Symposium Proceedings

Active Galaxies and Quasars

Six Years of the BALQSO Pair UM425

Tom Aldcroft, Paul J. Green (Harvard-Smithsonian Center for Astrophysics), Leopoldo Infante (Universidad Catolica de Chile), Sebastian Lopez (Universidad de Chile), Joshua N. Winn (Harvard-Smithsonian Center for Astrophysics),

We now report the latest chapter in the saga of UM425, which started with a snapshot survey in Chandra cycle 1, built steam with a deep 110 ksec observation in cycle 3, and finally concludes (until the next episode) with recent VLT spectroscopy revealing a foreground cluster of galaxies in the UM425 field. UM425 is a pair of QSOs at $z=1.47$ separated by 6.5 arcsec which show remarkably similar emission and broad absorption line (BAL) profiles in the optical/UV. We first observed this pair with Chandra during cycle 1, where we found that that high-ionization BAL QSOs appear in the X-rays to be normal QSOs with a moderate obscuring column of foreground gas. UM425A was seen to be one of the brightest known BAL QSOs in X-rays, so we proposed for a long 110 ksec followup observation in cycle 3. In addition to getting the best known X-ray spectrum of a BALQSO, we hoped to investigate this wide lens candidate by searching for foreground cluster emission. Despite deep searches, no lensing galaxy had been identified, leaving in doubt the lens interpretation. As hoped for, our deep observation revealed faint diffuse emission near the QSO pair, suggesting either the long-sought foreground lensing cluster or a high redshift cluster at the distance of UM425. Now from recent VLT spectroscopy we have discovered a foreground cluster of galaxies, and thereby interpret the diffuse X-ray emission as originating from $z=0.77$, rather than the quasar redshift. The mass of the cluster is consistent with the theoretical mass required for gravitational lensing, but UM425 would be an unusual gravitational lens, by virtue of the absence of a bright primary lensing galaxy.

X-ray Versus Optical Obscuration in AGNs: Hints from the XMM-BSS Sample

Valentina Braito (Johns Hopkins Univ.), Alessandro Caccianiga, Filomena Cocchia, Roberto Della Ceca, Tommaso Maccacaro, Paola Severgnini (INAF/Osservatorio Astronomico di Brera, Italy)

Extensive studies, in the 2-10 keV energy band, with Chandra and XMM-Newton have recently resolved the major part of CXB into discrete sources. Those studies have also shown the presence of exceptions to the widely accepted correlation between the optical obscuration and X-ray absorption, i.e. X-ray absorbed type 1 AGN. To investigate this issue we have selected, using the hardness ratio diagram, the sources of the 'XMM-Newton Bright Serendipitous Survey' sample that are located in regions in conflict with their optical classification. The good X-ray statistics, which characterize most of the sources in the XMM-BSS, combined with the relative brightness of their optical counterparts allows us to investigate the broad-band properties of these sources.

The Role of Absorption and Reflection in the X-ray Spectrum of Active Galactic Nuclei

Loic Chevallier, Suzy Collin, Anne-Marie Dumont (LUTH/OPM), Bozena Czerny (CAMK), Martine Mouchet (LUTH/OPM), Anabela C. Gonçalves (LUTH/OPM and CAAUL/OAL), Rene Goosmann (LUTH/OPM)

In the 2-10 keV range, the AGN continuum is generally well represented by a single power law but at smaller energies it displays an excess with respect to the extrapolation of this power law, called the soft X-ray excess; the nature of this component is still under discussion. Until now the soft X-ray excess was attributed either to reflection of the hard X-rays on the accretion disk, or to the presence of an additional comptonizing medium. An alternative solution, proposed by Gierlinski and Done (2004), is that this feature would be due to the absorption of an intrinsically steep power law source (whose origin is not clear) by a medium with a very large dispersion velocity (as a relativistic wind). Understanding the nature of the soft X-ray excess is essential for our knowledge of both the Warm Absorber and the primary spectrum, with consequences on the understanding of the accretion flow process. We have therefore examined the pros and cons of the reflection and absorption models. We conclude that the observed soft X-ray spectra can be probably modeled by absorption (for a strong excess) or by reflection (for a weak excess).

