Search

Start Over Subject quasars Capability Type Simple Cone Search
831. Variable quasar sample from SDSS
ID:
ivo://CDS.VizieR/J/ApJ/633/638
Title:
Variable quasar sample from SDSS
Short Name:
J/ApJ/633/638
Date:
21 Oct 2021
Publisher:
CDS
Description:
SDSS repeat spectroscopic observations have resulted in multiepoch spectroscopy for 2500 quasars observed more than 50 days apart. From this sample, calibrating against stars observed simultaneously, we identify 315 quasars that have varied significantly between observations (with respect to assumed nonvariable stars observed concurrently). These variable quasars range in redshift from 0.5 to 4.72. This is the first large quasar sample studied spectroscopically for variability and represents a potentially useful sample for future high-redshift reverberation mapping studies. This also marks the first time the precise wavelength dependence of quasar variability has been determined, allowing both the continuum and emission-line variability to be studied.
832. Variable stars of Draco dSph
ID:
ivo://CDS.VizieR/J/AJ/136/1921
Title:
Variable stars of Draco dSph
Short Name:
J/AJ/136/1921
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present a CCD survey of variable stars in the Draco dwarf spheroidal galaxy. This survey, which has the largest areal coverage since the original variable star survey by Baade & Swope (1961AJ.....66..300B), includes photometry for 270 RR Lyrae (RRL) stars, 9 anomalous Cepheids (ACs), 2 eclipsing binaries, and 12 slow, irregular red variables, as well as 30 background QSOs. Twenty-six probable double-mode RRL stars were identified. Observed parameters, including mean V and I magnitudes, V amplitudes, and periods, have been derived.
833. Variations in QSOs optical flux
ID:
ivo://CDS.VizieR/J/ApJ/698/895
Title:
Variations in QSOs optical flux
Short Name:
J/ApJ/698/895
Date:
21 Oct 2021
Publisher:
CDS
Description:
We analyze a sample of optical light curves, compiled from the literature, for 100 quasars, 70 of which have black hole mass estimates. Our sample is the largest and broadest used yet for modeling quasar variability. The sources in our sample have z<2.8, 10^42^<~{lambda}L_{lambda}_(5100{AA})<~10^46^, and 10^6^<~M_BH_/M_{sun}_<~10^10^. We model the light curves as a continuous time stochastic process, providing a natural means of estimating the characteristic timescale and amplitude of quasar variations. We employ a Bayesian approach to estimate the characteristic timescale and amplitude of flux variations; our approach is not affected by biases introduced from discrete sampling effects.
834. Variations of the fine-structure constant
ID:
ivo://CDS.VizieR/J/other/MmSAI/80.833
Title:
Variations of the fine-structure constant
Short Name:
J/other/MmSAI/80
Date:
21 Oct 2021
Publisher:
CDS
Description:
The Keck telescope's High Resolution Spectrograph (HIRES) has previously provided evidence for a smaller fine-structure constant, {alpha}, compared to the current laboratory value, in a sample of 143 quasar absorption systems: {Delta}{alpha}/{alpha}=(-0.57+/-0.11)10^-5^. The analysis was based on a variety of metal-ion transitions which, if alpha varies, experience different relative velocity shifts. This result is yet to be robustly contradicted, or confirmed, by measurements on other telescopes and spectrographs; it remains crucial to do so.
835. Velocity dispersion in AGN
ID:
ivo://CDS.VizieR/J/ApJ/585/647
Title:
Velocity dispersion in AGN
Short Name:
J/ApJ/585/647
Date:
21 Oct 2021
Publisher:
CDS
Description:
It has been proposed that the width of the narrow [O III] {lambda}5007 emission line can be used as a surrogate for the stellar velocity dispersion in active galaxies. This proposition is tested using the Sloan Digital Sky Survey (SDSS) Early Data Release (EDR) spectra of 107 low-redshift radio-quiet QSOs and Seyfert 1 galaxies by investigating the correlation between black hole mass, as determined from H{beta} FWHM and optical luminosity, and [O III] FWHM. The correlation is real, but the scatter is large. Without additional information or selection criteria, the [O III] width can predict the black hole mass to a factor of 5.
