We select a sample of 10 radio-quiet quasars with confirmed intranight optical variability and with available X-ray data. We compare the variability properties and the broad band spectral constraints to the predictions of intranight variability by three models: (i) irradiation of an accretion disk by a variable X-ray flux (ii) an accretion disk instability (iii) the presence of a weak blazar component. We concluded that the third model, e.g. the blazar component model, is the most promising if we adopt a cannonball model for the jet variable emission. In this case, the probability of detecting the intranight variability is within 20-80%, depending on the ratio of the disk to the jet optical luminosity. Variable X-ray irradiation mechanism is also possible but only under additional requirement: either the source should have a very narrow Hbeta line or occasional extremely strong flares should appear at very large disk radii.
We present new clues to the problem of the radio loudness dichotomy arising from an extensive search for intranight optical variability in seven sets of optically luminous radio-quiet quasars and (radio-loud) BL Lacertae objects, which are matched in optical luminosity and redshift. Our monitoring of radio-quiet quasars has for the first time clearly detected such intranight variability, with peak-to-peak amplitudes ~1%, occurring with a duty cycle of ~ 1/6. The matched BL Lacs have both higher variability amplitudes and duty cycles when observed in the same fashion. We show that the much less…
We present results of a programme of multi-epoch, intra-night optical monitoring of a sample of non-blazar type AGN, which includes seven radio-quiet QSOs (RQQs) and an equal number of radio-loud, lobe-dominated quasars (LDQs), covering a redshift range from about 0.2 to 2.0. These two sets of optically bright and intrinsically luminous QSOs are well matched in the redshift-optical luminosity ($z - M_B$) plane. Our CCD monitoring covered a total of 61 nights with an average of 6.1 hours of densely sampled monitoring of just a single QSO per night, thereby achieving a typical detection threshol…
We present results of a multi-epoch intra-night optical monitoring of eleven blazars consisting of six BL Lac objects and five radio core-dominated quasars (CDQs). These densely sampled and sensitive R-band CCD observations, carried out from November 1998 through May 2002 during a total of 47 nights with an average of 6.5 hours/night, have enabled us to detect variability amplitudes as low as $\sim 1%$ on intra-night time scales. A distinction is found for the first time between the intra-night optical variability (INOV) properties of the these two classes of relativistically beamed radio-loud…
All properties of the quasars known by the author (quietness of the BAL quasars, Lyman forest, iron in the high-z quasars...) are explained supposing that the kernel of the quasar is surrounded by a halo containing an accretion disk and magnetic satellites (no dark matter or fast jet, no variation of the fine structure constant, no strange source of iron...). The key is taking into account the coherent light-matter interactions.
We report the detection of the bright radio quiet quasar PG 1416-129 by the OSSE instrument on the Compton Gamma Ray Observatory (CGRO). This object had the hardest spectral index of all known AGN in the energy range 2-20 keV (Gamma=1.1 +- 0.1) when observed by Ginga. We find that the source is variable at energies >50 keV during the OSSE observation, and that the OSSE spectrum is steeper than the Ginga spectrum. The broad-band X-ray spectrum resembles that of Seyfert galaxies, supporting models which interpret radio quiet quasars as the high-luminosity counterparts of Seyferts.
We investigate the radio emitting structures of radio-quiet active galactic nuclei with an emphasis on radio-quiet quasars to study their connection to Seyfert galaxies. We present and analyse high-sensitivity VLA radio continuum images of 14 radio-quiet quasars and six Seyfert galaxies. Many of the low redshift radio-quiet quasars show radio structures that can be interpreted as jet-like outflows. However, the detection rate of extended radio structures on arcsecond scales among our sample decreases with increasing redshift and luminosity, most likely due to a lack of resolution. The morpholo…
Results of the X-ray spectral analysis of the high-redshift radio-quiet quasar Q1101-264 are presented. The ASCA spectrum suggests a marginal evidence of a FeK$α$ emission line at about 2 keV (observer's frame). Both the ASCA and ROSAT spectra are well fitted by a power law with spectral slope $Γ$$\sim$1.9. This is the first 0.3-30 keV spectrum (rest frame) of a z $>$ 2 radio-quiet quasar.
