High-redshift Radio-loud Quasars Research Articles

VLT/SINFONI study of black hole growth in high-redshift radio-loud quasars from the CARLA survey

ABSTRACT We present Very Large Telescope/Spectrograph for INtegral Field Observations in the Near Infrared (VLT/SINFONI) observations of 35 quasars at 2.1 < z < 3.2, the majority of which were selected from the Clusters Around Radio-Loud AGN (CARLA) survey. CARLA quasars have large C iv-based black hole masses (MBH > 109 M⊙) and powerful radio emission ($P_{500\, \rm MHz}$ > 27.5 W Hz−1). We estimate H α-based MBH, finding a scatter of 0.35 dex compared to C iv. We evaluate several recipes for correcting C iv-based masses, which reduce the scatter to 0.24 dex. The radio power of the radio-loud quasars is at most weakly correlated with the interconnected quantities H αwidth, L5100, and MBH, suggesting that it is governed by different physical processes. However, we do find a strong inverse correlation between C iv blueshift and radio power linked to higher Eddington ratios and L5100. Under standard assumptions, the black hole (BH) growth time is longer than the cosmic age for many CARLA quasars, suggesting that they must have experienced more efficient growth in the past. If these BHs were growing from seeds since the epoch of reionization, it is possible that they grew at the Eddington limit like the quasars at z ∼ 6–7, and then continued to grow at the reduced rates observed until z ∼ 2–3. Finally, we study the relation between MBH and environment, finding a weak positive correlation between MBH and galaxydensity measured by CARLA.

Host galaxies of high-redshift extremely red and obscured quasars

Abstract We present Hubble Space Telescope 1.4–1.6 $\mu$m images of the hosts of 10 extremely red quasars (ERQs) and six type 2 quasar candidates at z = 2–3. ERQs, whose bolometric luminosities range between 1047 and 1048 erg s−1, show spectroscopic signs of powerful ionized winds, whereas type 2 quasar candidates are less luminous and show only mild outflows. After performing careful subtraction of the quasar light, we clearly detect almost all host galaxies. The median rest-frame B-band luminosity of the ERQ hosts in our sample is $10^{11.2}\, \mathrm{L}_{\odot }$, or ∼4L* at this redshift. Two of the 10 hosts of ERQs are in ongoing mergers. The hosts of the type 2 quasar candidates are 0.6 dex less luminous, with 2/6 in likely ongoing mergers. Intriguingly, despite some signs of interaction and presence of low-mass companions, our objects do not show nearly as much major merger activity as do high-redshift radio-loud galaxies and quasars. In the absence of an overt connection to major ongoing gas-rich merger activity, our observations are consistent with a model in which the near-Eddington accretion and strong feedback of ERQs are associated with relatively late stages of mergers resulting in early-type remnants. These results are in some tension with theoretical expectations of galaxy formation models, in which rapid black hole growth occurs within a short time of a major merger. Type 2 quasar candidates are less luminous, so they may instead be powered by internal galactic processes.

On the nature of bright compact radio sources atz> 4.5

High-redshift radio-loud quasars are used to, among other things, test the predictions of cosmological models, set constraints on black hole growth in the early universe and understand galaxy evolution. Prior to this paper, 20 extragalactic radio sources at redshifts above 4.5 have been imaged with very long baseline interferometry (VLBI). Here we report on observations of an additional ten z>4.5 sources at 1.7 and 5 GHz with the European VLBI Network (EVN), thereby increasing the number of imaged sources by 50%. Combining our newly observed sources with those from the literature, we create a substantial sample of 30 z>4.5 VLBI sources, allowing us to study the nature of these objects. Using spectral indices, variability and brightness temperatures, we conclude that of the 27 sources with sufficient information to classify, the radio emission from one source is from star formation, 13 are flat-spectrum radio quasars and 13 are steep-spectrum sources. We also argue that the steep-spectrum sources are off-axis (unbeamed) radio sources with rest-frame self-absorption peaks at or below GHz frequencies and that these sources can be classified as gigahertz peaked-spectrum (GPS) and megahertz peaked-spectrum (MPS) sources.

