Some implications of high luminosity and fast variability in active galactic nuclei

An analysis of the variability timescale against bolometric luminosity for Active Galactic Nuclei shows that a number of sources violate the Eddington limit. The average ratio (L/L_E) is found to change according to group classification. Whilst Seyfert Galaxies have luminosites well within the Eddington limit, Quasars and BL Lac object tend to approach and exceed this limit. Furthermore, BL Lac objects may be further subdivided on the basis of their (L/L_E) ratio. The data on luminosity and variability timescale indicate the existence of two types of active galaxies, one having highly anisotropic emission, probably collimated into jets with pointing angles within few degrees to the line of sight, and the other relating to isotropic emission of photons from the nuclear region. The results are discussed in the light of the high γ-ray luminosity suggested by recent observations of active galaxies.     

Reverberation time lags in the high luminosity quasar PG 1247+267

Virial estimates of the black hole mass in the center of AGNs, derived from single-epoch observations of luminosity and emission line widths, are now available for several thousands of objects at all redshifts and luminosities, so that studies of the cosmological evolution of the AGN mass function are becoming possible. These estimates are based on the empirical luminosity-size relation measured through reverberation mapping at low redshifts and luminosities. For this reason the spectrophotometric monitoring of 4 luminous quasars was started in 2003, with the 1.8 m telescope of the Asiago Observatory. To measure the reverberation time lag we adopt a method, recently introduced by Zu et al. (2011), which takes advantage of the available statistical information on variability autocorrelation and makes simple assumptions on the continuum-emission line transfer function. We present the estimate of the broad line region size for the quasar PG 1247+267, which is the most luminous object with reverberation measures to date. Virial products computed for C iv and C iii] emission lines provide consistent results. A possible flattening of the size-luminosity relation for C iv at high luminosity is suggested.     

Quasars and their emission lines as cosmological probes

Quasars are the most luminous sources in the Universe. They are currently observed out to redshift z≈7$z\approx 7$ when the Universe was less than one tenth of its present age. Since their discovery 50 years ago astronomers have dreamed of using them as standard candles. Unfortunately quasars cover a very large range (8 dex) of luminosity making them far from standard. We briefly review several methods that can potentially exploit quasars properties and allow us to obtain useful constraints on principal cosmological parameters. Using our 4D Eigenvector 1 formalism we have found a way to effectively isolate quasars radiating near the Eddington limit. If the Eddington ratio is known, under several assumptions it is possible to derive distance independent luminosities. We discuss the main statistical and systematic errors involved, and whether these “standard Eddington candles” can be actually used to constrain cosmological models.     

Radio-faint BL Lac objects and their impact on the radio/gamma-ray connection

Radio and gamma-ray emissions in Active Galactic Nuclei (AGNs) are both related to the presence of relativistic particles in jets. With the advent of the Fermi Large Area Telescope (LAT), and thanks to its large sensitivity up to several GeV, many observational results are changing our understanding of these phenomena. BL Lac objects, which made up only a fraction of the known extragalactic gamma-ray source population before Fermi, have now become the most abundant class. However, since they are relatively weak radio sources, most of them are poorly known as far as their parsec scale structure and multi-wavelength properties are concerned. For this reason, we have selected a complete sample of 42 low redshift BL Lacs (independently of their gamma-ray properties) to study with a multi-wavelength (radio, optical, X-ray, gamma-ray) approach. Here, we present results and images of sources in the sample (most of which have never been observed before), using new VLBA observations at 8 and 15 GHz. Beyond this sample of BL Lacs, the population of gamma-ray AGNs has also dramatically enlarged in the Fermi era, permitting us to discuss the presence of a correlation between radio and gamma-ray properties with improved statistical significance. We explore the radio-gamma relation with several hundreds sources and using both simultaneous and archival radio data, thus tackling the impact of time variability.     

