2 results
The Variability of QSOs in the Optical Band
- D. Trèvese, A. Bunone, R.G. Kron
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- Journal:
- Symposium - International Astronomical Union / Volume 194 / 1999
- Published online by Cambridge University Press:
- 25 May 2016, pp. 184-186
- Print publication:
- 1999
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Constraints on the emission mechanism of AGNs can be provided by the variability of their spectral energy distribution (SED). Recently Di Clemente et al. (1996) have shown that the positive correlation of QSO variability with redshift can be due to a hardening of the spectrum in bright phases, coupled with the increase of the rest-frame frequency of the (fixed) observing band, for increasing redshift. Direct evidences of slope changes in the SEDs of a limited number of individual AGNs have been provided by Cutri et al. (1985), Edelson et al. (1990), and Kinney et al. (1991). In the following we present some preliminary results of a direct measure of variations of the SED slope in the complete, magnitude limited, sample of QSOs of the Selected Area 57 (Koo, Kron & Cudworth 1986; Trèvese et al. 1989 (T89)). The data are derived from two sets of prime focus plates of the SA 57, in the U, BJ, F and N bands, obtained with the 4-m telescope at the Kitt Peak National Observatory at two epochs separated by one year. Photometric methods and signal-to-noise optimization are described in T89 and Trèvese et al. (1994). The quasar sample, of < z > ⋍ 1.4, consists of 33 objects from T89, plus the brightest 3 members of the the Bershady, Trèvese and Kron (1998) sample of extended objects selected on the basis of variability. In Figure 1a Δα is reported versus the changes Δlog fv in the BJ band and shows positive correlation, indicating a hardening of the spectrum in the bright phase. A special care is needed to avoid spurious Δα – Δlog fv correlations (see Massaro & Trèvese 1996). The correlation coefficient is r=0.46 with a probability P(>r)=0.995, after the exclusion of one deviant point, whose inclusion would produce a higher correlation. The slope of the linear regression of ΔαΔlogv is b=1.9. Assuming that the spectra are, dominated by the big blue bump (BBB) in the sampled spectral region, around λ ≍ 2000 Å, we can use the simple approximation of a single black-body spectrum. To check the hypothesis that both the slope and brightness changes are caused by a temperature variations only, we derive the black-body temperatures from the SED slope as deduced from a linear fit of the log fv v.s. logv relation, based on the U, BJ and F data. The average slope (excluding the two highest and lowest z objects) is 〈α〉=-0.4±0.6 and the average temperature is T ≍ 25000 K. For a black-body of fixed emitting surface and varying temperature T, the changes of the local SED slope and the relevant luminosity variations are related by (dα/dT)/(dlogBv/dT) ≡ f(x), where x ≡ hv/kT. The slope b of the linear regression of Δα v.s. log fv of Figure la is compared with the function f(x) values of the sample determined by the dispersion of α. We can conclude that dα/dlogBv and α are consistent for x ≍ 3. For an average sampling wavelength < λ > ≍ 2000 Å of the sample, this corresponds to an average temperature T ≍ 2.5 × 104 of a black-body of fluctuating temperature.
A Search for Extended Objects with Variable Nuclei
- D. Trèvese, M.A. Bershady, R.G. Kron
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- Journal:
- Symposium - International Astronomical Union / Volume 186 / 1999
- Published online by Cambridge University Press:
- 25 May 2016, p. 488
- Print publication:
- 1999
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- Article
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A large sample of QSOs/AGNs with detectable host galaxies is important to study the relations between the properties of the host and nucleus. Relatively faint active nuclei are of particular interest since they represent the low luminosity part of the QSO luminosity function, whose cosmological evolution is still poorly known. The non-stellar colors criterion cannot be applied to extended objects since most galaxies appear as non-stellar in color space. We have selected a sample of candidate AGNs with extended image structure on the basis of their variability, extending a previous survey for variability in objects with stellar structure (Trèvese et al. 1989, A J, 98, 108; 1994, ApJ, 433, 494). The new sample allows a comparison with different selection techniques and increases the completeness of our previous survey. We add to the previously published spectroscopic observations new data providing the confirmation of some additional candidates. Presently we have 5 confirmed candidates from our primary sample of 16 objects brighter than BJ = 22 (Bershady, Trèvese, & Kron 1997, ApJ, in press). Since it is likely that our sample contains some additional bona fide AGNs, further spectroscopic work is desirable. We can put a lower limit of 106±20 deg−2 on the surface density of AGNs brighter than BJ=22.0 mag. The newly detected extended AGNs are at least ten times fainter than MB=-23 mag and represent 13 % of the total number of AGNs.