To build a large, optically bright, X-ray selected AGN sample we have correlated the ROSAT-FSC (<IX/29>) catalogue of X-ray sources with the USNO (<I/252>) catalogue limited to objects brighter than O=16.5 and then with the APS (<VII/214>) database. Each of the 3212 coincidences was classified using the slitless Hamburg spectra. 493 objects were found to be extended and 2719 starlike. Using both the extended objects and the galaxies known from published catalogues we built up a sample of 185 galaxies with O(APS)<17.0mag, which are high-probability counterparts of RASS-FSC X-ray sources. 130 galaxies have a redshift from the literature and for another 34 we obtained new spectra. The fraction of Seyfert galaxies in this sample is 20%. To select a corresponding sample of 144 high-probability counterparts among the starlike sources we searched for very blue objects in an APS-based color-magnitude diagram. Forty-one were already known AGN and for another 91 objects we obtained new spectra, yielding 42 new AGN, increasing their number in the sample to 83. This confirms that surveys of bright QSOs are still significantly incomplete. On the other hand we find that, at a flux limit of 0.02ct/s and at this magnitude, only 40% of all QSOs are detected by ROSAT.
We use Virtual Observatory (VO) tools to identify optically faint, obscured (i.e., type 2) active galactic nuclei (AGN) in the two Great Observatories Origins Deep Survey (GOODS) fields. By employing publicly available X-ray and optical data and catalogues we discover 68 type 2 AGN candidates.
We present a sample of 723 optically selected BL Lac candidates from the Sloan Digital Sky Survey Data Release 7 (SDSS DR7) spectroscopic database encompassing 8250deg^2^ of sky; our sample constitutes one of the largest uniform BL Lac samples yet derived. Each BL Lac candidate has a high-quality SDSS spectrum from which we determine spectroscopic redshifts for ~60% of the objects. Redshift lower limits are estimated for the remaining objects utilizing the lack of host galaxy flux contamination in their optical spectra; we find that objects lacking spectroscopic redshifts are likely at systematically higher redshifts.
With close pairs of quasars at different redshifts, a background quasar sight line can be used to study a foreground quasar's environment in absorption. We search 149 moderate-resolution background quasar spectra from Gemini, Keck, the MMT, and the SDSS to survey Lyman limit systems (LLSs) and damped Ly{alpha} systems (DLAs) in the vicinity of 1.8<z<4.0 luminous foreground quasars. A sample of 27 new quasar-absorber pairs is uncovered with column densities 10^17.2^cm^-2^<NH<10^20.9^cm^-2^ and transverse (proper) distances of 22h^-1^kpc<R<1.7h^-1^Mpc from the foreground quasars.
We present the Voigt profile analysis of 132 intervening CIV+CIII components associated with optically thin HI absorbers at 2.1<z<3.4 in the 19 high-quality UVES/VLT and HIRES/Keck QSO spectra. For logN_CIV_ in [11.7, 14.1], N_CIII_{propto}N_CIV_^1.42+/-0.11^ and <N_CIII}/N_CIV>=1.0+/-0.3 with a negligible redshift evolution. For 54 CIV components tied (aligned) with HI at logN_HI_ in [12.2, 16.0] and logN_CIV_ in [11.8, 13.8], the gas temperature T_b_ estimated from absorption line widths is well approximated to a Gaussian peaking at logT_b_~4.4+/-0.3 for logT_b_ in [3.5, 5.5], with a negligible non-thermal contribution. For 32 of 54 tied HI+CIV pairs, also tied with CIII at logN_CIII_ in [11.7, 13.8], we ran both photoionization equilibrium (PIE) and non-PIE (using a fixed temperature T_b_) cloudy models for the Haardt-Madau QSOs+galaxies 2012 UV background. We find evidence of bimodality in observed and derived physical properties. High-metallicity branch absorbers have a carbon abundance [C/H]temp>=-1.0, a line-of-sight length L_temp_<=20kpc and a total (neutral and ionized) hydrogen volume density logn_H,temp_ in [-4.5, -3.3] and logT_b_ in [3.9, 4.5]. Low-metallicity branch absorbers have [C/H]temp<=-1.0, L_temp_ in [20, 480]kpc and logn_H,temp_ in [-5.2, -4.3] and logT_b_~ 4.5. High-metallicity branch absorbers seem to be originated from extended discs, inner haloes or outflowing gas of intervening galaxies, while low-metallicity absorbers are produced by galactic haloes or the surrounding intergalactic medium filament.
We report the results of a study of optical microvariability in radio-quiet and radio-intermediate quasars. Observations were obtained on a total of seven objects: five radio-quiet quasars and two radio-intermediate quasars. No microvariability was detected in either the radio-quiet or radio-intermediate objects in our sample, despite intensive monitoring for several consecutive nights. In one object, PG 1257+346, evidence for interday variability was detected. We examined a sample of 117 radio-quiet objects found in the literature that have been studied for microvariability. This sample is discussed in terms of classification, redshift distribution, R (the ratio of the radio [5GHz] flux to the optical [4400{AA}] flux), optical magnitude, luminosity, and observing strategy.
The goal to this work is to observe a first set of 70 extragalactic sources at optical wavelengths that could achieve the link with the ICRF. Variations in the light curves of these targets are connected with astrophysical processes that could produce displacements of the optical photocenter. Such displacements, if they exist, are critical in the framework of the link of reference systems. Four telescope were used to observe the targets at optical wavelengths. Two of them are located in France, one in Chile, and the last in Australia. First observations were carried out during one year and a half in the R and V bands. A new method of characterizing the compactness of the targets was applied to the images obtained.
The results of the optical monitoring between November 1994 and November 1995 of twenty gamma-ray loud blazars included in the Torino blazar monitoring program are presented.
Results of long-term monitoring of the quasar 3C345 from 1984 to 1991 are presented. Observations has been conducted on the Byurakanskii station of the St. Petersburg Astronomical Institute since 1968 in the B band, and consist of 365 brightness estimates obtained over the course of 219 nights.
We present 426 epochs of optical monitoring data spanning 1000 days from 2003 December to 2006 June for the gravitationally lensed quasar SDSS J1004+4112. The time delay between the A and B images is {Delta}t_BA_=38.4+/-2.0days ({Delta}{chi}^2^=4) in the expected sense that B leads A and the overall time ordering is C-B-A-D-E. The measured delay invalidates all published models. The models probably failed because they neglected the perturbations from cluster member galaxies. Models including the galaxies can fit the data well, but conclusions about the cluster mass distribution should await the measurement of the longer, and less substructure sensitive, delays of the C and D images. For these images, a delay of {Delta}t_CB_~=681+/-15days is plausible but requires confirmation, while delays of {delta}t_CB_>560days and {delta}t_AD_>800 days are required. We clearly detect microlensing of the A/B images, with the delay-corrected flux ratios changing from mB-mA=0.44+/-0.01mag in the first season to 0.29+/-0.01mag in the second season and 0.32+/-0.01mag in the third season.
