ALBERT

Description: We present a study of the X-ray flaring activity of Sgr A * during all the 150 XMM–Newton and Chandra observations pointed at the Milky Way centre over the last 15 years. This includes the latest XMM–Newton and Chandra campaigns devoted to monitoring the closest approach of the very red Br emitting object called G2. The entire data set analysed extends from 1999 September through 2014 November. We employed a Bayesian block analysis to investigate any possible variations in the characteristics (frequency, energetics, peak intensity, duration) of the flaring events that Sgr A * has exhibited since their discovery in 2001. We observe that the total bright or very bright flare luminosity of Sgr A * increased between 2013 and 2014 by a factor of 2–3 (~3.5 significance). We also observe an increase (~99.9 per cent significance) from 0.27 ± 0.04 to 2.5 ± 1.0 d –1 of the bright or very bright flaring rate of Sgr A * , starting in late summer 2014, which happens to be about six months after G2's pericentre passage. This might indicate that clustering is a general property of bright flares and that it is associated with a stationary noise process producing flares not uniformly distributed in time (similar to what is observed in other quiescent black holes). If so, the variation in flaring properties would be revealed only now because of the increased monitoring frequency. Alternatively, this may be the first sign of an excess accretion activity induced by the close passage of G2. More observations are necessary to distinguish between these two hypotheses.

Description: This paper presents a survey of X-ray-selected active galactic nuclei (AGNs) with optical spectroscopic follow-up in a ~ 18 deg 2 area of the equatorial XMM-XXL north field. A sample of 8445 point-like X-ray sources detected by XMM–Newton above a limiting flux of $F_{\rm 0.5{\rm -}10\, keV} 〉 10^{-15} \rm \,erg\, cm^{-2}\, s^{-1}$ was matched to optical (Sloan Digital Sky Survey, SDSS) and infrared (IR; WISE ) counterparts. We followed up 3042 sources brighter than r = 22.5 mag with the SDSS Baryon Oscillation Spectroscopic Survey (BOSS) spectrograph. The spectra yielded a reliable redshift measurement for 2578 AGNs in the redshift range z = 0.02–5.0, with 0.5-2 keV luminosities ranging from 10 39 -10 46 erg s – 1 . This is currently the largest published spectroscopic sample of X-ray-selected AGNs in a contiguous area. The BOSS spectra of AGN candidates show a distribution of optical line widths which is clearly bimodal, allowing an efficient separation between broad- and narrow-emission line AGNs. The former dominate our sample (70 per cent) due to the relatively bright X-ray flux limit and the optical BOSS magnitude limit. We classify the narrow-emission line objects (22 per cent of the full sample) using standard optical emission line diagnostics: the majority have line ratios indicating the dominant source of ionization is the AGN. A small number (8 per cent of the full sample) exhibit the typical narrow line ratios of star-forming galaxies, or only have absorption lines in their spectra. We term the latter two classes ‘elusive’ AGN, which would not be easy to identify correctly without their X-ray emission. We also compare X-ray ( XMM–Newton ), optical colour (SDSS) and and IR ( WISE ) AGN selections in this field. X-ray observations reveal, by far, the largest number of AGN. The overlap between the selections, which is a strong function of the imaging depth in a given band, is also remarkably small. We show using spectral stacking that a large fraction of the X-ray AGNs would not be selectable via optical or IR colours due to host galaxy contamination. A substantial fraction of AGN may therefore be missed by these longer wavelength selection methods.

Description: The northern tile of the wide-area and shallow XMM-XXL X-ray survey field is used to estimate the average dark matter halo mass of relatively luminous X-ray-selected active galactic nucleus (AGN) [ $\rm log\, {\it L}_X (\rm 2{\rm -}10\,keV)= 43.6^{+0.4}_{-0.4}\,erg\,s^{-1}$ ] in the redshift interval z  = 0.5–1.2. Spectroscopic follow-up observations of X-ray sources in the XMM-XXL field by the Sloan telescope are combined with the VIMOS Public Extragalactic Redshift Survey spectroscopic galaxy survey to determine the cross-correlation signal between X-ray-selected AGN (total of 318) and galaxies (about 20 000). We model the large scales (2–25 Mpc) of the correlation function to infer a mean dark matter halo mass of $\log M / (\mathrm{M}_{{\odot }} \, h^{-1}) = 12.50 ^{+0.22} _{-0.30}$ for the X-ray-selected AGN sample. This measurement is about 0.5 dex lower compared to estimates in the literature of the mean dark matter halo masses of moderate-luminosity X-ray AGN [ L X (2-10 keV) 10 42 -10 43 erg s – 1 ] at similar redshifts. Our analysis also links the mean clustering properties of moderate-luminosity AGN with those of powerful ultraviolet/optically selected QSOs, which are typically found in haloes with masses few times 10 12 M . There is therefore evidence for a negative luminosity dependence of the AGN clustering. This is consistent with suggestions that AGN have a broad dark matter halo mass distribution with a high mass tail that becomes subdominant at high accretion luminosities. We further show that our results are in qualitative agreement with semi-analytic models of galaxy and AGN evolution, which attribute the wide range of dark matter halo masses among the AGN population to different triggering mechanisms and/or black hole fuelling modes.