A 1 Hour Quasi-period in the Seyfert Galaxy 3C 273

Catherine Espaillat, Joel Bregman, Philip Hughes, Edward Lloyd-Davies (Univ. of Michigan)

Quasi-periodic signals (QPOs) have yielded important constraints on the masses of black holes in galactic X-ray binaries, and here we extend that to active galactic nuclei (AGN). We analyze 19 observations of 10 AGN obtained with the $XMM$-$Newton$ EPIC PN camera, employing a continuous wavelet transform. We detect a statistically significant 3.1 ks quasi-period in the quasar 3C 273. If this period represents an orbital timescale originating near a last stable orbit of 3 $R_S$, it implies a central black hole mass of $6.8\times 10^6$ $M_{\odot}$. For a maximally rotating black hole with a last stable orbit of 0.6 $R_S$ we obtain $7.6\times 10^7$ $M_{\odot}$. A previous mass estimate obtained from reverberation mapping places the black hole mass of 3C 273 at $2.35\times 10^8$ $M_{\odot}$, substantially higher than both of our estimates. Assuming that the reverberation mass is correct, the X-ray quasi-period must be caused by a higher order oscillatory mode of the disk.

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The Origin of X-ray Emission in Low-Redshift Radio-Galaxy Nuclei

Daniel Evans (Harvard-Smithsonian Center for Astrophysics), Diana Worrall (Univ. of Bristol), Martin Hardcastle (Univ. of Hertfordshire), Ralph Kraft (Harvard-Smithsonian Center for Astrophysics), Mark Birkinshaw (Univ. of Bristol)

The physical origin of continuum X-ray emission in the cores of radio galaxies is widely debated. We present spectral results from Chandra and XMM-Newton observations of a sample of low-redshift (z $<$ 0.1) 3CRR radio galaxies, and consider whether the emission originates from the base of a relativistic jet, an accretion flow, or contains contributions from both. We find correlations between the unabsorbed X-ray, radio, and optical fluxes and luminosities of FRI-type radio galaxies, implying a common origin in the form of a jet. On the other hand, we find that the X-ray spectra of FRII-type radio galaxies is dominated by heavily absorbed emission which is likely to originate in an accretion flow. We discuss several models to account the differing nuclear properties of FRI- and FRII-type sources, and also demonstrate that both heavily obscured, accretion-related, and unobscured, jet-related components may be present at varying levels in all radio-galaxy nuclei.

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Nine Years in the X-Ray Life of NGC 4258

Antonella Fruscione, Lincoln J. Greenhill (Harvard-Smithsonian Center for Astrophysics), Alexei V. Filippenko (UCB), James M. Moran (Harvard-Smithsonian Center for Astrophysics), James R. Herrnstein (Rentec), Elizabeth Galle (Harvard-Smithsonian Center for Astrophysics)

We have analyzed X-ray (0.3-10 keV) observations of NGC 4258 obtained with the XMM-Newton and Chandra observatories. Including earlier observations by ASCA and Beppo-SAX, we present a new nine year time series of models fitted to the X-ray spectrum of NGC4258.

Our main conclusions are:

1. XMM and Chandra spectra are well fit by a multi-components model: a partially absorbed, hard (2 keV) power law, a soft thermal plasma, and a soft power law. The soft emission, some of which arises $<$70 pc from the central engine, does not vary appreciably from observation to observation.

2. XMM data indicate long-term time variability in the source count rate and absorbed flux over time scales of 6 months. No evidence of variability on individual $\sim$3 hour integrations.

3. From XMM data we detect a $\sim$60% increase in NH over $\sim$5 months, returning to a high level not reported since the ASCA observations in 1993, $NH~1.3 x 10^{23}$ cm-2.

4. Changes in NH and fX are not correlated, which indicates intrinsic variability of the central engine that is in one case 30% over 19 days (5-10 keV). We note that two of the largest estimates of unabsorbed luminosity are associated with the lowest estimates of NH, and we speculate that reductions in LX might affect the ionization state of the absorber.

5. The geometry and orientation of the accretion disk in NGC 4258 is well known from interferometric mapping of maser emission that arises in the accretion disk. The warped disk, a known source of H20 maser emission, is believed to cross the line of sight to the central engine. Assuming that the absorbing gas lies in the disk, we propose that the variations in NH arise from inhomogeneities sweeping across the line of sight in the rotating disk at the radius at which the disk crosses the line of sight. We estimate that the inhomogeneities are $\sim~10^{15}$ cm in size at the crossing radius of 0.29pc. This is the first direct confirmation that obscuration in type 2 AGNs may, in some cases, arise in thin, warped accretion disk rather than geometrically thick tori.

6. We do not detect Fe Kalpha line emission in any of the XMM or Chandra epochs, thus extending the disappearance of the line from the last ASCA detection in May 1999 May to May 2002. The inferred line emission region is comparable in size to the maser disk. If the line arises from the disk (e.g., by fluorescence), then it is difficult to understand the variability because the maser emission has not changed substantially.

