Ensemble spectral variability study of Active Galactic Nuclei from the XMM-Newton serendipitous source catalogue

The variability of the X-ray spectra of active galactic nuclei (AGN) usually includes a change of the spectral slope. This has been investigated for a small sample of local AGNs by Sobolewska and Papadakis, who found that slope variations are well correlated with flux variations, and that spectra are typically steeper in the bright phase (softer when brighter behaviour). Not much information is available for the spectral variability of high-luminosity AGNs and quasars. In order to investigate this phenomenon, we use data from the XMM-Newton Serendipitous Source Catalogue, Data Release 5, which contains X-ray observations for a large number of active galactic nuclei in a wide luminosity and redshift range, for several different epochs. This allows to perform an ensemble analysis of the spectral variability for a large sample of quasars. We quantify the spectral variability through the spectral variability parameter $\beta$, defined as the ratio between the change in spectral slope and the corresponding logarithmic flux variation. We find that the spectral variability of quasars has a softer when brighter behaviour, similarly to local AGNs.Comment: 5 pages, 2 figures. Prepared for the proceedings of the 6th Young Researchers Meeting, L'Aquil

The X-ray/UV ratio in Active Galactic Nuclei: dispersion and variability

The relation between the $\alpha_{OX}$ index and the optical/UV luminosity ($L_{UV}$), a by product of the X-ray - optical/UV luminosity relation, is affected by a relatively large dispersion, due to variability in the $\alpha_{OX}$ within single sources (intra-source dispersion) and variations of fundamental physical parameters from source to source (inter-source dispersion). We use archival data from the XMMSSC and from the XMMOM-SUSS3. We select a sub-sample in order to decrease the dispersion of the relation due to the presence of Radio-Loud and Broad Absorption Line objects, and to absorptions in both X-ray and optical/UV bands. We analyse the dependence of the residuals of the relation on various physical parameters in order to characterise the inter-source dispersion. We find a total dispersion of ~0.12 and, using the Structure Function, we find that intrinsic variability contributes for 56% of the variance of the relation. We find weak but significant dependences of the residuals of the relation on black-hole (BH) mass and on Eddington ratio, confirmed by a multivariate regression analysis of $\alpha_{OX}$ as a function of optical/UV luminosity and the above quantities. We find a weak positive correlation of both the $\alpha_{OX}$ and the residuals of the relation with inclination indicators (FWHM(H$\beta$) and EW[O$_{III}$]) suggesting a weak increase of X-ray/UV ratio with the viewing angle. Our results suggest the possibility of selecting a sample of objects, based on their viewing angle and/or BH mass and Eddington ratio, for which the $\alpha_{OX}-L_{UV}$ relation is as tight as possible, in light of the use of the X-ray - optical/UV luminosity relation to build a distance modulus (DM) - $z$ plane and estimate cosmological parameters.Comment: accepted for publication in A&

XMM-Newton observation of the cluster ZW 1305.4+2941 in the field SA 57

We report the details of an XMM observation of the cluster of galaxies ZW 1305.4+2941 at the intermediate redshift of z=0.241, increasing the small number of interesting X-ray constraints on properties of ~3 keV systems above z=0.1. Based on the 45 ks XMM observation, we find that within a radius of 228 kpc the cluster has an unabsorbed X-ray flux of 2.07 +/- 0.06 x 10^{-13} erg/cm^2/s, a temperature of kT = 3.17 +/-0.19 keV, in good agreement with the previous ROSAT determination, and an abundance of 0.93 (+0.24,-0.29} solar. Within r_500 = 723 +/- 6 kpc the rest-frame bolometric X-ray luminosity is L_X (r_500)= 1.25 +/- 0.16 x 10^{44} erg/s. The cluster obeys the scaling relations for L_X, T and the velocity dispersion derived at intermediate redshift for kT < 4 keV, for which we provide new fits for all literature objects. The mass derived from an isothermal NFW model fit is, M_vir = 2.77 +/- 0.21 x 10^{14} solar masses, with a concentration parameter, c = 7.9 +/- 0.5.Comment: 9 pages, 7 colour figures, accepted for publication in ApJ. Corrected typo on the fraction of blue galaxie

The MEXSAS2 Sample and the Ensemble X-ray Variability of Quasars

We present the second Multi-Epoch X-ray Serendipitous AGN Sample (MEXSAS2), extracted from the 6th release of the XMM Serendipitous Source Catalogue (XMMSSC-DR6), cross-matched with Sloan Digital Sky Survey quasar catalogues DR7Q and DR12Q. Our sample also includes the available measurements for masses, bolometric luminosities, and Eddington ratios. Analyses of the ensemble structure function and spectral variability are presented, together with their dependences on such parameters. We confirm a decrease of the structure function with the X-ray luminosity, and find a weak dependence on the black hole mass. We introduce a new spectral variability estimator, taking errors on both fluxes and spectral indices into account. We confirm an ensemble softer when brighter trend, with no dependence of such estimator on black hole mass, Eddington ratio, redshift, X-ray and bolometric luminosity.Comment: 6 pages, 3 figures, to appear in Frontiers in Astronomy and Space Science

X-ray analysis of the accreting supermassive black hole in the radio galaxy PKS 2251+11

