| 其他摘要 | Blazars are a special type of active galactic nuclei (AGN) whose multi-waveband emission is dominated by non-thermal radiation from relativistic jets which are directed close to the lines-of-sight of an observer. The ``blazar sequence'' means that blazars can be arranged in the order of their bolometric luminosities, in such a way that the higher the luminosity is, the lower the peak energy of the synchrotron emission component, and the larger the luminosity ratio between the high-energy inverse Compton component and the low-energy synchrotron component. Since its first suggestion in the 1990's (Fossati et al. 1998), the ``blazar sequence'' has been successful in describing well the collective properties of most of the blazars known to then. However, it starts to be challenged by recent observations on blazars. One important challenge is the recent finding of a claimed new population of Flat-Spectrum Radio Quasar (FSRQ) which have both high peak energy of the synchrotron emission and high bolometric luminosity---the so called High-energy peak FSRQ (HFSRQ), as results of identification of X-ray sources found in deep ROSAT observations (e.g. Padovani et al. 2003). However, whether or not exists such a population is still a matter of debate. On the other hand, recent studies of very radio-loud Narrow Line Seyfert\,1 (NLS1) galaxies indicated that a significant fraction of these objects have broad band SED similar to HFSRQ (zhou et al. 2007, Yuan et al. 2008). Questions arise as to whether there exist HFSRQ candidates, if so what the distribution of their line-width is and whether there are NLS1 galaxies. The SDSS spectroscopic survey provides us with a great opportunity to investigate these issues. In this thesis, we make use of the ROSAT-Green Bank (RGB) sample, which is a sample of X-ray sources detected in the ROSAT All-sky Survey with strong radio emission at 5\,GHz. The RGB sample is most suitable for selection HFSRQ candidates since the sample objects are bright in X-rays. We first present optical spectroscopic identification of the RGB sample objects which have SDSS spectra. Then we select HFSRQ candidates, followed by a brief study of their properties. We analyzed the SDSS optical spectra of 159 RGB objects making use of the algorithm developed at CfA, University of Science of Technology of China. We presented optical identification and detailed spectral classification of all these objects. Among them, 113 are found to be broad line AGN (83 QSOs and 27 broad line radio galaxies), out of which 3 are typical NLS1 AGN. There are 14 narrow line radio galaxies (NLRGs), among them, 2 Seyfert 2 galaxies, 12 LINERs. It is interesting to note a large fraction of LINERs found, for the first time, in NLRGs in the RGB sample, thanks to the high S/N and resolution of SDSS spectra. There are 10 BL Lac objects and 11 BL Lac object candidates. The remaining 11 objects are optically normal galaxies. The fractions of various AGN types are consistent with those in previous studies. We estimated the radio spectral indices of the identified sources using data from the 1.4GHz NVSS Radio Survey. We also constructed the very rough broad band SED using multi-waveband data. From the derived effective broad band spectral indices, we found that there are 19 (and perhaps 29) identified FSRQ having the SED within the HFSRQ region in the parameter space, depending on the exact definitions. We regard these objects as HFSRQ candidates. Therefore our result confirmed that of Padovani et al. The distribution of the width of broad emission line ranges from 1544\,km\,s$^{-1}$ to 20000\,km\,s$^{-1}$, with a median of 4217\,km\,s$^{-1}$. Among them, one is NLS1, making the NLS1 fraction to be $\sim5\%$. A comparison of the line-widths between HFSRQ and other radio quasars showed that the two distributions are statistically consistent. Among the RGB HFSRQ candidates, 5 have archival ROSAT PSPC observations and 1 has XMM-NEWTON observation. We analyzed their X-ray data, in an attempt to search for steep X-ray continuum, the postulated signature of HFSRQs. However, the results are inconclusive, due to the relatively poor data quality and the small sample size. More observations in X-rays are needed to perform such a test in the future. Also, extensive multi-waveband observations are needed to derive the synchrotron peak frequency in order to confirm their HFSRQ nature. |
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