We examine the recent claims that cosmic gamma-ray bursts are associated with either radio-quiet quasars or Abell clusters. These associations were based on positional coincidences between cataloged quasars or Abell clusters, and selected events from the BATSE 3B catalog of gamma-ray bursts. We use a larger sample of gamma-ray bursts with more accurate positions, obtained by the 3rd Interplanetary Network, to re-evaluate these possible associations. We find no evidence for either.
We present the first milli-arcsecond resolution radio images of a radio-quiet quasar, detecting a high brightness temperature core with data from the VLBA. On maps made with lower-frequency data from MERLIN and the VLA jets appear to emanate from the core in opposite directions, which correspond to radio-emission on arcsecond scales seen with the VLA at higher frequencies. These provide strong evidence for a black-hole--based jet-producing central engine, rather than a starburst, being responsible for the compact radio emission in this radio-quiet quasar.
I bring together evidence for the rapidity with which quasars' radio synchrotron lobe emission fades and for the intermittency with which jet plasma is ejected from individual quasars and radio galaxies and affirm the picture presented by Nipoti et al (2005) that the radio-loudness of quasars is a function of the epoch at which they are observed. I briefly illustrate this account with examples of successive episodes of jet activity where the axis along which jet plasma is launched appears to have precessed. A new model for the weak core radio emission from radio-quiet quasars, that is not any …
We study positional coincidences between gamma-ray bursts in the BATSE 3B catalogue and quasars/AGN taken from the Veron-Cetty & Veron compilation. For most classes of AGN, for BL Lac objects and for radio-loud quasars, we find no excess coincidences above random expectation, and we give upper limits for their burst emission rate. However, surprising evidence is found for a positional correlation between gamma-ray bursts and radio-quiet quasars. A total of 134 selected bursts with a position error radius <1.8d and the nearest and intrinsically brightest radio-quiet quasars (M_v<-24.2 and z<1.0…
We have quantified the galaxy environments around a sample of 0.5 < z < 0.8 radio-quiet quasars using the amplitude of the spatial galaxy--quasar correlation function, B_gq. The quasars exist in a wide variety of environments, some sources are located in clusters as rich as Abell class 1--2 clusters, whereas others exist in environments comparable to the field. We find that on average, the quasars prefer poorer clusters of approximately Abell class 0, which suggests that quasars are biased tracers of mass compared to galaxies. The mean B_gq for the sample is found to be indistinguishable from …
A database of radio-quiet quasars detected with ROSAT is presented containing 846 quasars seen in the All-Sky Survey and/or in pointed PSPC observations. About 70% of the objects have been detected in X-rays for the first time. We present the soft X-ray fluxes and spectra, if available. Using an optically selected subsample compiled from this database, we study the broad band properties of radio-quiet quasars with high statistical significance. We confirm that radio-quiet quasars have in general steeper soft X-ray spectra than radio-loud. The spectral differences persist to high redshifts. A s…
We present the results of a multi-epoch radio monitoring campaign measuring the milliarcsecond structure of the jet in the radio-quiet quasar PG1407+263. This is the highest-sensitivity, highest-resolution multi-year study of a distant active galaxy. The observations are naturally explained in terms of a beamed relativistic jet, some of whose fluctuations in flux density can be ascribed to interaction with the narrow-line region of the quasar. The optical properties of PG1407+263, in particular the low equivalent widths of the emission lines, may be related to the fact that we are viewing this…
Interactions between the radio jet and the optical emission of the narrow-line region (NLR) are a well known phenomenon in Seyfert galaxies. Here, we present the study of possible jet-NLR interactions in five radio-quiet PG quasars with double or triple radio structure. High spatial and spectral resolution observations were carried out in the Hbeta-[OIII]5007 wavelength range. In all cases, there is evidence for [OIII] profile substructure (shoulders, subpeaks, blueshifted ''broad'' components) with different clarity. To measure the velocity, line width, intensity, and location of these [OIII]…
We present infrared photometry and optical and infrared spectroscopy of the recently-discovered, extremely luminous nearby quasar PDS 456. A number of broad emission features are seen in the near-infrared which we are unable to identify. We measure a more accurate redshift from a narrow forbidden emission line and compare the optical-infrared spectrum to that of 3C 273. The close similarity suggests that PDS 456 is a radio-quiet analogue of 3C 273, although radio observations do not support this idea.
