# CV & Publications

2011
{Evans} D  A, {Summers} A  C, {Hardcastle} M  J, {Kraft} R  P, {Gandhi} P, {Croston} J  H, {Lee} J  C. {The Suzaku View of the Disk-Jet Connection in the Low-excitation Radio Galaxy NGC 6251}. \apjl. 2011;741 :L4.Abstract

{We present results from an 87 ks Suzaku observation of the canonical low-excitation radio galaxy (LERG) NGC 6251. We have previously suggested that LERGs violate conventional active galactic nucleus unification schemes: they may lack an obscuring torus and are likely to accrete in a radiatively inefficient manner, with almost all of the energy released by the accretion process being channeled into powerful jets. We model the 0.5-20 keV Suzaku spectrum with a single power law of photon index {$\Gamma$} = 1.82$^{+0.04}$ $_{- 0.05}$, together with two collisionally ionized plasma models whose parameters are consistent with the known galaxy- and group-scale thermal emission. Our observations confirm that there are no signatures of obscured, accretion-related X-ray emission in NGC 6251, and we show that the luminosity of any such component must be substantially sub-Eddington in nature. }

{Xiang} J, {Lee} J  C, {Nowak} M  A, {Wilms} J. {Using the X-Ray Dust Scattering Halo of Cygnus X-1 to Determine Distance and Dust Distributions}. \apj. 2011;738 :78.Abstract

{We present a detailed study of the X-ray dust scattering halo of the black hole candidate Cygnus X-1 based on two Chandra High Energy Transmission Gratings Spectrometer observations. Using 18 different dust models, including one modified by us (eponymously dubbed XLNW), we probe the interstellar medium between us and this source. A consistent description of the cloud properties along the line of sight (LOS) that describes at the same time the halo radial profile, the halo light curves, and the column density from source spectroscopy is best achieved with a small subset of these models. Combining the studies of the halo radial profile and the halo light curves, we favor a geometric distance to Cygnus X-1 of d = 1.81 {\plusmn} 0.09 kpc. Our study also shows that there is a dense cloud, which contributes \~{}50% of the dust grains along the LOS to Cygnus X-1, located at \~{}1.6 kpc from us. The remainder of the dust along the LOS is close to the black hole binary. }

2010
{Lal} D  V, {Kraft} R  P, {Forman} W  R, {Hardcastle} M  J, {Jones} C, {Nulsen} P  E  J, {Evans} D  A, {Croston} J  H, {Lee} J  C. {A Chandra Observation of 3C 288{\mdash}Reheating the Cool Core of a 3 keV Cluster from a Nuclear Outburst at z = 0.246}. \apj. 2010;722 :1735-1743.Abstract

{We present results from a 42 ks Chandra/ACIS-S observation of the transitional FR I/FR II radio galaxy 3C 288 at z = 0.246. We detect \~{}3 keV gas extending to a radius of \~{}0.5 Mpc with a 0.5-2.0 keV luminosity of 6.6 {\times} 10$^{43}$ erg s$^{-1}$, implying that 3C 288 lies at the center of a poor cluster. We find multiple surface brightness discontinuities in the gas indicative of either a shock driven by the inflation of the radio lobes or a recent merger event. The temperature across the discontinuities is roughly constant with no signature of a cool core, thus disfavoring either the merger cold front or sloshing scenarios. We argue therefore that the discontinuities are shocks due to the supersonic inflation of the radio lobes. If they are shocks, the energy of the outburst is \~{}10$^{60}$ erg, or roughly 30% of the thermal energy of the gas within the radius of the shock, assuming that the shocks are part of a front produced by a single outburst. The cooling time of the gas is \~{}10⁸ yr, so that the energy deposited by the nuclear outburst could have reheated and efficiently disrupted a cool core. }

{Evans} D  A, {Reeves} J  N, {Hardcastle} M  J, {Kraft} R  P, {Lee} J  C, {Virani} S  N. {The Hard X-ray View of Reflection, Absorption, and the Disk-Jet Connection in the Radio-loud AGN 3C 33}. \apj. 2010;710 :859-868.Abstract