BROADBAND OBSERVATIONS OF HIGH REDSHIFT BLAZARS

ABSTRACT We present a multi-wavelength study of four high redshift blazars, S5 0014+81 (z = 3.37), CGRaBS J0225+1846 (z = 2.69), BZQ J1430+4205 (z = 4.72), and 3FGL J1656.2−3303 (z = 2.40) using quasi-simultaneous data from the Swift, Nuclear Spectroscopic Telescope Array (NuSTAR) and the Fermi-Large Area Telescope (LAT) and also archival XMM-Newton observations. Other than 3FGL J1656.2−3303, none of the sources were known as γ-ray emitters, and our analysis of ∼7.5 yr of LAT data reveals the first time detection of statistically significant γ-ray emission from CGRaBS J0225+1846. We generate the broadband spectral energy distributions (SED) of all the objects, centering at the epoch of NuSTAR observations and reproduce them using a one-zone leptonic emission model. The optical−UV emission in all the objects can be explained by radiation from the accretion disk, whereas the X-ray to γ-ray windows of the SEDs are found to be dominated by inverse Compton scattering off the broad line region photons. All of them host black holes that are billions of solar masses. Comparing the accretion disk luminosity and the jet power of these sources with a large sample of blazars, we find them to occupy a high disk luminosity–jet power regime. We also investigate the X-ray spectral properties of the sources in detail with a major focus on studying the causes of soft X-ray deficit, a feature generally seen in high redshift radio-loud quasars. We summarize that this feature could be explained based on the intrinsic curvature in the jet emission rather than being due to the external effects predicted in earlier studies, such as host galaxy and/or warm absorption.

DISCLOSING THE RADIO LOUDNESS DISTRIBUTION DICHOTOMY IN QUASARS: AN UNBIASED MONTE CARLO APPROACH APPLIED TO THE SDSS-FIRST QUASAR SAMPLE

We investigate the dichotomy in the radio loudness distribution of quasars by modelling their radio emission and various selection effects using a Monte Carlo approach. The existence of two physically distinct quasar populations, the radio-loud and radio-quiet quasars, is controversial and over the last decade a bimodal distribution of radio loudness of quasars has been both affirmed and disputed. We model the quasar radio luminosity distribution with simple unimodal and bimodal distribution functions. The resulting simulated samples are compared to a fiducial sample of 8,300 quasars drawn from the SDSS DR7 Quasar Catalog and combined with radio observations from the FIRST survey. Our results indicate that the SDSS-FIRST sample is best described by a radio loudness distribution which consists of two components, with 12+/-1 % of sources in the radio-loud component. On the other hand, the evidence for a local minimum in the loudness distribution (bimodality) is not strong and we find that previous claims for its existence were probably affected by the incompleteness of the FIRST survey close to its faint limit. We also investigate the redshift and luminosity dependence of the radio loudness distribution and find tentative evidence that at high redshift radio-loud quasars were rarer, on average "louder", and exhibited a smaller range in radio loudness. In agreement with other recent work, we conclude that the SDSS-FIRST sample strongly suggests that the radio loudness distribution of quasars is not a universal function, and that more complex models than presented here are needed to fully explain available observations.

THE X-RAY PROPERTIES OF TYPICAL HIGH-REDSHIFT RADIO-LOUD QUASARS

We report spectral, imaging, and variability results from four new XMM-Newton observations and two new Chandra observations of high-redshift (z > 4) radio-loud quasars (RLQs). Our targets span lower, and more representative, values of radio loudness than those of past samples of high-redshift RLQs studied in the X-ray regime. Our spectral analyses show power-law X-ray continua with a mean photon index, \Gamma =1.74 +/- 0.11, that is consistent with measurements of lower redshift RLQs. These continua are likely dominated by jet-linked X-ray emission, and they follow the expected anti-correlation between photon index and radio loudness. We find no evidence of iron K\alpha ~ emission lines or Compton-reflection continua. Our data also constrain intrinsic X-ray absorption in these RLQs. We find evidence for significant absorption (N_H ~ 10^22 cm^-2) in one RLQ of our sample (SDSS J0011+1446); the incidence of X-ray absorption in our sample appears plausibly consistent with that for high-redshift RLQs that have higher values of radio loudness. In the Chandra observation of PMN J221-2719 we detect apparent extended (~ 14 kpc) X-ray emission that is most likely due to a jet; the X-ray luminosity of this putative jet is ~2% that of the core. The analysis of a 4.9 GHz VLA image of PMN J221-2719 reveals a structure that matches the X-ray extension found in this source. We also find evidence for long-term (450-460 days) X-ray variability by 80-100% in two of our targets.

