G. Risaliti

Occultation Measurement of the Size of the X-Ray-emitting Region in the Active Galactic Nucleus of NGC 1365

We present an occultation of the central X-ray-emitting region in the Seyfert galaxy NGC 1365. This extreme spectral variation (from Compton-thin to reflection-dominated and back to Compton-thin in 4 days) has been caught in a 10 day Chandra monitoring campaign consisting of six short (15 ks) observations performed every 2 days. We discuss the implications of this occultation within the scenario of a Compton-thick cloud crossing the line of sight of the X-ray source. We estimate a source size R ≤ 1014 cm and a distance of the cloud from the source D ≤ 1016 cm. This direct measurement confirms the theoretical expectations of an extremely compact X-ray source and shows that the Compton-thick circumnuclear gas is located at a distance from the center on the scale of the broad-line region.

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

We present two Rossi X-Ray Timing Explorer (RXTE) Proportional Counter Array (PCA) observations of the type 2 Seyfert galaxy NGC 4388 caught in an unusual low X-ray absorption state. The observations were triggered by a detection in the 1.5-3 keV band of the RXTE all-sky monitor. NGC 4388 was found at a somewhat high continuum level [f(2-10 keV) = 8 × 10-11 ergs cm-2 s-1] and with a column density NH ~ 3 × 1022 cm-2, a factor of ~10 lower than normal. The second PCA observation, 4 hr later, gave NH < 2 × 1021 cm-2 indicating, at the 3.1 σ level, variability so rapid it puts the absorber on a few 100 Schwarzschild radii scale, similar to the broad emission line region or smaller. This small scale creates difficulties for the parsec-scale obscuring torus paradigm of unified schemes for type 1 and type 2 active galactic nuclei.

Rapid NH changes in NGC 4151

We have analysed the two longest (elapsed time 3d )BeppoSAX observations of the X-ray brightest Seyfert galaxy, NGC 4151, to search for spectral variability on time-scales from a few tens of kiloseconds to years. We found in both cases highly significant spectral variability below ≈6 keV down to the shortest time-scales investigated. These variations can be naturally explained in terms of variations in the low energy cut-off due to obscuring matter along the line of sight. If the cut-off is modelled by two neutral absorption components, one fully covering the source and the second covering only a fraction of the source, the shortest time-scale of variability of a few days constrains the location of the obscuring matter to within 3.4 × 10 4 Schwarzschild radii from the central X-ray source. This is consistent with the distance of the broad emission-line region, as inferred from reverberation mapping, and difficult to reconcile with the parsec scale dusty molecular torus of Krolik & Begelman. We have also explored a more complex absorption structure, namely the presence of an ionized absorber. Although the behaviour of the ionization parameter is nicely consistent with the expectations, the results are not completely satisfactory from the statistical point of view. The overall absorption during the 2001 December observation is lower than in all other historical observations with similar 2‐10 keV flux. This suggests that absorption variability plays a crucial role in the observed flux variability of this source.

Enhanced star formation in narrow-line Seyfert 1 active galactic nuclei revealed by Spitzer

We present new low-resolution Spitzer mid-infrared spectroscopy of a sample of 20 ROSAT-selected local narrow-line Seyfert 1 galaxies (NLS1s). We detect strong active galactic nucleus (AGN) continuum in all and clear polycyclic aromatic hydrocarbon (PAH) emission in 70 per cent of the sources. The 6.2 μm PAH luminosity spans three orders of magnitude, from ∼10 39 to ∼10 42 erg s -1 providing strong evidence for intense ongoing star formation in the circumnuclear regions of these sources. Using the Infrared Spectrograph/Spitzer archive, we gathered a large number of additional NLS 1 s and their broad-line counterparts (BLS1s) and constructed NLS1 and BLS I subsamples to compare them in various ways. The comparison shows a clear separation according to full width at half-maximum (Hβ) [FWHM(Hβ)] such that objects with narrower broad Hβ lines are the strongest PAH emitters. We test this division in various ways trying to remove biases due to luminosity and aperture size. Specifically, we find that star formation activity around NLS1 AGN is larger than around BLS1 of the same AGN luminosity. The above result seems to hold over the entire range of distance and luminosity. Moreover, the star formation rate is higher in low black hole mass and high L/L Edd systems indicating that black hole growth and star formation are occurring simultaneously.

