Optical Spectral Variations of a Large Sample of Fermi Blazars
Optical Spectral Variations of a Large Sample of Fermi Blazars
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DOI：
10.3847/1538-4365/ac5a52
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发表时间：
2022-03
期刊：
影响因子：
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通讯作者：
Bing-Kai Zhang;Xiao-Yun Zhao;Qi Wu
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文献类型：
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作者：
Bing-Kai Zhang;Xiao-Yun Zhao;Qi Wu
我们研究了大样本费米耀变体（40个平谱射电类星体（FSRQ）和13个BL Lacertae天体（BL拉克））的光谱行为，发现了两个新的普遍光谱行为。在低状态下，光谱逐渐变得更软（陡峭）或更硬（平坦），但随着亮度的增加越来越慢，然后趋于稳定在高状态。这些行为分别被简称为更亮时更红稳定（RSWB）和更亮时更蓝稳定（BSWB）行为。34个FSRQ和7个BL拉克表现出明显的RSWB行为，2个FSRQ和5个BL拉克表现出明显的BSWB行为，这意味着FSRQ比BSWB更倾向于RSWB行为，而BL拉克在两种行为之间没有明显的偏好。我们提出了一个统一的非线性公式来定量描述FSRQs和BL拉克的光谱行为，它能很好地拟合这两种行为。我们认为，RSWB和BSWB行为起源于相同的机制，它们是耀变体的普遍光谱行为。经常观察到的更亮时更红和更亮时更蓝的趋势可以分别被认为是RSWB和BSWB行为的近似。很少观察到的变亮时稳定趋势也可以被视为RSWB或BSWB行为的近似或特殊情况。我们建立了一个包含两个常光谱指数分量的模型，不仅能很好地解释这两种光谱行为，而且成功地解释了FSRQs和BL拉克之间的差异行为。
We have investigated the optical spectral behavior of a large sample of Fermi blazars (40 flat-spectrum radio quasars (FSRQs) and 13 BL Lacertae objects (BL Lacs)), and found two new universal optical spectral behaviors. In the low state the optical spectrum gradually becomes softer (steeper) or harder (flatter), but more and more slowly as the brightness increases, and then tends to stabilize in the high state. These are briefly named the redder-stable-when-brighter (RSWB) and bluer-stable-when-brighter (BSWB) behaviors, respectively. Thirty-four FSRQs and 7 BL Lacs exhibit clear RSWB behavior, and 2 FSRQs and 5 BL Lacs show distinct BSWB behavior, which means that FSRQs favor more RSWB than BSWB behavior, while BL Lacs have no clear preference between both behaviors. We have put forward a unified nonlinear formula to quantitatively characterize the optical spectral behaviors of FSRQs and BL Lacs, which can fit both kinds of behaviors very well. We argue that the RSWB and BSWB behaviors originate from the same mechanism, and that they are the universal optical spectral behaviors for blazars. The frequently observed redder-when-brighter and bluer-when-brighter trends can be considered to be the approximations of the RSWB and BSWB behaviors, respectively. The rarely observed stable-when-brighter trend can also be viewed as an approximation or a special case of the RSWB or BSWB behaviors. We have developed a model with two constant-spectral-index components, which cannot only explain the two kinds of optical spectral behaviors well, but also successfully interpret the differential behaviors between FSRQs and BL Lacs.