7. We do not observe evidence for absorption lines in any XMM or Chandra spectra.

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Highlights from a Chandra Survey of Quasar Jets

Jonathan M. Gelbord, Herman L. Marshall (MIT Kavli Institute), Dan A. Schwartz (Harvard-Smithsonian Center for Astrophysics), Diana M. Worrall, Mark Birkinshaw (Univ. of Bristol), Eric S. Perlman, Markos Georganopoulos (UMBC), Jim E. J. Lovell (CSIRO), Sebastian Jester (FNAL), Dave W. Murphy (JPL), Dave L. Jauncey, Leith Godfrey (CSIRO)

We are conducting a Chandra survey of quasars with extended radio structure. The majority of these are found to have X-ray bright jets. This emission is often interpreted as inverse Compton scattering of cosmic microwave background photons, but the expected redshift dependence of this mechanism is marginally excluded by our data.

From this sample we have selected a handful of interesting cases for multiwavelength followup study, to better understand the emission mechanisms and physical conditions at various points within these systems. Of particular interest is PKS 1421-490, in which we discovered an unusual feature 5.9 arcsec from the radio peak that appears to be a unique, optically-dominated jet knot. New, deeper Chandra observations coupled with HST and VLBI data provide new insight into this object. We also report recent Chandra and HST observations of PKS 1055+201. X-rays are found throughout the 23 arcsec (=170 kpc in the plane of the sky at z=1.11) length of the jet, making this one of the longest quasar jets seen in X-rays. Additionally, diffuse X-ray emission is detected surrounding the jet, which may be part of the jet structure or the effect of the jet upon the surrounding medium. Similar diffuse emission spans the gap between the core and the counter-lobe, provides direct evidence of an otherwise-unseen counter jet.

Partial support for this work was provided by the NASA through the SAO contract SV3-73016 to MIT for Support of the Chandra X-Ray Center, which is operated by SAO for and on behalf of NASA under contract NAS8-03060. Support has also come from SAO grant GO4-5124 and directly from NASA under contract NAS8-39073.


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A Constant Pressure Model for the Warm Absorber in NGC 3783

Anabela C. Gonçalves (LUTH/OPM and CAAUL/OAL), Suzy Collin, Anne-Marie Dumont (LUTH/OPM), Agata Rozanska (CAMK), Martine Mouchet, Loic Chevallier, Rene Goosmann (LUTH/OPM)

Many AGN exhibit X-ray absorption features caused by the presence of highly ionized gas located on the line of sight of the central continuum. Such a material is called Warm Absorber (WA) and should be stratified, displaying zones of different density, temperature and ionization. Our approach to the study of the WA relies on the assumption of pressure equilibrium, resulting in the natural stratification of the medium, which allows to explain the presence of lines from different ionization states in many AGN observed by Chandra and XMM-Newton. Among the best WA observations available are those of NGC 3783, which we have analyzed. We have used the TITAN code, developed by our team, to calculate a constant pressure grid of models dedicated to fit the WA in NGC 3783. Our study shows that the WA can be modelled under constant pressure conditions. Finally, this work provides a good example of the application of the TITAN code to the study of the Warm Absorber in AGN and opens perspectives for the future use of the code by a larger community, through a larger grid of models to be made available.

Monitoring the M87 Jet for 4 years with Chandra: The Outburst of Knot HST-1

D. E. Harris (SAO), C. C. Cheung (NRAO; Stanford Univ.), J. A. Biretta, W. Sparks (STScI), W. Junor (LANL), E. S. Perlman, A. S. Wilson (Univ. of Maryland)

The X-ray intensity of knot HST-1, 0.85 $^{\prime\prime}$ (65pc, projected) from the nucleus of the radio galaxy M87, has increased by more than a factor of 50 during the last 5 years. The optical increase is similar and our more limited radio data indicate a commensurate activity. We give the primary results of our Chandra X-ray Observatory monitoring program and consider some of the implications of this extreme variability in a relativistic jet. We find that the data support a modest beaming synchrotron model as indicated in our earlier papers. Based on this model, the decay of the X-ray lightcurve appears to be dominated by light travel time across the emitting region rather than the synchrotron loss timescales.

