Person:

Wilkes, Belinda

We present the results of the VLA radio observations at 1.475 GHz (20 cm) of the active galactic nuclei (AGN) in the Cambridge-Cambridge ROSAT Serendipity Survey (CRSS), a sample of 123 faint X-ray sources with fx(0.5−2.0keV)≥2×10−14ergs−1cm−2 fx(0.5−2.0keV)≥2×10−14ergs−1cm−2 . Of the 80 AGN in the sample, seven show radio emission at the 5 σ level and only two ( 2.5+4.0–1.7 2.5–1.7+4.0 per cent) qualify as radio-loud (RL) objects (αro ≥ 0.35). This result, compared with 13 per cent RL in the Einstein Observatory Extended Medium Sensitivity Survey (EMSS) sample of AGN [flux limit fx(0.3−3.5keV)∼2×10−13ergs−1cm−2 fx(0.3−3.5keV)∼2×10−13ergs−1cm−2 ], confirms that the fraction of X-ray-selected RLAGN drops rapidly as the X-ray flux limit is lowered. Combining the CRSS AGN sample with that extracted from the EMSS we study the X-ray luminosity function (XLF) and evolutionary properties for radio-quiet (RQ) and RLAGN separately. We find that the RQ and RLAGN populations show the same cosmological evolution within the errors. In fact, when the luminosity evolution is parametrized with a power law of the form L∗x(z)=L∗x(0)(1+z)k Lx∗(z)=Lx∗(0)(1+z)k , we find k = 2.43 ± 0.26 and 2.71 ± 0.10 for RL and RQ AGN populations respectively. In addition, the shapes of the de-evolved XLFs of the two classes appear to be different at both the low-luminosity (Lx < 1044 erg s–1) and high-luminosity ends. These results are robust for different cosmological models (using q0 = 0.0 and 0.5) and for different values of the threshold αro used to distinguish between RQ and RL objects. Finally, we find that the differences in the shapes of the XLFs of RQ and RLAGN can be explained by introducing an X-ray beaming model to separate the observed X-ray luminosity of radio quasars into relativistically beamed and isotropic contributions.

The unification model for powerful radio galaxies (RGs) and radio-loud quasars postulates that these objects are intrinsically the same but viewed along different angles. Herschel Space Observatory data permit the assessment of that model in the far-infrared spectral window. We analyze photometry from Spitzer and Herschel for the distant 3CR hosts, and find that RGs and quasars have different mid-infrared, but indistinguishable far-infrared colors. Both these properties, the former being orientation dependent and the latter orientation invariant, are in line with expectations from the unification model. Adding powerful radio-quiet active galaxies and typical massive star-forming (SF) galaxies to the analysis, we demonstrate that infrared colors not only provide an orientation indicator, but can also distinguish active from SF galaxies.

Within 40 years of the detection of the first extrasolar X-ray source in 1962,NASA's Chandra X-ray Observatory has achieved an increase in sensitivity of 10 orders of magnitude, comparable to the gain in going from naked-eye observations to the most powerful optical telescopes over the past 400 years. Chandra is unique in its capabilities for producing sub-arcsecond X-ray images with 100-200 eV energy resolution for energies in the range 0.08<E<10 keV, locating X-ray sources to high precision, detecting extremely faint sources, and obtaining high resolution spectra of selected cosmic phenomena. The extended Chandra mission provides a long observing baseline with stable and well-calibrated instruments, enabling temporal studies over time-scales from milliseconds to years. In this report we present a selection of highlights that illustrate how observations using Chandra, sometimes alone, but often in conjunction with other telescopes, have deepened, and in some instances revolutionized, our understanding of topics as diverse as protoplanetary nebulae; massive stars; supernova explosions; pulsar wind nebulae; the superfluid interior of neutron stars; accretion flows around black holes; the growth of supermassive black holes and their role in the regulation of star formation and growth of galaxies; impacts of collisions, mergers, and feedback on growth and evolution of groups and clusters of galaxies; and properties of dark matter and dark energy.