X-ray redshifts with the International X-ray Observatory (IXO)

We explore the capabilities of the future space science mission IXO (International X-ray Observatory) for obtaining cosmological redshifts of distant Active Galactic Nuclei (AGNs) using the X-ray data only. We first find in which regions of the X-ray luminosity (L_X) versus redshift (z) plane the weak but ubiquitous Fe Kα narrow emission line can deliver an accurate redshift (δz < 5%) as a function of exposure time, using a CCD-based Wide Field Imager (IXO/WFI) as the one baselined for IXO. Down to a 2–10 keV X-ray flux of 10⁻¹⁴ erg cm⁻² s⁻¹ IXO/WFI exposures of 100 ks, 300 ks and 1 Ms will deliver 20%, 40% and 60% of the redshifts. This means that in a typical 18′ × 18′ IXO/WFI field of view, 4, 10 and 25 redshifts will be obtained for free from the X-ray data alone, spanning a wide range up to z ∼ 2–3 and fairly sampling the real distribution. Measuring redshifts of fainter sources will indeed need spectroscopy at other wavebands.     

An X-ray survey of a complete sample of 3CR radio galaxies

Imaging X-ray observations of normal spiral galaxies show extended and complex x-ray emission, easily explainable with a complex of unresolved X-ray sources. A variety of nuclear sources, including starburst nuclei and miniature active nuclei are seen. The total (0.5–3.0 keV) luminosities are in the range of Lx 10³⁸ - 10⁴⁰ erg s⁻¹. The X-ray luminosity is linearly correlated with the optical luminosity. It is also correlated with the radio continuum luminosity at 21cm, but following a power law relationship with an exponent α = 0.6. This latter relationship might have implications on the Population I X-ray binary formation models and/or on the origin of the radio continuum emission in spiral galaxies     

Indicators of black-hole mass and Eddington accretion ratio from QSO X-ray and UV spectra

The evolution of luminous QSOs is linked to the evolution of massive galaxies. We know this because the relic black-holes found locally have masses dependent on the properties of the host galaxy’s bulge. An important way to explore this evolution would be to measure dependences of black-hole masses and Eddington accretion ratios over a range of redshifts, i.e., with cosmological age. For low redshift QSOs (and their lower luminosity Seyfert galaxy counterparts) it has been possible to infer black-hole masses from the luminosities and velocity dispersions of their host-galaxy bulges. These masses agree with those virial black-hole masses calculated from the Doppler widths of the broad Hβ emission lines. The latter method can then be extended to more distant and luminous QSOs, up to redshifts of 0.6 with ground-based optical observations. We discuss ways to extend these explorations to higher redshifts – up to 3 using the widths of QSOs’ broad UV emission lines, and in principle, and to redshifts near 4 from ground-based infrared observations of rest-frame Hβ at 2.5 μm. We discuss the possibility of investigating the accretion history of the higher redshift QSOs using measures of Eddington accretion ratio – the soft X-ray spectral index and the eigenvectors of Principal Components Analyses of QSOs’ UV emission-line spectra.     

Radio galaxies and the acceleration of the universe beyond a redshift of one

Radio galaxies provide a means to determine the coordinate distance, the luminosity distance, the dimension-less luminosity distance, or the angular size distance to sources with redshifts as large as two. Dimensionless coordinate distances for 55 supernovae and 20 radio galaxies are presented and discussed here. The radio galaxy results are consistent with those obtained using supernovae, suggesting that neither method is plagued by unknown systematic errors. The acceleration parameter q(z) and the expansion rate H(z) or dimensionless expansion rate E(z) can be determined directly from the data without having to make assumptions regarding the nature or evolution of the “dark energy”. The expansion rate E(z) can be determined from the first derivative of the dimensionless coordinate distance, (dy/dz)⁻¹, and the acceleration parameter can be determined from a combination of the first and second derivatives of the dimensionless coordinate distance. A model-independent determination of E(z) will allow the properties and redshift evolution of the “dark energy” to be determined, and a model-independent determination of q(z) will allow the redshift at which the universe transitions from acceleration to deceleration to be determined directly. Determination of E(z) and q(z) may also elucidate possible systematic errors in the determinations of the dimensionless coordinate distances.     