Relationships Between the X-ray and UV properties of Radio-Quiet Quasars

Brandon Kelly, Jill Bechtold (Univ. of Arizona), Aneta Siemiginowska, Tom Aldcroft, Martin Elvis (Harvard-Smithsonian Center for Astrophysics), Malgorzata Sobolewska (Nicolaus Copernicus Astronomical Center),

We present results of our work comparing the X-ray properties with the UV properties using a sample of $\sim 75$ radio-quiet, non-BAL quasars over a broad range in redshift and luminosity. All but 5 of the sources were detected by Chandra. I performed a multivariate regression analysis to compare $\alpha_X = 1 - \Gamma_X$ and $\alpha_{\rm ox}$ with $\log_{10} \lambda L_{\lambda} (2500\AA), \log_{10} (1 + z), \alpha_{\rm
uv}, \log_{10} EW_{\rm C IV}, \mu_{C IV}$ (the C IV line centroid), and $\log_{10} FWHM_{\rm C IV}$. Subset selection was carried out for both cases to select which set of the aforementioned non-X-ray parameters were related to $\alpha_X$ and $\alpha_{\rm ox}$. For $\alpha_X$, the best subset was selected using 10-fold cross-validation, and for $\alpha_{\rm ox}$ an approximation to the posterior probabilities of the models based on the Bayesian Information Criterion (BIC) was used. The best fit for $\alpha_X$ had $\alpha_X$ correlated with both UV luminosity and C IV $EW$, and anti-correlated with $\alpha_{\rm uv}$, although there is considerable model uncertainty. In addition, it is likely that $\alpha_X$ is related to $EW_{\rm C IV}$ and $\alpha_{\rm uv}$, and there is weak evidence that $\alpha_X$ is also related to UV luminosity. For the case of $\alpha_{\rm ox}$, the most probable model is that $\alpha_{\rm ox}$ is anti-correlated with UV luminosity, and correlated with C IV line centroid and redshift. This model is $\approx
3.5$ times more likely than the next most probable model. In addition, the posterior probabilities that $\alpha_{\rm ox}$ is related to UV luminosity, redshift, and $\mu_{\rm C IV}$ are 0.9999, 0.9668, and 0.9060, respectively. The data give weak evidence that $\alpha_{\rm ox}$ is unrelated to $\alpha_{\rm uv}$, $EW_{\rm C IV}$, and $FWHM_{C IV}$. These correlations cannot be explained as being the results of correlations among the non-X-ray parameters, as the multivariate regression already takes into account any collinearity among the covariates.

X-ray Properties of the GigaHertz-Peaked and Compact Steep Spectrum Sources

Stephanie LaMassa, Aneta Siemiginowska, Thomas L. Aldcroft (Harvard-Smithsonian Center for Astrophysics), Matteo Guainazzi (European Space Astronomy Center of ESA), Jill Bechtold (Steward Observatory, Univ. of Arizona), Martin Elvis (Harvard-Smithsonian Center for Astrophysics)

Giga-Hertz Peaked Spectrum (GPS) radio sources are powerful radio and X-rays emitters. Their radio properties have been extensively studied leading to two possible explanations of the compact nature of the GPS sources: (1) frustrated source scenario in which the expansion of the radio source is confined by a dense environment; (2) evolution scenario in which the source is at an early stage of its expansion to a typical large scale radio source. Measurements of the expansion velocity of the radio components in several GPS sources (Compact Symmetric Objects) suggest that these sources are young, while there has been no evidence for a dense medium required for the source confinement. Here we consider a sample GPS sources, containing both galaxies and quasars, observed with Chandra and XMM-Newton. Chandra observations allow for detailed studies of the source morphology on arcsec scale and we discuss different types of observed X-ray morphology for our sample. Spectral modeling of Chandra and XMM-Newton data indicate that the GPS galaxies are more obscured than quasars. We discuss the implication of this finding on our understanding of the nature of GPS sources.

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Fe K Emission and Absorption in the Bright Seyfert IC 4329a

Alex Markowitz (NASA/GSFC), James Reeves (NASA/GSFC and JHU)

We present a re-analysis of the XMM-EPIC long-look of the X-ray bright Seyfert AGN IC 4329a, complementary to the RGS/EPIC analysis of Steenbrugge et al (2005). The Fe K bandpass is dominated by emission consistent with moderately-broadened Fe K alpha and beta. We detect, at high significance (confirmed with Monte Carlo simulations), a narrow absorption feature at 7.7 keV; one possible candidate for this feature's origin is highly blueshifted Fe K XXVI. Time-resolved spectrscopy of the XMM data as well as RXTE monitoring data, covering time scales from minutes to 2 years, show there is little variability in continuum flux or photon index on any time scale. There is no evidence for variability of the Fe K alpha line on any time scale, consistent with an origin far from the black hole (e.g., as discussed by previous works).

