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تسجيل مستخدم جديدThe near-IR shape of the big blue bump emission from quasars: under the hot dust emission
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Makoto Kishimoto
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One primary difficulty in understanding the nature of the putative accretion disk in the central engine of AGNs is that some of its key intrinsic spectral signatures are buried under the emissions from the surrounding regions, i.e. the broad line region (BLR) and the obscuring torus. We argue here that these signatures can be revealed by using optical and near-IR polarization. At least in some quasars, the polarization is seen only in the continuum and is not shared by emission lines. In this case, the polarized flux is considered to show the intrinsic spectrum interior to the BLR, removing off the emissions from the BLR and torus. We have used this polarization to reveal the Balmer-edge feature and near-IR spectral shape of the central engine, both of which are important for testing the fundamental aspects of the models.
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The near-infrared shape of the big blue bump component in quasar spectra has been essentially unknown. It usually cannot be observed directly, due to the strong hot dust emission which dominates quasar spectra longward of ~1micron. However this is qu
ite an important part of the spectrum theoretically. At least bare disk models provide quite a robust prediction for the overall continuum shape in the near-infrared. Self-gravity should become important in the outer, near-infrared emitting regions of the putative disk, possibly leaving a signature of disk truncation in the near-infrared. We propose here that this important part of the spectrum can be revealed for the first time by observing polarized flux from normal quasars. At least in some polarized quasars, the emission lines are all unpolarized and so the polarized flux should originate interior to the broad line region, and therefore also interior to the dust emitting region. This can then be used to eliminate the dust emission. We present the results of near-infrared polarimetry for such three quasars (Ton202, 4C37.43, B2 1208+32). The data for Ton202 have the highest S/N, and the near-infrared polarized flux in this case is measured to have quite a blue shape, nu^+0.42+-0.29 in F_nu, intriguingly consistent with the simple multi-temperature black body, bare disk prediction of nu^+1/3. All these data, although still with quite low S/N for the other two objects, demonstrate the unique potential of the technique with future better data. We also present similar data for other quasars and radio galaxies, and briefly discuss the nature of the polarization.
Thermal models for the quasar Big Blue Bump generally lead to bound-free continuum features, which may be in absorption or emission. Searches for Lyman edges attributable to quasar atmospheres (in particular accretion disk atmospheres) have been ambi
guous at best, but various relativistic and non-LTE effects may make them hard to detect. The Balmer edge features tentatively predicted by such models might be easier to detect since theyd form farther out in the potential wells. These can be sought in certain cases using spectropolarimetry to remove the effects of atomic emission (i.e. the Small Blue Bump) from the spectrum. We do find apparent Balmer edges in absorption in several quasars using this method! Although the features we see are believable and apparently common, more data and more modeling are needed to verify that they are best interpreted as atmospheric Balmer edges.
We report observations of three SDSS z>6 QSOs at 250 GHz (1.2mm) using the 117-channel Max-Planck Millimeter Bolometer (MAMBO-2) array at the IRAM 30-meter telescope. J1148+5251 (z=6.41) and J1048+4637 (z=6.23) were detected with 250 GHz flux densiti
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