类星体X射线辐射的性质

本文对247个类星体的X射线性质进行了统计分析,这些X射线资料是由爱因斯坦卫星在0.5—4.5keV的能带内观测到的。主要结果是:在各种视亮度和红移(或距离)的已知1619个类星体中,都有百分比相近的部分为X射线源。Ⅰ型Seyferc星系的发射线H_β与[OⅢ]的等值宽度比R_EW同X射线光度L_x强相关,这可作为Ⅰ型Seyfert星系的X射线光度的一个新指标。光学辐射以及2.5GHz以上的高频射电辐射都同X射线辐射相关,说明这三种辐射的区域依次相邻而且机制也相似。色指数U-B和B-V都同X射线辐射不相关,但U-B同B-V弱相关。      

X-ray observations of high-z radio loud/quiet quasars

It is widely recognized that a knowledge of the X-ray properties of high-z radio loud/quiet quasars (RLQs/RQQs) would have important consequences for probing the conditions of the early Universe, studying the evolution of quasars, and assessing quasar contributions to the CXB. In distant active galactic nuclei (AGN) we can detect only the quasars having high luminosities or collimated beams with current X-ray astronomy satellites. We have also detected more than ten RLQs above z=2 with ASCA. We have observed 8 high z RQQs above z=2, but have not detected the higher z RQQs, above z=2.5. A considerable fraction of the high z RLQs show excess absorption over Galactic column densities, while most high z RQQs do not indicate excess absorption, although the samples are small. The X-ray photon index of RLQs is smaller than that of RQQs. The two RLQs above z=4 are very flat, suggesting that they are blazar-like AGN. A long term observation of RQQ, H1400#10 (z=2.078) did not indicate an Fe Kα emission line, giving a weaker upper limit than a typical line intensity of Seyfert 1 galaxies.     

GRB host galaxies: An unbiased sample

We describe the current status and recent results from our Swift/VLT legacy survey, a VLT Large Programme aimed at characterizing the host galaxies of a homogeneously selected sub-sample of Swift   GRBs. The immediate goals are to determine the host luminosity function, study the effects of reddening, determine the fraction of Lyα$\mathrm{Ly}\mathrm{\alpha }$ emitters in the hosts, and obtain redshifts for targets without a reported one. The main effort so far has been the definition of a very carefully selected sample, obeying strict and well-defined criteria: 68 targets in total. Among the preliminary results is a large optical detection rate, the lack of extremely red objects (only one possible case in the sample) and an update of the Swift   GRB redshift distribution with 〈z〉∼2.0$〈z〉\sim 2.0$.     

X-ray spectra and time variability of active galactic nuclei

The X-ray spectra of broad line active galactic nuclei of all types (Seyfert I's, NELG's, Broadline radio galaxies) is well fit by a power law in the .5–100 keV band of mean energy slope α = .68±.15. There is, as yet, no strong evidence for time variability of this slope in a given object. The constraints that this places on simple models of the central energy source are discussed. BL Lac objects have quite different X-ray spectral properties and show pronounced X-ray spectral variability.On time scales longer than 12 hours most radio quiet AGN do not show strong, ΔI/I > .5, variability. The probability of variability of these AGN seems to be inversely related to their luminosity. However characteristic timescales for variability have not been measured for many objects. This general lack of variability may imply that most AGN are well below the Eddington limit. Radio bright AGN tend to be more variable on long, τ 6 month, timescales.     

S5 0716+714: GeV variability study

The GeV observations by Fermi-LAT give us the opportunity to characterize the high-energy emission (100 MeV–300 GeV) variability properties of the BL Lac object S5 0716+714. In this study, we performed flux and spectral analysis of more than 3 year long (August 2008 to April 2012) Fermi-LAT data of the source. During this period, the source exhibits two different modes of flux variability with characteristic timescales of ∼75 and ∼140 days, respectively. We also notice that the flux variations are characterized by a weak spectral hardening. The GeV spectrum of the source shows a clear deviation from a simple power law, and is better explained by a broken power law. Similar to other bright Fermi blazars, the break energy does not vary with the source flux during the different activity states. We discuss several possible scenarios to explain the observed spectral break.     

A survey of radio jets with Chandra and HST

Relativistic jets are a common property of radio-loud Active Galactic Nuclei (AGN). Understanding jet physical properties is an essential precursor to understanding the mechanisms of energy transport, and ultimately, how energy is extracted from the central black hole. In this paper, I highlight recent developments from Chandra and HST observations of kpc-scale jets in AGN, with particular emphasis on our survey of 17 radio jets in a sample of FRII radio galaxies. These observations show that (1) X-ray and optical emission is common from kpc-scale jets, (2) a large fraction of the bolometric luminosity is emitted at X-rays, and (3) in most sources, a candidate emission process for the X-rays is inverse Compton scattering of the Cosmic Microwave Background off the relativistic electrons in the jet. If the latter scenario holds, the implication is that jets are still relativistic on kpc scales.     