X-ray spectral properties of high-redshift radio-loud quasars beyond redshift 4 - first results

We present the results of X-ray spectroscopic observations with XMM–Newton for four high-redshift radio-loud quasars at z > 4. Among these, three objects, namely GB B1508+5714, PMN J0324−2918 and PKS B1251−407, do not show soft X-ray spectral flattening; the derived upper limits on assumed intrinsic absorption are (3.3–17.3) × 1021 cm−2, the least of which is among the most stringent limits for z > 4 quasars. There is a tentative indication for soft X-ray spectral flattening in PMN J1451-1512 at z= 4.76, though the significance is not high. These observations more than double the number of z > 4 radio-loud quasars that have X-ray spectroscopic data to seven, which compose a significant subset of a flux-limited sample of z > 4 radio-loud quasars. Based on this subset, we show, in the second part of this paper, some preliminary results on the overall X-ray spectral properties of the sample. Soft X-ray spectral flattening, which is thought to arise from intrinsic X-ray absorption, was found in about half of the sample (3/7 or 4/7). We give a preliminary distribution of the absorption column density NH. For those with detected X-ray absorption, the derived NH values fall into a very narrow range (around a few times 1022 cm−2 for ‘cold’ absorption), suggesting a possible common origin of the absorber. This NH distribution is consistent with that in the redshift range of 2–4, though the data are sparse. Those that do not show X-ray absorption are constrained to have upper limits on the NH broadly consistent in general with the lower end of the distribution of the detected NH. Compared to lower redshift samples at z < 2, there is an extension, or a systematic shift, towards higher values in the intrinsic NH distribution at z > 4, and an increase of the fraction of radio-loud quasars showing X-ray absorption towards high redshifts. These results indicate a cosmic evolution effect, which seems to be the strongest at redshifts around 2. There is a tentative tendency that objects showing X-ray absorption have X-ray fluxes systematically higher than those showing apparently no absorption. After the spectral flattening is accounted for, the rest frame 1–50 keV continua have photon indices with a mean of 1.64 and a standard deviation of 0.11 (or a mean of 1.67 and a standard deviation of 0.14 for a Gaussian fit). Variability appears to be common on time-scales from a few months to years in the quasar rest frame, sometimes in both fluxes and spectral slopes.

Radio and X-ray observations of an exceptional radio flare in the extreme z= 4.72 blazar GB B1428+4217

We report on the extreme behaviour of the high redshift blazar GB B1428+4217 at z=4.72. A continued programme of radio measurements has revealed an exceptional flare in the lightcurve, with the 15.2 GHz flux density rising by a factor ~3 from ~140 mJy to ~430 mJy in a rest-frame timescale of only ~4 months -- much larger than any previous flares observed in this source. In addition to new measurements of the 1.4-43 GHz radio spectrum we also present the analysis and results of a target-of-opportunity X-ray observation using XMM-Newton, made close to the peak in radio flux. Although the X-ray data do not show a flare in the high energy lightcurve, we are able to confirm the X-ray spectral variability hinted at in previous observations. GB B1428+4217 is one of several high-redshift radio-loud quasars that display a low energy break in the X-ray spectrum, probably due to the presence of excess absorption in the source. X-ray spectral analysis of the latest XMM-Newton data is shown to be consistent with the warm absorption scenario which we have hypothesized previously. Warm absorption is also consistent with the observed X-ray spectral variability of the source, in which the spectral changes can be successfully accounted-for with a fixed column density of material in which the ionization state is correlated with hardness of the underlying power-law emission.

