Fabrizio Nicastro

Ubiquitous Variability of X-Ray-absorbing Column Densities in Seyfert 2 Galaxies

We present a study of the variations in the absorbing column density of 25 X-ray-defined Seyfert 2 galaxies, as inferred from hard X-ray observations, on timescales from months to several years. We show that a significant variation of NH (from 20% to 80%) is observed in almost all (22 of 25) of the sources with multiple X-ray observations, although X-ray absorption never vanishes. For a subsample of 11 sources observed at least five times, the typical variation time, as defined by a structure function, is less than 1 yr for both heavily absorbed (NH ~ 1023 cm-2) and moderately absorbed (NH ~ 1022 cm-2) sources. These variations rule out the simplest version of the unified models, based on a homogeneous obscuring torus, and suggest the presence of clumpy circumnuclear material on a scale well below a parsec. We propose a modification of the torus model in which an overabundance of slightly dusty broad emission-line region (BELR) clouds obscures the BELR. The BELR needs, like the torus, to have an axisymmetric structure. This model is closely related to that of Elvis for type 1 active galactic nuclei (AGNs). For lightly obscured AGNs (NH ~ 1022 cm-2), the structure function shows an increase at a timescale of ~5 yr, indicating a second absorber, most probably on a 5-10 pc scale associated with the host galaxy.

Chandra Discovery of a Tree in the X-Ray Forest toward PKS 2155–304: The Local Filament?

We present the first X-ray detection of resonant absorption from warm/hot local gas either in our Galaxy, or in the intergalactic space surrounding our Galaxy, along the line of sight toward the blazar PKS 2155-304. The Chandra HRCS/LETG spectrum of this z = 0.116 source clearly shows, at ≥5 σ level, unresolved (FWHM ≤ 800 km s-1 at a 2 σ confidence level) O VII Kα and Ne IX Kα resonant absorption lines at 21.603 and 13.448 A (i.e., cz = 14 km s-1 in the rest frame, from the O VII Kα line). O VIII Kα and O VII Kβ from the same system are also detected at a lower significance level (i.e., ~3 σ), while upper limits are set on O VIII Kβ, Ne X Kα, and Ne IX Kβ. The Far Ultraviolet Spectroscopic Explorer spectrum of this source shows complex O VI 2s→2p absorption at the same redshift as the X-ray system, made by at least two components: one relatively narrow (FWHM = 106 ± 9 km s-1) and slightly redshifted (cz = 36 ± 6 km s-1), and one broader (FWHM = 158 ± 26 km s-1) and blueshifted (cz = -135 ± 14 km s-1). We demonstrate that the physical states of the UV and X-ray absorbers are hard to reconcile with a single, purely collisionally ionized, equilibrium plasma. We propose instead that the X-ray and at least the broader and blueshifted UV absorber are produced in a low-density intergalactic plasma, collapsing toward our Galaxy, consistent with the predictions of a warm-hot intergalactic medium from numerical simulations. We find that any reasonable solution requires overabundance of Ne compared to O by a factor of ~2, with respect to the solar value. We propose several scenarios to account for this observation.

Is RX J1856.5?3754 a Quark Star?

Deep Chandra Low Energy Transmission Grating and High Resolution Camera spectroscopic observations of the isolated neutron star candidate RX J1856.5-3754 have been analyzed to search for metallic and resonance cyclotron spectral features and for pulsation behavior. As found from earlier observations, the X-ray spectrum is well represented by an ~60 eV (7 × 105 K) blackbody. No unequivocal evidence of spectral line or edge features has been found, arguing against metal-dominated models. The data contain no evidence for pulsation, and we place a 99% confidence upper limit of 2.7% on the unaccelerated pulse fraction over a wide frequency range from 10-4 to 100 Hz. We argue that the derived interstellar medium neutral hydrogen column density of 8 × 1019 cm-2 ≤ NH ≤ 1.1 × 1020 cm-2 favors the larger distance from two recent Hubble Space Telescope parallax analyses, placing RX J1856.5-3754 at ~140 pc instead of ~60 pc and in the outskirts of the R CrA dark molecular cloud. That such a comparatively rare region of high interstellar matter (ISM) density is precisely where an isolated neutron star reheated by accretion of ISM would be expected is either entirely coincidental or current theoretical arguments excluding this scenario for RX J1856.5-3754 are premature. Taken at face value, the combined observational evidence—a lack of spectral and temporal features and an implied radius of R∞ = 3.8-8.2 km that is too small for current neutron star models—points to a more compact object, such as allowed for quark matter equations of state.

Broad Emission Line Regions in Active Galactic Nuclei: The Link with the Accretion Power.