836. VEXAS DR2 catalogs
ID:
ivo://CDS.VizieR/II/369
Title:
VEXAS DR2 catalogs
Short Name:
II/369
Date:
22 Feb 2022
Publisher:
CDS
Description:
We present the second public data release of the VISTA EXtension to Auxiliary Surveys (VEXAS DR2), where we classify objects into stars, galaxies and quasars based on an ensemble of machine learning algorithms. The aim of VEXAS is to build the widest multi-wavelength catalogue, providing reference magnitudes, colours and morphological information for a large number of scientific uses. We apply an ensemble of thirty-two different machine learning models, based on three different algorithms and on different magnitude sets, training samples and classification problems (two or three classes) on the three VEXAS Data Release 1 (DR1) optical+infrared (IR) tables. The tables were created in DR1 cross-matching VISTA near-infrared data with Wide-field Infrared Survey Explorer far-infrared data and with optical magnitudes from the Dark Energy Survey (VEXAS-DESW), the Sky Mapper Survey (VEXAS-SMW), and the Panoramic Survey Telescope and Rapid Response System Survey (VEXAS-PSW). We assemble a large table of spectroscopically confirmed objects (VEXAS-SPEC-GOOD, 415 628 unique objects), based on the combination of six different spectroscopic surveys that we use for training. We develop feature imputation to classify also objects for which magnitudes in one or more bands are missing. We classify in total 90106 objects in the Southern Hemisphere. Among these, ~62.9x10^6^ (~52.6x10^6^) are classified as 'high confidence' ('secure') stars, ~920000 (~750000) as 'high confidence' ('secure') quasars and ~34.8 (~34.1) millions as 'high confidence' ('secure') galaxies, with pclass>=0.7 (pclass>=0.9). The DR2 tables update the DR1 with the addition of imputed magnitudes and membership probabilities to each of the three classes. The density of high-confidence extragalactic objects varies strongly with the survey depth: at pclass>0.7; there are 111/deg^2^ quasars in the VEXAS-DESW footprint and 103/deg^2^ in the VEXAS-PSW footprint, while only 10.7/deg^2^ in the VEXASSM footprint. Improved depth in the midIR and coverage in the optical and nearIR are needed for the SM footprint that is not already covered by DESW and PSW.
837. VLA Images of Extragalactic Objects
ID:
ivo://CDS.VizieR/J/ApJS/124/285
Title:
VLA Images of Extragalactic Objects
Short Name:
J/ApJS/124/285
Date:
21 Oct 2021
Publisher:
CDS
Description:
Maps of 212 extragalactic radio sources at 4.9 GHz are shown in Stokes I along with linear polarization vectors. The objects have a declination range of 0 degrees to -35 degrees and were chosen from the Hewitt & Burbidge quasar catalog with a spectral index less than -0.5 and the NVSS survey with a minimum 1.4 GHz flux density greater than 300 mJy. The observations were made with the Very Large Array in its B configuration, and the images have a typical resolution of 2 arcseconds. One hundred ninety-five objects were resolved; 108 of the sources are quasars, and another 51 have been identified as galaxies.
