Results for "D. R. Ballantyne"

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The Contribution of Active Galactic Nuclei to the Microjansky Radio PopulationApr 06 2009A X-ray background synthesis model is used to calculate the contribution of Active Galactic Nuclei (AGNs) to the 1.4 GHz number counts between 100 nJy and 10 mJy. The number counts are broken down into contributions from radio-quiet and radio-loud AGNs, ... More
The average 0.5-200 keV spectrum of local active galactic nuclei and a new determination of the 2-10 keV luminosity function at z \approx 0Oct 28 2013The broadband X-ray spectra of AGNs contains information about the nuclear environment from Schwarzschild radii scales to distances of ~1 pc. In addition, the average shape of the X-ray spectrum is an important input into X-ray background synthesis models. ... More
Clustering, Cosmology and a New Era of Black Hole Demographics -- II. The Conditional Luminosity Functions of Type 2 and Type 1 Active Galactic NucleiApr 27 2016Sep 14 2016The orientation-based unification model of active galactic nuclei (AGNs) posits that the principle difference between obscured (Type 2) and unobscured (Type 1) AGNs is the line-of-sight into the central engine. If this model is correct than there should ... More
Obscuring Active Galactic Nuclei with Nuclear Starburst DisksJun 17 2008We assess the potential of nuclear starburst disks to obscure the Seyfert-like AGN that dominate the hard X-ray background at z~1. Over 1200 starburst disk models, based on the theory developed by Thompson et al., are calculated for five input parameters: ... More
The Integrated Relativistic Iron Line from Active Galactic Nuclei: Chasing the Spin Evolution of Supermassive Black HolesMay 04 2010The spin of a supermassive black hole (SMBH) is directly related to the radiative efficiency of accretion on to the hole, and therefore impacts the amount of fuel required for the black hole to reach a certain mass. Thus, a knowledge of the SMBH spin ... More
The Distribution and Cosmic Density of Relativistic Iron Lines in Active Galactic NucleiOct 20 2009X-ray observations of several active galactic nuclei show prominent iron K-shell fluorescence lines that are sculpted due to special and general relativistic effects. These observations are important because they probe the space-time geometry close to ... More
The Accretion Geometry in Radio-Loud Active GalaxiesJul 20 2007We review the latest attempts to determine the accretion geometry in radio-loud active galactic nuclei (AGN). These objects, which comprise ~10-20% of the AGN population, produce powerful collimated radio jets that can extend thousands of parsecs from ... More
Reflection spectra from an accretion disc illuminated by a neutron star X-ray burstFeb 25 2004Recent time-resolved X-ray spectra of a neutron star undergoing a superburst revealed an Fe K line and edge consistent with reprocessing from the surrounding accretion disc. Here, we present models of X-ray reflection from a constant density slab illuminated ... More
Clustering, Cosmology and a New Era of Black Hole Demographics -- I. The Conditional Luminosity Function of Active Galactic NucleiApr 27 2016Sep 14 2016Deep X-ray surveys have provided a comprehensive and largely unbiased view of active galactic nuclei (AGN) evolution stretching back to $z \sim 5$. However, it has been challenging to use the survey results to connect this evolution to the cosmological ... More
A complete view of the broad-line radio galaxy 4C+74.26 with XMM-NewtonJun 27 2005This paper presents a timing study and broadband spectral analysis of the broad-line radio galaxy 4C+74.26 based on a 35 ks XMM-Newton observation. As found in previous datasets, the source exhibits no evidence for rapid variability, and its 0.2-10 keV ... More
On the Contribution of AGN to the Cosmic Background RadiationJan 26 2007We present the results of calculations of the cosmic AGN background spectrum from 3 keV (4\times 10^{-4} microns) to 1000 microns. These computations make use of the measured X-ray luminosity function and its evolution, as well as fits from synthesis ... More
Revealing the accretion disc corona in Mrk 335 with multi-epoch X-ray spectroscopyAug 27 2015Dec 12 2015Active galactic nuclei host an accretion disc with an X-ray producing corona around a supermassive black hole. In bright sources, such as the Seyfert 1 galaxy Mrk 335, reflection of the coronal emission off the accretion disc has been observed. Reflection ... More
Constraining Radiatively Inefficient Accretion Flows with PolarizationMay 17 2007The low-luminosity black hole Sgr A* provides a testbed for models of Radiatively Inefficient Accretion Flows (RIAFs). Recent sub-millimeter linear polarization measurements of Sgr A* have provided evidence that the electrons in the accretion flow are ... More
The stellar remnants of high redshift nuclear starburst discs: a potential origin for nuclear star clusters?Feb 27 2019Nuclear starburst discs (NSDs) are very compact star-forming regions in the centers of galaxies that have been studied as a possible origin for the absorbing gas around a central active galactic nucleus. NSDs may be most relevant at $z\sim 1$ when obscured ... More
