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Register a new userComposite spectra of quasars with different UV spectral index
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The composite spectra of quasars are widely used as templates for redshift determination, as well as for measurements of the mean transmission in Lyalpha-forest studies, and for investigation of general spectral properties of quasars. Possibility of composite spectra utilisation in these fields is related to remarkable similarity of quasar spectra in UV-optical range. But despite of general similarity in spectral shapes, they differ in several parameters, one of which is the spectral index. In the present paper we study the possible effects, related to neglect of this difference. We compiled 16 composite spectra from subsamples of individual SDSS DR7 quasar spectra with different spectral indices alpha_lambda within the wavelength range 1270-1480 AA, and show that (i) the redshifts measured for a test sample of high signal-to-noise ratio quasar spectra using these composites as templates appear to be systematically higher than those calculated with a traditional template, compiled from spectra with different alpha_lambda, with 1.5 times smaller errors in the former case; (ii) the difference in alpha_lambda in individual spectra used for compilation of composites can yield the mean transmission uncertainty up to 20%; (iii) a number of emission lines indistinguishable in ordinary composites, but seen in individual high-resolution spectra, can be detected in such composites. It is also shown, that there is no dependence of alpha_lambda on quasar luminosity in SDSS u, g, r and i bands, and monochromatic luminosity at 1450 AA.
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We present the composite optical spectrum for the largest sample of giant radio quasars (GRQs). They represent a rare subclass of radio quasars due to their large projected linear sizes of radio structures, which exceed 0.7 Mpc. To construct the composite spectrum, we combined 216 GRQs optical spectra from Sloan Digital Sky Survey (SDSS). As a result, we obtained the composite spectrum covering the wavelength range from 1400 {AA} to 7000 {AA}. We calculated the power-law spectral slope for GRQs composite, obtaining $alpha_{lambda}=-1.25$ and compared it with that of the smaller-sized radio quasars, as well as with the quasar composite spectrum obtained for large sample of SDSS quasars. We obtained that the GRQs continuum is flatter (redder) than the continuum of comparison quasar samples. We also show that the continuum slope depends on core and total radio luminosity at 1.4 GHz, being steeper for higher radio luminosity bin. Moreover, we found the flattening of the continuum with an increase of the projected linear size of radio quasar. We show that $alpha_{lambda}$ is orientation-dependent, being steeper for a higher radio core-to-lobe flux density ratio which is consistent with AGN unified model predictions. For two GRQs, we fit the spectral energy distribution using X-CIGALE code to compare the consistency of results obtained in the optical part of the electromagnetic spectrum with broad-band emission. The parameters obtained from the SED fitting confirmed the larger dust luminosity for the redder optical continuum.
Quasi-stellar object (QSO) spectral templates are important both to QSO physics and for investigations that use QSOs as probes of intervening gas and dust. However, combinations of various QSO samples obtained at different times and with different instruments so as to expand a composite and to cover a wider rest frame wavelength region may create systematic effects, and the contribution from QSO hosts may contaminate the composite. We have constructed a composite spectrum from luminous blue QSOs at 1 < z < 2.1 selected from the Sloan Digital Sky Survey (SDSS). The observations with X-shooter simultaneously cover ultraviolet (UV) to near- infrared (NIR) light, which ensures that the composite spectrum covers the full rest-frame range from Ly$beta$ to 11350 $AA$ without any significant host contamination. Assuming a power-law continuum for the composite we find a spectral slope of $alpha_lambda$ = 1.70+/-0.01, which is steeper than previously found in the literature. We attribute the differences to our broader spectral wavelength coverage, which allows us to effectively avoid fitting any regions that are affected either by strong QSO emissions lines (e.g., Balmer lines and complex [Fe II] blends) or by intrinsic host galaxy emission. Finally, we demonstrate the application of the QSO composite spectrum for evaluating the reddening in other QSOs.
We present the study on the X-ray emission for a sample of radio-detected quasars constructed from the cross-matches between SDSS, FIRST catalogs and XMM-Newton archives. A sample of radio-quiet SDSS quasars without FIRST radio detection is also assembled for comparison. We construct the optical and X-ray composite spectra normalized at rest frame $4215,rm AA$ (or $2200,rm AA$) for both radio-loud quasars (RLQs) and radio-quiet quasars (RQQs) at $zle3.2$, with matched X-ray completeness of 19%, redshift and optical luminosity. While the optical composite spectrum of RLQs is similar to that of RQQs, we find that RLQs have higher X-ray composite spectrum than RQQs, consistent with previous studies in the literature. By dividing the radio-detected quasars into radio loudness bins, we find the X-ray composite spectra are generally higher with increasing radio loudness. Moreover, a significant correlation is found between the optical-to-X-ray spectral index and radio loudness, and there is a unified multi-correlation between the radio, X-ray luminosities and radio loudness in radio-detected quasars. These results could be possibly explained with the corona-jet model, in which the corona and jet are directly related.
We report the first comparative study of strong MgII absorbers seen towards radio-loud quasars of core-dominated (CDQs) and lobe-dominated (LDQs) types and normal QSOs. The MgII associated absorption systems having a velocity offset v < 5000km/s from the systemic velocity of the background quasar were also excluded. Existing spectroscopic data for redshift-matched sightlines of 3975 CDQs and 1583 LDQs, covering a emission redshift range 0.39-4.87, were analysed and 864 strong MgII absorbers were found, covering the redshift range 0.45-2.17. The conclusions reached using this well-defined large dataset of strong MgII absorbers are: (i) The number density, dN/dz, towards CDQs shows a small, marginally significant excess over the estimate available for QSOs; (ii) In the redshift space, this difference is reflected in terms of a 1.6sigma excess of dN/dz over the QSOs, within the narrow redshift interval 1.2-1.8; (iii) The dN/dbeta distribution (with beta=v/c) for CDQs shows a significant excess over the distribution found for a redshift and luminosity matched sample of QSOs, at beta in the range 0.05-0.1. This leads us to infer that a significant fraction of strong MgII absorption systems seen in this offset velocity range are probably associated with the CDQs and might be accelerated into the line of sight by their powerful jets and/or due to the accretion-disk outflows close to our direction. Support to this scenario comes from a consistency check in which we only consider the spectral range corresponding to beta > 0.2. The computed redshift distribution for strong MgII absorbers towards CDQs now shows excellent agreement with that known for QSOs, as indeed is expected for purely intervening absorption systems. Thus, it appears that for CDQs the associated strong MgII absorbers can be seen at much larger velocities relative to the nucleus than the commonly adopted upper limit of 5000km/s.
We present the review of some new problems in cosmology and physics of stars in connection with future launching of WSO. We discuss three problems. UV observations of distant z > 6 quasars allow to obtain information on the soft < 1 KeV X-ray radiation of the accretion disk around a supermassive black hole because of its cosmological redshift. Really the region of X-ray radiation is insufficiently investigated because of high galactic absorption. In a result one will get important information on the reionization zone of the Universe. Astronomers from ESO revealed the effect of alignment of electric vectors of polarized QSOs. One of the probable mechanism of such alignment is the conversion of QSO radiation into low mass pseudoscalar particles (axions) in the extragalactic magnetic field. These boson like particles have been predicted by new SUSY particle physics theory. Since the probability of such conversion is increasing namely in UV spectral range one can expect the strong correlation between UV spectral energy distribution of QSO radiation and polarimetric data in the optical range. In the stellar physics one of the interesting problems is the origin of the X-ray sources with super Eddington luminosities. The results of UV observations of these X-ray sources will allow to find the origin of these sources as accreting intermediate mass black holes.
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