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X-ray Spectral Variability Study Using RXTE Active Galaxies

Barbara Mattson (UMD/NASA GSFC), Kimberly Weaver (NASA), Christopher Reynolds (UMD)

We report early results of a systematic X-ray spectral variability study of bright Seyfert galaxies observed by the Rossi X-Ray Timing Explorer (RXTE). The RXTE public archive contains data for 40 Seyfert galaxies with suitable temporal coverage to perform variability studies covering short (days/weeks) to long (years) timescales. We have developed a data pipeline to automate the data reduction. The pipeline produces a series of spectra for each source divided temporally in such a way that each has at least 125,000 net photons to ensure a good spectral fit. To fit the spectra, we have assumed an intrinsic powerlaw X-ray spectrum produced close to the central black hole that is reprocessed and absorbed by material around the black hole. We fit each X-ray spectrum with a model of this reflected emission, including fluorescent Fe K$\alpha$ emission, Compton reflection component and absorption. The goal of this research is first to build a database of spectral parameters for a large sample of AGN, and then to seek correlations between spectral parameters and those specific to various classes of AGN (e.g. Seyfert type or radio-loudness). The results will constrain the geometry of the nuclear region, providing tests for current reflection and unification models of AGN.

Chandra and XMM Observations of Type II Quasars from the SDSS

Andrew Ptak (JHU), Nadia Zakamska, Michael Strauss (Princeton), Julian Krolik, Timothy Heckman (JHU), Donald Schneider (PSU), Jon Brinkmann (Apache Point Observatory)

We are carrying out sensitive X-ray observations with Chandra and XMM of type II quasars selected from the Sloan Digital Sky Survey based on their optical emission line properties. We present observations of four objects at redshifts 0.4 $<$ z $<$ 0.8 and an analysis of the archival data for four additional objects in the same redshift range. Six of the eight were detected in X-rays; five of them have sufficient signal to derive spectral information. All of the detected sources have intrinsic luminosities L(2-10 keV) $> 5 x 10^{43}$ erg $\rm {s^{-1}}$. The five with sufficient counts for spectral fitting show evidence for significant absorption ($N_H >\sim$ a few x $10^{22} \rm {cm^{-2}}$). At least three of the objects likely have $N_H > 10^{23} \rm {cm^{-2}}$; some may be Compton-thick ( $N_H > 10^{24} \rm {cm^{-2}}$). In the five objects for which we could fit spectra, the slopes tend to be significantly flatter than is typically observed in AGN; it is possible that this is due either to reprocessing of the nuclear emission or to a line of sight that passes through patchy absorption.

Synchrotron-Loss Spectral Breaks in Jets and Pulsar-Wind Nebulae

Stephen Reynolds (North Carolina State Univ.)

Flows of synchrotron-emitting material can be found in several astrophysical contexts, including extragalactic jets and pulsar-wind nebulae. For X-ray synchrotron emission, flow times are often longer than electron radiative lifetimes, so the effective source size at a given X-ray energy is the distance electrons radiating at that energy can convect before they burn off. Since synchrotron losses vary strongly with electron energy, the source size drops with increasing X-ray energy, resulting in a steepening of the synchrotron spectrum. For homogeneous sources, this burnoff produces the well-known result of a steepening by 0.5 in the source's integrated spectral index. However, for inhomogeneous sources, different amounts of steepening are possible. I exhibit a simple phenomenological picture of an outflow, with transverse flow-tube radius, magnetic-field strength, matter density, and flow velocity all varying as different powers of distance from the injection point. For such a picture, I calculate the value of the spectral index above the break as a function of the power-law indices, and show the possible range of steepenings. I show that these simple calculations are confirmed by full integrations of source luminosity. In many cases, extragalactic jets show X-ray synchrotron emission steeper by more than 0.5 than the radio emission; the same phenomenon is exhibited by many pulsar-wind nebulae. It is possible that source inhomogeneities are responsible in at least some cases, so that the amount of spectral steepening becomes a diagnostic for source dynamical or geometrical properties.

