Observations Of Quasars Research Articles

Looking for a varying α in the cosmic microwave background

We perform a likelihood analysis of the recently released BOOMERanG and MAXIMA data, allowing for the possibility of a time-varying fine-structure constant. We find that in general these data prefer a value of \ensuremath{\alpha} that was smaller in the past (which is in agreement with measurements of \ensuremath{\alpha} from quasar observations). However, there are some interesting degeneracies in the problem which imply that strong statements about \ensuremath{\alpha} cannot be made using this method until independent accurate determinations of ${\ensuremath{\Omega}}_{b}{h}^{2}$ and ${H}_{0}$ are available. We also show that a preferred lower value of $\ensuremath{\alpha}$ comes mainly from the data points around the first Doppler peak, whereas the main effect of the high-$l$ data points is to increase the preferred value for ${\ensuremath{\Omega}}_{b}{h}^{2}$ (while also tightening the constraints on ${\ensuremath{\Omega}}_{0}$ and ${H}_{0}).$ We comment on some implications of our results.

The Earliest Luminous Sources and the Damping Wing of the Gunn-Peterson Trough

Recent observations of high-redshift galaxies and quasars indicate that the hydrogen component of the intergalactic medium (IGM) must have been reionized at some redshift z 6. Prior to complete reionization, sources of ultraviolet radiation will be seen behind intervening gas that is still neutral, and their spectra should show the red damping wing of the Gunn-Peterson trough. While this characteristic feature may, in principle, totally suppress the Lyα emission line in the spectra of the first generation of objects in the universe, we show here that the IGM in the vicinity of luminous quasars will be highly photoionized on several megaparsec scales as a result of the source emission of Lyman continuum photons. If the quasar lifetime is shorter than the expansion and gas recombination timescales, the volume ionized will be proportional to the total number of photons produced above 13.6 eV; the effect of this local photoionization is to greatly reduce the scattering opacity between the redshift of the quasar and the boundary of its H II region. We find that the transmission on the red side of the Lyα resonance is always greater than 50% for sources radiating a total of 1069.5 ionizing photons into the IGM. The detection of a strong Lyα emission line in the spectra of bright quasi-stellar objects shining for 107 yr cannot then be used, by itself, as a constraint on the reionization epoch. The first signs of an object radiating prior to the transition from a neutral to an ionized universe may be best searched for in the spectra of luminous sources with a small escape fraction of Lyman continuum photons into the IGM or sources with a short duty cycle.

The Incidence of the Host Galaxy in Microvariability Observations of Quasars

The incidence of a host galaxy in aperture photometry of active galactic nuclei is studied by means of actual and simulated CCD observations. Our goal is to evaluate the importance of spurious variations, introduced by seeing —uctuations during the observations, in the diUerential light curves used to study optical microvariability. Repeated CCD observations during two consecutive nights were used to obtain time-resolved aperture photometry for the BL Lac object PKS 2316(423, which is located at the center of a conspicuous elliptical galaxy, and for several —eld stars. The blazar seems to be variable according to standard variability criteria; however we show that the observed diUerential magnitude variations are strongly correlated with seeing —uctuations during the nights. Moreover, another galaxy within the same CCD —eld shows nearly identical variations, clearly indicating that such variations are artifacts of the aperture photometry. Simulated observations of quasars within host galaxies of diUerent morphologies and spanning a wide range of luminosities were also used to evaluate the eUects of changing seeing con- ditions. The results show that spurious diUerential magnitude variations due to seeing —uctuations are larger for active nuclei within brighter hosts, particularly when small photometric apertures (about the seeing FWHM in radius) are used. According to our results, several recommendations are given to future observers.

Numerical Data-Processing Simulations of Microarcsecond Classical and Relativistic Effects in Space Astrometry

