标题： NGC 4736中致密核X射线辐射的探测 显示英文标题

标题： Detection of Compact Nuclear X-Ray Emission in NGC 4736

摘要： 我们报告了一次对LINER星系NGC 4736的深度ROSAT PSPC观测结果。检测到两个明亮源，它们相距仅约1'，较亮的一个与星系中心重合。在这两个源中，没有一个在ROSAT能段的分钟到小时时间尺度上显示出明显的X射线变化。典型的AGN X射线光谱所用简单幂律模型对核源观测到的X射线光谱拟合效果较差。加入Raymond-Smith组分显著改善了拟合效果。这与核区存在温度kT~0.3 keV的热气体以及一个紧凑核源一致。然而，仔细检查残差显示低能（< 0.25 keV）处有明显特征。我们发现，在约0.22 keV处添加一条窄发射线显著改善了光谱参数化。我们根据PSPC光谱校准的准确性重新审视这些结果。得到的光子指数约为2.3，与在ROSAT能量范围内测量的塞弗特1型星系的光子指数相似。另一方面，紧凑源在0.1-2 keV波段的光度仅为约3.4×10^{39} erg/s，远低于典型塞弗特星系。我们讨论了这些结果对LINER和AGN之间联系的意义。离心源本质上是暂现的。它具有硬X射线光谱，光子指数约为1.5，因此很可能是X射线双星。关于它与该星系的关联仍有一些不确定性。如果它确实位于星系内，则0.1-2 keV波段的光度将大于5.1×10^{38} erg/s，使其成为恒星级质量黑洞候选体。 显示英文摘要

摘要： We report the results from a deep ROSAT PSPC observation of LINER galaxy NGC 4736. Two bright sources are detected, separated by only about 1', with the brighter one coinciding with the center of the galaxy. Neither source shows apparent X-ray variability on time-scales of minutes to hours in the ROSAT band. Simple power-law models, typical of AGN X-ray spectra, produce poor fits to the observed X-ray spectrum of the nuclear source. The addition of a Raymond-Smith component improves the fits significantly. This is consistent with the presence of hot gas in the nuclear region with kT=~0.3 keV, in addition to a compact nuclear source. However, a careful examination of the residuals reveal apparent features at low energies (< 0.25 keV). We find that the addition of a narrow emission line at about 0.22 keV is a significant improvement to the parameterization of the spectrum. We examine the results in the light of the accuracy of the PSPC spectral calibration. The derived photon index is about 2.3, which is similar to those for Seyfert 1 galaxies measured in the ROSAT energy range. On the other hand, the 0.1-2 keV luminosity for the compact source is only about 3.4x10^{39} erg/s, much fainter than typical Seyfert galaxies. We discuss the implications of these results on the connection between LINERs and AGNs. The off-center source is transient in nature. It has a hard X-ray spectrum, with a photon index of about 1.5, so is likely to be an X-ray binary. There is still some ambiguity regarding its association with the galaxy. If it is indeed located in the galaxy, the 0.1-2 keV luminosity would be greater than 5.1x10^{38} erg/s, making it a stellar-mass black hole candidate.