AChandraSnapshot Survey of Representative High-Redshift Radio-Loud Quasars from the Parkes-MIT-NRAO Sample

We present the results of Chandra ACIS-S snapshot observations of six radio-loud quasars (RLQs) at z ≈ 3.5-4.7. These observations sample luminous RLQs with moderate to high radio loudness (R ≈ 200-9600) and aim to connect the X-ray properties of radio-quiet quasars (R 1000) at high redshift. This work extends a study by Bassett and coworkers that used similar methods to examine z > 4 RLQs with moderate radio loudness (R ≈ 40-400). All of our targets are clearly detected. A search for extended X-ray emission associated with kiloparsec-scale radio jets revealed only limited evidence for X-ray extension in our sample: three sources showed no evidence of X-ray extension, and the other three had 3%-30% of their total X-ray fluxes extended >1'' away from their X-ray cores. In addition, we do not observe any systematic flattening of the optical-to-X-ray spectral index (αox) compared to low-redshift quasars. These results suggest that kiloparsec-scale X-ray jet emission is not dominated by inverse Compton scattering of cosmic microwave background seed photons off jet electrons. We measured X-ray continuum shapes and performed individual and joint spectral fits of our data combined with eight archival RLQs. A single power-law model acceptably fitted the data, with best-fit photon indices of 1.72 for moderate-R sources and 1.47 for high-R sources. We added an intrinsic absorption component to our model, and neither the moderate-R nor the high-R fits set a lower bound on NH. The upper limits were NH < 3.0 × 1022 cm-2 and NH < 2.8 × 1022 cm-2, respectively. Our spectral results suggest that intrinsic absorption does not strongly depend on radio loudness, and high-R sources have flatter power laws than moderate-R sources. Overall, our high-redshift RLQs have basic X-ray properties consistent with similar RLQs in the local universe.

XMM-NewtonObservations of High-Redshift Quasars

We report on our XMM observations of the high redshift quasars BR 2237--0607 (z=4.558) and BR 0351--1034 (z=4.351), together with 14 other z>4 objects found in the XMM public archive. Contrary to former reports, we do not find high redshift radio-loud quasars to be more absorbed than their radio-quiet counterparts. We find that the optical to X-ray spectral index alpha-ox is correlated with the luminosity density at 2500 A, but does not show a correlation with redshift. The mean 2-10 keV power-law slope of the 9 high redshift radio-quiet quasars in our sample for which a spectral analysis can be performed is alpha-x1.23+-0.48, similar to alpha-x=1.19 found from the ASCA observations of low redshift Narrow-Line Seyfert 1 galaxies (NLS1s), and significantly different from alpha-x=0.78 found for low redshift Broad-Line Seyfert galaxies. While the optical/UV spectra of low to high redshift quasars look remarkably similar, we find a first indication of a difference in their X-ray spectrum. The steep X-ray spectral index suggests high Eddington ratios L/L_Edd. These observations give credence to the hypothesis of Mathur (2000) that NLS1s are low luminosity cousins of high redshift quasars, both likely to be in their early evolutionary stage.

ChandraObservations of Radio-Loud Quasars atz &gt; 4: X-Rays from the Radio Beacons of the Early Universe

We present the results of Chandra observations of six radio-loud quasars (RLQs) and one optically bright radioquiet quasar (RQQ) at z 4.1‐4.4. These observations cover a representative sample of RLQs with moderate radio-loudness (R 40‐400), filling the X-ray observational gap between optically selected radio-quiet quasars (predominantly R 2‐10) and the five known blazars at z 4 (R 800‐27000), where R= f5 GHz f4400 ˚ A (rest frame). We study the relationship between X-ray luminosity and radio-loudness for quasars at high redshift and constrain RLQ X-ray continuum emission and absorption. From a joint spectral fit of nine moderate-R RLQs observed by Chandra, we find tentative evidence for absorption above the Galactic NH , with a best-fit neutral intrinsic column density of NH 2 4 2 0 1 8 10 22 cm 2 , consistent with earlier claims of increased absorption toward high-redshift RLQs. We also search for evidence of an enhanced jet-linked component in the X-ray emission due to the increased energy density of the cosmic microwave background (CMB) at high redshift, but we find neither spatial detections of X-ray jets nor a significant enhancement in the X-ray emission relative to comparable RLQs at low-to-moderate redshifts. Overall, the z 4‐5 RLQs have basic X-ray properties consistent with comparable RLQs in the local universe, suggesting that the accretion/jet mechanisms of these objects are similar as well. Subject headings: galaxies: active — galaxies: high-redshift — galaxies: jets — galaxies: nuclei — quasars: general

XMM-NewtonObservations of Two High-Redshift Quasars: RX J1028-0844 and BR 0351-1034