{We present results from Suzaku and Swift observations of the nearby radio galaxy 3C 33, and investigate the nature of absorption, reflection, and jet production in this source. We model the 0.5-100 keV nuclear continuum with a power law that is transmitted either through one or more layers of pc-scale neutral material, or through a modestly ionized pc-scale obscurer. The standard signatures of reflection from a neutral accretion disk are absent in 3C 33: there is no evidence of a relativistically blurred Fe K{$\alpha$} emission line, and no Compton reflection hump above 10 keV. We find the upper limit to the neutral reflection fraction is R łt} 0.41 for an e-folding energy of 1 GeV. We observe a narrow, neutral Fe K{$\alpha$} line, which is likely to originate at least 2000 R $_{s}$ from the black hole. We show that the weakness of reflection features in 3C 33 is consistent with two interpretations: either the inner accretion flow is highly ionized, or the black-hole spin configuration is retrograde with respect to the accreting material. }

{Lee} J  C. {X-ray Spectroscopy of Astrophysical Dust. A High Spectral Resolution (Re)View and Look to the Future}. 2010;157 :93-101.Abstract

{High spectral resolution X-ray instruments on powerful X-ray satellites (e.g. Chandra, XMM-Newton) pointed through dust and gas at bright black holes and neutron stars can be used to study dust and intervening material in unique ways. With the new subfield of Condensed Matter Astrophysics as its goal, I will discuss current efforts to combine techniques and knowledge from condensed matter physics and astrophysics to determine the species-specific quantity and composition of interstellar gas and dust in the ISM and ionized environments. Prospects for improving on this work in future X-ray missions with higher throughput and spectral resolution are also presented in the context of spectral resolution goals for gratings and calorimeters. }

{Lee} J  C. {X-ray Spectroscopy of Astrophysical Dust. A High Spectral Resolution (Re)View and Look to the Future}. 2010;157 :93-101.Abstract

{High spectral resolution X-ray instruments on powerful X-ray satellites (e.g. Chandra, XMM-Newton) pointed through dust and gas at bright black holes and neutron stars can be used to study dust and intervening material in unique ways. With the new subfield of Condensed Matter Astrophysics as its goal, I will discuss current efforts to combine techniques and knowledge from condensed matter physics and astrophysics to determine the species-specific quantity and composition of interstellar gas and dust in the ISM and ionized environments. Prospects for improving on this work in future X-ray missions with higher throughput and spectral resolution are also presented in the context of spectral resolution goals for gratings and calorimeters. }

2009
Noble MS, Li J, Young, A., Lee JC. Parallelizing the XSTAR Photoionization Code. Astronomical Data Analysis Software and Systems XVIII [Internet]. 2009;411 :301. Publisher's VersionAbstract

We describe two means by which XSTAR, a code which computes physical conditions and emission spectra of photoionized gases, has been parallelized. The first is pvmxstar, a wrapper which can be used in place of the serial xstar2xspec script to foster concurrent execution of the XSTAR command line application on independent sets of parameters. The second is pmodel, a plugin for the Interactive Spectral Interpretation System (ISIS) which allows arbitrary components of a broad range of astrophysical models to be distributed across processors during fitting and confidence limits calculations, by scientists with little training in parallel programming. Plugging the XSTAR family of analytic models into pmodel enables multiple ionization states (e.g., of a complex absorber/emitter) to be computed simultaneously, alleviating the often prohibitive expense of the traditional serial approach. Initial performance results indicate that these methods substantially enlarge the problem space to which XSTAR may be applied within practical timeframes.
{Hanke} M, {Wilms} J, {Nowak} M A, {Pottschmidt} K, {Schulz} N S, {Lee} J C. {Chandra X-Ray Spectroscopy of the Focused Wind in the Cygnus X-1 System. I. The Nondip Spectrum in the Low/Hard State}. The Astrophysical Journal. 2009;690 :330-346.Abstract

