高空和低空狩猎：XMM对食极HU宝瓶座的监测
Hunting high and low: XMM monitoring of the eclipsing polar HU Aquarii
摘要：
我们要研究的时间和光谱行为的HU Aqr在X射线域在不同的吸积状态。我们从四个不同的XMM-牛顿点覆盖中间和低状态的光谱和光变曲线。X射线观测伴随着高时间分辨率光度测量，使用Optima和ULTRACAM仪器获得。在2002年5月和2003年5月的两个场合，HU Aqr被发现处于中间状态，与早期的高状态测量相比，吸积率降低了50倍。中间态的X射线谱可以用一个包含黑体成分和热等离子体的模型来描述。与高态相反，软X射线通量和硬X射线通量之间的比率接近平衡。与先前的测量结果一致，我们观察到吸积点和流向连接两颗恒星的线迁移。HU Aqr在2003年10月和2005年5月的两个低态期间亮度进一步降低了80倍，在那里它被检测到的光度只有L_X = 4.7 * 10^（28）erg/sec。这种光度与活跃的日冕发射体非常吻合，但相对较高的等离子体温度（3.5和2.0 keV）与残余吸积更相容。我们根据在各个波段测量的吸积点的日食出口更新了HU Aqr的日食星历。所观测到的吸积点食定时的（O-C）图揭示了与线性趋势的复杂偏差，这可以通过恒定或周期性的周期变化或其组合来解释。二次项意味着周期以\dot{P}_o rb =-7.的速率减小。11 * 10^（-12）秒/秒。如果观测到的周期变化反映了真正的角动量损失，这将是引力辐射的30倍。
英文摘要：
We want to study the temporal and spectral behaviour of HU Aqr in the X-ray domain during different accretion states. We obtained spectra and light curves from four different XMM-Newton pointings covering intermediate and low states. The X-ray observations were accompanied with high time resolution photometry obtained with the Optima and ULTRACAM instruments. On two occasions in May 2002 and 2003 HU Aqr was found in an intermediate state with the accretion rate reduced by a factor of 50 compared to earlier high state measurements. X-ray spectra in the intermediate state can be described by a model containing a blackbody component and hot thermal plasma. Contrary to the high state the ratio between soft and hard X-ray flux is nearly balanced. In agreement with previous measurements we observed a migration of the accretion spot and stream towards the line connecting both stars. The brightness of HU Aqr was further reduced by a factor of 80 during two low states in October 2003 and May 2005, where it was detected at a luminosity of only L_X = 4.7 * 10^(28) erg/sec . This luminosity would fit well with an active coronal emitter, but the relatively high plasma temperatures of 3.5 and 2.0 keV are more compatible with residual accretion. We updated the eclipse ephemeris of HU Aqr based on the eclipse egress of the accretion spot measured in various wavelength bands. The (O-C)-diagram of the observed accretion spot eclipse timings reveals complex deviations from a linear trend, which can be explained by a constant or cyclic period change or a combination thereof. The quadratic term implies a period decrease at a rate of \dot{P}_orb = -7..-11 * 10^(-12) sec/sec. In case the observed period change reflects a true angular momentum loss, this would be a factor of 30 larger than given by gravitational radiation.
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