On the Measurements of D/H in QSO Absorption Systems Closing in on the primordial abundance of deuterium

We present our measurements of the deuterium to hydrogen ratio (D/H) in QSO absorption systems, which give D/H = 3.40 ± 0.25 × 10-5 based on analysis of four independent systems. We discuss the properties of two systems which provide the strongest constraints on D/H. We outline the systematic effects involved in measurements of D/H and introduce a sophisticated method of analysis which properly accounts for these effects.     

Keck observations of ROSAT ultra-luminous X-ray source candidates

We present results of optical follow-up observations of candidate ultra-luminous X-ray sources (ULXs). Using Keck optical spectroscopy, 17 of the candidates from the Colbert and Ptak [Colbert, E.J.M., Ptak, A.F. A catalog of candidate intermediate-luminosity X-ray objects. ApJS 143, 25–45, 2002] catalog have been identified; this is one of the largest sets of optical identifications of such objects thus far. Fifteen are background active galactic nuclei (AGN); two are foreground stars in our Galaxy. These findings are consistent with background and foreground object expectations, as derived from log N–log S relations. Also, the results are briefly discussed in terms of the spiral-galaxy/ULX connection.     

RATAN-600 and RadioAstron reveal the neutrino-associated blazar TXS 0506+056 as a typical variable AGN

The possible association with the high-energy neutrino event IceCube-170922A has sparked interest in the blazar TXS 0506+056. We present 72 instantaneous 1–22 GHz spectra measured over the past 20 years with the RATAN-600 telescope and compare them with the results of observations of 700 variable Active Galactic Nuclei (AGN) studied within the same program. The recent radio flare of TXS 0506+056 started from a minimum in 2013 and reached its first peak in December 2017 and a second peak in May-June 2018. This was the third strong radio flare in this source since 1997. The spectrum remains nearly flat during the flares. The spectral shape and variability pattern observed in TXS 0506+056 are typical for variable AGN. RadioAstron Space VLBI observations in 2013–2015 did not detect TXS 0506+056 on space-ground baselines of more than 9 Earth diameters. However, an observation on 23 September 2015 resulted in the detection of interferometric signal on 6 Earth diameter baselines at 18 cm close to the detection limit. We consider the possibility that TXS 0506+056 and other AGN may accelerate relativistic protons more efficiently than electrons. Relativistic protons are necessary to produce both the high-energy neutrinos observed in the IceCube Observatory and the high AGN brightness temperatures implied by the RadioAstron detection. They may also provide the main contribution to the observed synchrotron radiation of parsec-scale AGN jets. This supports the suggestion that relativistic protons may play a much more important part in extragalactic astrophysics than earlier anticipated.     

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.     

A multi-epoch study of the C IV absorption variability in the broad absorption line quasar APM 08279+5255

Broad absorption line (BAL) variability potentially represents a powerful tool to investigate the physical nature and the structure of gas outflows in active galactic nuclei. Most existing BAL variability studies rely on observations taken at a few epochs for samples of tens of BAL QSOs. In this study we present the first “monitoring” of a single object, APM 08279+5255, which has been observed more than 20 times since 2003. All available spectra from the literature have also been analysed, including two high resolution spectra, extending the time interval from 1998 to 2012. A relative stability of the shape of the absorption profile is found. At the same time significant variations of the equivalent width are observed. A correlation of the BAL equivalent width with the QSO luminosity is found for the first time. These results suggest that changes in the ionisation state of the gas are causing opacity changes.     

Low ionization lines in high luminosity quasars: The calcium triplet

In order to investigate where and how low ionization lines are emitted in quasars we are studying a new collection of spectra of the CaII triplet at λ8498, λ8542, λ8662 observed with the Very Large Telescope (VLT) using the Infrared Spectrometer And Array Camera (ISAAC). Our sample involves luminous quasars at intermediate redshift for which CaII observations are almost nonexistent. We fit the CaII triplet and the OI λ8446 line using the Hβ profile as a model. We derive constraints on the line emitting region from the relative strength of the CaII triplet, OI λ8446 and Hβ.     

Optical and X-ray aspects of quasars

We review the present knowledge on the cosmological evolution of quasars, by discussing some of the recent results obtained from studies of optically selected objects. Despite the fast development of prism survey tecniques, the color selection still appears to be the best tecnique for constructing the complete samples which are necessary for statistical studies. It is shown, however, that even the best available complete samples of quasars selected on the basis of ultraviolet excess (z < 2.2) are not sufficient to univocally determine the “correct” evolutionary model. Moreover, some preliminary results suggest that the evolution law derived from quasars with m_B<20 and z<2.2 can not be extrapolated to fainter magnitudes and higher redshifts. On the basis of what is known today about the optical and X-ray properties of quasars, we then discuss some of teh possible results, relevant to cosmology, which can be achieved with future coordinated optical and X-ray observations of quasars.     

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.     

Numerical Simulations of the Warm-Hot Intergalactic Medium

In this paper we review the current predictions of numerical simulations for the origin and observability of the warm hot intergalactic medium (WHIM), the diffuse gas that contains up to 50 per cent of the baryons at z∼0. During structure formation, gravitational accretion shocks emerging from collapsing regions gradually heat the intergalactic medium (IGM) to temperatures in the range T∼10⁵–10⁷ K. The WHIM is predicted to radiate most of its energy in the ultraviolet (UV) and X-ray bands and to contribute a significant fraction of the soft X-ray background emission. While O vi and C iv absorption systems arising in the cooler fraction of the WHIM with T∼10⁵–10^5.5 K are seen in FUSE and Hubble Space Telescope observations, models agree that current X-ray telescopes such as Chandra and XMM-Newton do not have enough sensitivity to detect the hotter WHIM. However, future missions such as Constellation-X and XEUS might be able to detect both emission lines and absorption systems from highly ionised atoms such as O vii, O viii and Fe xvii.