Cosmological constraints from the Hubble diagram of quasars at high redshifts

The concordance (LambdaCDM) model reproduces the main current cosmological observations assuming the validity of general relativity at all scales and epochs, the presence of cold dark matter, and of a cosmological constant, equivalent to a dark energy with constant density in space and time. However, the LambdaCDM model is poorly tested in the redshift interval between the farthest observed Type Ia supernovae5 and that of the Cosmic Microwave background (CMB). We present new measurements of the expansion rate of the Universe in the range 0.5<z<5.5 based on a Hubble diagram of quasars. The quasar distances are estimated from their X-ray and ultraviolet emission, following a method developed by our group. The distance modulus-redshift relation of quasars at z<1.4 is in agreement with that of supernovae and with the concordance model. Yet, a deviation from the LambdaCDM model emerges at higher redshift, with a statistical significance of ~4 sigma. If an evolution of the dark energy equation of state is allowed, the data suggest a dark energy density increasing with time.Comment: To appear in Nature Astronomy. This is the re-submitted version in the authors' custom format. The complete data table is available upon reques

The SDSS/XMM-newton quasar sample. I. first results

We have searched in the XMM-Newton public archive for quasars in the Sloan Digital Sky Survey First Data Release and found 55 lying in the field of an XMM-Newton observation with exposure times >20 ks (as of 2004 August). The 35 quasars that yielded good X-ray spectra span redshifts from 0.5 to 2.5. The large collecting area of XMM-Newton allows us to investigate the dependence of the X-ray spectra of quasars on luminosity, redshift, and optical colors. We find (1) no evolution of X-ray slope (Γ) with either redshift or luminosity; (2) no correlation between Γ (or absorbing column density) and the optical-to-X-ray ratio, αOX; and (4) no relation between αOX and optical colors. The two latter results suggest that obscuration is not the dominant cause of the spread in X-ray slope or optical color. We find four unusual quasars (i.e., 10% of the sample): three are absorbed (NH > 1022 cm-2), of which one has high luminosity (1.5 × 1044 ergs s-1); the fourth has Γ = 0.6 ± 0.2, which is far flatter than the typical value of 1.8-2.0, and a strong emission line (EW = 1.2 ± 0.4 keV), which, if Fe K, implies a redshift of ~1.4

The effects of X-ray absorption variability in NGC 4395

We present a new X-ray analysis of the dwarf Seyfert galaxy NGC 4395, based on two archival XMM-Newton and Suzaku observations. This source is well known for a series of remarkable properties: one of the smallest estimated black hole masses among Active Galactic Nuclei (of the order of ~10^5 M_sun), intense flux variability on very short time-scales (a few tens of seconds), an unusually flat X-ray continuum (Gamma ~ 1.4 over the 2-10 keV energy range). NGC 4395 is also characterized by significant variations of the X-ray spectral shape, and here we show that such behaviour can be explained through the partial occultation by circumnuclear cold absorbers with column densities of ~10^22-10^23 cm^-2. In this scenario, the primary X-ray emission is best reproduced by means of a power law with a standard Gamma ~ 1.8 photon index, consistent with both the spectral slope observed at higher energies and the values typical of local AGN.Comment: 7 pages, 8 figures, 2 tables. Accepted for publication in MNRA

The structure of AGNs from X-ray absorption variability

We present new evidence of X-ray absorption variability on time scales from a few hours to a few days for several nearby bright AGNs. The observed N_H variations imply that the X-ray absorber is made of clouds eclipsing the X-ray source with velocities in excess of 10^3 km/s, and densities, sizes and distances from the central black hole typical of BLR clouds. We conclude that the variable X-ray absorption is due to the same clouds emitting the broad emission lines in the optical/UV. We then concentrate on the two highest signal-to-noise spectra of eclipses, discovered in two long observations of NGC 1365 and Mrk 766, and we show that the obscuring clouds have a cometary shape, with a high density head followed by a tail with decreasing N_H. Our results show that X-ray time resolved spectroscopy can be a powerful way to directly measure the physical and geometrical properties of BLR clouds.Comment: 8 Pages, 5 figures. Proceedings of IAU Symposium 267 "Co-Evolution of Central Black Holes and Galaxies

AGN Obscuration and the Unified Model

Unification Models of Active Galactic Nuclei postulate that all the observed differences between Type 1 and Type 2 objects are due to orientation effects with respect to the line-of-sight to the observer. The key ingredient of these models is the obscuring medium, historically envisaged as a toroidal structure on a parsec scale. However, many results obtained in the last few years are clearly showing the need for a more complex geometrical distribution of the absorbing media. In this paper we review the various pieces of evidence for obscuring media on different scales, from the vicinity of the black hole to the host galaxy, in order to picture an updated unification scenario explaining the complex observed phenomenology. We conclude by mentioning some of the open issues.Comment: 14 pages, 8 figures, review article accepted for publication on the special issue of Advances in Astronomy "Seeking for the Leading Actor on the Cosmic Stage: Galaxies versus Supermassive Black Holes

The physical relation between disc and coronal emission in quasars.