We report the results of XMM-Newton observations of two high-redshift quasars, one radio-loud, RX J1028.6-0844 (z = 4.276) and one radio-quiet, BR 0351-1034 (z = 4.351). We find that the evidence for strong excess absorption toward RX J1028-0844 is marginal at best, contrary to previous claims. The superior sensitivity and broader, softer energy range of XMM-Newton (0.2–10 keV) allows better determination of spectral parameters than much deeper ASCA observations (0.8–7 keV). Our XMM-Newton observations call into question several other ASCA results of strong absorption toward high-redshift radio-loud quasars. RX J1028.6-0844 occupies the same parameter space in broadband spectral properties as the low-redshift BL Lac objects, showing no obvious evolution with redshift. The radio-quiet quasar BR 0351-1024 became fainter between ROSAT and XMM observations by a factor of at least 5, but with the present data we cannot determine whether there is an associated spectral change. These observations do not support previous claims of weaker X-ray emission from high-redshift, radio-quiet quasars. The soft X-ray spectral slope required to reconstruct the ROSAT PSPC hardness ratio of BR 0351-1034 is about αX = 3.5, the steepest X-ray slope ever observed in a high-redshift quasar and similar to that of low-redshift narrow-line Seyfert 1 galaxies.

Overdensities of Extremely Red Objects in the Fields of High-Redshift Radio-Loud Quasars

We have examined the occurrence of extremely red objects (EROs) in the fields of 13 luminous quasars (11 radio-loud and two radio-quiet) at 1.8 < z < 3.0. The average surface density of Ks ≤ 19 mag EROs is 2–3 times higher than in large, random-field surveys, and the excess is significant at the ≈3 σ level even after taking into account that the ERO distribution is highly inhomogeneous. This is the first systematic investigation of the surface density of EROs in the fields of radio-loud quasars above z ≈ 2 and shows that a large number of the fields contain clumps of EROs, similar to what is seen only in the densest areas in random-field surveys. The high surface densities and angular distribution of EROs suggest that the excess originates in high-redshift galaxy concentrations, possibly young clusters of galaxies. The fainter EROs at Ks ≳ 19 mag show some evidence of being more clustered in the immediate 20'' region around the quasars, suggesting an association with the quasars. Comparing with predictions from spectral synthesis models, we find that if the Ks ≈ 19 mag ERO excess is associated with the quasars at z ≈ 2, their magnitudes are typical of ≳L* passively evolving galaxies formed at z ≈ 3.5 (Ωm = 0.3, ΩΛ = 0.7, and H0 = 70 km s-1 Mpc-1). Another interpretation of our results is that the excess originates in concentrations of galaxies at z ≈ 1 lying along the line of sight to the quasars. If this is the case, the EROs may be tracing massive structures responsible for a magnification bias of the quasars.

Serendipitous active galactic nuclei in the XMM--Newton fields of Markarian 205 and QSO 0130-403

The X-ray spectra of serendipitously observed active galactic nuclei (AGN) in the XMM-Newton fields of Mrk 205 and QSO 0130-403 are analysed. The sample consists of 23 objects, none of which is detected at radio frequencies, with a median X-ray luminosity of ∼4 x 10 4 4 erg s - 1 and redshifts ranging from ∼0.1 to just over 3. The mean photon index was found to be 1.89 ′ 0.04. In contrast with past ASCA and ROSAT observations of high-redshift radio-loud quasars, we find little evidence for excess intrinsic absorption in these radio-quiet objects, with only three sources requiring a column density in excess of the Galactic value. Comparing the measured spectral indices over the redshift range, we also find there is no X-ray spectral evolution of quasi-stellar objects (QSOs) with time, up to redshift of 3. Within the sample there is no evidence for evolution of the optical to X-ray spectral index, α o x , with redshift, the mean value being -1.66 ′ 0.04. However, upon comparing the values from the Bright Quasar Survey at low redshift (z 4), a slight steepening of α o x is noted for the more distant objects. In most of the sources there is no significant requirement for a soft excess, although a weak thermal component (≤10 per cent of L X ) cannot be excluded. There is an indication of spectral flattening (by ΔΓ = 0.2) at higher energies (>3 keV, QSO rest frame) for the sample as a whole. This is consistent with the presence of a Compton reflection component in these radio-quiet AGN, with the scattering medium (such as an accretion disc or molecular torus) occupying a solid angle of 2π sr to the X-ray source.

