在软X射线瞬态窄线Seyfert 1星系WPVS 007中首次探测到硬X射线光子：Swift观测到的X射线光子分布
First detection of hard X-ray photons in the soft X-ray transient Narrow-Line Seyfert 1 galaxy WPVS 007: The X-ray photon distribution observed by Swift
摘要：
本文报道了在ROSAT全天巡天期间首次探测到的X射线瞬态窄线Seyfert 1星系WPVS 007中的硬X射线光子（E> 2.5keV），该星系是X射线谱最软的活动星系核。在2007年9月的50 ks观测中，Swift在0.3-10.0 keV波段清晰地探测到了AGN，其能级约为2 x 10^{-17} W m^{-2}。这是自1990年ROSAT全天巡天观测以来第一次有可能通过将迄今为止进行的所有Swift观测结果（总共85.5 k s）相加得出X射线光子分布。这种光子分布与AGN的X射线光谱一致，该AGN具有部分覆盖吸收体，柱密度约为1 x 10^{23} cm^{-2}，覆盖率约为90%。与2002年的钱德拉数据相比，WPVS 007的亮度增加了约4倍。Swift的数据还表明，导致当前低状态的吸收体可能已经开始消失。这种消失是由一个显着的变化，在硬度比从一个非常软的X射线状态在2005年10月至2007年1月的观测到一个相当硬的X射线状态在2007年9月的观测。在紫外线中，WPVS 007在过去两年中似乎变得更暗，最高可达0.5等。由光谱能量分布得到的光学-X射线光谱斜率为α ox =2.5，将WPVS 007归类为X射线弱活动星系核。校正红化和X射线吸收后，α ox变为1.9，大蓝凸块中的光度为log LBBB=37.7 [W]，换算成爱丁顿比L/LEdd约为1.0。
英文摘要：
We report on the first detection of hard X-ray photons (E>2.5 keV) in the X-ray transient Narrow-Line Seyfert 1 galaxy WPVS 007 which was the AGN with the softest X-ray spectrum during the ROSAT All-Sky Survey. The AGN is clearly detected at a level of about 2 x 10^{-17} W m^{-2} in the observed 0.3-10.0 keV band by Swift in a 50 ks observation in 2007 September. For the first time since the ROSAT All-Sky Survey observation in 1990 it was possible to derive an X-ray photon distribution by adding together all Swift observations that have been performed so far (85.5 ks in total). This photon distribution is consistent with an X-ray spectrum of an AGN with a partial covering absorber with a column density in the order of ~ 1 x 10^{23} cm^{-2} and a covering fraction of about 90%. A comparison with the 2002 Chandra data suggests that WPVS 007 has become brighter by a factor of about 4. The Swift data also suggest that the absorber which is causing the current low-state may have started to disappear. This disappearance is indicated by a significant change in the hardness ratio from a very soft X-ray state during the 2005 October to 2007 January observations to a rather hard X-ray state in the 2007 September observations. In the UV, WPVS 007 seems to become fainter by up to 0.5 mag over the last two years. The optical to X-ray spectral slope derived from the spectral energy distribution is alpha-ox=2.5 which classifies WPVS 007 as an X-ray weak AGN. After correcting for reddening and X-ray absorption, alpha-ox becomes 1.9 and the luminosity in the Big-Blue-Bump is log LBBB=37.7 [W], which translates into an Eddington ratio L/LEdd ~ 1.0.
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