The response of the Fe Kαline to changes in the X-ray illumination of accretion discsJan 25 2002X-ray reflection spectra from photoionized accretion discs in active galaxies are presented for a wide range of illumination conditions. The energy, equivalent width (EW) and flux of the Fe K line are shown to depend strongly on the ratio of illuminating ... More
The Contribution of Particle Impact to the Production of Fe K Emission from Accreting Black HolesApr 11 2003The iron K line is perhaps the most important spectral diagnostic available in the study of accreting black holes. The line is thought to result from the reprocessing of external X-rays by the surface of the accretion disk. However, as is observed in ... More
Relativistic ionized accretion disc models of MCG--6-30-15Apr 20 2001Oct 11 2001We present results from fitting ionized accretion disc models to three long ASCA observations of the Seyfert 1 galaxy MCG--6-30-15. All three datasets can be fit by a model consisting of ionized reflection from the inner region of the accretion disc (with ... More
On the Dynamics of Suddenly Heated Accretion Disks around Neutron StarsFeb 22 2005Type I X-ray bursts and superbursts on neutron stars release sudden and intense radiation fields into their surroundings. Here, we consider the possible effects of these powerful explosions on the structure of the accretion disk. The goal is to account ... More
Accretion disk signatures in Type I X-ray Bursts: prospects for future missionsMay 19 2016Type I X-ray bursts and superbursts from accreting neutron stars illuminate the accretion disk and produce a reflection signal that evolves as the burst fades. Examining the evolution of reflection features in the spectra will give insight into the burst-disk ... More
The Evolution and Eddington Ratio Distribution of Compton Thick Active Galactic NucleiApr 05 2010Apr 27 2010Previous studies of the active galactic nuclei (AGN) contribution to the cosmic X-ray background (CXB) consider only observable parameters such as luminosity and absorbing column. Here, for the first time, we extend the study of the CXB to physical parameters ... More
Balancing the Cosmic Energy Budget: The Cosmic X-ray Background, Blazars and the Compton Thick AGN FractionOct 26 2009At energies ~>2 keV, active galactic nuclei (AGN) are the source of the cosmic X-ray background (CXB). For AGN population synthesis models to replicate the peak region of the CXB (~30 keV), a highly obscured and therefore nearly invisible class of AGN, ... More
Evidence for an Untruncated Accretion Disk in the Broad-Line Radio Galaxy 4C+74.26Feb 22 2005We present evidence for a broad, ionized Fe K line in the XMM-Newton spectrum of the broad-line radio galaxy (BLRG) 4C+74.26. This is the first indication that the innermost regions of the accretion flow in BLRGs contain thin, radiatively efficient disks. ... More
The Evolution of the Accretion Disk around 4U 1820-30 During a SuperburstJan 09 2004Accretion from a disk onto a collapsed, relativistic star -- a neutron star or black hole -- is the mechanism widely believed to be responsible for the emission from compact X-ray binaries. Because of the extreme spatial resolution required, it is not ... More
The evolving X-ray spectrum of active galactic nuclei: evidence for an increasing reflection fraction with redshiftApr 12 2019The cosmic X-ray background (XRB) spectrum and active galaxy number counts encode essential information about the spectral evolution of active galactic nuclei (AGNs) and have been successfully modeled by XRB synthesis models for many years. Recent measurements ... More
Continuum Acceleration of Black Hole WindsSep 16 2004Motivated by recent observations of high-velocity, highly ionized winds in several QSOs, models of purely continuum-driven winds launched from approximately 200 GM_{BH}/c^2 are presented. Launching conditions are investigated, as well as the observational ... More
Modelling the time dependence of the TeV Gamma-ray source at the Galactic CentreAug 16 2010The physical mechanism behind the TeV gamma-ray source observed at the centre of the Galaxy is still unknown. One intriguing possibility is that the accretion flow onto the central supermassive black hole is responsible for accelerating protons to TeV ... More
Soft X-ray emission lines from photoionized accretion discs: constraints on their strength and widthFeb 07 2002Jun 28 2002We consider the properties of soft X-ray emission lines in the reprocessed emission from a photoionized accretion disc. Observations of these lines will be important in determining the ionization state and metallicity of the innermost regions of active ... More
Fe Kαemission from photoionized slabs: the impact of the iron abundanceDec 07 2001Iron K\alpha emission from photoionized and optically thick material is observed in a variety of astrophysical environments including X-ray binaries, active galactic nuclei, and possibly gamma-ray bursts. This paper presents calculations showing how the ... More