{We present analyses of a 50 ks observation of the supergiant X-ray binary system Cygnus X-1 (Cyg X-1)/HDE226868 taken with the Chandra High Energy Transmission Grating Spectrometer (HETGS). Cyg X-1 was in its spectrally hard state and the observation was performed during superior conjunction of the black hole, allowing for the spectroscopic analysis of the accreted stellar wind along the line of sight. A significant part of the observation covers X-ray dips as commonly observed for Cyg X-1 at this orbital phase, however, here we analyze only the high count rate nondip spectrum. The full 0.5-10 keV continuum can be described by a single model consisting of a disk, a narrow and a relativistically broadened Fe K{$\alpha$} line, and a power-law component, which is consistent with simultaneous Rossi X-Ray Timing Explorer broadband data. We detect absorption edges from overabundant neutral O, Ne, and Fe, and absorption line series from highly ionized ions and infer column densities and Doppler shifts. With emission lines of He-like Mg XI, we detect two plasma components with velocities and densities consistent with the base of the spherical wind and a focused wind. A simple simulation of the photoionization zone suggests that large parts of the spherical wind outside of the focused stream are completely ionized, which is consistent with the low velocities (łt}200 km s$^{-1}$) observed in the absorption lines, as the position of absorbers in a spherical wind at low projected velocity is well constrained. Our observations provide input for models that couple the wind activity of HDE 226868 to the properties of the accretion flow onto the black hole. }

{Neilsen} J, {Lee} J C. {Accretion disk winds as the jet suppression mechanism in the microquasar GRS 1915+105}. Nature [Internet]. 2009;458 :481-484. Publisher's VersionAbstract

{Stellar-mass black holes with relativistic jets, also known as microquasars, mimic the behaviour of quasars and active galactic nuclei. Because timescales around stellar-mass black holes are orders of magnitude smaller than those around more distant supermassive black holes, microquasars are ideal nearby laboratories' for studying the evolution of accretion disks and jet formation in black-hole systems. Whereas studies of black holes have revealed a complex array of accretion activity, the mechanisms that trigger and suppress jet formation remain a mystery. Here we report the presence of a broad emission line in the faint, hard states and narrow absorption lines in the bright, soft states of the microquasar GRS 1915+105. (Hard' and `soft' denote the character of the emitted X-rays.) Because the hard states exhibit prominent radio jets, we argue that the broad emission line arises when the jet illuminates the inner accretion disk. The jet is weak or absent during the soft states, and we show that the absorption lines originate when the powerful radiation field around the black hole drives a hot wind off the accretion disk. Our analysis shows that this wind carries enough mass away from the disk to halt the flow of matter into the radio jet. }

{Xiang} J, {Lee} J C, {Nowak} M A, {Wilms} J, {Schulz} N S. {The Accretion Disk Corona and Disk Atmosphere of 4U 1624-490 as Viewed by the Chandra-High Energy Transmission Grating Spectrometer}. The Astrophysical Journal. 2009;701 :984-993.Abstract

{We present a detailed spectral study (photoionization modeling and variability) of the ''Big Dipper'' 4U 1624-490 based on a Chandra-High Energy Transmission Grating Spectrometer (HETGS) observation over the \~{}76 ks binary orbit of 4U 1624-490 . While the continuum spectrum can be modeled using a blackbody plus power law, a slightly better fit is obtained using a single {$\Gamma$} = 2.25 power-law partially (71%) covered by a local absorber of column density N\_H Local=8.1\_$\{$-0.6$\}$\^{}$\{$+0.7$\}${\times} 10\^{}$\{$22$\}$ cm\^{}$\{$-2$\}$. The data show a possible quasi-sinusoidal modulation with period 43$^{+13}$ $_{-9}$ ks that might be due to changes in local obscuration. Photoionization modeling with the XSTAR code and variability studies of the observed strong Fe XXV and Fe XXVI absorption lines point to a two-temperature plasma for their origin: a highly ionized component of ionization parameter {$\xi$}$_{hot}$ {\ap} 10$^{4.3}$ erg cm s$^{-1}$ (T \~{} 3.0 {\times} 10⁶ K) associated with an extended accretion disk corona of radius R \~{} 3 {\times} 10$^{10}$ cm, and a less-ionized more variable component of {$\xi$} {\ap} 10$^{3.4}$ erg cm s$^{-1}$ (T \~{} 1.0 {\times} 10⁶ K) and {$\xi$} {\ap} 10\^{}$\{$3.1$\}$ erg cm s\^{}$\{$-1$\}$ (T \~{} 0.9 {\times} 10⁶ K) coincident with the accretion disk rim. We use this, with the observed Fe XXV and Fe XXVI absorption line variations (in wavelength, strength, and width) to construct a viewing geometry that is mapped to changes in plasma conditions over the 4U 1624-490 orbital period. }