AbstractThe accuracy of astrometric observations conducted via a space-borne optical interferometer orbiting the Earth is expected to approach a few microarcseconds. Data processing of such extremely high-precision measurements requires access to a rigorous relativistic model of light ray propagation developed in the framework of General Relativity. The data-processing of the space interferometric observations must rely upon the theory of general-relativistic transformations between the spacecraft, geocentric, and solar barycentric reference systems allowing unique and unambiguous interpretation of the stellar aberration and parallax effects. On the other hand, the algorithm must also include physically adequate treatment of the relativistic effect of light deflection caused by the spherically-symmetric (monopole-dependent) part of the gravitational field of the Sun and planets as well as the quadrupole- and spin-dependent counterparts of it. In some particular cases the gravitomagnetic field induced by the translational motion of the Sun and planets should be also taken into account for unambigious prediction of the light-ray deflection angle. In the present paper we describe the corresponding software program to take into account all classical (proper motion, parallax, etc.) and relativistic (aberration, deflection of light) effects up to the microarcsecond threshold and demonstrate, using numerical simulations, how observations of stars and/or quasars conducted on board a space optical interferometer orbiting the Earth can be processed and disentangled. For numerical simulations the spacecraft orbital parameters and the telescope optical-system-characteristics have been taken to be similar to those in the Hipparcos mission. The performed numerical data analysis verifies that the relativistic algorithm chosen for data processing is convergent and can be used in practice to determine astronomical coordinates and proper motions of stars (quasars) with the required microarcsecond precision. Results shown in the paper have been obtained with a rather small number of stars (a few thousand). Simulations based on a much larger number of stars, e.g., from the Guide Star Catalogue used to model original observations will give more complete information about potential abilities of the space astrometric missions.

Extreme scattering of pulsars

AbstractExtreme Scattering Events are radio-wave lensing events caused by AU-sized concentrations of ionised gas. Although they were discovered more than a decade ago we still have no clear picture of the physical nature of the lenses. To discriminate between the various models, we need to amass more information on multiple imaging phenomena. Pulsars are perfect targets for such studies: they offer six- to ten-times the information content of quasar observations, and their small angular sizes make them sensitive to distant lenses. In addition, small source-size means that multiple imaging can be studied even when there is little change in the total source flux, because the (weak) secondary images interfere with the primary and create periodic fringes in the spectrum.

Further observations of quasars in ESO/SERC field 927

We present the spectra and redshifts of 62 quasars, from observations made with the Cerro Tololo Inter-American Observatory (CTIO) 4-m Blanco Telescope. These quasars form part of a total sample of 118 (with 56 having been published previously), which is being used for analysis of structure in the early universe. Quasars of particular interest are noted, including eight broad absorption line (BAL) quasars and two quasars with unusual emission spectra. Finally, we include a short summary of the present status of the large quasar group (LQG) that was discovered by Clowes & Campusano from the earlier observations. The quasars are from an area∼25.3 deg2 of ESO/SERC field 927, which is centred at (1950) 10h40m00s, 05°00′00′′.

ASCA Observation of a Radio-Loud Quasar and a New Candidate for a Cluster of Galaxies

Abstract During an X-ray observation of the distant galaxy cluster CL 0107+31 (z = 0.69) with ASCA, an extended X-ray source was detected in the field of view. Based on an image analysis in two different energy bands of 1–3 keV and 3–10 keV, we found that this source consists of a hard point-like component and an additional soft diffuse X-ray source extended over ≥ 3′ in radius. However, due to poor photon statistics, we could not distinguish whether their X-ray spectra are thermal or non-thermal. The peak position of the central X-ray source coincides with a radio-loud quasar, NRAO 58. NRAO 58 is found to have a power-law spectrum with a photon index $1.6^{+0.3}_{-0.2}$ and a (2–10 keV) luminosity of ∼ 4×46 erg s−1 (in the quasar rest frame). The extended component is likely to be a candidate for a nearby galaxy group which accidentally coincides with the quasar.

Near infrared observations of quasars with extended ionized envelopes

We have observed a sample of 15 and 8 quasars with redshifts between 0.11 and 0.87 (mean value 0.38) in the J and bands respectively. Eleven of the quasars were previously known to be associated with extended emission line regions. After deconvolution of the image, substraction of the PSF when possible, and identification of companions with the help of HST archive images when available, extensions are seen for at least eleven quasars. However, average profiles are different from that of the PSF in only four objects, for which a good fit is obtained with an law, suggesting that the underlying galaxies are ellipticals. Redshifts were available in the literature for surrounding objects in five quasar fields. For these objects, one to five companion galaxies were found. One quasar even belongs to a richness class 1 cluster. Most other quasars in our sample have nearby galaxies in projection which may also be companions. Environmental effects are therefore probably important to account for the properties of these objects.

