ALBERT

Description: We present a new catalogue of ∼2400 optically selected quasars with spectroscopic redshifts and X-ray observations from either Chandra or XMM–Newton. The sample can be used to investigate the non-linear relation between the ultraviolet (UV) and X-ray luminosity of quasars as well as to build a Hubble diagram up to a redshift of z ∼ 7.5. We selected sources that are neither reddened by dust in the optical and UV nor obscured by gas in the X-rays, and whose X-ray fluxes are free from flux-limit-related biases. After checking for any possible systematics, we confirm, in agreement with our previous works, that the X-ray to UV relation provides distance estimates matching those from supernovae up to z ∼ 1.5, and its slope shows no redshift evolution up to z ∼ 5. We provide a full description of the methodology for testing cosmological models, further supporting a trend whereby the Hubble diagram of quasars is well reproduced by the standard flat cold dark matter model up to z ∼ 1.5–2, but strong deviations emerge at higher redshifts. Since we have minimised all non-negligible systematic effects and proven the stability of the LX − LUV relation at high redshifts, we conclude that an evolution of the expansion rate of the Universe should be considered as a possible explanation for the observed deviation, rather than some systematic (redshift-dependent) effect associated with high-redshift quasars.

Description: We present the analysis of a Chandra High-Energy Transmission Grating (HETG) observation of the local Seyfert galaxy NGC 1365. The source, well known for its dramatic X-ray spectral variability, was caught in a reflection-dominated, Compton-thick state. The high spatial resolution afforded by Chandra allowed us to isolate the soft X-ray emission from the active nucleus, neglecting most of the contribution from the kpc-scale starburst ring. The HETG spectra thus revealed a wealth of He- and H-like lines from photoionized gas, whereas in larger aperture observations these are almost exclusively produced through collisional ionization in the circumnuclear environment. Once the residual thermal component is accounted for, the emission-line properties of the photoionized region close to the hard X-ray continuum source indicate that NGC 1365 has some similarities to the local population of obscured active galaxies. In spite of the limited overall data quality, several soft X-ray lines seem to have fairly broad profiles (~800–1300 km s –1 full width at half-maximum), and a range of outflow velocities (up to ~1600 km s –1 , but possibly reaching a few thousand km s –1 ) appears to be involved. At higher energies, the Kα fluorescence line from neutral iron is resolved with 〉99 per cent confidence, and its width of ~3000 km s –1 points to an origin from the same broad-line region clouds responsible for eclipsing the X-ray source and likely shielding the narrow-line region.

Description: We report on a comprehensive X-ray spectral analysis of the nearby radio-quiet quasar MR 2251–178, based on the long-look (~400 ks) XMM–Newton observation carried out in 2011 November. As the properties of the multiphase warm absorber (thoroughly discussed in a recent, complementary work) hint at a steep photoionizing continuum, here we investigate into the nature of the intrinsic X-ray emission of MR 2251–178 by testing several physical models. The apparent 2–10 keV flatness as well as the subtle broad-band curvature can be ascribed to partial covering of the X-ray source by a cold, clumpy absorption system with column densities ranging from a fraction to several x 10 23  cm –2 . As opposed to more complex configurations, only one cloud is required along the line of sight in the presence of a soft X-ray excess, possibly arising as Comptonized disc emission in the accretion disc atmosphere. On statistical grounds, even reflection with standard efficiency off the surface of the inner disc cannot be ruled out, although this tentatively overpredicts the observed ~14–150 keV emission. It is thus possible that each of the examined physical processes is relevant to a certain degree, and hence, only a combination of high-quality, simultaneous broad-band spectral coverage and multi-epoch monitoring of X-ray spectral variability could help disentangling the different contributions. Yet, regardless of the model adopted, we infer for MR 2251–178 a bolometric luminosity of ~5–7 10 45  erg s –1 , implying that the central black hole is accreting at ~15–25 per cent of the Eddington limit.