This paper reports the X-ray spectral analysis of 5 high-redshift (z \ge 2) radio-quiet quasars observed by ASCA. A simple power law continuum plus cold Galactic absorption model well fits all the spectra (tipically between $\sim$ 2-30 keV in the sources frame). Neither the X-ray spectral hardening, attributed to a reflection component and observed in Seyfert galaxies, nor the excess absorption previously detected in high-redshift radio-loud quasars, have been revealed. Only a marginal evidence of a neutral or mildly ionized FeKαline is found in one of the quasars. The average spectral slope i…
We report optical monitoring observations of 20 high-luminosity AGN, 12 of which are radio-quiet quasars (RQQs). Intra-night optical variability (INOV) was detected for 13 of the 20 objects, including 5 RQQs. The variations are distinctly stronger and more frequent for blazars than for the other AGN classes. By combining these data with results obtained earlier in our program, we have formed an enlarged sample consisting of 9 BL Lacs, 19 RQQs and 11 lobe-dominated radio-loud quasars. The moderate level of rapid optical variability found for both RQQs and radio lobe-dominated quasars argues aga…
We present supportive evidence that the Boroson and Green eigenvector 1 is not driven by source orientation. Until recently it was generally accepted that eigenvector 1 does not depend on orientation as it strongly correlates with [OIII]5007 emission, thought to be an isotropic property. However, recent studies of radio-loud AGN have questioned the isotropy of [OIII] emission and concluded that [OII]3727 emission is isotropic. In this paper we investigate the relation between eigenvector 1 and [OII] emission in radio-quiet BQS (Bright Quasar Survey) quasars, and readdress the issue of orientat…
We report the detection of emission from dust and carbon monoxide in the radio-quiet quasar BR1202--0725 at redshift z=4.7. Maps of these emissions reveal two objects separated by a few arc seconds, which could indicate either the presence of a companion to the quasar or gravitational lensing of the quasar itself. Regardless of the precise interpretation of the maps, the detection of carbon monoxide confirms the presence of a large mass of molecular gas in one of the most distant galaxies known, and shows that conditions conducive to huge bursts of star formation existed in the early Universe.