Description: We report the discovery of a new ‘changing-look' quasar, SDSS J101152.98+544206.4, through repeat spectroscopy from the Time Domain Spectroscopic Survey. This is an addition to a small but growing set of quasars whose blue continua and broad optical emission lines have been observed to decline by a large factor on a time-scale of approximately a decade. The 5100 Å monochromatic continuum luminosity of this quasar drops by a factor of 〉9.8 in a rest-frame time interval of 〈9.7 yr, while the broad Hα luminosity drops by a factor of 55 in the same amount of time. The width of the broad Hα line increases in the dim state such that the black hole mass derived from the appropriate single-epoch scaling relation agrees between the two epochs within a factor of 3. The fluxes of the narrow emission lines do not appear to change between epochs. The light curve obtained by the Catalina Sky Survey suggests that the transition occurs within a rest-frame time interval of approximately 500 d. We examine three possible mechanisms for this transition suggested in the recent literature. An abrupt change in the reddening towards the central engine is disfavoured by the substantial difference between the time-scale to obscure the central engine and the observed time-scale of the transition. A decaying tidal disruption flare is consistent with the decay rate of the light curve but not with the prolonged bright state preceding the decay; nor can this scenario provide the power required by the luminosities of the emission lines. An abrupt drop in the accretion rate on to the supermassive black hole appears to be the most plausible explanation for the rapid dimming.

Description: We present X-shooter at Very Large Telescope observations of a sample of 10 luminous, X-ray obscured quasi-stellar objects (QSOs) at z  ~ 1.5 from the XMM -COSMOS survey, expected to be caught in the transitioning phase from starburst to active galactic nucleus (AGN)-dominated systems. The main selection criterion is X-ray detection at bright fluxes ( L X   10 44  erg s –1 ) coupled to red optical-to-near-infrared-to-mid-infrared colours. Thanks to its large wavelength coverage, X-shooter allowed us to determine accurate redshifts from the presence of multiple emission lines for five out of six targets for which we had only a photometric redshift estimate, with an 80 per cent success rate, significantly larger than what is observed in similar programs of spectroscopic follow-up of red QSOs. We report the detection of broad and shifted components in the [O iii ] 5007, 4959 complexes for six out of eight sources with these lines observable in regions free from strong atmospheric absorptions. The full width at half-maximum (FWHM) associated with the broad components are in the range FWHM ~ 900–1600 km s –1 , larger than the average value observed in Sloan Digital Sky Survey type 2 AGN samples at similar observed [O iii ] luminosity, but comparable to those observed for QSO/ultraluminous infrared galaxies systems for which the presence of kpc scale outflows has been revealed through integral field unit spectroscopy. Although the total outflow energetics (inferred under reasonable assumptions) may be consistent with winds accelerated by stellar processes, we favour an AGN origin for the outflows given the high outflow velocities observed ( v  〉 1000 km s –1 ) and the presence of strong winds also in objects undetected in the far-infrared.

Description: Massive black hole binaries are naturally predicted in the context of the hierarchical model of structure formation. The binaries that manage to lose most of their angular momentum can coalesce to form a single remnant. In the last stages of this process, the holes undergo an extremely loud phase of gravitational wave emission, possibly detectable by current and future probes. The theoretical effort towards obtaining a coherent physical picture of the binary path down to coalescence is still underway. In this paper, for the first time, we take advantage of observational studies of active galactic nuclei evolution to constrain the efficiency of gas-driven binary decay. Under conservative assumptions we find that gas accretion towards the nuclear black holes can efficiently lead binaries of any mass forming at high redshift (2) to coalescence within the current time. The observed ‘downsizing’ trend of the accreting black hole luminosity function further implies that the gas inflow is sufficient to drive light black holes down to coalescence, even if they bind in binaries at lower redshifts, down to z   0.5 for binaries of ~10 7 M , and z   0.2 for binaries of ~10 6 M . This has strong implications for the detection rates of coalescing black hole binaries of future space-based gravitational wave experiments.

Description: We present a census of the active black hole population at 1 〈  z  〈 2, by constructing the bivariate distribution function of black hole mass and Eddington ratio, employing a maximum likelihood fitting technique. The study of the active black hole mass function (BHMF) and the Eddington ratio distribution function (ERDF) allows us to clearly disentangle the active galactic nuclei (AGN) downsizing phenomenon, present in the AGN luminosity function, into its physical processes of black hole mass downsizing and accretion rate evolution. We are utilizing type-1 AGN samples from three optical surveys (VVDS, zCOSMOS and SDSS), that cover a wide range of 3 dex in luminosity over our redshift interval of interest. We investigate the cosmic evolution of the AGN population as a function of AGN luminosity, black hole mass and accretion rate. Compared to z  = 0, we find a distinct change in the shape of the BHMF and the ERDF, consistent with downsizing in black hole mass. The active fraction or duty cycle of type-1 AGN at z  ~ 1.5 is almost flat as a function of black hole mass, while it shows a strong decrease with increasing mass at z  = 0. We are witnessing a phase of intense black hole growth, which is largely driven by the onset of AGN activity in massive SMBHs (supermassive black holes) towards z  = 2. We finally compare our results to numerical simulations and semi-empirical models and while we find reasonable agreement over certain parameter ranges, we highlight the need to refine these models in order to match our observations.

