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arXiv 1602.00702astro-ph.HEastro-ph.COastro-ph.GA
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IC 751：NuSTAR发现的一种新的外观变化的AGN

IC 751: a new changing-look AGN discovered by NuSTAR

C. Ricci, F. E. Bauer, P. Arevalo, S. Boggs, W. N. Brandt, F. E. Christensen, W. W. Craig, P. Gandhi, C. J. Hailey, F. A. Harrison, M. Koss, C. B. Markwardt, D.… 展开作者

C. Ricci, F. E. Bauer, P. Arevalo, S. Boggs, W. N. Brandt, F. E. Christensen, W. W. Craig, P. Gandhi, C. J. Hailey, F. A. Harrison, M. Koss, C. B. Markwardt, D. Stern, E. Treister, W. W. Zhang

备注：
Accepted for publication in ApJ, 11 pages, 6 figures
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摘要：

我们提出了五个NuSTAR观测IC 751中的2型活动星系核（AGN）的结果，其中三个是同时进行的XMM-牛顿或雨燕/XRT。我们发现，在$\lesssim 3$个月的时间尺度上，核X射线源经历了从康普顿厚态（$N_{\rm\,H}\simeq 2\times 10^{24}\rm\,cm^{-2}$）到康普顿薄态（$N_{\rm\,H}\simeq 4\times 10^{23}\rm\,cm^{-2}$）的明显跃迁，这使得IC 751成为NuSTAR发现的第一个外观变化的活动星系核.在$\sim2σ$水平下的视线柱密度的变化也在$\sim 48$小时（$ΔN_{\rm\,H}\sim 10^{23}\rm\,cm^{-2}$）的时间尺度上发现。从$\sim 100$ KS的时间尺度上的光谱变异性的缺乏，我们推断，不同的吸收体位于超出宽线区域的发射加权平均半径，因此可能与外部的宽线区域或cluberous分子环面。通过采用物理环面X射线谱模型，我们能够从变化的云[$N_{\rm\,H}\sim(1-150)\times10^{22}\rm\,cm^{-2}$]中分离出不变吸收体（$N_{\rm\,H}\sim 3.8\times 10^{23}\rm\,cm^{-2}$）的柱密度，并约束负责再处理X射线辐射（$N_{\rm\,H} \sim 6 \times 10^{24}\rm\,cm^{-2}$）的材料的柱密度。我们发现的证据显着的内在的X射线的变化，与通量变化的5个因素的时间尺度上的几个月在2 - 10和10 - 50千电子伏波段。

英文摘要：

We present the results of five NuSTAR observations of the type 2 active galactic nucleus (AGN) in IC 751, three of which were performed simultaneously with XMM-Newton or Swift/XRT. We find that the nuclear X-ray source underwent a clear transition from a Compton-thick ($N_{\rm\,H}\simeq 2\times 10^{24}\rm\,cm^{-2}$) to a Compton-thin ($N_{\rm\,H}\simeq 4\times 10^{23}\rm\,cm^{-2}$) state on timescales of $\lesssim 3$ months, which makes IC 751 the first changing-look AGN discovered by NuSTAR. Changes of the line-of-sight column density at a $\sim2σ$ level are also found on a time-scale of $\sim 48$ hours ($ΔN_{\rm\,H}\sim 10^{23}\rm\,cm^{-2}$). From the lack of spectral variability on timescales of $\sim 100$ ks we infer that the varying absorber is located beyond the emission-weighted average radius of the broad-line region, and could therefore be related either to the external part of the broad-line region or a clumpy molecular torus. By adopting a physical torus X-ray spectral model, we are able to disentangle the column density of the non-varying absorber ($N_{\rm\,H}\sim 3.8\times 10^{23}\rm\,cm^{-2}$) from that of the varying clouds [$N_{\rm\,H}\sim(1-150)\times10^{22}\rm\,cm^{-2}$], and to constrain that of the material responsible for the reprocessed X-ray radiation ($N_{\rm\,H} \sim 6 \times 10^{24}\rm\,cm^{-2}$). We find evidence of significant intrinsic X-ray variability, with the flux varying by a factor of five on timescales of a few months in the 2-10 and 10-50 keV band.

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