{Neilsen} J, {Lee} J  C, {Nowak} M  A, {Dennerl} K, {Vrtilek} S  D. {Spectroscopic Signatures of the Superorbital Period in the Neutron Star Binary LMC X-4}. \apj. 2009;696 :182-191.Abstract

{We present the first high-resolution X-ray study of emission line variability with superorbital phase in the neutron star binary LMC X-4. Our analysis provides new evidence from X-ray spectroscopy confirming accretion disk precession as the origin of the superorbital period. The spectra, obtained with the Chandra High-Energy Transmission Grating Spectrometer and the XMM-Newton Reflection Grating Spectrometer, contain a number of emission features, including lines from hydrogen-like and helium-like species of N, O, Ne, and Fe, a narrow O VII radiative recombination continua (RRCs), and fluorescent emission from cold Fe. We use the narrow RRC and the He{$\alpha$} triplets to constrain the temperature and density of the (photoionized) gas. By comparing spectra from different superorbital phases, we attempt to isolate the contributions to line emission from the accretion disk and the stellar wind. There is also evidence for highly ionized iron redshifted and blueshifted by \~{}25,000 km s$^{-1}$. We argue that this emission originates in the inner accretion disk and show that the emission line properties in LMC X-4 are natural consequences of accretion disk precession. }

{Lee} J  C, {Xiang} J, {Ravel} B, {Kortright} J, {Flanagan} K. {Condensed Matter Astrophysics: A Prescription for Determining the Species-specific Composition and Quantity of Interstellar Dust Using X-rays}. \apj. 2009;702 :970-979.Abstract

{We present a new technique for determining the quantity and composition of dust in astrophysical environments using łt}6 keV X-rays. We argue that high-resolution X-ray spectra as enabled by the Chandra and XMM-Newton gratings should be considered a powerful and viable new resource for delving into a relatively unexplored regime for directly determining dust properties: composition, quantity, and distribution. We present initial cross section measurements of astrophysically likely iron-based dust candidates taken at the Lawrence Berkeley National Laboratory Advanced Light Source synchrotron beamline, as an illustrative tool for the formulation of our technique for determining the quantity and composition of interstellar dust with X-rays. (Cross sections for the materials presented here will be made available for astrophysical modeling in the near future.) Focused at the 700 eV Fe L$_{ III }$ and L$_{ II }$ photoelectric edges, we discuss a technique for modeling dust properties in the soft X-rays using L-edge data to complement K-edge X-ray absorption fine structure analysis techniques discussed by Lee {\amp} Ravel. The paper is intended to be a techniques paper of interest and useful to both condensed matter experimentalists and astrophysicists. For the experimentalists, we offer a new prescription for normalizing relatively low signal-to-noise ratio L-edge cross section measurements. For astrophysics interests, we discuss the use of X-ray absorption spectra for determining dust composition in cold and ionized astrophysical environments and a new method for determining species-specific gas and dust ratios. Possible astrophysical applications of interest, including relevance to Sagittarius A*, are offered. Prospects for improving on this work in future X-ray missions with higher throughput and spectral resolution are also presented in the context of spectral resolution goals for gratings and calorimeters, for proposed and planned missions such as Astro-H and the International X-ray Observatory. }

{Lee} J  C, {Smith} R  K, {Canizares} C  R, {Costantini} E, {de Vries} C, {Drake} J, {Dwek} E, {Edgar} R, {Juett} A  M, {Li} A, et al. {Solid State Astrophysics: Probing Interstellar Dust and Gas Properties with X-rays}, in astro2010: The Astronomy and Astrophysics Decadal Survey. Vol 2010. ; 2009 :178.Abstract