Description: In this paper we describe and publicly release a catalogue consisting of 8445 point-like X-ray sources detected in the XMM-XXL north survey. For the 2512 AGN which have reliable spectroscopy from SDSS-III/BOSS, we present the X-ray spectral fitting which has been computed with a Bayesian approach. We have also applied an X-ray spectral stacking method to different sub-samples, selected on the basis of the AGN physical properties ( L 2–10 keV , z, M BH , Edd and N H ). We confirm the well-known Iwasawa–Taniguchi effect in our luminosity–redshift sub-samples, and argue that such an effect is due to a decrease in the covering factor of a distant obscuring ‘torus’ with increasing X-ray luminosity. By comparing the distribution of the reflection fraction, the ratio of the normalization of the reflected component to the direct radiation, we find that the low-luminosity, low-redshift sub-sample had systematically higher reflection fraction values than the high-redshift, high-luminosity one. On the other hand, no significant difference is found between samples having similar luminosity but different redshift, suggesting that the structure of the torus does not evolve strongly with redshift. Contrary to previous works, we do not find evidence for an increasing photon index at high Eddington ratio. This may be an indication that the structure of the accretion disc changes as the Eddington ratio approaches unity. Comparing our X-ray spectral analysis results with the optical spectral classification, we find that ~20 per cent of optical type-1 AGN show an X-ray absorbing column density higher than 10 21.5 cm – 2 , and about 50 per cent of type-2 AGN have an X-ray absorbing column density less than 10 21.5 cm – 2 . We suggest that the excess X-ray absorption shown in the high-luminosity optical type-1 AGN can be due to small-scale dust-free gas within (or close to) the broad-line region, while in the low-luminosity ones it can be due to a clumpy torus with a large covering factor.

Description: We present the X-ray spectral analysis of the 390 brightest extragalactic sources in the Chandra -Cosmic Evolution Survey catalogue, showing at least 70 net counts in the 0.5–7 keV band. This sample has a 100 per cent completeness in optical–infrared identification, with ~75 per cent of the sample having a spectroscopic redshift and ~25 per cent a photometric redshift. Our analysis allows us to accurately determine the intrinsic absorption, the broad-band continuum shape () and intrinsic L 2–10 distributions, with an accuracy better than 30 per cent on the spectral parameters for 95 per cent of the sample. The sample is equally divided in type 1 (49.7 per cent) and type 2 active galactic nuclei (48.7 per cent) plus few passive galaxies at low z . We found a significant difference in the distribution of of type 1 and type 2, with small intrinsic dispersion, a weak correlation of with L 2–10 and a large population (15 per cent of the sample) of high luminosity, highly obscured (QSO2) sources. The distribution of the X-ray/Optical flux ratio (Log( F X / F i )) for type 1 is narrow (0 〈 X/O 〈 1), while type 2 are spread up to X/O = 2. The X/O correlates well with the amount of X-ray obscuration. Finally, a small sample of Compton-thick candidates and peculiar sources is presented. In the appendix, we discuss the comparison between Chandra and XMM–Newton spectra for 280 sources in common. We found a small systematic difference, with XMM–Newton spectra that tend to have softer power laws and lower obscuration.

Description: SPIDERS (The SPectroscopic IDentification of eROSITA Sources) is a programme dedicated to the homogeneous and complete spectroscopic follow-up of X-ray active galactic nuclei and galaxy clusters over a large area (~7500 deg 2 ) of the extragalactic sky. SPIDERS is part of the Sloan Digital Sky Survey (SDSS)-IV project, together with the Extended Baryon Oscillation Spectroscopic Survey and the Time-Domain Spectroscopic Survey. This paper describes the largest project within SPIDERS before the launch of eROSITA : an optical spectroscopic survey of X-ray-selected, massive (~10 14 –10 15 M ) galaxy clusters discovered in ROSAT and XMM–Newton imaging. The immediate aim is to determine precise ( z  ~ 0.001) redshifts for 4000–5000 of these systems out to z  ~ 0.6. The scientific goal of the program is precision cosmology, using clusters as probes of large-scale structure in the expanding Universe. We present the cluster samples, target selection algorithms and observation strategies. We demonstrate the efficiency of selecting targets using a combination of SDSS imaging data, a robust red-sequence finder and a dedicated prioritization scheme. We describe a set of algorithms and work-flow developed to collate spectra and assign cluster membership, and to deliver catalogues of spectroscopically confirmed clusters. We discuss the relevance of line-of-sight velocity dispersion estimators for the richer systems. We illustrate our techniques by constructing a catalogue of 230 spectroscopically validated clusters (0.031 〈  z  〈 0.658), found in pilot observations. We discuss two potential science applications of the SPIDERS sample: the study of the X-ray luminosity-velocity dispersion ( L X –) relation and the building of stacked phase-space diagrams.