽上的吸积质量之间的对应关系似乎支持这一假设。
We study the behaviour of the cyclotron resonant scattering feature (CRSF) of the high mass X-ray binary Vela X-1 using the long-term hard X-ray monitoring performed by the Burst Alert Telescope (BAT) on board Swift. High statistics, intensity selected spectra were built along 11 years of BAT survey. While the fundamental line is not revealed, the second harmonic of the CRSF can be clearly detected in all the spectra, at an energy varying between $\sim 53$ keV and $\sim 58$ keV, directly correlated with the luminosity. We have further investigated the evolution of the CRSF in time, by studying the intensity selected spectra built along four 33-month time intervals along the survey. For the first time we find in this source a secular variation in the CRSF energy: independent of the source luminosity, the CRSF second harmonic energy decreases by $\sim 0.36$ keV/year between the first and the third time interval, corresponding to an apparent decay of the magnetic field of $\sim 3\times 10^{10}$ G/year. The intensity-cyclotron energy pattern is consistent between the third and the last time intervals. A possible interpretation for this decay could be the settling of an accreted mound that produces either a distortion of the poloidal magnetic field on the polar cap or a geometrical displacement of the line forming region. This hypothesis seems supported by the correspondance between the rate of the line shift per unit accreted mass and the mass accreted on the polar cap per unit area in Vela X-1 and Her X-1, respectively.