{The abundances of gas and dust (solids and complex molecules) in the interstellar medium (ISM) as well as their composition and structures impact practically all of astrophysics. Fundamental processes from star formation to stellar winds to galaxy formation all scale with the number of metals. However, significant uncertainties remain in both absolute and relative abundances, as well as how these vary with environment, e.g., stellar photospheres versus the interstellar medium (ISM). While UV, optical, IR, and radio studies have considerably advanced our understanding of ISM gas and dust, they cannot provide uniform results over the entire range of column densities needed. In contrast, X-rays will penetrate gas and dust in the cold (3K) to hot (100,000,000K) Universe over a wide range of column densities (log NH=20-24 cm\^{}-2), imprinting spectral signatures that reflect the individual atoms which make up the gas, molecule or solid. *X-rays therefore are a powerful and viable resource for delving into a relatively unexplored regime for determining gas abundances and dust properties such as composition, charge state, structure, and quantity via absorption studies, and distribution via scattering halos.* }

{Lee} J  C, {Xiang} J, {Ravel} B, {Kortright} J, {Flanagan} K. {Condensed Matter Astrophysics: A Prescription for Determining the Species-specific Composition and Quantity of Interstellar Dust Using X-rays}. \apj. 2009;702 :970-979.Abstract

{We present a new technique for determining the quantity and composition of dust in astrophysical environments using łt}6 keV X-rays. We argue that high-resolution X-ray spectra as enabled by the Chandra and XMM-Newton gratings should be considered a powerful and viable new resource for delving into a relatively unexplored regime for directly determining dust properties: composition, quantity, and distribution. We present initial cross section measurements of astrophysically likely iron-based dust candidates taken at the Lawrence Berkeley National Laboratory Advanced Light Source synchrotron beamline, as an illustrative tool for the formulation of our technique for determining the quantity and composition of interstellar dust with X-rays. (Cross sections for the materials presented here will be made available for astrophysical modeling in the near future.) Focused at the 700 eV Fe L$_{ III }$ and L$_{ II }$ photoelectric edges, we discuss a technique for modeling dust properties in the soft X-rays using L-edge data to complement K-edge X-ray absorption fine structure analysis techniques discussed by Lee {\amp} Ravel. The paper is intended to be a techniques paper of interest and useful to both condensed matter experimentalists and astrophysicists. For the experimentalists, we offer a new prescription for normalizing relatively low signal-to-noise ratio L-edge cross section measurements. For astrophysics interests, we discuss the use of X-ray absorption spectra for determining dust composition in cold and ionized astrophysical environments and a new method for determining species-specific gas and dust ratios. Possible astrophysical applications of interest, including relevance to Sagittarius A*, are offered. Prospects for improving on this work in future X-ray missions with higher throughput and spectral resolution are also presented in the context of spectral resolution goals for gratings and calorimeters, for proposed and planned missions such as Astro-H and the International X-ray Observatory. }

2008
{Evans} D  A, {Fong} W-F, {Hardcastle} M  J, {Kraft} R  P, {Lee} J  C, {Worrall} D  M, {Birkinshaw} M, {Croston} J  H, {Muxlow} T  W  B. {A Radio through X-Ray Study of the Jet/Companion-Galaxy Interaction in 3C 321}. \apj. 2008;675 :1057-1066.Abstract

{We present a multiwavelength study of the nucleus, environment, jets, and hot spots of the nearby FR II radio galaxy 3C 321, using new and archival data from MERLIN, the VLA, Spitzer, HST, and Chandra. An initially collimated radio jet extends northwest from the nucleus of its host galaxy and produces a compact knot of radio emission adjacent (in projection) to a companion galaxy, after which it dramatically flares and bends, extending out in a diffuse structure 35 kpc northwest of the nucleus. We argue that the simplest explanation for the unusual morphology of the jet is that it is undergoing an interaction with the companion galaxy. Given that the northwest hot spot that lies {\gt}\~{}250 kpc from the core shows X-ray emission, which likely indicates in situ high-energy particle acceleration, we argue that the jet-companion interaction is not a steady state situation. Instead, we suggest that the jet has been disrupted on a timescale less than the light-travel time to the end of the lobe, \~{}10⁶ yr, and that the jet flow to this hot spot will only be disrupted for as long as the jet-companion interaction takes place. The host galaxy of 3C 321 and the companion galaxy are in the process of merging, and each hosts a luminous AGN. As this is an unusual situation, we investigate the hypothesis that the interacting jet has driven material on to the companion galaxy, triggering its AGN. Finally, we present detailed radio and X-ray observations of both hot spots, which show that there are multiple emission sites, with spatial offsets between the radio and X-ray emission. }

