Radio-selected Quasars Research Articles

The optical polarization properties of quasars

Optical polarimetry is presented for 163 radio-selected quasars, including a complete sample of 90 sources stronger than 2 Jy at 5 GHz. Synchrotron components are detected in over 30 percent of these flat spectrum quasars, with an optical polarization threshold of p greater than 3 percent. The fraction of polarized quasars is a strong function of the compactness of the radio emission, as measured on VLBI scales. Including the duty cycle correction, essentially every quasar with S(core) greater than S(ext) at radio wavelengths has a prominent blazar component at optical wavelengths. Virtually all of the radio sources with weak emission lines (or BL Lac objects) are highly polarized. Optical and radio polarization are not correlated, and there is no difference in the alpha(ro) distributions of high- and low-polarization quasars, or sources with strong and weak emission lines. Redshift information is incomplete, but BL Lac objects appear to have lower redshifts than strong-lined quasars. 76 refs.

VLA observations of objects in the Palomar Bright Quasar Survey

view Abstract Citations (1058) References (37) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS VLA Observations of Objects in the Palomar Bright Quasar Survey Kellermann, K. I. ; Sramek, R. ; Schmidt, M. ; Shaffer, D. B. ; Green, R. Abstract All 114 objects from the Palomar Bright Quasar Survey (BQS) have been observed using the VLA at 5 GHz (6 cm) wavelength with 18 arcsec resolution. The new radio observations typically reach a level of 0. 1 in the ratio R of radio-to-optical luminosity which allow radio emission to be detected from most of the BQS objects. Ninety six (84%) of the BQS were detected as radio sources above a 3 σ level of 0.2 mJy, and 75 (82%) of the complete sample of 92 quasars with M_B_ < -23 were detected. There appears to be no difference in the radio properties of AGNs (M_B_ > 23) and quasars (M_B_ < 23). The distribution of radio flux density for the BQS sample appears bimodal. Most of the BQS quasars are "radio quiet" and have a radio flux density close to that of the optical flux density, but 15% - 20% are "radio loud" and are much brighter at radio than at optical wavelengths. Only a few percent of the BQS sample overlap radio-selected quasars in the ratio of radio-to-optical flux density. Most of the sources detected with 18 arcsec resolution were reobserved with a resolution of 0.5 arcsec. For the optically selected quasars, we find an increasing fraction of compact sources with increasing redshift. We derive the radio luminosity function, assuming that it is either (a) independent of optical luminosity, or (b) a function of the ratio R of radio-to-optical luminosity. The statistics of radio detections of optically selected quasars in this and in other published studies are equally well represented by the two cases. The radio emission from high-redshift quasars (z > 0.5) is dominated by compact components; quasars with R > 100 mostly have small redshifts. The distinction between the radio-loud and radio-quiet quasars from this optically selected sample cannot be explained simply by the effects of geometry and relativistic beaming for objects having a fixed ratio of intrinsic radio and optical luminosity. Publication: The Astronomical Journal Pub Date: October 1989 DOI: 10.1086/115207 Bibcode: 1989AJ.....98.1195K Keywords: Astronomical Catalogs; Quasars; Radio Sources (Astronomy); Very Large Array (Vla); Centimeter Waves; Data Sampling; Luminosity; Radiant Flux Density; Astrophysics; QUASARS full text sources ADS | data products NED (114) SIMBAD (113)