Two rival hypotheses have been proposed for the origin of the compact radio flux observed in radio-quiet quasars (RQQs). It has been suggested that the radio emission in these objects, typically some two or three orders of magnitude less powerful than in radio-loud quasars (RLQs), represents either emission from a circumnuclear starburst or is produced by radio jets with bulk kinetic powers 10^3 times lower than those of RLQs with similar luminosity ratios in other wavebands. We describe the results of high resolution (parsec-scale) radio-imaging observations of a sample of 12 RQQs using the V…
The majority of quasars are weak in their radio emission, with flux densities comparable to those in the optical, and energies far lower. A small fraction, about 10%, are hundreds to thousands of times stronger in the radio. Conventional wisdom holds that there are two classes of quasars, the radio quiets and radio louds, with a deficit of sources having intermediate power. Are there really two separate populations, and if so, is the physics of the radio emission fundamentally different between them? This paper addresses the second question, through a study of radio variability across the full…
Although well established for BL Lac objects and radio-loud quasars, the occurrence of intra-night optical variability (INOV) in radio-quiet quasars is still debated, primarily since only a handful of INOV events with good statistical significance, albeit small amplitude, have been reported so far. This has motivated us to continue intra-night optical monitoring of bona-fide radio-quiet quasars (RQQs). Here we present the results for a sample of 11 RQQs monitored by us on 19 nights. On 5 of these nights a given RQQ was monitored simultaneously from two well separated observatories. In all, two…
We observed 17 optically-selected, radio-quiet high-redshift quasars with the Chandra Observatory ACIS, and detected 16 of them. The quasars have redshift between 3.70 and 6.28 and include the highest redshift quasars known. When compared to low-redshift quasars observed with ROSAT, these high redshift quasars are significantly more X-ray quiet. We also find that the X-ray spectral index of the high redshift objects is flatter than the average at lower redshift. These trends confirm the predictions of models where the accretion flow is described by a cold, optically-thick accretion disk surrou…
Several independent lines of evidence now point to a correlation between black hole mass, M_bh, and radio-luminosity. In this paper we discuss the correlation for quasars from the FIRST Bright Quasar Survey (FBQS), using black hole mass estimates from Hbeta linewidths. The FBQS objects fill in the gap between the radio-loud and radio-quiet quasars in the radio-luminosity - optical-luminosity plane, and we find that they fill the corresponding gap in the M_bh - radio luminosity correlation. There is thus a continuous variation of radio luminosity with M_bh, and no evidence for a ``switch'' at s…
We match quasars discovered in a multi-color survey centered on the northern Hubble Deep Field (HDF) with radio sources from an ultra-deep radio survey. Although 3 out of 12 quasars are detected at a level below 0.2 mJy at 1.4 GHz, all of the quasars in the search area are radio-quiet by the criterion L_r < 10^{25} h^{-1}_{50} W/Hz. We combine this information with other radio surveys of quasars so as to break the degeneracy between redshift and luminosity. In the redshift range 0.02 < z < 3.64, the radio-loud fraction increases with increasing optical luminosity, consistent with some degree o…
Starting from a sample of SDSS quasars appearing also in the 2MASS survey, we study the continuum properties of about 1000 objects observed in 8 bands, from NIR to UV. We construct the mean spectral energy distribution (SED) and compare and contrast the continua of radio loud (RLQ) and radio quiet (RQQ) objects. The SEDs of the two populations are significantly different in the sense that RLQs are redder, with power law spectral indices <alpha(RLQ)>=-0.55+/-0.04 and <alpha(RQQ)>=-0.31+/-0.01 in the spectral range between 10^14.5 and 10^15.35 Hz. This difference is discussed in terms of differe…
The observation that the extremely broad, blueshifted absorption troughs which characterize broad absorption line quasars (BALQs) occur exclusively in radio-quiet quasars (RQQs) suggests that this class of active galactic nuclei (AGN) may offer important clues to the radio-loud/radio-quiet (RL/RQ) dichotomy in quasars. Interestingly, there is also substantial observational evidence for similar, but lower velocity intrinsic absorption outflows in some Seyferts and radio-loud quasars (RLQs) as well. Theoretically, however, it is difficult to interpret this broad range of mass ejection phenomena …
Here we report the X-ray spectral analysis of 5 high-redshift ($z \geq 2$) radio-quiet quasars (RQQs) observed with the ASCA satellite. A simple power law with Galactic absorption model well fits the data. There is no evidence of a reflection component. The average spectral slope in the observed 0.5--10 keV energy range, $< Γ> = 1.68\pm{0.09}$ (dispersion $σ\sim 0.11$), places the high-z objects of the present sample in the lower-Γtail of the RQQs spectral indices distribution.