{Evans} D  A, {Hardcastle} M  J, {Lee} J  C, {Kraft} R  P, {Worrall} D  M, {Birkinshaw} M, {Croston} J  H. {XMM-Newton Observations of the Nuclei of the Radio Galaxies 3C 305, DA 240, and 4C 73.08}. \apj. 2008;688 :844-851.Abstract

{We present new XMM-Newton EPIC observations of the nuclei of the nearby radio galaxies 3C 305, DA 240, and 4C 73.08, and investigate the origin of their nuclear X-ray emission. The nuclei of the three sources appear to have different relative contributions of accretion- and jet-related X-ray emission, as expected based on earlier work. The X-ray spectrum of the FR II narrow-line radio galaxy (NLRG) 4C 73.08 is modeled with the sum of a heavily absorbed power law that we interpret to be associated with a luminous accretion disk and circumnuclear obscuring structure, and an unabsorbed power law that originates in an unresolved jet. This behavior is consistent with other narrow-line radio galaxies. The X-ray emission of the low-excitation FR II radio galaxy DA 240 is best modeled as an unabsorbed power law that we associate with a parsec-scale jet, similar to other low-excitation sources that we have studied previously. However, the X-ray nucleus of the narrow-line radio galaxy 3C 305 shows no evidence for the heavily absorbed X-ray emission that has been found in other NLRGs. It is possible that the nuclear optical spectrum in 3C 305 is intrinsically weak-lined, with the strong emission arising from extended regions that indicate the presence of jet-environment interactions. Our observations of 3C 305 suggest that this source is more closely related to other weak-lined radio galaxies. This ambiguity could extend to other sources currently classified as NLRGs. We also present XMM-Newton and VLA observations of the hot spot of DA 240, arguing that this is another detection of X-ray synchrotron emission from a low-luminosity hot spot. }

2007
{Gibson} R  R, {Canizares} C  R, {Marshall} H  L, {Young} A  J, {Lee} J  C. {Line Variability in the High-Resolution X-Ray Spectrum of MCG -6-30-15}. \apj. 2007;655 :749-761.Abstract

{The recent 540 ks Chandra HETGS spectrum of the well-studied, variable active galactic nucleus (AGN) MCG -6-30-15 shows strong 1s-2p absorption lines from many ions. The spectrum was obtained over a period of about 10 days, and the large number of counts in the spectrum makes it ideal for testing variability on short timescales. We apply quantitative tests for line variability to the 1s-2p absorption lines of H- and He-like Ne, Mg, Si, and S. We find significant correlations and anticorrelations between lines as a function of time, much as we would expect if ionization levels in the absorber were varying. We also find evidence for variation in at least one 1s-2p resonance absorption line as a function of luminosity. We consider several possibilities to explain the line variation. First we consider factors that could change ionization levels in the absorber: radial motion, density variation, luminosity variation, and continuum shape variation. None of these individually can explain the line variation, although we cannot completely constrain continuum shape variation without simultaneous knowledge of the ultraviolet (UV) continuum. Other factors, considered individually, are also unable to explain all the variation: multiple changing continuum components, variable obscuration, and changes in velocity dispersion. Changes in line emission are an unlikely cause of significant variation in absorption-line measurements, but we are unable to fully constrain them. Variability could be due to a changing line of sight through a structured absorber. Modeling such scenarios should produce useful constraints on continuum emission mechanisms and absorber structure. }

{Xiang} J, {Lee} J  C, {Nowak} M  A. {Using the X-Ray Dust Scattering Halo of 4U 1624-490 to Determine Distance and Dust Distributions}. \apj. 2007;660 :1309-1318.Abstract