The X-ray spectra of high-redshift quasars

view Abstract Citations (68) References (30) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The X-Ray Spectra of High-Redshift Quasars Canizares, Claude R. ; White, Julia L. Abstract We present mean spectral parameters for various ensembles of quasars observed with the Einstein Observatory Imaging Proportional Counter (IPC). Our sample contains 71 optically or radio-selected quasars with 0.1 < z < 3.5, Galactic N_H_ < 10^21^ cm^-2^, total counts of 30-500, and IPC gain < 19. Data for each object are fitted to a power law with absorption in neutral material. Quasars are grouped into ensembles according to radio properties (flat radio spectrum [FRS], steep radio spectrum [SRS], or radio-quiet [RQ]), and either redshift or X-ray luminosity, l_x_. For each ensemble, composite χ^2^ contours for the two interesting parameters, N_H_ and α (the energy power-law index), are found by adding the individual χ_2_ grids appropriately shifted to remove the Galactic contribution to N_H_ for each object. All ensembles are consistent with little or no additional absorption beyond that expected in the Galaxy, so we have considered the limits on α assuming this is the case. We find a clear correlation between radio properties and α. FRS quasars have α ~ 0.4, SRS quasars have α ~ 0.7, and RQ quasars have α ~ 1-1.4. There is no evidence for a dependence of α on z or, for the FRS and SRS ensembles, on l_x_, over nearly three decades. FRS quasars with 2.0 < z < 3.5 have just as flat mean spectra as those with low z, implying that a single power law, which is flatter than the canonical one with α ~ 0.65, continues into the 1-10 keV band (in which the observed softer X-rays were emitted). Unfortunately, the results for high-redshift and high-l_x_ RQ quasars are ambiguous because of systematic uncertainties in the ensemble means. Thus we cannot test the two-component spectral hypothesis of Wilkes and Elvis for these objects. SRS X-ray spectra could be steeper than FRS spectra because of the mixing of two components, although a single intrinsically steeper spectrum is easier to reconcile with the absence of z-dependence. The uncertainty in α for RQ quasars with high z leaves open the important question of their contribution to the cosmic X-ray background. Publication: The Astrophysical Journal Pub Date: April 1989 DOI: 10.1086/167273 Bibcode: 1989ApJ...339...27C Keywords: Quasars; Red Shift; X Ray Spectra; Absorption Spectra; Heao 2; Luminosity; Proportional Counters; Radio Spectra; Astrophysics; QUASARS; X-RAYS: SOURCES full text sources ADS | data products SIMBAD (71)

Emission-line properties of optically and radio-selected complete quasar samples

view Abstract Citations (148) References (37) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Emission-Line Properties of Optically and Radio-selected Complete Quasar Samples Baldwin, J. A. ; Wampler, E. Joseph ; Gaskell, C. Martin Abstract We present spectrophotometry of two complete samples of quasars, one found from the flat radio spectra of the objects and the other from their optical colors. We confirm that the equivalent widths of the major ultraviolet emission lines decrease with increasing continuum luminosity (the "Baldwin effect"), and that the (Lyα + N V)/C IV and λ1909/C IV intensity ratios are also luminosity dependent by virtue of the C IV luminosity dependence. We also show that the Mg II/C IV intensity ratio and possibly the strengths of the blue Fe II emission features and the FWHM of the λ1909 blend are luminosity dependent. These results generally support the Mushotzky and Ferland interpretation of the Baldwin effect in terms of a weak inverse correlation between the continuum luminosity and ionization parameter. Except for the possibility of evolution with look-back time, these luminosity-dependent effects still appear to be a valid way of calibrating quasar luminosities for cosmological tests. Possible differences in the optical spectra which might correlate with the very different radio properties of these two quasar samples are masked by these luminosity-dependent effects. Any future comparison between optical and radio-selected quasars should be made using a fainter sample of optically selected quasars which have luminosities comparable to those of the radio-selected sample. Publication: The Astrophysical Journal Pub Date: March 1989 DOI: 10.1086/167224 Bibcode: 1989ApJ...338..630B Keywords: Emission Spectra; Quasars; Stellar Spectrophotometry; Luminosity; Radio Spectra; Red Shift; Astrophysics; COSMOLOGY; QUASARS; RADIO SOURCES: GENERAL; SPECTROPHOTOMETRY full text sources ADS | data products SIMBAD (112) NED (107) MAST (1)