We have used the Very Long Baseline Array (VLBA) to image five radio-quiet quasars (RQQs) at milliarcsecond resolution, at frequencies between 1.4 and 5 GHz. These quasars have typical total flux densities of a few millijansky at gigahertz frequencies, and are compact on arcsecond scales. The VLBA images reveal that four of the quasars are dominated by unresolved radio cores, while the fifth has an apparent two-sided jet. Typical core brightness temperatures range from 10^8 K to at least 10^9 K. The compact radio morphologies and X-ray luminosities of many objects in the RQQ sample seem to ind…
Although radio-quiet quasars (RQQs) constitute >90% of optically-identified quasar samples their radio properties are only poorly understood. In this paper we present the results of a multi-frequency VLA study of 27 low-redshift RQQs. We detect radio emission from 20 objects, half of which are unresolved (< 0.24''). In cases where significant structure can be resolved, double, triple and linear radio sources on scales of a few kpc are found. The radio emission (typically) has a steep spectrum (alpha ~ 0.7), and high brightness temperatures (T_B > 10^5 K) are measured in some of the radio compo…
We report on two surveys of radio-weak AGN to look for radio variability. We find significant variability with an RMS of 10-20% on a timescale of months in radio-quiet and radio-intermediate quasars. This exceeds the variability of radio cores in radio-loud quasars (excluding blazars), which vary only on a few percent level. The variability in radio-quiet quasars confirms that the radio emission in these sources is indeed related to the AGN. The most extremely variable source is the radio-intermediate quasar III Zw 2 which was recently found to contain a relativistic jet. In addition we find l…
We present Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) data on the large-scale environments of 16 0.39 < z < 0.51 quasars from the Large Bright Quasar Survey (LBQS). The LBQS quasars are representative of the radio-quiet population, and this is the first look at their large-scale environments. We compare the LBQS environments with the environments of 27 0.15 < z < 0.55 quasars selected from the HST Archive. The majority of the Archive quasars are from the PG and PKS surveys, and these quasars are more luminous on average than the LBQS. By comparing the LBQS and Archive e…
We will present the results of a NICMOS H-band imaging survey of a small sample of z ~ 2 to 3 radio-quiet quasars. We have resolved extension in at least 4 of 5 objects and find evidence for a wide range in the morphologies and magnitudes of these hosts. The host galaxy luminosities range from sub-L* to about 4 L*, with most of the hosts having luminosities about L*. These host galaxies have magnitudes and sizes consistent with those of the Ly break galaxies at similar redshifts and at similar rest wavelengths, but are about a magnitude fainter than the comparable 6C radio galaxies. One residu…
Although radio-quiet quasars (RQQs), which constitute the majority of optically-identified quasar samples, are by no means radio silent the properties of their radio emission are only poorly understood. We present the results of a multi-frequency VLA study of 27 low-redshift RQQs. In general, we find that the properties of the radio sources in RQQs are consistent with them being weak, small-scale (~1 kpc) jets similar to those observed in nearby Seyfert galaxies. We conclude that a significant fraction of the radio emission in RQQs is directly associated with the central engine and is not a re…
Accretion disk models still do not provide a satisfactory explanation of the optical/ultraviolet continuum observed in Seyferts and quasars. Substantial theoretical progress has been made in understanding one aspect of the problem: the dearth of spectral features at the Lyman limit. Promising solutions have also been proposed to explain the surprising observations of large polarization in the Lyman continuum observed in some sources. I review the recent progress in this field, and try to point out future research directions which would be fruitful in trying to obtain a complete, self-consisten…
BeppoSAX and ASCA observations of the nearby (z=0.184), high-luminosity, radio-quiet quasar PDS456 are presented. The X-ray spectrum is characterized by a prominent ionized edge at 8-9 keV (originally discovered by RXTE, Reeves et al. 2000) and by a soft excess below 1.5 keV. The lack of any significant iron K alpha emission line suggests for the edge an origin from line-of-sight material rather than from reflection from a highly ionized accretion disc. The hard X-ray continuum is indeed well modelled by transmission through a highly-ionized medium with a large column density (N_H warm = 4.5 x…