Description: Abstract Localization processes in the viscous lower crust generate ductile shear zones over a broad range of scales affecting long‐term lithosphere deformation and the mechanical response of faults during the seismic cycle. Here we use centimeter‐scale numerical models in order to gain detailed insight into the processes involved in strain localization and rheological weakening in viscously deforming rocks. Our 2‐D Cartesian models are benchmarked to high‐temperature and high‐pressure torsion experiments on Carrara marble samples containing a single weak Solnhofen limestone inclusion. The models successfully reproduce bulk stress‐strain transients and final strain distributions observed in the experiments by applying a simple, first‐order softening law that mimics rheological weakening. We find that local stress concentrations forming at the inclusion tips initiate strain localization inside the host matrix. At the tip of the propagating shear zone, weakening occurs within a process zone, which expands with time from the inclusion tips toward the matrix. Rheological weakening is a precondition for shear zone localization, and the width of this shear zone is found to be controlled by the degree of softening. Introducing a second softening step at elevated strain, a high strain layer develops inside the localized shear zone, analogous to the formation of ultramylonite bands in mylonites. These results elucidate the transient evolution of stress and strain rate during inception and maturation of ductile shear zones.

Description: We present a detailed analysis of a recent, 2013 Suzaku campaign on the nearby ( z  = 0.184) luminous ( L bol  ~ 10 47 erg s –1 ) quasar PDS 456. This consisted of three observations, covering a total duration of ~1 Ms and a net exposure of 455 ks. During these observations, the X-ray flux was unusually low, suppressed by a factor of 〉10 in the soft X-ray band when compared to previous observations. We investigated the broad-band continuum by constructing a spectral energy distribution (SED), making use of the optical/UV photometry and hard X-ray spectra from the later simultaneous XMM–Newton and NuSTAR campaign in 2014. The high-energy part of this low-flux SED cannot be accounted for by physically self-consistent accretion disc and corona models without attenuation by absorbing gas, which partially covers a substantial fraction of the line of sight towards the X-ray continuum. At least two layers of absorbing gas are required, of column density log ( N H,low /cm –2 ) = 22.3 ± 0.1 and log ( N H,high /cm –2 ) = 23.2 ± 0.1, with average line-of-sight covering factors of ~80 per cent (with typical ~5 per cent variations) and 60 per cent (±10–15 per cent), respectively. During these observations PDS 456 displays significant short-term X-ray spectral variability, on time-scales of ~100 ks, which can be accounted for by variable covering of the absorbing gas along the line of sight. The partial covering absorber prefers an outflow velocity of $v_{\rm pc} = 0.25^{+0.01}_{-0.05}\,c$ at the 〉99.9 per cent confidence level over the case where v pc  = 0. This is consistent with the velocity of the highly ionized outflow responsible for the blueshifted iron K absorption profile. We therefore suggest that the partial covering clouds could be the denser, or clumpy part of an inhomogeneous accretion disc wind. Finally estimates are placed upon the size-scale of the X-ray emission region from the source variability. The radial extent of the X-ray emitter is found to be of the order ~15–20 R g , although the hard X-ray (〉2 keV) emission may originate from a more compact or patchy corona of hot electrons, which is typically ~6–8 R g in size.

Description: We present an X-ray spectral analysis of a large sample of 25 ‘bare’ active galactic nuclei (AGN), sources with little or no complicating intrinsic absorption, observed with Suzaku . Our work focuses on studying the potential contribution from relativistic disc reflection and examining the implications of this interpretation for the intrinsic spectral complexities frequently displayed by AGN in the X-ray bandpass. During the analysis, we take the unique approach of attempting to simultaneously undertake a systematic analysis of the whole sample, as well as a detailed treatment of each individual source, and find that disc reflection has the required flexibility to successfully reproduce the broad-band spectrum observed for all of the sources considered. Where possible, we use the reflected emission to place constraints on the black hole spin for this sample of sources. Our analysis suggests a general preference for rapidly rotating black holes, which if taken at face value is most consistent with the scenario in which supermassive black hole growth is dominated by prolonged, ordered accretion. However, there may be observational biases towards AGN with high spin in the compiled sample, limiting our ability to draw strong conclusions for the general population at this stage. Finally, contrary to popular belief, our analysis also implies that the dichotomy between radio-loud/radio-quiet AGN is not solely related to black hole spin.

