Optical Microvariability in Quasars: Spectral Variability

We present a method that we developed to discern where the optical microvariability (OM) in quasars originates: in the accretion disk (related to thermal processes) or in the jet (related to non-thermal processes). Analyzing nearly simultaneous observations in three different optical bands of continuum emission, we are able to determine the origin of several isolated OM events. In particular, our method indicates that from nine events reported by Ramirez et al. (2009), three of them are consistent with a thermal origin, three to non-thermal, and three cannot be discerned. The implications for the emission models of OM are briefly discussed.Comment: Accepted for publication in the Astrophysical Journa

Spectroscopy of the neighboring massive clusters Abell 222 and Abell 223

We present a spectroscopic catalog of the neighboring massive clusters Abell 222 and Abell 223. The catalog contains the positions, redshifts, R magnitudes, V-R color, as well as the equivalent widths for a number of lines for 183 galaxies, 153 of them belonging to the A 222 and A 223 system. We determine the heliocentric redshifts to be z=0.2126+/-0.0008 for A 222 and z=0.2079+/-0.0008 for A 223. The velocity dispersions of both clusters in the cluster restframe are about the same: sigma = 1014^{+90}_{-71} km/s and sigma = 1032^{+99}_{-76} km/s for A 222 and A 223, respectively. While we find evidence for substructure in the spatial distribution of A 223, no kinematic substructure can be detected. From the red cluster sequence identified in a color--magnitude--diagram we determine the luminosity of both clusters and derive mass--to--light ratios in the R--band of (M/L)_A222 = (202+/-43) h_70 M_{su}n/L_{sun} and (M/L)_A223 = (149+/-33) h_70 M_{sun}/L_{sun}. Additionally we identify a group of background galaxies at z ~ 0.242.Comment: Accepted for publication in A&A, 10 pages, 9 figures, full version of table 2 included in source distribution, version with higher quality images available from http://www.astro.uni-bonn.de/~dietrich

Optical variability of PKS 0736+017

We present BVR photometric observations of the blazar PKS 0736+017. These observations were carried out with three telescopes in Mexico and two in Spain between December 1998 and April 2003. PKS 0736+017 shows remarkable variation at different timescales and amplitudes. Maximum brightness was detected on December 19, 2001 (B=14.90+/-0.01, V=14.34+/-0.01, and R=13.79+/-0.01). A peculiar tendency to redden with increased brightness was detected throughout our observations. Moreover, in one season a good correlation between flux level and spectral slope is shown. This "anomalous" behaviour cannot be described by common flare models of blazars. The flux vs. spectral slope correlation observed in this and other blazars is worth further study.Comment: 8 pages, 6 figures, accepted for publication in Astronomy & Astrophysic

Optical Variability of Faint QSOs and AGNs

We report some results of a new analysis (Trèvese et al. 1993, ApJ (submitted) (T93)) of the variability of the faint QSO sample of SA57 (Koo, Kron and Cudworth, 1986, PASP, 98, 285), concerning the intrinsic time scales of variability and the dependence of the amplitude of variability on absolute magnitude and redshift. Prime focus plates of SA57 have been obtained with the Mayall 4-m telescope at the Kitt Peak National Observatory since 1974. The present analysis is based on 14 BJ plates spanning 15 years at 11 independent epochs. The digitization, object selection, image classification and photometry are described in Koo (1986, ApJ, 311, 651) and Trèvese et al. (1989, AJ, 98, 108 (T89)). The magnitude limit of the sample is BJ = 22.5 and stellar objects are selected with a threshold in image size. This criteria give a sample of 694 objects in the field, 34 of which are QSOs whose spectra have been measured with the Mayall 4-m telescope. The r.m.s. error in the BJ band is 0.05 mag at BJ Ã 22 mag. All but one of these QSOs appear to be variable according to T93.</jats:p

The Variability of QSOs in the Optical Band

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 &amp; 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 &lt; z &gt; ⋍ 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 &amp; Trèvese 1996). The correlation coefficient is r=0.46 with a probability P(&gt;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 &lt; λ &gt; ≍ 2000 Å of the sample, this corresponds to an average temperature T ≍ 2.5 × 104 of a black-body of fluctuating temperature.</jats:p

A Search for Extended Objects with Variable Nuclei

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, &amp; 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.</jats:p

Matter Distribution in the Galaxy Clusters A 539 and A 2319

We performed a combined X-ray and optical analysis of the two clusters A539 and A2319, based on ROSAT PSPC 0.4-2.4 keV images of the public archive and F band photometry from microdensitometric scans of Palomar 48 inch plates (Trèvese et al. 1992, A&amp;AS, 94, 327). Assuming spherical symmetry and following the methods adopted in Cirimele, Nesci, and Trèvese (1997, ApJ, 475, 11 (CNT97)) we derived the radial distribution of gas and galaxy densities ρgasand ρgaland we have computed the morphological parameter βxo≡dln ρgas(r)/dln ρgal(r), introduced in CNT97. This allows to check the validity of the hydrostatic equilibrium condition, which reads, for an isotropic and uniform velocity distribution of r.m.s. dispersion σr. In the case of A539, adopting σr=629 km s−1from Fadda et al. (1996, ApJ, 473, 670) and T=1.57 keV David et al. (1996, ApJ, 473, 692), we obtained marginally consistent values of βspec= 1.54±0.50 and βxo=1.08±0.11. In the case of A2319 we took into account the presence of the secondary component A2319B (Oegerle et al. 1995, AJ, 110, 32) and the temperature gradient (Markevitch M. 1996, ApJ, 465, L1). The resulting radial increase of βspecis consistent with that of (βxo(r) +dlnT(r)/dln ρgal), suggesting that the hydrostatic equilibrium holds also in the presence of a temperature gradient. The radial distribution of the total binding mass, the mass in galaxies and intergalactic gas show that in both clusters the gas mass profile is steeper than galaxies and total masses consistently with our previous results (CNT97). Adopting a constant gas temperature, the relevant baryon fractions are larger than 20 %, adding new evidence to the “baryon catastrophe”. Taking into account the radial decrease of gas temperature, the baryon fraction is further increased. This implies that either Ωo&lt; 0.25, or that large halos of dark matter surround galaxy clusters, as suggested by White &amp; Fabian (1995, MNRAS, 273, 72).</jats:p

Properties of nearby clusters of galaxies

We present F band photometry from digitized plates obtained with 48-inch Palomar Schmidt for 1074 galaxies brighter than m3+3 in 10 clusters. For each galaxy equatorial coordinates, magnitude, size, ellipticity and orientation are given. We provide the galaxy surface density maps for each cluster