Multiwaveband Observations of Quasars with Flat Radio Spectra and Strong Millimeter‐Wave Emission

We present multiwaveband observations of a well-selected sample of 28 quasars and two radio galaxies with flat radio spectra and strong millimeter-wave emission (referred to here as FSRQs). The data are analyzed to determine the radio to infrared and X-ray to γ-ray properties of FSRQs and the relationships between them. Specifically, the synchrotron self-Compton (SSC) process is examined as a likely common radiation mechanism. For most sources, the broadband spectra are still incomplete, especially in the far-infrared and ultraviolet range. Therefore, precise analysis, such as model fitting of spectra, is not usually possible. To compensate partially for this, we have taken a statistical approach and examine the relationship between high- and low-energy emission by using the data set for the entire sample.

Geodetic VLBI Observations of EGRET Blazars

We present VLBI observations of the EGRET quasars 0202+149, CTA 26, and 1606+106, as well as additional analysis of VLBI observations of 1156+295 presented in Piner & Kingham (1997b). We have produced 8 and 2 GHz VLBI images at 11 epochs, 8 epochs, and 12 epochs, spanning the years 1989 to 1996, of 0202+149, CTA 26, and 1606+106 respectively. The VLBI data have been taken from the Washington VLBI correlator's geodetic database. We have measured the apparent velocities of the jet components and find that CTA 26 and 1606+106 are superluminal sources, with average apparent speeds of 8.9 and 2.9 h^{-1}c respectively (H_{0}=100h km s^{-1} Mpc^{-1}, q_{0}=0.5). The components in 0202+149 are stationary, and we identify this source as a compact F double. These sources all have apparently bent jets, and we detected non-radial motion of components in CTA 26 and 1156+295. We have not yet detected any components emerging subsequent to the gamma-ray flares in CTA 26, 1156+295, and 1606+106, and we derive lower limits on the ejection times of any such components. The misalignment angle distribution of the EGRET sources is compared to the distribution for blazars as a whole, and we find that EGRET sources belong preferentially to neither the aligned nor the misaligned population. We also compare the average values for the apparent velocities and Doppler beaming factors for the EGRET and non-EGRET blazars, and find no significant differences. We thus find no indication, within the measurement errors, that EGRET blazars are any more strongly beamed than their counterparts which have not been detected in gamma-rays.

Reionization of the Intergalactic Medium and the Damping Wing of the Gunn‐Peterson Trough

Observations of high-redshift quasars show that the intergalactic medium (IGM) must have been reionized at some redshift z > 5. If a source of radiation could be observed at the rest-frame Lyα wavelength, at a sufficiently high redshift where some of the IGM in the line of sight was not yet reionized, the Gunn-Peterson trough should be present. Longward of the Lyα wavelength, a damping wing should be observed, caused by the neutral IGM whose absorption profile can be predicted. Measuring the shape of this damping wing would provide irrefutable evidence of the observation of the IGM before reionization and a determination of the density of the neutral IGM. This measurement might be hindered by the possible presence of a dense absorption system associated with the source. Shortward of the Lyα wavelength, absorption should be seen from the patchy structure of the IGM in the process of reionization, intersected in the line of sight. We show that a complete Gunn-Peterson trough is most likely to continue to be observed through the epoch where the IGM is partially ionized. The damping wings of the neutral patches around an ionized region should overlap in the spectrum if the proper path length through the ionized region is less than 1 h-1 Mpc; even in larger ionized regions, the characteristic background intensity should be low enough to yield a very high optical depth due to the residual neutral fraction, although occasionally some flux may be transmitted through large, underdense voids within an ionized region. In the case of the He II reionization, the ionization fronts are much thicker than in the case of hydrogen, and the profile of this front determines the shape of the absorption at the edge of a He III region. Analogous to the case of hydrogen, windows of transmitted flux are not likely to be observed until after the low-density IGM has been completely reionized. Therefore, the observation of these transmission windows by Reimers et al. at z 2.85 suggests that the helium reionization was complete by this redshift. The recently discovered afterglows of gamma-ray bursts might soon be observed at the very high redshifts required for these observations. Their featureless continuum spectrum and high luminosities make them ideal sources for studying absorption by the IGM.

At the Border of Eternity

The modern epoch of great astronomical discoveries began in 1962 with x-ray observations of the sky (1), followed by observations of quasars, microwave background radiation, neutron stars as pulsars and x-ray sources, and gamma ray bursts (2). Then came hints of the most intriguing objects in the universe: black holes. To be more precise, the first indications for the existence of black holes were obtained in 1972 from the UHURU satellite x-ray observations (3) of Cyg X-1, a binary system in which the mass of the compact star exceeds the limiting mass of a stable superdense neuron star (about three solar masses). At this mass, the star should be compressed to the state of a black hole. At a recent conference (4) in Calcutta, new data from x-ray, ultraviolet, optical, and radio observations gave convincing proof of these objects, which were predicted theoretically 60 years ago (5).