The role of nuclear activity as the power source of ultraluminous infrared galaxies

We present the results of a 5–8xa0μm spectral analysis performed on the largest sample of local ultraluminous infrared galaxies (ULIRGs) selected so far, consisting of 164 objects up to a redshift of ∼0.35. The unprecedented sensitivity of the Infrared Spectrograph onboard Spitzer allowed us to develop an effective diagnostic method to quantify the active galactic nucleus (AGN) and starburst (SB) contribution to this class of objects. The large AGN over SB brightness ratio at 5–8xa0μm and the sharp difference between the spectral properties of AGN and SB galaxies in this wavelength range make it possible to detect even faint or obscured nuclear activity, and disentangle its emission from that of star formation. By defining a simple model we are also able to estimate the intrinsic bolometric corrections for both the AGN and SB components, and obtain the relative AGN/SB contribution to the total luminosity of each source. Our main results are the following. n n n n1 nThe AGN detection rate among local ULIRGs amounts up to 70 per cent, with 113/164 convincing detections within our sample, while the global AGN/SB power balance is ∼1/3. n n2 nA general agreement is found with optical classification; however, among the objects with no spectral signatures of nuclear activity, our IR diagnostics find a subclass of elusive, highly obscured AGN. n n3 nWe analyse the correlation between nuclear activity and IR luminosity, recovering the well-known trend of growing AGN significance as a function of the overall energy output of the system: the sources exclusively powered by star formation are mainly found at LIR < 1012.3xa0 L⊙, while the average AGN contribution rises from ∼10 to ∼60 per cent across the ULIRG luminosity range. n n4 nFrom a morphological point of view, we confirm that the AGN content is larger in compact systems, but the link between activity and evolutionary stage is rather loose. n n5 nBy analysing a control sample of IR-luminous galaxies around z∼ 1, we find evidence for only minor changes with redshift of the large-scale spectral properties of the AGN and SB components. This underlines the potential of our method as a straightforward and quantitative AGN/SB diagnostic tool for ULIRG-like systems at high redshift as well, and hints to possible photometric variants for fainter sources.

THE SLOAN DIGITAL SKY SURVEY/XMM-NEWTON QUASAR SURVEY: CORRELATION BETWEEN X-RAY SPECTRAL SLOPE AND EDDINGTON RATIO

We present a correlation between the 2-10 keV spectral slope {gamma} {sub X} and the Eddington ratio L/L{sub EDD} in a sample of {approx}400 Sloan Digital Sky Survey (SDSS) quasars with available hard X-ray spectra from XMM-Newton serendipitous observations. We find that the {gamma} {sub X}-L/L {sub EDD} correlation is strongest in objects with black hole (BH) masses determined from the H{beta} line, and weaker (but still present) for those based on Mg II. An empirical nonlinear correction of the Mg II-based masses, obtained by comparing the mass estimates in SDSS quasars having both H{beta} and Mg II measurements, significantly increases the strength of the correlation. No correlation is found among objects with BH masses derived from C IV, confirming that this line is not a reliable indicator of the BH mass. No significant correlation is found with the bolometric luminosity, while a {gamma} {sub X}-M {sub BH} relation is present, though with a lower statistical significance than between {gamma} {sub X} and L/L {sub EDD}. Our results imply a physical link between the accretion efficiency in the (cold) accretion disk of active galactic nuclei and the physical status of the (hot) corona responsible for the X-ray emission.

Highly Ionized Iron Absorption Lines from Outflowing Gas in the X-Ray Spectrum of NGC 1365

We present the discovery of four absorption lines in the X-ray spectrum of the Seyfert galaxy NGC 1365, at energies between 6.7 and 8.3 keV. The lines are detected with high statistical confidence (from >20 σ for the strongest to ~4 σ for the weakest) in two XMM-Newton observations 60 ks long. We also detect the same lines, with a lower signal-to-noise ratio (but still >2 σ for each line), in two previous shorter (~10 ks) XMM-Newton observations. The spectral analysis identifies these features as Fe XXV and Fe XXVI Kα and Kβ lines, outflowing with velocities varying between ~1000 and ~5000 km s-1 among the observations. These are the highest quality detections of such lines so far. The high equivalent widths [EW(Kα) ~ 100 eV] and the Kα/Kβ ratios imply that the lines are due to absorption of the AGN continuum by a highly ionized gas with column density NH ~ 5 × 1023 cm-2 at a distance of ~(50-100)RS from the continuum source.