We present sensitive 850 micron SCUBA photometry of a statistically-complete sample of six of the most luminous, z>4 radio-quiet quasars, reaching noise levels comparable with the deep blank sky surveys. One quasar (BR2237-0607; z=4.55) is detected at 850micron with a flux of 5.0mJy (4.5sigma), whilst a second (BR0019-1522; z=4.52) has a detection at the 2 sigma level. When combined with our previous millimetre measurements of z>4 quasars, we find that there is a large range (5--10) in far infrared (FIR) luminosity at fixed UV luminosity, and that the typical quasar has a L_FIR and mass of coo…
We present spectroscopic observations obtained with the FORS1 instrument on the ESO VLT under good seeing conditions of the radio-quiet z_em = 3.04 quasar Q1205-30 and its associated extended Lyman-alpha emission. The extended Lyman-alpha emission was originally found in a deep narrow band image targeting a z_abs = z_em Lyman-limit system in the spectrum of the QSO. Using spectral point-spread function fitting to subtract the QSO spectrum, we clearly detect the extended Lyman-alpha emission as well as two foreground galaxies at small impact parameters (2.12 +- 0.04 and 2.77 +- 0.07 arcsec). Th…
Continuum observations at radio, millimetre, infrared and soft X-ray energies are presented for a sample of 22 quasars, consisting of flat and steep spectrum radio loud, radio intermediate and radio quiet objects. The primary observational distinctions, among the different kinds of quasars in the radio and IR energy domains are studied using large observational datasets provided by ISOPHOT on board the Infrared Space Observatory, by the IRAM interferometer, by the sub-millimetre array SCUBA on JCMT, and by the European Southern Observatory (ESO) facilities IRAC1 on the 2.2 m telescope and SEST…
We present the results of a deep K-band imaging study which reveals the host galaxies around a sample of luminous radio-quiet quasars. The K-band images, obtained at UKIRT, are of sufficient quality to allow accurate modelling of the underlying host galaxy. Initially, the basic structure of the hosts is revealed using a modified Clean deconvolution routine optimised for this analysis. 2 of the 14 quasars are shown to have host galaxies with violently disturbed morphologies which cannot be modelled by smooth elliptical profiles. For the remainder of our sample, 2D models of the host and nuclear…
We have made radio observations of 87 optically selected quasars at 5 GHz with the VLA in order to measure the radio power for these objects and hence determine how the fraction of radio-loud quasars varies with redshift and optical luminosity. The sample has been selected from the recently completed Edinburgh Quasar Survey and covers a redshift range of 0.3 < z < 1.5 and an optical absolute magnitude range of -26.5 < M_{B} < -23.5 (h, q_{0} = 1/2). We have also matched up other existing surveys with the FIRST and NVSS radio catalogues and combined these data so that the optical luminosity-red…
I review our knowledge of the properties of the host galaxies of radio-loud and radio-quiet quasars, both in comparison to each other and in the context of the general galaxy population. It is now clear that the hosts of radio-loud and radio-quiet quasars with M_V < -23.5 are virtually all massive elliptical galaxies. The masses of these spheroids are as expected given the relationship between black-hole and spheroid mass found for nearby quiescent galaxies, as is the growing prevalence of disc components in the hosts of progressively fainter AGN. There is also now compelling evidence that qua…
The detection of rapid variability on a time-scale of hours in radio-quiet quasars (RQQSOs) could be a powerful discriminator between starburst, accretion disc and relativistic jet models of these sources. This paper contains an account of a dedicated search for rapid optical variability in RQQSOs. The technique used differential photometry between the RQQSO and stars in the same field of view of the CCD. The 23 RQQSOs that were observed all have high luminosities (-27 < Mv <- 30), and 22 of these sources are at redshifts z > 1. The total amount of observation time was about 60 hours and these…