Description: In the current framework, the standard parametrization of our Universe is the so-called Lambda cold dark matter (ΛCDM) model. Recently, a ∼4σ tension with the ΛCDM model was shown to exist via a model-independent parametrization of a Hubble diagram of type Ia supernovae (SNe Ia) from the JLA survey and quasars. Model-independent approaches and independent samples over a wide redshift range are key to testing this tension and any possible systematic errors. Here we present an analysis of a combined Hubble diagram of SNe Ia, quasars, and gamma-ray bursts (GRBs) to check the agreement of the quasar and GRB cosmological parameters at high redshifts (z 〉  2) and to test the concordance flat ΛCDM model with improved statistical accuracy. We build a Hubble diagram with SNe Ia, quasars, and GRBs, where quasars are standardised through the observed non-linear relation between their ultraviolet and X-ray emission and GRBs through the correlation between the spectral peak energy and the isotropic-equivalent radiated energy (the so-called Amati relation). We fit the data with cosmographic models consisting of a fourth-order logarithmic polynomial and a fifth-order linear polynomial, and compare the results with the expectations from a flat ΛCDM model. We confirm the tension between the best-fit cosmographic parameters and the ΛCDM model at ∼4σ with SNe Ia and quasars, at ∼2σ with SNe Ia and GRBs, and at 〉 4σ with the whole SNe Ia+quasars+GRB data set. The completely independent high-redshift Hubble diagrams of quasars and GRBs are fully consistent with each other, strongly suggesting that the deviation from the standard model is not due to unknown systematic effects but to new physics.

Description: We investigated the interstellar medium (ISM) properties of the disc and outflowing gas in the central regions of nine nearby Seyfert galaxies, all characterised by prominent conical or biconical outflows. These objects are part of the Measuring Active Galactic Nuclei Under MUSE Microscope (MAGNUM) survey, which aims to probe their physical conditions and ionisation mechanism by exploiting the unprecedented sensitivity of the Multi Unit Spectroscopic Explorer (MUSE), combined with its spatial and spectral coverage. Specifically, we studied the different properties of the gas in the disc and in the outflow with spatially and kinematically resolved maps by dividing the strongest emission lines in velocity bins. We associated the core of the lines with the disc, consistent with the stellar velocity, and the redshifted and the blueshifted wings with the outflow. We measured the reddening, density, ionisation parameter, and dominant ionisation source of the emitting gas for both components in each galaxy. We find that the outflowing gas is characterised by higher values of density and ionisation parameter than the disc, which presents a higher dust extinction. Moreover, we distinguish high- and low-ionisation regions across the portion of spatially resolved narrow-line region (NLR) traced by the outflowing gas. The high-ionisation regions characterised by the lowest [N II]/Hα and [S II]/Hα line ratios generally trace the innermost parts along the axis of the emitting cones where the [S III]/[S II] line ratio is enhanced, while the low-ionisation regions follow the cone edges and/or the regions perpendicular to the axis of the outflows, also characterised by a higher [O III] velocity dispersion. A possible scenario to explain these features relies on the presence of two distinct populations of line emitting clouds: one is optically thin to the radiation and is characterised by the highest excitation, while the other is optically thick and is impinged by a filtered, and thus harder, radiation field which generates strong low-excitation lines. The highest values of [N II]/Hα and [S II]/Hα line ratios may be due to shocks and/or a hard filtered radiation field from the active galactic nucleus.