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121.
122.
Shan-Jie Qian Xi-Zhen Zhang T P Krichbaum National Astronomical Observatories Chinese Academy of Sciences Beijing Max-Planck-Institut für Radioastronomie Auf dem Hügel Bonn Germany 《中国天文和天体物理学报》2004,4(3):231-246
The spectral energy distribution (SED) of the γ-ray flare observed in July 1997 in BL Lacertae is re-considered. It is pointed out that the optical observations made by Webb et al. showed the associated optical flare has a hard spectrum (the average spectral index αopt~0.48, Fν∝ν^-α), and the ASCA observations made by Tanihata et al. showed very steep spectra in the soft X-ray band (0.7-1.5 keV) (αx~3-4). We find that the flux densities and spectral indices in both the optical and soft X-ray bands are closely consistent with a ‘canonical‘ synchrotron spectrum emitted by relativistic electrons of a power-law energy distribution with a high energy cutoff, and thus the peak of the SED of the synchrotron radiation (in representation of νFν) is located in the EUV - soft X-ray bands. Therefore, the GeV γ-ray emission observed in the July 1997 outburst may be mainly due to the synchrotron self-Compton (SSC) process, contrasting with the current explanations in terms of external radiation Compton (ERC) process, in which the seed photons are mostly taken to be the UV emission from the clouds of the broad emission line region. We argue that the hard optical spectra observed during the γ-ray outburst may be an important signature for the acceleration of high energy electrons (γe-10^4) in the γ-ray emitting region. 相似文献
123.
A. Caccianiga M.J. Marchã S. Antón K.-H. Mack M.J. Neeser 《Monthly notices of the Royal Astronomical Society》2002,329(4):877-889
This paper presents the optical properties of the objects selected in the CLASS blazar survey. Because an optical spectrum is now available for 70 per cent of the 325 sources present in the sample, a spectral classification, based on the appearance of the emission/absorption lines, is possible. A wide variety of optical spectral types is found. Besides 'classical' BL Lacs (42), BL Lac candidates (5) and high-power flat spectrum radio quasars (67), a significant number of 'passive' elliptical galaxies (41) is also found. Moreover, 33 broad emission line objects with a low radio power are discovered, suggesting that at least a fraction (∼ per cent) of low-power blazars have a broad line region. Finally, 34 objects showing only narrow emission lines, either as a result of some starburst activity in the host galaxy or as a result of the presence of an active galactic nucleus, appear in the sample. 相似文献
124.
M. Chiaberge † A. Capetti † A. Celotti † 《Monthly notices of the Royal Astronomical Society》2001,324(4):L33-L37
We have observed emission from the nucleus of the closest radio galaxy, Centaurus A, from the radio to the gamma-ray band. We construct, for the first time, its overall spectral energy distribution (SED) which appears to be intriguingly similar to those of blazars, showing two broad peaks located in the far-infrared band and at ∼0.1 MeV respectively. The whole nuclear emission of Centaurus A is successfully reproduced with a synchrotron self-Compton model. The estimated physical parameters of the emitting source are similar to those of BL Lacs, except for a much smaller beaming factor, as qualitatively expected when a relativistic jet is orientated at a large angle to the line of sight. These results represent strong evidence that Centaurus A is indeed a misoriented BL Lac, and provide strong support in favour of the unification scheme for low-luminosity radio-loud active galactic nuclei. Modelling of the SED of Centaurus A also provides further and independent indications of the presence of velocity structures in sub-parsec-scale jets. 相似文献
125.
D.C. Gabuzda J.L. Gómez I. Agudo 《Monthly notices of the Royal Astronomical Society》2001,328(3):719-725
VLBI total intensity and linear polarization images of the BL Lacertae object 0735+178 have been obtained at using the VLA as a ground array together with the HALCA orbiting antenna, and at about a month later using the VLBA. The most surprising result to come from a comparison of these two images, which have nearly equivalent resolution, is that the path of the jet appears to be appreciably different at the two wavelengths. We have interpreted this as evidence for free–free absorption to the east of the core, near a bend of about 90° in the 2-cm image where the emission is very weak in the 6-cm image. Overall, the polarization structures seen in corresponding regions at the two wavelengths are in good agreement. We find tentative evidence for enhanced rotation measures near two sharp bends in the VLBI jet. 相似文献
126.
Paolo Padovani †‡ Luigi Costamante Paolo Giommi Gabriele Ghisellini rea Comastri Anna Wolter Laura Maraschi Gianpiero Tagliaferri C. Megan Urry 《Monthly notices of the Royal Astronomical Society》2001,328(3):931-943
We present new BeppoSAX observations of seven BL Lacertae objects selected from the 1-Jy sample plus one additional source. The collected data cover the energy range (observer's frame), reaching ∼50 keV for one source (BL Lac). All sources characterized by a peak in their multifrequency spectra at infrared/optical energies (i.e., of the low-energy peaked BL Lac type, LBL) display a relatively flat X-ray spectrum, which we interpret as inverse Compton emission. Four objects (two-thirds of the LBLs) show some evidence for a low-energy steepening, which is probably due to the synchrotron tail merging into the inverse Compton component around ∼ . If this were generally the case with LBLs, it would explain why the ROSAT spectra of our sources are systematically steeper than the BeppoSAX ones . The broad-band spectral energy distributions fully confirm this picture, and a synchrotron inverse Compton model allows us to derive the physical parameters (intrinsic power, magnetic field, etc.) of our sources. Combining our results with those obtained by BeppoSAX on BL Lacs covering a wide range of synchrotron peak frequency, ν peak , we confirm and clarify the dependence of the X-ray spectral index on ν peak originally found in ROSAT data. 相似文献
127.
128.
W. J. Chaplin Y. Elsworth G. R. Isaak B. A. Miller R. New B. Pintér † 《Monthly notices of the Royal Astronomical Society》2005,359(2):607-614
We report monitoring observations of 20 high-luminosity active galactic nuclei (AGN), 12 of which are radio-quiet quasars (RQQs). Intranight optical variability (INOV) was detected for 13 of the 20 objects, including 5 RQQs. The variations are distinctly stronger and more frequent for blazars than for the other AGN classes. By combining these data with results obtained earlier in our programme, we have formed an enlarged sample consisting of 9 BL Lacs, 19 RQQs and 11 lobe-dominated radio-loud quasars (RLQs). The moderate level of rapid optical variability found for both RQQs and radio lobe-dominated quasars (LDQs) argues against a direct link between INOV and radio loudness. We supplemented the present observations of 3 BL Lacs with additional data from the literature. In this extended sample of 12 well observed BL Lacs, stronger INOV is found for the EGRET detected subset. 相似文献
129.
130.