We present two XMM-Newton observations of the luminous (L_x > 10^46 erg/s), radio-quiet quasar, PG 1407+265, separated by eleven months. The data indicate two distinct states: a highly variable, bright state (first epoch); and a quiescent, low-flux one (second epoch). During the low-flux state the spectrum is consistent with a single, unabsorbed power law. However, during the brighter state a highly variable, steep component is statistically required. Contemporaneous UV data from the Optical Monitor allow an estimate of the optical-to-X-ray spectral index (alpha_ox), which appears typical of r…
We report new Chandra observations of seven optically faint, z \sim 4 radio-quiet quasars. We have combined these new observations with previous Chandra observations of radio-quiet quasars to create a sample of 174 sources. These sources have 0.1 < z < 4.7, and 10^{44} ergs s^{-1} < nu L_{nu} (2500 Å) < 10^{48} ergs s^{-1}. The X-ray detection fraction is 90%. We find that the X-ray loudness of radio-quiet quasars decreases with UV luminosity and increases with redshift. The model that is best supported by the data has a linear dependence of optical-to-X-ray ratio, alpha_{ox}, on cosmic time, …
We investigate the $\mbh- σ_*$ relation for radio-loud quasars with redshift $z<0.83$ in Data Release 3 of the Sloan Digital Sky Survey (SDSS). The sample consists of 3772 quasars with better model of H$β$ and \oiii lines and available radio luminosity, including 306 radio-loud quasars, 3466 radio-quiet quasars with measured radio luminosity or upper-limit of radio luminosity (181 radio-quiet quasars with measured radio luminosity). The virial supermassive black hole mass (\mbh) is calculated from the broad \hb line, the host stellar velocity dispersion ($σ_*$) is traced by the core \oiii gase…
We report the first results of a spectroscopic search for Lyman alpha, envelopes around three z=4.5 radio-quiet quasars. Our observational strategy uses the FORS2 spectrograph attached to the UT1 of the Very Large Telescope (VLT) in the multi-slit mode. This allows us to observe simultaneously the quasars and several PSF stars. The spectra of the latter are used to remove the point-like quasar from the data, and to unveil the faint underlying Lyman alpha, envelopes associated with the quasars with unprecedented depth. We clearly detect an envelope around two of the three quasars. These envelop…
The radio emission in radio loud quasars originates in a jet carrying relativistic electrons. In radio quiet quasars (RQQs) the relative radio emission is ~10^3 times weaker, and its origin is not established yet. We show here that there is a strong correlation between the radio luminosity (L_R) and X-ray luminosity (L_X) with L_R~10^-5L_X, for the radio quiet Palomar-Green (PG) quasar sample. The sample is optically selected, with nearly complete radio and X-ray detections, and thus this correlation cannot be due to direct selection biases. The PG quasars lie on an extension of a similar corr…
We present an analysis of the effects of luminosity on the shape of the mid-infrared spectral energy distributions (SEDs) of 234 radio-quiet quasars originally presented by Richards et al. In quasars without evident dust extinction, the spectrally integrated optical and infrared luminosities are linearly correlated over nearly three decades in luminosity. We find a significant (>99.99% confidence) correlation between the 1.8-8.0 micron spectral index and infrared luminosity that indicates an enhancement of the mid-infrared continuum with increasing luminosity. Coupled with strong evidence for …
We modeled the spectral energy distribution (SED) of high redshift radio quiet quasars (high-z RQQs). We computed spectra in a patchy corona geometry where an accretion disk extends to the last stable orbit and the Comptonizing active regions (hot clouds) are distributed above the disk. We explored the model parameter space to find theoretical parameters that give spectra with the optical/UV luminosity, the X-ray loudness, and the X-ray photon index compatible with those of high-z RQQs observed with Chandra. We found that a range of solutions is possible, from high-$kT_{\rm e}$ low-$τ$ to low-…
A statistical study is presented of (a) the frequency of narrow CIV lambda 1549 absorption lines in 1.5 <~ z <~ 3.6 radio-quiet and radio-loud quasars, and of (b) the UV and radio properties of the absorbed quasars. The quasar sample is unbiased with respect to absorption properties and the radio-quiet and radio-loud subsamples are well matched in redshift and luminosity. A similarly high incidence (>~50%) of narrow CIV absorbers is detected for the radio-quiet and radio-loud quasars, and a constant ~25% of all the quasars, irrespective of radio type display associated CIV absorbers stronger t…