Revealing the Structure of the Absorber in NGC 1365 through Extreme X-ray Variability and Iron Absorption

G. Risaliti, M. Elvis, G. Fabbiano, A. Baldi, A. Zezas (Harvard-Smithsonian Center for Astrophysics), S. Bianchi, G. Matt

I present multiple XMM-Newton observations revealing extreme X-ray properties of the Seyfert Galaxy NGC 1365, which provide an unique opportunity to study the complex circumnuclear absorber. In particular, we observed (a) changes of the X-ray state from transmission-dominated to reflection dominated in time intervals as short as three weeks (b) changes of the column density of the cold absorber in transmission-dominated states, in time intervals of a few 10,000 sec; (c) the presence of four strong absorption lines in the 6.7-8.3 keV energy range, identified as FeXXV and FeXXVI $K\alpha$ and $K\beta$, implying the presence of a hot absorber with column density of the order of a few $10^{23}$ cm$^{-2}$. These observations suggest an extremely compact (of the order of the broad line region size) and composite structure of the circumnuclear absorber, consisting of at least two components, one cold and clumpy, and one extremely ionized.

The Ionization Structure of the Wind in NGC 5548

Katrien Steenbrugge (Harvard-Smithsonian Center for Astrophysics), Jelle Kaastra (SRON), Mike Crenshaw (Department of Physics and Astronomy, Georgia State Univ.), Steve Kraemer (Catholic Univ. of America), Nahum Arav (CASA), Ian George (Joint Center for Astrophysics, Univ. of Maryland), Duane Liedahl (Physics Dept., Lawrence Livermore National Lab.), Frits Paerels (Columbia Univ.), Jane Turner (Joint Center for Astrophysics, Univ. of Maryland), Tahir Yaqoob (Laboratory for High Energy Astrophysics)

Combining the one week long Chandra and the 140 ks XMM-Newton observations of NGC 5548, we are able to study the velocity and ionization structure of the warm absorber. Using simultaneous HST STIS observations, we show that the X-ray and UV absorber have the same kinematics and have overlapping ionization parameters. In the X-ray spectra we detect a lower ionized component, which has a similar ionization parameter as the UV absorber. This component can not be in pressure equilibrium with the higher ionized components of the outflow, a requirement of most warm absorber theories. A continuous ionization parameter distribution, assuming a power-law function for the total hydrogen column density versus ionization parameter, equally well describes the data. We determine upper limits of the opening angle for such a outflow.

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The X-ray/Optical Properties of Active Galaxies Over Wide Luminosity and Redshift Ranges

Aaron Steffen, Iskra Strateva, Niel Brandt (Penn State), Dave Alexander (IoA), Anton Koekemoer (STScI), Bret Lehmer, Don Schneider (Penn State), John Silverman (Harvard-Smithsonian Center for Astrophysics), Christian Vignali (INAF)

We present the results of a partial correlation analysis testing the strength of the relationships between monochromatic 2500 Å and 2 keV luminosities ( $l_{\mbox{\scriptsize 2500 \AA}}$ and $l_{\mbox{\scriptsize 2 keV}}$, respectively), $\alpha_{\mbox{\tiny
OX}}$, and redshift for optically-selected AGNs. We extend the work of Strateva et al. (2005) to include 54 moderate-luminosity, optically-selected AGNs from the COMBO-17 survey and corresponding X-ray observations from the Extended Chandra Deep Field-South. We also include recently published optical/X-ray observations of 18 high-redshift, optically-selected AGNs, and 46 luminous, low-redshift AGNs from the Bright Quasar Survey, which brings our total sample to 339 AGNs, 299 ($88\%$) of which have X-ray detections. We confirm that $\alpha_{\mbox{\tiny
OX}}$ is strongly anticorrelated with $l_{\mbox{\scriptsize 2500 \AA}}$ ($11.8\sigma$), and find no significant correlation exists between $\alpha_{\mbox{\tiny
OX}}$ and redshift ($0.2\sigma$). We take advantage of the high X-ray detection fraction of our sample to measure the correlation between $\alpha_{\mbox{\tiny
OX}}$ and $l_{\mbox{\scriptsize 2 keV}}$. We find a moderately significant ($2.5\sigma$) anticorrelation between $\alpha_{\mbox{\tiny
OX}}$ and $l_{\mbox{\scriptsize 2 keV}}$.