We investigate the dichotomy between jetted and non-jetted Active Galactic Nuclei (AGNs), focusing on the fundamental differences of these two classes in the accretion physics onto the central supermassive black hole (SMBH). Our aim is to study and constrain the structure, kinematics and physical state of the nuclear environment in the Broad Line Radio Galaxy (BLRG) PKS 2251+11. The high X-ray luminosity and the relative proximity make such AGN an ideal candidate for a detailed analysis of the accretion regions in radio galaxies. We performed a spectral and timing analysis of a $\sim$64 ks observation of PKS 2251+11 in the X-ray band with XMM-Newton. We modeled the spectrum considering an absorbed power law superimposed to a reflection component. We performed a time-resolved spectral analysis to search for variability of the X-ray flux and of the individual spectral components. We found that the power law has a photon index $\Gamma=1.8\pm 0.1$, absorbed by an ionized partial covering medium with a column density $N_H=(10.1\pm 0.8) \times 10^{23}$ cm$^{-2}$, a ionization parameter $\log{\xi}=1.3\pm 0.1$ erg s$^{-1}$ cm and a covering factor $f\simeq90\%$. Considering a density of the absorber typical of the Broad Line Region (BLR), its distance from the central SMBH is of the order of $r\sim 0.1$ pc. An Fe K$\alpha$ emission line is found at 6.4 keV, whose intensity shows variability on time scales of hours. We derived that the reflecting material is located at a distance $r\gtrsim600r_s$, where $r_s$ is the Schwarzschild radius. Concerning the X-ray properties, we found that PKS 2251+11 does not differ significantly from the non-jetted AGNs, confirming the validity of the unified model in describing the inner regions around the central SMBH, but the lack of information regarding the state of the very innermost disk and SMBH spin still leave unconstrained the origin of the jet

The NuSTAR view of the Seyfert Galaxy HE 0436-4717

We present the multi epoch spectral analysis of HE 0436-4717, a bright Seyfert 1 galaxy serendipitously observed by the high energy satellite NuSTAR four times between December 2014 and December 2015. The source flux shows modest variability within each pointing and among the four observations. Spectra are well modelled in terms of a weakly variable primary power law with constant photon index ($\Gamma$=2.01$\pm$0.08). A constant narrow \ion{Fe} K$\alpha$ emission line suggests that this feature has an origin far from the central black hole, while a broad relativistic component is not required by the data. The Compton reflection component is also constant in flux with a corresponding reflection fraction R=0.7$^{+0.2}_{-0.3}$. The iron abundance is compatible with being Solar (A$_{Fe}$=1.2$^{+1.4}_{-0.4}$), and a lower limit for the high energy cut-off E$_c$>280 keV is obtained. Adopting a self-consistent model accounting for a primary Comptonized continuum, we obtain a lower limit for the hot corona electron temperature kT$_e$>65 keV and a corresponding upper limit for the coronal optical depth of $\tau_e$<1.3. The results of the present analysis are consistent with the locus of local Seyfert galaxies in the kT$_e$-$\tau_e$ and temperature-compactness diagrams.Comment: accepted for publication in A&

Spectral Slope Variability of BL Lac Objects in the Optical Band

Light curves of eight BL Lac objects in the BVRI bands have been analyzed. All of the objects tend to be bluer when brighter. However spectral slope changes differ quantitatively from those of a sample of QSOs analyzed in a previous paper (Trevese & Vagnetti 2002) and appear consistent with a different nature of the optical continuum. A simple model representing the variability of a synchrotron component can explain the spectral changes. Constraints on a possible thermal accretion disk component contributing to the optical luminosity are discussed.Comment: 15 pages, 5 figures, to appear in ApJ, June 200

BeppoSAX Observations of 2 Jy Lobe-dominated Broad-Line Sources: the Discovery of a Hard X-ray Component

We present new BeppoSAX LECS, MECS, and PDS observations of five lobe-dominated, broad-line active galactic nuclei selected from the 2 Jy sample of southern radio sources. These include three radio quasars and two broad-line radio galaxies. ROSAT PSPC data, available for all the objects, are also used to better constrain the spectral shape in the soft X-ray band. The collected data cover the 0.1 - 10 keV energy range, reaching 40 keV for one source. Detailed spectral fitting shows that all sources have a flat hard X-ray spectrum with energy index alpha_x ~ 0.75 in the 2 - 10 keV energy range. This is a new result, which is at variance with the situation at lower energies where these sources exhibit steeper spectra. Spectral breaks ~0.5 at 1 - 2 keV characterize the overall X-ray spectra of our objects. The flat, high-energy slope is very similar to that displayed by flat-spectrum/core-dominated quasars, which suggests that the same emission mechanism (most likely inverse Compton) produces the hard X-ray spectra in both classes. Contrary to the optical evidence for some of our sources, no absorption above the Galactic value is found in our sample. Finally, a (weak) thermal component is also present at low energies in the two broad-line radio galaxies included in our study.Comment: 4 pages, LateX, 3 figures. Uses espcrc2.sty. To appear in: "The Active X-ray Sky: Results from BeppoSAX and Rossi-XTE", Rome, Italy, 21-24 October, 1997, Eds.: L. Scarsi, H. Bradt, P. Giommi and F. Fior

Quasar Spectral Slope Variability in the Optical Band

We performed a new analysis of B and R light curves of a sample of PG quasars. We confirm the variability-redshift correlation and its explanation in terms of spectral variability, coupled with the increase of rest-frame observing frequency for quasars at high redshift. The analysis of the instantaneous spectral slope for the whole quasar samples indicates both an inter-QSO and intra-QSO variability-luminosity correlation. Numerical simulations show that the latter correlation cannot be entirely due to the addition of the host galaxy emission to a nuclear spectrum of variable luminosity but constant shape, implying a spectral variability of the nuclear component. Changes of accretion rate are also insufficient to explain the amount of spectral variation, while hot spots possibly caused by local disk instabilities can explain the observations.Comment: 20 pages, 6 figures, to appear in Ap.J., January 200