{We present X-ray dust scattering halo results based on our 76 ks Chandra ACIS-S/HETGS observation of the LMXB dipping source 4U 1624-490. Through analysis of the halo light curves with 2-6 keV spectra over the persistent and dipping periods, we estimate a geometric distance of \~{}15 kpc to 4U 1624-490. We also fit halo radial profiles with different ISM dust grain models to assess the location, uniformity, and density of the halo. Our analysis shows that the dust spatial distribution is not uniform along the line of sight; rather, it is consistent with the spiral arm structure mapped in H II. The large difference between the absorption hydrogen column (N$^{abs}$$_{H}\~{}8{\times}10^{22} cm^{-2}; probes all gas along the line of sight) derived from broadband spectral fitting and the scattering hydrogen column (N^{sca}$$_{H}$\~{}4{\times}10$^{22}$ cm$^{-2}$; probes only Galactic gas) derived from our studies of the 4U 1624-490 X-ray halo suggests that a large fraction of the column is local to the X-ray binary. We also present (and apply) a new method for assessing the Chandra point-spread function at large ({\gt}50'') angles, through use of the time delays from the observed dips. }

{Evans} D  A, {Lee} J  C, {Turner} T  J, {Weaver} K  A, {Marshall} H  L. {Probing Unification with Chandra HETGS and XMM-Newton EPIC and RGS Spectroscopy of the Narrow Emission Line Galaxy NGC 2110}. \apj. 2007;671 :1345-1354.Abstract

{We present results from Chandra HETGS (250 ks over two epochs) and XMM-Newton EPIC and RGS (60 ks) observations of NGC 2110, which has been historically classified as a narrow emission line galaxy. Our results support the interpretation that the source is a Seyfert 2 viewed through a patchy absorber. The nuclear X-ray spectrum of the source is best described by a power law of photon index {$\Gamma$}\~{}1.7, modified by absorption from multiple layers of neutral material at a large distance from the central supermassive black hole. We report the strong detections of Fe K{$\alpha$} and Si K{$\alpha$} lines, which are marginally resolved with the Chandra HETGS, and we constrain the emission radius of the fluorescing material to {\gt}\~{}1 pc. There is some evidence for modest additional broadening at the base of the narrow Fe K{$\alpha$} core with a velocity \~{}4500 km s$^{-1}$. We find tentative evidence for ionized emission (O VIII Ly{$\alpha$}, an O VIII RRC feature, and possibly a Ne IX forbidden line) in the Chandra MEG and XMM-Newton RGS spectra, which could be associated with the known extended X-ray emission that lies \~{}160 pc from the nucleus. We suggest that the 10$^{23}$ cm$^{-2}$ partially covering absorber originates in broad-line region clouds in the vicinity of the AGN, and that the 3{\times}10$^{22}$ cm$^{-2}$ coverer is likely to have a more distant origin and have a flattened geometry in order to allow the small-scale radio jet to escape. }

{Xiang} J, {Lee} J  C, {Nowak} M  A. {Using the X-Ray Dust Scattering Halo of 4U 1624-490 to Determine Distance and Dust Distributions}. \apj. 2007;660 :1309-1318.Abstract

{We present X-ray dust scattering halo results based on our 76 ks Chandra ACIS-S/HETGS observation of the LMXB dipping source 4U 1624-490. Through analysis of the halo light curves with 2-6 keV spectra over the persistent and dipping periods, we estimate a geometric distance of \~{}15 kpc to 4U 1624-490. We also fit halo radial profiles with different ISM dust grain models to assess the location, uniformity, and density of the halo. Our analysis shows that the dust spatial distribution is not uniform along the line of sight; rather, it is consistent with the spiral arm structure mapped in H II. The large difference between the absorption hydrogen column (N$^{abs}$$_{H}\~{}8{\times}10^{22} cm^{-2}; probes all gas along the line of sight) derived from broadband spectral fitting and the scattering hydrogen column (N^{sca}$$_{H}$\~{}4{\times}10$^{22}$ cm$^{-2}$; probes only Galactic gas) derived from our studies of the 4U 1624-490 X-ray halo suggests that a large fraction of the column is local to the X-ray binary. We also present (and apply) a new method for assessing the Chandra point-spread function at large ({\gt}50'') angles, through use of the time delays from the observed dips. }