Discovery of radio-loud quasars with redshifts above 4 from the PMN sample

We present results of a new, large survey for high-redshift radio-loud quasars, which targets quasars with . The survey is based on the PMN and NVSS radio surveys, optically identified using digitised UKST B, R and I plates. Six new flat-spectrum QSOs have been discovered, and one previously known QSO rediscovered, based on their red optical colours. The QSOs discovered in this survey are bright in both radio and optical bands; in particular PMN J1451-1512 (, , ) and PMN J0324-2918 (, ) are very luminous. PMN J1451-1512 at is also now the most distant radio-selected quasar. In addition, 9 new quasars with were discovered during the survey. We present spectra and finding charts for the new quasars. We also derive a surface density of for flat-spectrum QSOs with mJy and mag.

HUBBLE SPACE TELESCOPE Imaging of the Host Galaxies of High‐Redshift Radio‐loud Quasars

We present rest-frame UV and Lyα images of spatially resolved structures (hosts) around five high-redshift radio-loud quasars obtained with the WFPC2 camera on the Hubble Space Telescope (HST). The quasars were imaged with the PC1 through the F555W (V-band) filter, which at the redshifts of the quasars (2.1 < z < 2.6) have central wavelengths of λrest ≈ 1500-1800 A, and at rest-frame Lyα using appropriately chosen narrowband filters with the WFC2. The objects were selected from ground-based imaging surveys. Those had shown that many radio-loud quasars at high redshift have prominent host galaxies that appeared to have properties similar to those of high-redshift radio galaxies. Our HST observations allow a more detailed investigation of quasar host morphologies and a comparison with similar HST studies of radio galaxies by others. Using several methods to measure and quantify the host properties we find that all five quasars are extended and that this fuzz contains ≈5%-40% of the total continuum flux and 15%-65% of the Lyα flux within a radius of about 15. The rest-frame UV luminosities of the hosts are log λPλ ≈ 11.9-12.5 L☉ (assuming no internal dust extinction), comparable to the luminous radio galaxies at similar redshifts and a factor 10 higher than both radio-quiet field galaxies at z ~ 2-3 and the most UV-luminous low-redshift starburst galaxies. The Lyα luminosities of the hosts are log LLyα ≈ 44.3-44.9 ergs s-1, which are also similar to the those of luminous high-redshift radio galaxies and considerably larger than the Lyα luminosities of high-redshift field galaxies. To generate the Lyα luminosities of the hosts would require roughly a few percent of the total observed ionizing luminosity of the quasar. The UV continuum morphologies of the hosts appear complex and knotty at the relatively high surface brightness levels of our exposures (about 24 V mag arcsec-2). In two quasars we find evidence for foreground galaxies that confuse the host galaxy morphologies and that are responsible for some of the perceived radio/optical misalignments observed in ground-based imaging data. We do find good alignment between the extended Lyα and the radio sources, strong evidence for jet-cloud interactions in two cases, again resembling radio galaxies, and what is possibly the most luminous radio-UV synchrotron jet in one of the hosts at z = 2.110. We discuss the significance of jet-cloud collisions in radio-loud quasars and their influence on radio morphologies in the framework of simple orientation-based quasar/radio galaxy unification schemes. Our observations suggest that the host galaxies of radio-loud steep-spectrum quasars are similar to those of radio galaxies and strengthen previous conclusions based on ground-based data that both types of objects are probably members of the same parent population.

Millimeter-wave continuum observation of high-z radio quasars

Millimeter-wave continuum observations of high redshift (z3) radio loud quasars (RLQs) and radio intermediate quasars (RIQs) have been performed with the 45 m telescope of the Nobeyama Radio Observatory. Sixteen RLQs with S 5GHz > 200 mJy and nine RIQs with 200 mJy > S 5GHz > 20 mJy were observed at four millimeter-wave frequencies. All the observed quasars have synchrotron spectra and their possible dust emission component is obscured by the synchrotron emission in millimeter-wave frequencies, which can be explained by their strong AGN activities. Observed quasars are classified into three spectral classes, according to their millimeter-wave spectral index as steep spectrum, millimeter turn over and extreme flat spectrum quasars. Extreme flat spectrum quasars have relatively flat spectra up to 150 GHz, where the rest frequency is higher than 600 GHz. This is an indication that the objects are in very young stage of quasar evolution and shows higher nuclear activity than lower redshift quasars. One of the QSOs, 2358+189 previously known as a RIQ, is found to have extreme flat spectrum, and is now classified as RLQs.