Near-Infrared Spectroscopic Observation of the High-z Quasar PKS 0201+113

A near-infrared spectroscopic observation of a high-redshift quasar, PKS 0201+113, made with the OH-airglow suppressor spectrograph (OHS) is presented. This quasar is known to have a damped Lyα system in the line of sight. Distinctive characteristics shown in the low-resolution spectrum of the quasar in the J- and H-bands are a large redshift difference between the Mg II emission line and high-excitation lines, the presence of Fe II emission features and a possibly reddened continuum spectrum. The difference in the velocity between the C IV and Mg II lines is consistent with a simple picture of the separated distribution of broad line regions made of two regions: i) high-excited clouds having a high outflow velocity and ii) relatively quiescent low-excited clouds surrounding the central hot region. The presence of Fe II emission implies that the metalicity of the central environmental gas of the high-redshift quasar is still similar to that of low-redshift quasars. A softening of the spectral slope toward short wavelengths could be explained if we take reddening by the damped Lyα system into account.

Multicolor Survey For High Redshift Quasars By Kiso Schmidt Telescope

Several groups have been successful in the survey of high-redshift (z > 4) quasars. For example, Schneider et al. (1994) and Schmidt et al. (1995) were successful in detecting 90 quasars with 2.75 < z < 4.75, R < 21 for 62deg2 using Palomar Transit Grism. Among them, there are 9 quasars with z > 4.0. Storrie-Lombardi et al. (1996) were detected 31 quasars with z > 4.0, mT < 19 for 2500deg2 by APM photographic multicolor survey using UK Schmidt telescope. Survey observations of quasars up to now have to select one of the following conditions; wide field surveys using photographic Schmidt plate which cover large areas but do not go faint enough, or deep surveys using large telescopes which go faint magnitude but which cover relatively small areas.

Do the Broad Emission Line Clouds See the Same Continuum That We See?

Recent observations of quasars, Mrk 335 and the HST quasar composite spectrum, have indicated that many of them have remarkably soft ionizing continua (f� / � 2 , 13.6 eV – 100 eV). We point out that the number of h� > 54.4 eV photons is insufficient to create the observed strengths of the He ii emission lines. While the numbers of photons which energize C iv �1549 and O vi �1034 are sufficient, even the most efficiently emitting clouds f or these two lines must each cover at least 20% – 40% of the source. If the typical quasar ionizing continuum is indeed this soft, then we must conclude that the broad emission line clouds must see a very different (harder) continuum than we see. The other viable possibility is that the UV – EUV SED is double-peaked, with the second peaking near 54 eV, its Wien tail the observed soft X-ray excess. Subject headings: line: formation — quasars: emission lines and general — ultraviolet

The Soft X‐Ray Properties of a Complete Sample of Optically Selected Quasars. II. Final Results

We present the results of ROSAT position sensitive proportional counter (PSPC) observations of 10 quasars. These objects are part of our ROSAT program to observe a complete sample of optically selected quasars. This sample includes all 23 quasars from the bright quasar survey with a redshift z less than or = 0.400 and a Galactic H I column density N(sup Gal sub H I) less than 1.9 x 10(exp 20)/sq cm. These selection criteria, combined with the high sensitivity and improved energy resolution of the PSPC, allow us to determine the soft (approximately 0.2-2 keV) X-ray spectra of quasars with about an order of magnitude higher precision compared with earlier soft X-ray observations. The following main results are obtained: Strong correlations are suggested between the soft X-ray spectral slope alpha(sub x) and the following emission line parameters: H beta Full Width at Half Maximum (FWHM), L(sub O III), and the Fe II/H beta flux ratio. These correlations imply the following: (1) The quasar's environment is likely to be optically thin down to approximately 0.2 keV. (2) In most objects alpha(sub x) varies by less than approximately 10% on timescales shorter than a few years. (3) alpha(sub x) might be a useful absolute luminosity indicator in quasars. (4) The Galactic He I and H I column densities are well correlated. Most spectra are well characterized by a simple power law, with no evidence for either significant absorption excess or emission excess at low energies, to within approximately 30%. We find mean value of alpha(sub x) = -1.50 +/- 0.40, which is consistent with other ROSAT observations of quasars. However, this average is significantly steeper than suggested by earlier soft X-ray observations of the Einstein IPC. The 0.3 keV flux in our sample can be predicted to better than a factor of 2 once the 1.69 micrometer(s) flux is given. This implies that the X-ray variability power spectra of quasars flattens out between f approximately 10(exp -5) and f approximately 10(exp -8) Hz. A steep alpha(sub x) is mostly associated with a weak hard X-ray component, relative to the near-IR and optical emission, rather than a strong soft excess, and the scatter in the normalized 0.3 keV flux is significantly smaller than the scatter in the normalized 2 keV flux. This argues against either thin or thick accretion disks as the origin of the soft X-ray emission. Further possible implications of the results found here are briefly discussed.