Beaming and the X-ray, optical and radio properties of quasars

We examine the correlations of X-ray and optical luminosities with radio properties for radio-selected quasars. A remarkably strong correlation is found to exist between the total radio luminosity (irrespective of whether it comes from a compact core or extended lobes) and the X-ray luminosity. A weaker correlation is found between the radio luminosity and the optical luminosity. The results are consistent with a two-component origin for the optical/X-ray emission, one isotropic and one relativistically beamed. This model also accounts for the change of slope with radio structure for the radio/X-ray luminosity correlation noted by Kembhavi et al. Quasars with flat radio spectra are found to have smaller emission-line equivalent widths than do steep-spectrum quasars, again in the sense predicted by the beaming hypothesis.

X-ray and radio core emission in radio quasars

In order to investigate the physical relationship between X-ray and radio core emission in radio-selected quasars, 35 radio quasars have been observed with the VLA at 6 and 20 cm. The sample was chosen from a list of radio quasars with known X-ray luminosity but poorly known radio properties. Including data gathered from the literature, radio core detections or upper limits at 6 cm have been obtained for 127 radio quasars which have published Einstein X-ray data. A statistical association is sought between radio core luminosity and X-ray luminosity, and it is found that there is a strong correlation. The slope of the relation of L(x) to L(Gamma)-alpha is alpha = 0.71 + or - 0.07 for unresolved quasars with flat radio spectra. The slope decreases as quasars with extended radio regions are considered. This is traced to the presence of radio emission which is unrelated to the X-ray emission, in the presently unresolved cores of quasars.

Radio and X-ray emission in radio-selected quasars

Radio and X-ray emission in radio-selected quasars

The Einstein Observatory Medium Sensitivity Survey - Optical identifications for a complete sample of X-ray sources

It is suggested that virtually all the X-ray sources in the Einstein Observatory's Medium Sensitivity Survey flux range can be identified with objects visible on the POSS, on the basis of the complete identification of all sources north of -25 deg declination. There is no evidence for a significant population of 'blank field' X-ray sources at this flux level, and therefore no evidence for any new X-ray source class with very high L(x)/L(v). Most of the quasars detected in the present survey are spectroscopically similar to optical or radio-selected quasars. About 25 percent of the quasar sample, however, had reddish colors, and permitted lines dominated by a narrow-line component. These objects form a second sequence of active galactic nuclei, distinct in their optical properties from the broad line objects.

The angular size of the high-redshift quasar Q0000–398

THE optically selected quasar1 –398 has a redshift2 z = 2.827. This quasar has been identified with a faint double radio source whose angular extent is 134 ± 40 arc s. As all known high-redshift radio-selected quasars have much smaller angular sizes, it has been suggested that the upper envelope of the angular size–redshift diagram of radio quasars is depressed by an anticorrelation of linear size and radio luminosity3. We show here, first that the radio source associated with –398 is compact and centred on the optical quasar, and second that there is no evidence in our data on other quasars for the suggested anticorrelation.

