[论文解读] Broad band energy distribution of ROSAT detected quasars II: Radio-quiet objects

Wei Yuan, W. Brinkmann+2 more|ArXiv.org|May 4, 1998

Galaxies: Formation, Evolution, Phenomena被引用 4

一句话总结

本研究基于ROSAT全源天区巡天与指向观测，对846个射电宁静类星体进行了超宽波段能谱分布分析，证实这些天体的软X射线谱比射电loud类星体更陡峭；在红移z ≈ 2以内，谱型趋于平坦，而在z > 2.5时，X射线谱无显著演化；同时发现X射线与紫外光度之间存在紧密相关性，且在高红移区域无显著超额吸收。

ABSTRACT

A database of radio-quiet quasars detected with ROSAT is presented containing 846 quasars seen in the All-Sky Survey and/or in pointed PSPC observations. About 70% of the objects have been detected in X-rays for the first time. We present the soft X-ray fluxes and spectra, if available. Using an optically selected subsample compiled from this database, we study the broad band properties of radio-quiet quasars with high statistical significance. We confirm that radio-quiet quasars have in general steeper soft X-ray spectra than radio-loud. The spectral differences persist to high redshifts. A spectral flattening with increasing redshift is confirmed up to redshift around 2, beyond which the spectral slopes seem to be independent of redshift. The spectral slopes of the ROSAT radio-quiet quasars at z>2.5 are consistent with those found for nearby quasars in the medium energy band (2-10keV), implying that the X-ray spectral evolution is not important. We show that there is, in a statistical sense, little or no excess absorption for most of the radio-quiet objects at z>2. The existence of a correlation between the X-ray luminosity and the luminosity at 2500A is confirmed. Individual objects show a large scatter from this correlation. The X-ray loudness alpha_ox appears to be independent of z, but the analyses indicate a slight increase of alpha_ox with optical luminosity. However, this behavior is likely not caused by physical properties inherent to the quasars but is the result of the intrinsic dispersion. Finally, we find a small fraction of objects which appear to be relatively `X-ray quiet' compared to the bulk of the other quasars.

研究动机与目标

• 利用ROSAT全源天区巡天与指向观测，表征射电宁静类星体的超宽波段能谱分布。
• 研究射电宁静类星体在宽红移范围内软X射线谱的性质。
• 考察X射线谱指数与光度相关性随红移和光学光度的演化。
• 评估高红移射电宁静类星体中是否存在超额吸收。
• 确定X射线响度参数（α_ox）对红移和光学光度的依赖关系。

提出的方法

• 汇编846个在ROSAT全源天区巡天与指向PSPC观测中探测到的射电宁静类星体数据库，其中约70%为首次在X射线波段被探测到。
• 选取一个光学选源子样本，以实现对超宽波段性质的高显著性统计分析。
• 对软X射线数据（0.1–2.4 keV）进行谱拟合，以推导光子指数并评估谱的硬软程度。
• 通过X射线通量与2500 Å处紫外光度的交叉相关性，研究X射线响度参数α_ox。
• 利用统计建模方法，对谱指数与光度相关性进行红移依赖性分析。
• 通过谱拟合与预期本征吸收模型的比较，评估超额吸收情况。

实验结果

研究问题

• RQ1 射电宁静类星体在整个宇宙时空中是否表现出比射电loud类星体更陡的软X射线谱？
• RQ2 软X射线谱指数如何随红移演化，特别是在z ≈ 2以上区域？
• RQ3 高红移射电宁静类星体（z > 2）是否存在显著的超额吸收？
• RQ4 X射线光度与2500 Å紫外光度之间的相关性在不同红移下是否保持一致？
• RQ5 X射线响度参数α_ox是否依赖于红移或光学光度？

主要发现

• 射电宁静类星体表现出显著比射电loud类星体更陡的软X射线谱，且该差异在高红移区域依然存在。
• 谱指数随红移增加而变平，直至z ≈ 2，此后趋于稳定，不再发生进一步演化。
• 在z > 2.5时，X射线谱指数与附近类星体在2–10 keV波段的谱指数一致，表明无显著X射线谱演化。
• 统计分析显示，射电宁静类星体在z > 2时几乎不存在或仅有极少量超额吸收，与部分早期推测相反。
• X射线光度与2500 Å紫外光度之间的紧密相关性得到证实，尽管个别天体存在较大离散度。
• X射线响度参数α_ox对红移无显著依赖性，但随光学光度略有增加，这可能源于内在弥散而非物理因果关系。