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A Chandra X-Ray Survey of Ultraluminous Infrared Galaxies

Stacy Teng, A. S. Wilson, S. Veilleux (Univ. of Maryland), A. J. Young (MIT), D. B. Sanders (IfA, Univ. Hawaii)

We present results from Chandra observations of 14 ultraluminous infrared galaxies (ULIRGs; log( $\rm {L_{IR}/L_\odot) \geq 12}$) with redshifts between 0.04 and 0.16. The goals of the observations were to investigate any correlation between infrared color or luminosity and the properties of the X-ray emission and to attempt to determine whether these objects are powered by starbursts or active galactic nuclei (AGNs). The sample contains approximately the same number of high and low luminosity objects and "warm" and "cool" ULIRGs. All 14 galaxies were detected by Chandra. Our analysis shows that the X-ray emission of the two Seyfert 1 galaxies in our sample are dominated by AGN. The remaining 12 sources are too faint for conventional spectral fitting to be applicable. Hardness ratios were used to estimate the spectral properties of these faint sources. The photon indices, $\Gamma$'s, for our sample plus the Chandra-observed sample from Ptak peak in the range of 1.0-1.5, consistent with expectations for X-ray binaries in a starburst, an absorbed AGN, or hot bremsstrahlung from a starburst or AGN. The values of $\Gamma$ for the objects in our sample classified as Seyferts (type 1 or 2) are larger than 2, while those classified as HII regions or LINERs tend to be less than 2. The hard X-ray to far-infrared ratios for the 12 weak sources are similar to those of starbursts, but we cannot rule out the possibility of absorbed, possibly Compton-thick, AGNs in some of these objects. Two of these faint sources were found to have X-ray counterparts to their double optical and infrared nuclei.

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A Chandra Search for Hidden AGN in H II Nuclei

Panayiotis Tzanavaris, Ioannis Georgantopoulos (National Observatory Athens), Antonis Georgakakis (Imperial College London)

We identified nine, early-type spirals, namely NGC 278, 891, 2146, 2782, 3310, 4102, 4217 and 4303, from the Palomar survey sample of Ho, Fillipenko and Sargent (1997), classified as normal, H II nuclei, which fall within archived Chandra ACIS-S fields. We carried out a detailed search of the Chandra fields to identify possible X-ray counterparts, and obtained the X-ray luminosity, $L_X$, hardness ratio, HR, and radial profiles of X-ray counterparts in order to search for hidden AGN. Our results suggest that two of the H II galaxies may harbour a low-luminosity AGN.

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The Extended Chandra Deep Field-South Survey: Optical & NIR Properties of X-ray Detected Sources

C. Megan Urry, Shanil Virani, Ezequiel Treister (Yale Univ.), Edward Taylor (Leiden), J. Van Duyne, Eric Gawiser, Pieter van Dokkum (Yale Univ.), MUSYC Collaboration

The Extended Chandra Deep Field-South survey consists of 4 Chandra ACIS-I pointings and covers $\sim$1100 square arcminutes ($\sim0.3 deg^2$) surrounding the original CDF-S field, to a depth of approximately 228 ks. This is the largest Chandra survey ever conducted at such depth. In our X-ray catalog of this field (Virani et al. 2005, astro-ph/0506551), we detect 651 unique sources, of which 561 are detected in the full 0.5-8.0 keV band, 529 in the soft 0.5-2.0 keV band, and 335 in the hard 2.0-8.0 keV band. In this paper, we present the optical and near-IR counterparts to these X-ray detected sources obtained as part of the MUltiwavelength Survey by Yale/Chile (MUSYC; Gawiser et al. 2005, astro-ph/0509202). Of these 651 X-ray sources, $\sim$75% of these sources have optical and near-IR counterparts in deep MUSYC imaging of this field. We present the optical and near-infrared properties of these sources (ie, magnitude distributions and colors), as well as 7 new extreme X-ray-to-optical flux ratio objects (EXOs) found in the E-CDF-S field.


The 2MASS Red AGN: the Missing Link between Type 2 and Type 1 AGN?

Belinda Wilkes (SAO) Gary Schmidt, Paul Smith (Arizona), Ken Pounds (Leicester), Roc Cutri (IPAC), Kim Page (Leicester), Himel Ghosh (OSU), Dean Hines (Space Science Institute), Brant Nelson (IPAC), Joanna Kuraskiewicz (SAO)