BeppoSAX observations of quasars

We present results from recent BeppoSAX observations of low redshift (z<0.4, PG sample) and high redshift (2<z<4.5) radio-quiet and radio-loud quasars. Significant curvature has been detected in the spectra of the observed PGs: the spectrum flattens by 0.5 above 2 keV. The possible presence of narrow features in the MECS spectra is discussed. Intrinsic absorption has been measured in the z=3.9 radio-loud quasar 1745+624. The z=2.3 radio-quiet quasar HE1104-1805 has been found at a very low flux level, in comparison with previous ROSAT and ASCA observations, implying large (factor of ∼ 4) variability on years timescales.

A search for high-redshift quasars among GB/FIRST flat-spectrum radio sources

We present the method and first results of a survey for high-redshift (z > 3) radio-loud quasars, which is based on optical identifications of 2902 flat-spectrum radio sources with S5GHz ≥ 25 mJy. The radio sample was defined over a 1600 deg2 region using the 5-GHz Green Bank survey and the 1.4-GHz VLA FIRST survey. 560 sources were identified to a limit of E= 19.5 on APM scans of POSS-I plates, and 337 of these optical counterparts are unresolved. From these a complete sample of 73 sources for spectroscopic follow-up was defined based on criteria of red (O − E ≥ 1.2) optical colour. We have obtained spectra for 36 of these, and an additional 14 have redshifts in the literature, thus 70 per cent of the spectroscopic sample is completed. Six objects in the sample were found to be radio-loud quasars with z > 3, of which two were previously known. The efficiency of the spectroscopic phase of the survey is therefore about 1 in 9, whereas without the colour selection criterion the efficiency would have been 1 in sim; 40. The six z > 3 quasars were found in an effective area of 1100 deg2, implying a surface density of one flat-spectrum z > 3 radio-loud quasar per 190 deg2 to limits of E= 19.5 and S5GHz ≥ 25 mJy. This survey has also produced the first known radio-loud BAL quasar, 1556+3517 with z = 1.48, which has been reported in an earlier paper. This object has a redder optical colour (O − E = 2.56) than all the z > 3 quasars found in this survey to date. In addition, we have obtained spectra of 22 GB/FIRST sources that are not part of the complete sample. We give positions, E (red) magnitudes, O − E colours, radio fluxes, radio spectral indices and redshifts where possible for objects for which we have obtained spectra. We give spectra and finding charts for the z > 3 quasars.

X‐Ray Spectral Survey of WGACAT Quasars. I. Spectral Evolution and Low‐Energy Cutoffs

We have used the WGA catalog of ROSAT Position Sensitive Proportional Counter (PSPC) X-ray sources to study the X-ray spectrum of about 500 quasars in the redshift interval 0.1-4.1, detected with a signal-to-noise ratio better than 7. We have parameterized the PSPC spectrum in terms of two effective energy spectral indices, αS (0.1-0.8 keV), and αH (0.4-2.4 keV), which allows for the different Galactic NH along the quasars' line of sight. We have used these data to explore the questions raised by the initial PSPC high-redshift quasar studies, and in particular the occurrence of low-energy X-ray cutoffs in high-redshift radio-loud quasars. We have also studied the emission spectra of a large sample of radio-loud and radio-quiet quasars and studied their differences. We find that low-energy X-ray cutoffs are more commonly (and perhaps exclusively) found in radio-loud quasars. Therefore, the low-energy X-ray cutoffs are physically associated with the quasars, and not with intervening systems, since those would affect radio-quiet and radio-loud equally. We suggest that photoelectric absorption is a likely origin of these cutoffs. The number of cutoffs in radio-loud quasars significantly increases with redshift rather than with luminosity. A partial correlation analysis confirms that αS is truly anticorrelated with redshift at the 99.9% confidence level, indicating evolution with cosmic epoch, and not a luminosity effect. Conversely, for αH the observed anticorrelation with redshift is mostly due to a strong dependence on luminosity. In radio-quiet quasars, we find marginal evidence for a flattening of αH (P = 4.5%), going from z 2.