We have carried out a survey with the Chandra X-Ray Observatory of a sample of 10 bright broad absorption line (BAL) quasars (QSOs). Eight of 10 sources are detected. The six brightest sources have only high-ionization BALs (hiBALs), while the four faintest all show low-ionization BALs (loBALs). We perform a combined spectral fit for hiBAL QSOs (384 counts total; 0.5-6 keV) to determine the mean spectral parameters of this sample. We derive an underlying best-fit power-law slope Γ = 1.8 ± 0.35, which is consistent with the mean slope for radio-quiet QSOs from ASCA, but BAL QSOs require a (rest-frame) absorbing column of 6.5 × 1022 cm-2, with a partial covering fraction of ~80%. The optical-to-X-ray spectral slope (αox from 2500 Å to 2 keV) varies from 1.7 to 2.4 across the full sample, consistent with previous results that BAL QSOs appear to be weak soft X-ray emitters. Removing the absorption component from our best-fit spectral model yields a range of αox from 1.55 to 2.28. All six hiBAL QSOs have deabsorbed X-ray emission consistent with non-BAL QSOs of similar luminosity. The spectral energy distributions of the hiBAL QSOs—both the underlying power-law slope and αox—provide the first conclusive evidence that BAL QSOs have appeared to be X-ray weak because of intrinsic absorption and that their underlying emission is consistent with non-BAL QSOs. By contrast, the removal of the best-fit absorption column detected in the hiBAL QSOs still leaves the four loBAL QSOs with values of αox > 2 that are unusually X-ray faint for their optical luminosities, which is consistent with other evidence that loBALs have higher column density, dustier absorbers. Important questions of whether BAL QSOs represent a special line of sight toward a QSO nucleus or rather an early evolutionary or high-accretion phase in a QSO lifetime remain to be resolved, and the unique properties of loBAL QSOs will be an integral part of that investigation.

3C 351 (z = 0.371), and X-ray-'quiet' quasar, is one of the few quasars showing signs of a 'warm absorber' in its X-ray spectrum; i.e., partially ionized absorbing material in the line of sight whose opacity depends on its ionization structure. The main feature in the X-ray spectrum is a K-edge due to O VII or O VIII. 3C 351 also shows unusually strong, blueshifted, associated, absorption lines in the ultraviolet (Bahcall et al. 1993) including O VI (lambda lambda 1031, 1037). This high ionization state strongly suggests an identification with the X-ray absorber and a site within the active nucleus. In this paper we demonstrate that the X-ray and UV absorption is due to the same material. This is the first confirmed UV/X-ray absorber. Physical conditions of the absorber are determined through the combination of constraints derived from both the X-ray and UV analysis. This highly ionized, outflowing, low-density, high-column density absorber situated outside the broad emission line region (BELR) is a previously unknown component of nuclear material. We rule out the identification of the absorber with a BELR cloud as the physical conditions in the two regions are inconsistent with one another. The effect of the X-ray quietness and IR upturn in the 3C 351 continuum on the BELR is also investigated. The strengths of the high-ionization lines of C IV lambda-1549 and O VI lambda-1034 with respect to Lyman-alpha are systematically lower (up to a factor of 10) in the material ionized by the 3C 351 continuum as compared to those produced by the 'standard' quasar continuum, the strongest effect being on the strength of O VI lambda-1034. We find that for a 3C 351-like continuum, C III) lambda-1909 ceases to be a density indicator.

We present a survey of serendipitous extended X-ray sources and optical cluster candidates from the Chandra Multiwavelength Project (ChaMP). Our main goal is to make an unbiased comparison of X-ray and optical cluster detection methods. In 130 archival Chandra pointings covering 13 deg2, we use a wavelet decomposition technique to detect 55 extended sources, of which 6 are nearby single galaxies. Our X-ray cluster catalog reaches a typical flux limit of about ~10-14 ergs cm-2 s-1, with a median cluster core radius of 21''. For 56 of the 130 X-ray fields, we use the ChaMP's deep NOAO 4 m MOSAIC g', r', and i' imaging to independently detect cluster candidates using a Voronoi tessellation and percolation (VTP) method. Red-sequence filtering decreases the galaxy fore- and background contamination and provides photometric redshifts to z ~ 0.7. From the overlapping 6.1 deg2 X-ray/optical imaging, we find 115 optical clusters (of which 11% are in the X-ray catalog) and 28 X-ray clusters (of which 46% are in the optical VTP catalog). The median redshift of the 13 X-ray/optical clusters is 0.41, and their median X-ray luminosity (0.5-2 keV) is LX = img1.gif × 1043 ergs s-1. The clusters in our sample that are only detected in our optical data are poorer on average (~4 σ) than the X-ray/optically matched clusters, which may partially explain the difference in the detection fractions.

