arXiv 1202.21212012-02-09astro-ph.HE

使用XMM-牛顿法对RX J1856 .5-3754进行光谱监测。EPIC-pn数据分析

Spectral monitoring of RX J1856.5-3754 with XMM-Newton. Analysis of EPIC-pn data

N. Sartore, A. Tiengo, S. Mereghetti, A. De Luca, R. Turolla, F. Haberl

备注：: 11 pages, 6 figures. Accepted for publication in Astronomy and Astrophysics
DOI：: 10.1051/0004-6361/201118489

摘要：

利用大量的XMM-牛顿观测数据，我们研究了孤立的中子星RX J1856.5-3754在2002年4月至2011年10月这段时间内的光谱和通量变化。这是附近一小群热发射孤立中子星的最明亮和最广泛观测的来源，其中至少有一个成员（RX J0720.4-3125，Hohle等人，2010年，显示了长期的变化。在0.15-1.2 keV能量范围内用EPIC-pn相机获得的数据的详细分析揭示了用单个黑体拟合导出的温度的小变化（在kT^inf \sim 61 eV附近的1%的量级）。这种变化与源在检测器上的位置相关，并且可以归因于仪器效应，最有可能是通道与能量关系的空间依赖性。对于采样的仪器坐标，我们将这种效应量化为增益斜率和偏移分别为\sim 4%和\sim 15 eV的变化。只选择一个均匀的观测子集，源成像在相同的探测器位置，我们没有发现从2005年3月到现在的RX J1856.5-3754的光谱或通量变化的证据，极限为Delta kT^inf < 0.5%和Delta f_X < 3%（0.15-1.2 keV），置信度为3 sigma。2002年4月测得的温度略高（kT^inf \sim 61.5 eV，而kT^ \inf \sim 61 eV）。如果这种差异不是仪器造成的，那么它意味着2002年4月至2005年3月期间\sim的变化率为\sim -0.15 eV yr^-1。来自所选观测的高统计谱与两个黑体模型的和最好地拟合，其中温度kT_h^inf = 62.4_{-0.4}^{+0.6} eV和kT_s^ \inf = 38.9_{-2.9}^{+4.9} eV，这解释了在光学带中看到的通量。没有显著的光谱特征被检测到，其等效宽度的上限为6 eV。

英文摘要：

Using a large set of XMM-Newton observations we searched for long term spectral and flux variability of the isolated neutron star RX J1856.5-3754 in the time interval from April 2002 to October 2011. This is the brightest and most extensively observed source of a small group of nearby, thermally emitting isolated neutron stars, of which at least one member (RX J0720.4-3125, Hohle et al., 2010) has shown long term variability. A detailed analysis of the data obtained with the EPIC-pn camera in the 0.15-1.2 keV energy range reveals small variations in the temperature derived with a single blackbody fit (of the order of 1% around kT^inf \sim 61 eV). Such variations are correlated with the position of the source on the detector and can be ascribed to an instrumental effect, most likely a spatial dependence of the channel to energy relation. For the sampled instrumental coordinates, we quantify this effect as variations of \sim 4% and \sim 15 eV in the gain slope and offset, respectively. Selecting only a homogeneous subset of observations, with the source imaged at the same detector position, we find no evidence for spectral or flux variations of RX J1856.5-3754 from March 2005 to present-day, with limits of Delta kT^inf < 0.5% and Delta f_X < 3% (0.15-1.2 keV), with 3sigma confidence. A slightly higher temperature (kT^inf \sim 61.5 eV, compared to kT^\inf \sim 61 eV) was instead measured in April 2002. If this difference is not of instrumental origin, it implies a rate of variation \sim -0.15 eV yr^-1 between April 2002 and March 2005. The high-statistics spectrum from the selected observations is best fitted with the sum of two blackbody models, with temperatures kT_h^inf = 62.4_{-0.4}^{+0.6} eV and kT_s^\inf = 38.9_{-2.9}^{+4.9} eV, which account for the flux seen in the optical band. No significant spectral features are detected, with upper limits of 6 eV on their equivalent width.

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