838. VLA imaging of obscured WISE+NVSS QSOs
ID:
ivo://CDS.VizieR/J/ApJ/896/18
Title:
VLA imaging of obscured WISE+NVSS QSOs
Short Name:
J/ApJ/896/18
Date:
07 Mar 2022 07:27:35
Publisher:
CDS
Description:
We present new subarcsecond-resolution Karl G. Jansky Very Large Array (VLA) imaging at 10GHz of 155 ultraluminous (Lbol~10^11.7^-10^14.2^L_{sun}_) and heavily obscured quasars with redshifts z~0.4-3. The sample was selected to have extremely red mid-infrared-optical color ratios based on data from the Wide-Field Infrared Survey Explorer (WISE) along with a detection of bright, unresolved radio emission from the NRAO VLA Sky Survey (NVSS) or Faint Images of the Radio Sky at Twenty cm Survey. Our high-resolution VLA observations have revealed that the majority of the sources in our sample (93 out of 155) are compact on angular scales <0.2 (<=1.7kpc at z~2). The radio luminosities, linear extents, and lobe pressures of our sources are similar to young radio active galactic nuclei (e.g., gigahertz-peaked spectrum [GPS] and compact steep-spectrum [CSS] sources), but their space density is considerably lower. Application of a simple adiabatic lobe expansion model suggests relatively young dynamical ages (~10^4-7yr^), relatively high ambient ISM densities (~1-10^4^cm^-3^), and modest lobe expansion speeds (~30-10000km/s). Thus, we find our sources to be consistent with a population of newly triggered, young jets caught in a unique evolutionary stage in which they still reside within the dense gas reservoirs of their hosts. Based on their radio luminosity function and dynamical ages, we estimate that only ~20% of classical large-scale FR I/II radio galaxies could have evolved directly from these objects. We speculate that the WISE-NVSS sources might first become GPS or CSS sources, of which some might ultimately evolve into larger radio galaxies.
839. VLBA 5GHz observations of Fermi/LAT AGNs
ID:
ivo://CDS.VizieR/J/ApJ/744/177
Title:
VLBA 5GHz observations of Fermi/LAT AGNs
Short Name:
J/ApJ/744/177
Date:
21 Oct 2021
Publisher:
CDS
Description:
The radio properties of blazars detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope have been observed contemporaneously by the Very Long Baseline Array (VLBA). In total, 232 sources were observed with the VLBA. Ninety sources that were previously observed as part of the VLBA Imaging and Polarimetry Survey (VIPS) have been included in the sample, as well as 142 sources not found in VIPS. This very large, 5GHz flux-limited sample of active galactic nuclei (AGNs) provides insights into the mechanism that produces strong {gamma}-ray emission. In particular, we see that {gamma}-ray emission is related to strong, uniform magnetic fields in the cores of the host AGN. Included in this sample are non-blazar AGNs such as 3C84, M82, and NGC 6251. For the blazars, the total VLBA radio flux density at 5GHz correlates strongly with {gamma}-ray flux. The LAT BL Lac objects tend to be similar to the non-LAT BL Lac objects, but the LAT flat-spectrum radio quasars (FSRQs) are significantly different from the non-LAT FSRQs. Strong core polarization is significantly more common among the LAT sources, and core fractional polarization appears to increase during LAT detection.
840. VLBA images of High Frequency Peakers
ID:
ivo://CDS.VizieR/J/A+A/450/959
Title:
VLBA images of High Frequency Peakers
Short Name:
J/A+A/450/959
Date:
21 Oct 2021
Publisher:
CDS
Description:
We propose a morphological classification based on the parsec scale structure of fifty-one High Frequency Peakers (HFPs) from the "bright" HFP sample. VLBA images at two adjacent frequencies (chosen among 8.4, 15.3, 22.2 and 43.2GHz) have been used to investigate the morphological properties of the HFPs in the optically thin part of their spectrum. We confirm that there is quite a clear distinction between the pc-scale radio structure of galaxies and quasars: the 78% of the galaxies show a "Double/Triple" morphology, typical of Compact Symmetric Objects (CSOs), while the 87% of the quasars are characterised by Core-Jet or unresolved structure. This suggests that most HFP candidates identified with quasars are likely blazar objects in which a flaring self-absorbed component at the jet base was outshining the remainder of the source at the time of the selection based on the spectral shape. Among the sources classified as CSOs or candidates it is possible to find extremely young radio sources with ages of about 100 years or even less.