The red active galactic nuclei (AGNs) being revealed by the Two Micron All Sky Survey (2MASS) have a surface density similar to, or greater than, previously known AGN, suggesting that a large fraction of the population has been missed by earlier surveys. Similar red AGN are being found in other IR and X-ray surveys, reinforcing this suggestion, although the size and diversity of the previously missed population(s) remain uncertain. The 2MASS red AGN sample includes broad, intermediate and narrow lined AGN with the latter in the minority by a factor of 3. Optical observations indicate high levels of polarisation suggesting a significant contribution from scattered light. Their emission lines have properties similar to normal, broad-lined or intermediate AGN but their optical-IR spectral energy distributions (SEDs) are unusually red. Chandra observations show that they are generally X-ray faint and have hard spectra. although not directly related in individual AGN, both X-ray and optical-IR SEDs suggest absorption at the level, log Nh 21-23. Higher S/N XMM-Newton data show significant complexity in the X-ray spectra, indicating large uncertainty in the X-ray absorption levels deduced from low S/N data. The combination of all these properties combines to suggest that red AGN are viewed at an intermediate angle such that our line-of-sight passes through the surface of the absorbing material postulated by Unification models, perhaps the wind in the currently popular disk/wind models. They are also candidates for the absorbed AGN population required by models to explain the Cosmic X-ray Background (CXRB), reducing the need for a, so far largely undiscovered, type 2 quasar population. We estimate that their contribution to the CXRB may be as high as 30% or as low as 3% depending on their intrinsic SEDs, their intrinsic X-ray flux and their evolution with redshift.

Strong Gravity Effects in the High Luminosity Quasar E1821+643

Tahir Yaqoob (JHU/GSFC)

A narrow, redshifted absorption line superimposed on the red wing of a broad Fe K line was detected with the Chandra HETGS in the high luminosity quasar E1821+643 (z=0.297). We argue that the absorption line (at $\sim$6.2 keV in the quasar frame) could be due to gravitationally redshifted Fe XXV or Fe XXVI resonance absorption within $\sim$10-20 gravitational radii of the putative central black hole, although inflow is not ruled out (requiring velocities up to $\sim$0.1c). An interesting possibility is that the absorption line arises in a low-velocity outflow ($\sim$0.0003-0.003c), as found in many Seyfert galaxies, but in this case much closer to the black hole. The structure and peak energy of the Fe K emission complex is variable, from a comparison with non-comptemoraneous ASCA and Chandra LETG observations. The full range of ionization states of Fe may be contributing to the line emission with variable relative strengths at different times. The line at $\sim$6.4 keV may arise in a relativistic accretion disk and its EW brings into question the validity of the so-called X-ray Baldwin effect, which would predict that a quasar with a luminosity as high as E1821+643 should have little or no Fe K line emission.

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A Chandra HETGS Study of the LLAGN M81*

Andrew Young, Claude Canizares, Teddy Cheung, Sebastian Heinz, Mario Jimenez-Garate, Sera Markoff, Herman Marshall, Mike Nowak (MIT)

The spiral galaxy M81 harbors a low luminosity active galactic nucleus (LLAGN), the luminosity of which is only a small fraction (less than one per cent) of the Eddington limit. Such low Eddington fraction accretion flows are thought to be qualitatively different to the higher Eddington fraction accretion flows found in more luminous active galactic nuclei (AGN), and are of considerable theoretical interest. We have obtained a deep (300 ksec) Chandra High Energy Transmission Grating Spectrometer (HETGS) observation of M81* to study the X-ray spectrum of the nucleus. The sub-arcsecond spatial resolution of Chandra isolates the nucleus from the surrounding point sources and diffuse gas. A number of emission lines are seen in the spectrum, including Si K alpha fluorescence, Si XIV, Si XIII (resonance and forbidden lines), Mg XII, Ne X and O VIII. Some of these lines are resolved, with FWHM of approximately 1700 km/s. Furthermore, we do not detect any narrow iron K alpha fluorescence lines from either neutral or ionized iron. The He-like ion triplets, broadened lines and lack of strong iron K alpha fluorescence provide important constraints for models of the accretion flow. A contemporaneous multi-waveband radio to X-ray spectrum of M81* and theoretical modeling of the accretion flow is the subject of a separate talk by Sera Markoff and collaborators.

Probing the Extended Emission Surrounding Seyfert Galaxies

Jianning Zeng (JCA, UMBC), Ian George (JCA, UMBC & NASA/GSFC), Steven Kraemer (The CUA & NASA/GSFC), Tracey Turner (JCA, UMBC & NASA/GSFC), Henrique Schmitt (NRAO)

We are conducting a search for, and analysis of the extended diffuse X-ray emission surrounding nearby Seyfert nuclei using Chandra X-ray Observatory (CXO). Our primary motivation is to study and parameterize the X-ray emission on the smallest resolvable scale in order to investigate how it may be related to other AGN-related phenomena such as the NLR & other orientation indicators, the central mass/luminosity etc. Our ultimate goal is to test unification schemes. Here we present results for a sub-set nearby Seyfert galaxies observed using CXO ACIS-S3 chip. The spectroscopy of the extended X-ray emission will be modeled to illustrate and contrast the characteristics of the extended X-ray emission seen in our sample.