THE X-RAY ENERGY DEPENDENCE OF THE RELATION BETWEEN OPTICAL AND X-RAY EMISSION IN QUASARS

We develop a new approach to the well-studied anti-correlation between the optical-to-X-ray spectral index, αox, and the monochromatic optical luminosity, l opt. By cross-correlating the Sloan Digital Sky Survey DR5 quasar catalog with the XMM-Newton archive, we create a sample of 327 quasars with X-ray signal-to-noise ratio (S/N) > 6, where both optical and X-ray spectra are available. This allows αox to be defined at arbitrary frequencies, rather than the standard 2500 A and 2 keV. We find that while the choice of optical wavelength does not strongly influence the αox-l opt relation, the slope of the relation does depend on the choice of X-ray energy. The slope of the relation becomes steeper when αox is defined at low (~1 keV) X-ray energies. This change is significant when compared to the slope predicted by a decrease in the baseline over which αox is defined. The slopes are also marginally flatter than predicted at high (~10 keV) X-ray energies. Partial correlation tests show that while the primary driver of αox is l opt, the Eddington ratio correlates strongly with αox when l opt is taken into account, so accretion rate may help explain these results. We combine the αox-l opt and Γ-L bol/L Edd relations to naturally explain two results: (1) the existence of the Γ-lx relation as reported in Young et al., and (2) the lack of a Γ-l opt relation. The consistency of the optical/X-ray correlations establishes a more complete framework for understanding the relation between quasar emission mechanisms. We also discuss two correlations with the hard X-ray bolometric correction, which we show correlates with both αox and Eddington ratio. This confirms that an increase in accretion rate correlates with a decrease in the fraction of up-scattered disk photons.

THE FIFTH DATA RELEASE SLOAN DIGITAL SKY SURVEY/XMM-NEWTON QUASAR SURVEY

We present a catalog of 792 Fifth Data Release Sloan Digital Sky Survey quasars with optical spectra that have been observed serendipitously in the X-rays with the XMM-Newton. These quasars cover a redshift range of z = 0.11-5.41 and a magnitude range of i = 15.3-20.7. Substantial numbers of radio-loud (70) and broad absorption line (51) quasars exist within this sample. Significant X-ray detections at ≥2σ account for 87% of the sample (685xa0quasars), and 473xa0quasars are detected at ≥6σ, sufficient to allow X-ray spectral fits. For detected sources, ~60% have X-ray fluxes between F 2–10 keV = (1-10) ×10–14 erg cm–2 s–1. We fit a single power law, a fixed power law with intrinsic absorption left free to vary, and an absorbed power-law model to all quasars with X-ray signal-to-noise ratio ≥ 6, resulting in a weighted mean photon index Γ = 1.91 ± 0.08, with an intrinsic dispersion σΓ = 0.38. For the 55xa0sources (11.6%) that prefer intrinsic absorption, we find a weighted mean NH = 1.5 ± 0.3 × 1021 cm–2. We find that Γ correlates significantly with optical color, Δ(g – i), the optical-to-X-ray spectral index (αox), and the X-ray luminosity. While the first two correlations can be explained as artifacts of undetected intrinsic absorption, the correlation between Γ and X-ray luminosity appears to be a real physical correlation, indicating a pivot in the X-ray slope.

Final verdict from XMM–Newton: the X‐ray obscured Seyfert galaxy NGC 5506 has a broad Fe Kα line

We present the first unambiguous evidence of a broad (Gaussian width ∼330 eV) component of the iron Kα fluorescent emission line in the X-ray obscured Narrow Line Seyfert 1 Galaxy NGC 5506. This is the main results of a spectroscopic monitoring campaign on this source performed with the XMM-Newton observatory between February 2001 and January 2009. The broad line lacks extreme redwards skewness. If modelled with a relativistic component, the profile of the line is consistent with a flat emissivity radial dependence (α ≃1.9). The disk inclination (≃40◦) is nominally larger then typically observed in unobscured AGN, in agreement with most measurements of broadened iron lines in Seyfert 2 galaxies. The quality of the data allows us to decompose the full iron emission line complex, and to study its long-term (timescales of weeks to years) variability pattern. The intensity of the neutral and narrow iron Kα core remains constant during the monitoring campaign. This indicates that the optically thick gas responsible for the non-relativistic reprocessing of the primary AGN continuum in NGC 5506 is probably located in the torus rather than in the optical Broad Line Region.