We present a model which relates the width of the Broad Emission Lines of AGN to the Keplerian velocity of an accretion disk at a critical distance from the central black hole. This critical distance falls in a region bounded on the inward side by the transition radius between the radiation pressure and the gas pressure dominated region of the accretion disk and on the outward side by the maximum radius below which a stabilizing, radially accreting and vertically outflowing corona exists. We show that in the framework of this picture the observed range of Hβ FWHM from Broad Line to Narrow Line type 1 AGN is well reproduced as a function of the accretion rate. This interval of velocities is the only permitted range and goes from ∼ 20, 000 km s for sub-Eddington accretion rates, to ∼ 1, 000 km s for Eddington accretion rates. Subject headings: accretion, accretion disks — galaxies: active — quasars: emission linesWe present a model that relates the width of the broad emission lines of active galactic nuclei (AGNs) to the Keplerian velocity of an accretion disk at a critical distance from the central black hole. This critical distance falls in a region bounded on the inward side by the transition radius between the radiation pressure- and the gas pressure-dominated region of the accretion disk and on the outward side by the maximum radius below which a stabilizing, radially accreting and vertically outflowing corona exists. We show that in the framework of this picture, the observed range of Hbeta FWHMs from broad-line to narrow-line type 1 AGNs is well reproduced as a function of the accretion rate. This interval of velocities is the only permitted range and goes from approximately 20,000 km s-1 for sub-Eddington accretion rates to approximately 1000 km s-1 for Eddington accretion rates.

Toward a Self-Consistent Model of the Ionized Absorber in NGC 3783

We present a detailed model for the ionized absorbing gas evident in the 900 ks Chandra HETGS spectrum of NGC 3783. The analysis was carried out with PHASE, a new tool designed to model X-ray and UV absorption features in ionized plasmas. The 0.5-10 keV intrinsic continuum of the source is well represented by a single power law (Γ = 1.53) and a soft blackbody component (kT ~ 0.1 keV). The spectrum contains over 100 features, which are well fitted by PHASE with just six free parameters. The model consists of a simple two-phase absorber with a difference of ≈35 in the ionization parameter and a difference of ≈4 in the column density of the phases. The two absorption components turned out to be in pressure equilibrium and are consistent with a single outflow (≈750 km s-1), a single turbulent velocity (300 km s-1), and solar elemental abundances. The main features of the low-ionization phase are an Fe M-shell unresolved transition array (UTA) and the O VII lines. The O VII features, usually identified with the O VIII and a warm absorber, are instead produced in a cooler medium that also produces O VI lines. The UTA sets tight constraints on the ionization degree of the absorbers, making the model more reliable. The high-ionization phase is required by the O VIII and the Fe L-shell lines, and there is evidence for an even more ionized component in the spectrum. A continuous range of ionization parameters is disfavored by the fits, particularly to the UTA. Our model indicates a severe blending of the absorption and emission lines, as well as strong saturation of the most intense O absorption lines. This is in agreement with the O VII (τλ = 0.33) and O VIII (τλ = 0.13) absorption edges required to fit the spectrum. The low-ionization phase can be decomposed into three subcomponents on the basis of the outflow velocity, FWHM, and H column densities found for three of the four UV absorbers detected in NGC 3783. However, the ionization parameters are systematically smaller in our model than those derived from UV data, indicating a lower degree of ionization. Finally, our model predicts a Ca XVI line for the feature observed at around 21.6 A (a feature formerly identified as O VII), constraining the contribution from a zero-redshift absorber.

The mass of the missing baryons in the X-ray forest of the warm–hot intergalactic medium

Recent cosmological measurements indicate that baryons comprise about four per cent of the total mass-energy density of the Universe, which is in accord with the predictions arising from studies of the production of the lightest elements. It is also in agreement with the actual number of baryons detected at early times (redshifts z > 2). Close to our own epoch (z < 2), however, the number of baryons detected add up to just over half (∼ 55 per cent) of the number seen at z > 2 (refs 6–11), meaning that about ∼45 per cent are ‘missing’. Here we report a determination of the mass-density of a previously undetected population of baryons, in the warm–hot phase of the intergalactic medium. We show that this mass density is consistent, within the uncertainties, with the mass density of the missing baryons.

The XMM Deep survey in the CDF-S - I. First results on heavily obscured AGN

We present the first results of the spectroscopy of distant, o bscured AGN as obtained with the ultra‐deep (�3.3 Ms) XMM‐Newton survey in the Chandra Deep Field South (CDF‐S). One of the primary goals of the project is to characterize the X‐ray spectral properties of obscured and heavily obscured Compton‐thick AGN over the range of redhifts and luminosities that are relevant in terms of their contribution to the X‐ray background. The ultra‐deep exposure, coupled with the XMM detector’s spectral throughput, allowed us to accumulate good quality X‐ray spectra for a large number of X‐ray sources and, in particular, for heavily obscured AGN at cosmological redshifts. Specifically we present the X ‐ray spectral properties of two high‐redshift ‐ z= 1.53 and z=3.70 ‐

The unprecedented optical outburst of the quasar 3C 454.3 : The WEBT campaign of 2004-2005

Context. The radio quasar 3C 454.3 underwent an exceptional optical outburst lasting more than 1 year and culminating in spring 2005. The maximum brightness detected was

An Unveiling Event in the Type 2 Active Galactic Nucleus NGC 4388:A Challenge for a Parsec-Scale Absorber

R=12.0

A swan song: the disappearance of the nucleus of NGC 4051 and the echo of its past glory

, which represents the most luminous quasar state thus far observed (