Using the large multiwavelength data set in the Chandra/SWIRE Survey (0.6 deg2 in the Lockman Hole), we show evidence for the existence of highly obscured (Compton-thick) AGNs, estimate a lower limit to their surface density, and characterize their multiwavelength properties. Two independent selection methods based on the X-ray and infrared spectral properties are presented. The two selected samples contain (1) five X-ray sources with hard X-ray spectra and column densities gsim1024 cm-2 and (2) 120 infrared sources with red and AGN-dominated infrared SEDs. We estimate a surface density of at least 25 Compton-thick AGNs deg-2 detected in the infrared in the Chandra/SWIRE field, of which ~40% show distinct AGN signatures in their optical/near-infrared SEDs, the remaining being dominated by the host galaxy emission. Only ~33% of all Compton-thick AGNs are detected in the X-rays at our depth [F(0.3-8 keV) > 10-15 ergs cm-2 s-1]. We report the discovery of two sources in our sample of Compton-thick AGNs, SWIRE J104409.95+585224.8 (z = 2.54) and SWIRE J104406.30+583954.1 (z = 2.43), which are the most luminous Compton-thick AGNs at high z currently known. The properties of these two sources are discussed in detail with an analysis of their spectra, SEDs, luminosities, and black hole masses.

The Chandra Multiwavelength Project (ChaMP) is a wide-area (~14 deg2) survey of serendipitous Chandra X-ray sources, aiming to establish fair statistical samples covering a wide range of characteristics (such as absorbed active galactic nuclei [AGNs] and high-z clusters of galaxies) at flux levels (fX ~ 10-15 to 10-14 ergs s-1 cm-2) intermediate between the Chandra Deep Field surveys and previous missions. We present the first results of X-ray source properties obtained from the initial sample of 62 observations. The data have been uniformly reduced and analyzed with techniques specifically developed for the ChaMP and then validated by visual examination. Utilizing only near-on-axis X-ray-bright sources (to avoid problems caused by incompleteness and the Eddington bias), we derive the log N- log S relation in soft (0.5-2 keV) and hard (2-8 keV) energy bands. The ChaMP data are consistent with previous results of ROSAT, ASCA, and Chandra Deep Field surveys. In particular, our data nicely fill in the flux gap in the hard band between the Chandra Deep Field data and the previous ASCA data. We check whether there is any systematic difference in the source density between cluster and noncluster fields and also search for field-to-field variation, both of which have been previously reported. We found no significant field-to-field cosmic variation in either test within the statistics (~1 σ) across the flux levels included in our sample. In the X-ray color-color plot, most sources fall in the location characterized by photon index = 1.5-2 and NH = a few × 1020 cm2, suggesting that they are typical broadline AGNs. There also exist a considerable number of sources with peculiar X-ray colors (e.g., highly absorbed, very hard, very soft). We confirm a trend that on average the X-ray color hardens as the count rate decreases. Since the hardening is confined to the softest energy band (0.3-0.9 keV), we conclude that it is most likely due to absorption. We cross-correlate the X-ray sources with other catalogs and describe their properties in terms of optical color, X-ray-to-optical luminosity ratio, and X-ray colors.

We present the final installment of an intensive 13 year study of variations of the optical continuum and broad Hβ emission line in the Seyfert 1 galaxy NGC 5548. The database consists of 1530 optical continuum measurements and 1248 Hβ measurements. The Hβ variations follow the continuum variations closely, with a typical time delay of about 20 days. However, a year-by-year analysis shows that the magnitude of emission-line time delay is correlated with the mean continuum flux. We argue that the data are consistent with the simple model prediction between the size of the broad-line region and the ionizing luminosity, r ∝ Lion. Moreover, the apparently linear nature of the correlation between the Hβ response time and the nonstellar optical continuum Fopt arises as a consequence of the changing shape of the continuum as it varies, specifically Fopt ∝ FUV.

We report on Chandra observations of the three quasars SDSSp J083643+005453, SDSSp J103027+052455, and SDSSp J130608+035626 at redshifts 5.82, 6.28, and 5.99, respectively. All three sources are clearly detected in the X-ray band, up to rest-frame energies of ~55 keV. These observations demonstrate the unprecedented sensitivity of Chandra to detect faint sources in relatively short exposure times (5.7-8.2 ks). The broadband X-ray properties of these highest redshift quasars do not appear to be any different from their lower redshift cousins, implying little or no evolution for these few sources out to z ~ 6, at redshifts where the optical space density is falling. Spectra of the sources could not be determined with only a few counts detected. Observations with XMM-Newton will be able to constrain the spectral shapes, if they are simple. Determination of complex spectra in a reasonable amount of time, however, will have to await the next generation of X-ray missions.