We present the first results from a major HST WFPC2 imaging study aimed at providing the first statistically meaningful comparison of the morphologies, luminosities, scalelengths and colours of the host galaxies of radio-quiet quasars, radio-loud quasars, and radio galaxies. We describe the design of this study and present the images which have been obtained for the first half of our 33-source sample. We find that the hosts of all three classes of luminous AGN are massive elliptical galaxies, with scalelengths ~=10 kpc, and R-K colours consistent with mature stellar populations. Most important…
Over the past few years, we have been collecting data with the Nordic Optical Telescope (NOT) on the galaxy environments around active galactic nuclei (AGN). Here we present some results from a sample of 21 radio-loud and 20 radio-quiet quasars in the redshift range 0.5 < z < 0.8. We find a few quasars in very rich environments, perhaps as rich as Abell class 1-2 clusters, but more often the quasars seem to prefer groups and poorer clusters. We also find that on average the galaxy environments around radio-loud and radio-quiet quasars are indistinguishable, consistent with the findings that bo…
We have performed quasi-simultaneous radio flux density measurements at 2.7 and 10 GHz for all PG quasars with radio flux densities between 4-200 mJy. We find that a large fraction of these sources are variable, flat-spectrum quasars. This brings the total fraction of flat-spectrum quasars with a ratio between radio and optical flux of R>10 - a value previously used to define a radio-loud quasar - to 40% in the PG quasar sample. We also find that the median R-parameter of these flat-spectrum quasars is lower than those of steep-spectrum radio-loud quasars. This contradicts the predictions of t…
We present the results from a study of the timing properties and the energy spectrum of the radio-quiet quasar PG0804+761, based on monitoring RXTE PCA observations that lasted for a year. This is a systematic study of the X-ray variations on time scales of weeks/months of the most luminous radio-quiet quasar studied so far. We detect significant variations in the 2-10 keV band of an average amplitude of ~15%. The excess variance of the light curve is smaller than that of Seyfert galaxies, entirely consistent with the relationship between variability amplitude and luminosity defined from the S…
(abridged) The radio-quiet quasar PG1416-129 (z=0.129) exhibits atypical optical and X-ray properties. Between 1990 and 2000, in response to its optical continuum decrease, the ``classical'' broad component of Hbeta almost completely disappeared, with a factor of 10 decrease in the line flux. In the X-ray band, this object was observed by Ginga in 1988 to have the hardest quasar photon index, with Gamma=1.1+/-0.1. We present an XMM/EPIC observation of PG1416-129 performed in July 2004. We analyze the time-averaged pn spectrum of this quasar, as well as perform time-resolved spectroscopy. We fi…
We present a model for the radio emission from radio-quiet quasar nuclei. We show that a thermal origin for the high brightness temperature, flat spectrum point sources (known as radio ``cores'') is possible provided the emitting region is hot and optically-thin. We hence demonstrate that optically-thin bremsstrahlung from a slow, dense disk wind can make a significant contribution to the observed levels of radio core emission. This is a much more satisfactory explanation, particularly for sources where there is no evidence of a jet, than a sequence of self-absorbed synchrotron components whic…
Using a sample of 3814 quasars from the Sloan Digital Sky Survey, we confirm that high-ionization, broad, emission lines such as CIV are significantly blueshifted with respect to low-ionization, broad, emission lines such as MgII. CIV emission line peaks have a range of shifts from a redshift of 500 km/s to blueshifts well in excess of 2000 km/s as compared to MgII. We confirm previous results that suggest an anti-correlation between the shift of the CIV emission line peak and the rest equivalent width of the CIV emission line. Furthermore, using composite spectra, we are able to show that the…