The heart-shaped supernova remnant 3C 391 viewed in multi-bands

Using Chandra X-ray, Spitzer mid-IR, and 1.5 GHz radio data, we examine the spatial structure of SNR 3C 391. The X-ray surface brightness is generally anti-correlative with the IR and radio brightness. The multiband data clearly exhibit a heart-shaped morphology and show the multi-shell structure of the remnant. A previously unseen thin brace-like shell on the south detected at 24 μm is projected outside the radio border and confines the southern faint X-ray emission. The leading 24 μm knot on the SE boundary appears to be partly surrounded by soft X-ray emitting gas. The mid-IR emission is dominated by the contribution of the shocked dust grains, which may have been partly destroyed by sputtering.     

Formation of the first stars and quasars

We examine various observable signatures of the first generation of stars and low-luminosity quasars, including the metal enrichment, radiation background, and dust opacity/emission that they produce. We calculate the formation history of collapsed baryonic halos, based on an extension of the Press-Schechter formalism, incorporating the effects of pressure and H₂-dissociation. We then use the observed C/H ratio at z=3 in the Lyman-α forest clouds to obtain an average the star formation efficiency in these halos. Similarly, we fit the efficiency of black-hole formation, and the shape of quasar light curves, to match the observed quasar luminosity function (LF) between z=2−4, and use this fit to extrapolate the quasar LF to faint magnitudes and high redshifts. To be consistent with the lack of faint point-sources in the Hubble Deep Field, we impose a lower limit of ∼ 75 km s⁻¹ for the circular velocities of halos harboring central black holes.We find that in a ΛCDM model, stars reionize the IGM at z_reion=9−13, and quasars at z=12. Observationally, z_reion can be measured by the forthcoming MAP and Planck Surveyor satellites, via the damping of CMB anisotropies by ∼10% on small angular scales due to electron scattering. We show that if reionization occurs later, at 5 ≲ z_reion ≲ 10, then it can be measured from the spectra of individual sources. We also find that the Next Generation Space Telescope will be able to directly image about 1–40 star clusters, and a few faint quasars, from z >10 per square arcminute. The amount of dust produced by the first supernovae has an optical depth of τ=0.1−1 towards high redshift sources, and the reprocessed UV flux of stars and quasars distorts the cosmic microwave background radiation (CMB) by a Compton y-parameter comparable to the COBE limit, y ∼ 1.5 × 10⁻⁵.     

Gamma-ray blazars: The view from AGILE

During the first 3 years of operation the Gamma-Ray Imaging Detector onboard the AGILE satellite detected several blazars in a high γ-ray activity: 3C 279, 3C 454.3, PKS 1510-089, S5 0716+714, 3C 273, W Comae, Mrk 421, PKS 0537-441 and 4C +21.35. Thanks to the rapid dissemination of our alerts, we were able to obtain multiwavelength data from other observatories such as Spitzer, Swift, RXTE, Suzaku, INTEGRAL, MAGIC, VERITAS, and ARGO as well as radio-to-optical coverage by means of the GASP Project of the WEBT and the REM Telescope. This large multifrequency coverage gave us the opportunity to study the variability correlations between the emission at different frequencies and to obtain simultaneous Spectral Energy Distributions of these sources from radio to γ-ray energy bands, investigating the different mechanisms responsible for their emission and uncovering in some cases a more complex behavior with respect to the standard models. We present a review of the most interesting AGILE results on these γ-ray blazars and their multifrequency data.     

QSO number counts to B < 24.5 from hubble space telescope WFPC2 images

We have conducted a survey for faint quasars to B < 24.5, z < 2.1 based on archival high-latitude Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) images covering 0.04 deg² of the sky in three filters spanning 2300–9600 Å. Taking advantage of the superb 0.1″ resolution of WFPC2, we have developed an approach to select quasar candidates based on their morphology and position in multicolor space. We derive cumulative surface densities and compare them with our previous survey in the Groth–Westphal Strip and other faint quasar surveys that utilize very different selection approaches. In most cases the agreement with these surveys is good, however, we find approximately twice as many quasar candidates to B = 24.5 as the COMBO-17 survey. Although we are confident that our approach eliminates most contaminating stars and galaxies, spectroscopic follow-up will be required to determine the level of residual contamination and confirm or refute this excess.