We propose a modified version of the observed non-linear relation between the X-ray (2 keV) and the ultraviolet (2,500 Å) emission in quasars (i.e., LX∝LγUV) which involves the full width at half-maximum, FWHM, of the broad emission line, i.e., LX∝LγˆUV FWHMβˆ. By analyzing a sample of 550 optically selected non-jetted quasars in the redshift range of 0.36–2.23 from the Sloan Digital Sky Survey cross matched with the XMM-Newton catalog 3XMM-DR6, we found that the additional dependence of the observed LX − LUV correlation on the FWHM of the MgII broad emission line is statistically significant. Our statistical analysis leads to a much tighter relation with respect to the one neglecting FWHM, and it does not evolve with redshift. We interpret this new relation within an accretion disc corona scenario where reconnection and magnetic loops above the accretion disc can account for the production of the primary X-ray radiation. For a broad line region size depending on the disc luminosity as Rblr∝L0.5disc, we find that LX∝L4/7UV FWHM4/7, which is in very good agreement with the observed correlation

Una nuova prova della teoria della relatività generale con i buchi neri rotanti

Le osservazioni del telescopio spaziale NuSTAR della NASA hanno permesso di rivelare per la prima volta la “firma” caratteristica di un buco nero rotante nella radiazione X emessa dal suo disco di accrescimento. Il risultato offre informazioni fondamentali per migliorare la nostra comprensione del ruolo dei buchi neri giganti nell’evoluzione delle galassie di cui fanno parte.Observations performed with the newest NASA space telescope, NuSTAR, have revealed for the first time the signature of a rotating black hole on the X-ray radiation emitted by its accretion disk. This result yields fundamental new insights into the role of giant black holes in the evolution of their host galaxies

The XMM-Newton long look of NGC 1365: lack of a high/soft state in its ultraluminous X-ray sources

Based on our long (~ 300 ks) 2007 XMM-Newton observation of the Seyfert galaxy NGC 1365, we report here on the spectral and timing behaviour of two ultraluminous X-ray sources, which had previously reached isotropic X-ray luminosities L_X ~ 4 x 10^{40} erg/s (0.3-10 keV band). In 2007, they were in a lower state (L_X ~ 5 x 10^{39} erg/s, and L_X ~ 1.5 x 10^{39} erg/s for X1 and X2, respectively). Their X-ray spectra were dominated by power-laws with photon indices Gamma ~ 1.8 and Gamma ~ 1.2, respectively. Thus, their spectra were similar to those at their outburst peaks. Both sources have been seen to vary by a factor of 20 in luminosity over the years, but their spectra are always dominated by a hard power-law; unlike most stellar-mass BHs, they have never been found in a canonical high/soft state dominated by a standard disk. The lack of a canonical high/soft state seems to be a common feature of ULXs. We speculate that the different kind of donor star and/or a persistently super-Eddington accretion rate during their outbursts may prevent accretion flows in ULXs from settling into steady standard disks.Comment: 9 pages, accepted by Ap

ERRATUM: "The Fifth Data Release Sloan Digital Sky Survey/XMM-Newton Quasar Survey" (2009, ApJS, 183, 17)

We have discovered an error in Column 9 of Table 2 in the original paper. This column reports the fit flags, indicating which model a source prefers: a simple power-law (SPL; flag = 1), a fixed power-law plus intrinsic absorption (FPL; flag = 2), or an absorbed power-law (APL; flag = 3). The table of the original paper mistakenly reports all flag = 3 sources as having flag = 2, and all flag = 2 sources as having flag = 1, so that only 32 sources prefer an absorption model. We have updated Table 2 to print out the correct fit flags for each source, resulting in 55 sources that prefer an absorption model. Since the fit flags determine which numbers are reported for the remaining columns of Table 2, these numbers are updated as well. The abstract and text of the original paper report the correct number of absorbed sources, so the conclusions are unaffected. In addition, a minor rounding error was found in the SDSS names of some objects, and so we replace both Tables 1 and 2 with corrected versions

Spatially resolved Fe K spectroscopy of NGC 4945

We present the imaging and spectroscopic analysis of the combined Chandra ACIS-S observations of the Compton-thick Seyfert 2 galaxy NGC 4945. We performed a spatially-resolved spectroscopy of the circumnuclear environment of the source, picturing the innermost 200 parsecs around the highly absorbed nucleus. The additional 200 ks ACIS-S data with respect to the previous campaign allowed us to map with even greater detail the central structure of this source and to discover an enhanced iron emission in the innermost nuclear region, with respect to the associated Compton reflection continuum. We revealed that the Equivalent Width of the iron K$\alpha$ line is spatially variable (ranging from 0.5 to 3 keV), on scales of tens of parsecs, likely due to the ionization state and orientation effects of the reprocessing material, with respect to the central X-ray illuminating source. A clump of highly ionized Fe XXV He-$\alpha$ is also detected, 40 parsecs east to the nucleus. When observations taken years apart are considered, the central unresolved reflected emission is found to remain constant.Comment: 10 pages, 6 figures, 4 tables, accepted for publication in MNRA