Quasars: Observed properties of optically-selected objects at large redshifts

Abstract I present a description of the recent use of slitless spectroscopy to discover most of the known, high-redshift (z > 2) quasi-stellar objects by direct detection of Ly-α emission and then review the observed properties of these objects in four sections: 1. (a)| Space Distribution and Luminosity Function : If we assume the redshifts are cosmological, the co-moving space density of QSOs probably continues to increase from our locality out to the most distant objects so far observed; there is no evidence for any cutoff in the distribution either near z∼2.5 or z∼3.3. Estimates for the surface density of emission-line QSOs on the sky at B=19.5 magnitude, over all redshifts currently vary from about 2 to 15 deg−2. This surface density increases steeply as a function of magnitude to beyond magnitude 20, which is quite distinct from the observed behaviour of radio-selected quasars. Work is beginning on the degree of isotropy in the distribution of optically-selected QSOs, now that hundreds of high redshift objects are being found on a single Schmidt plate. The luminosity function probably scales by a factor ∼4–8 per magnitude and there is a range of at least 5 magnitudes in the optical continuum flux from QSOs near z=2. The first confirmed, optically selected, high-redshift object with broad absorption and no obvious emission lines has recently been found using objective-prism Schmidt plates. 2. (b)| Emission-Line Spectrum : The widths of the emission lines observed in high-redshift objects are comparable to those of permitted lines in class 1 Seyfert galaxies. It is probable that many aspects of Seyfert galaxy models are directly relevant to the quasar phenomenon. Several recent observational estimates of the ratio I(Ly-α)/I(Hβ) give values ∼2–8 in contradiction to straightforward application of recombination theory (which predicts values ∼22–40); a moderate amount of reddening by dust confined to the line-emitting region may provide a satisfactory explanation of the discrepancy. As in radio-selected quasars, the emitting region is likely to consist of optically thick condensations which do not completely cover the source; of 14 optically selected objects studied to date, none show obvious Ly-β emission, and yet at most one or two show a significant discontinuity in the Lyman continuum that must unambiguously be associated with the emission-line region. The average ratios OVI/Ly-α and CIV/Ly-α agree with the values predicted by simple photoionisation models, but the observed NV/Ly-α and (SiIV+OIV])/Ly-α ratios may be too large by a factor of about 5. The detection of Hell λ1640 in several of the objects argues against a very low helium abundance. Possible differences in the optical properties of radio-loud and radio-quiet QSOs are: (i) Radio-loud QSOs tend to have higher intrinsic polarisation (P 3. (c)| Absorption-Line Spectra : Statistical studies of absorption-line systems in well-defined samples of optically selected QSOs have recently begun, at resolutions ranging from 0.3A to 40A; their purpose is to try to elucidate the nature and location of most of the absorbing material. The density of lines per unit wavelength interval seems to be a steep function of redshift, so a reasonably small number of objects may suffice for a suitable statistical sample. In some extreme cases, there is little doubt as to where the absorption arises. Very wide lines seen in objects such as PHL5200 and Q1246-057 provide information on material most likely to be associated with the QSO. The recently discovered sharp-lined systems with metallic lines and velocities relative to the QSO exceeding 0.6c are perhaps more likely to be associated with intervening galaxies, but the case is by no means clear cut. 4. (d)| Radio Properties : Only ∼7–12% of various samples of optically selected QSOs have been detected as radio emitters (e.g. down to a 6cm flux limit of ∼20mJy). The fraction of radio sources seems to be highest among the brightest optical objects; weak radio sources (e.g. in the 20–100mJy range at 6cm) are surprisingly rare in all radio surveys of optically-selected objects. Samples of optically-selected QSOs allow an estimate of the distribution of spectral indices, free of bias associated with radio selection techniques. I have assumed for most of this article that redshift is an indicator of distance in a zero-pressure cosmology with zero cosmological constant and zero deceleration parameter. Some derived parameters depend strongly on these assumptions. I have not included material which came to my attention after 15th May 1978.

On the emission-line spectra of radio and optically selected quasars of high redshift

view Abstract Citations (39) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the emission-line spectra of radio and optically selected quasars of high redshift. Osmer, P. S. Abstract The question is raised of whether Smith's (1975) color-independent objective-prism technique produces a sample of unusually strong-lined or otherwise atypical quasars. SIT vidicon observations of eight previously known radio-selected quasars with redshifts greater than 1.9 are presented, and the objects' properties are compared with those of the quasars from the objective-prism survey. The two groups are found to have similar distributions of line strengths, strong-line equivalent widths, and strong-line intensity ratios. It is concluded that the objective-prism technique does not produce a sample of quasars with anomalously strong emission lines. Publication: The Astrophysical Journal Pub Date: May 1977 DOI: 10.1086/155224 Bibcode: 1977ApJ...214....1O Keywords: Line Spectra; Quasars; Spectral Emission; Astronomical Models; Radio Spectra; Red Shift; Spectral Line Width; Tables (Data); Astrophysics full text sources ADS | data products NED (7)