Compact Radio Cores in the Galactic Center and Elsewhere

AbstractCompact radio cores are not only common in radio galaxies and quasars but also in many nearby galaxies with low-active, supermassive black holes. One famous example is the Galactic Center source Sgr A*. Recent studies of proper motions and radial velocities of stars in the inner parsec of the Galaxy convincingly demonstrate the presence of a compact dark mass of 2.5 · 106M⊙in the nucleus of the Milky Way. Millimeter VLBI and submm observations of Sgr A*thus probe a region of only a few Schwarzschild radii in diameter. In this paper I will review our current theoretical and observational knowledge of this source and compare it to some famous LINER galaxies like NGC 4258, NGC 3079, and NGC 6500. In all cases these radio cores can be well explained by a standard AGN jet model, and, with the exception of Sgr A*, large scale outflows are observed that have powers comparable to those inferred from the radio cores. Recent VLBI observations of radio-weak quasars and HST observations of Seyfert galaxies indicate that these AGN also produce powerful jets which, however, have relatively less luminous radio cores than radio-loud quasars and the LINERs discussed here. Therefore, jets and compact radio cores appear to be natural constituents of an AGN, but the reason why apparently some jets are radio-loud and others not remains a mystery.

A polarization study of quasar 4C71.07

Parsec-scale observations of quasars have shown that the magnetic fields along the jets do not always lie parallel to the jet direction but there are some cases of misalignment. Preliminary results from dual frequency polarization VLBI are presented for quasar 4C71.07 (0836+710). Our results show that Faraday rotation is not the origin of the misalignment. The magnetic field is found to be misaligned to the structural jet direction by a maximum of 28 ± 4°. The consequences for the magnetic field geometry and the interaction with the narrow line region are discussed.

Measurement of the opacity of ionized helium in the intergalactic medium

DETERMINATION of the density and ionization state of diffuse primordial gas in the intergalactic medium (IGM) is an important step towards understanding the nature of dark matter and the formation of structure in the Universe. Absorption by neutral hydrogen atoms in the IGM should produce a large flux deficit in the spectra of high-redshift quasars at wavelengths below that of the H I Lyman-α line (the Gunn–Peterson effect1). But observations of quasars at low spectral resolution have revealed only relatively small deficits, and at higher spectral resolution these appear as numerous discrete absorption features (the Lyman-α forest). This implies that intergalactic hydrogen is both highly ionized and non-uniformly distributed2. Similar absorption is expected from intergalactic helium3, although its detection is more challenging and evidence for its existence was obtained only recently4,5. Here we report the observation of absorption from singly ionized helium (He ii) in the spectrum of the quasar HS1700 + 64. We measure a mean flux deficit over the redshift range 2.2 < z < 2.6 that yields an effective optical depth of 1.00 (±0.07). Comparing this result with the H I absorption towards this quasar, we argue that most of the measured He II absorption occurs in the lowest-density regions of the IGM.

The Radio and Optical Properties of Quasars

The relationship between the radio and optical emission of quasars was first studied using radio-selected objects, which generally had high radio luminosities due to the relatively low sensitivity limits of the surveys. More recently, radio follow-up observations have been made of optical surveys surveys in other wavebands. Taken together, the two survey methods have detected quasars with a range of over 6 orders of magnitude in radio luminosity. We report here new observations with the Very Large Array (VLA) of 103 quasars from the Large Bright Quasar Survey (LBQS), with absolute magnitudes in the range −25 &lt; MB &lt; −23 and redshifts in the range 0.2 &lt; z &lt; 1.1. This can be combined with our previous LBQS observations (Paper I, Paper II) to yield 359 quasars, the largest sample of sensitive radio observations of optically-selected quasars in a single survey.