Ionized Iron Lines in X-ray Reflection SpectraFeb 15 2001May 26 2001We present results from new calculations of the X-ray reflection spectrum from ionized accretion discs. These computations improve on our previous models by including the condition of hydrostatic balance in the vertical direction, following the work of ... More
Multiple X-ray reflection from ionized slabsJun 11 2002Multiple reflection of X-rays may be important when an accretion disc and its hot corona have a complicated geometry, or if returning radiation due to gravitational light bending is important, or in emission from a funnel such as proposed in some gamma-ray ... More
On the hard X-ray spectra of radio-loud active galaxiesJan 24 2002Mar 19 2002Over the last few years X-ray observations of broad-line radio galaxies (BLRGs) by ASCA, RXTE and BeppoSAX have shown that these objects seem to exhibit weaker X-ray reflection features (such as the iron K\alpha line) than radio-quiet Seyferts. This has ... More
X-ray Reflection By Photoionized Accretion DiscsFeb 02 2001May 29 2001We present the results of reflection calculations that treat the relevant physics with a minimum of assumptions. The temperature and ionization structure of the top five Thomson depths of an illuminated disc are calculated while also demanding that the ... More
Reflection Spectra from Photoionized Accretion DiscsApr 16 2002We review recent progress on the modeling and use of reflection spectra from irradiated and ionized accretion discs. On the computational side, calculations of reflection spectra from discs with non-uniform density structure have shown that thermal instabilities ... More
The HII Region KR 140: Spontaneous Formation of a High Mass StarMar 15 2000We have used a multiwavelength data set from the Canadian Galactic Plane Survey (CGPS) to study the Galactic HII region KR 140, both on the scale of the nebula itself and in the context of the star forming activity in the nearby W3/W4/W5 complex of molecular ... More
Radio stacking reveals evidence for star formation in the host galaxies of X-ray selected active galactic nuclei at z<1Oct 05 2011Nuclear starbursts may contribute to the obscuration of active galactic nuclei (AGNs). The predicted star formation rates are modest, and, for the obscured AGNs that form the X-ray background at z < 1, the associated faint radio emission lies just beyond ... More
A Possible Link Between the Galactic Center HESS Source and Sgr A*Jan 24 2007Feb 13 2007Recently, HESS and other air Cerenkov telescopes have detected a source of TeV gamma-rays coincident with the Galactic center. It is not yet clear whether the gamma-rays are produced via leptonic or hadronic processes, so it is important to consider possible ... More
Exploring the disc/jet interaction in the radio-loud quasar 4C +74.26 with SuzakuMay 23 2008We report on a 90 ks Suzaku observation of the radio-loud quasar 4C +74.26. The source was observed in its highest flux state to date, and we find that it brightened by about 20 per cent during the observation. We see evidence of spectral hardening as ... More
X-ray Reflection from Inhomogeneous Accretion Disks: I. Toy Models and Photon BubblesNov 17 2003Numerical simulations of the interiors of radiation dominated accretion disks show that significant density inhomogeneities can be generated in the gas. Here, we present the first results of our study on X-ray reflection spectra from such heterogeneous ... More
A two-component ionized reflection model of MCG--6-30-15Feb 14 2003Ionized reflection has often been considered as the explanation for the unusual Fe K variability observed in MCG--6-30-15. In this paper, we test this model using a 325 ks observation of MCG--6-30-15 by XMM-Newton and BeppoSAX. The data are fit between ... More
Evidence for Ionised Accretion Discs in Five Narrow-Line Seyfert 1 GalaxiesNov 20 2000(Abridged) We present the results of fitting ASCA spectra of six narrow-line Seyfert 1 (NLS1) galaxies with the ionised reflection models of Ross & Fabian (1993). We find that five of the galaxies (TONS 180, PKS 0558-504, Ark 564, Mrk 335 and PG 1244+026) ... More
Connecting Galaxy Evolution, Star Formation and the X-ray BackgroundSep 13 2005Jan 26 2006As a result of deep hard X-ray observations by Chandra and XMM-Newton a significant fraction of the cosmic X-ray background (CXRB) has been resolved into individual sources. These objects are almost all active galactic nuclei (AGN) and optical followup ... More
The Star Formation Laws of Eddington-Limited Star-Forming DisksJan 29 2013Two important avenues into understanding the formation and evolution of galaxies are the Kennicutt-Schmidt (KS) and Elmegreen-Silk (ES) laws. These relations connect the surface densities of gas and star formation (\sigmagas\ and \sigmastar, respectively) ... More
X-ray spectral diagnostics of the immediate environment of GRB 991216Jun 07 2002The recent report of iron line features in the afterglow of the gamma-ray burst (GRB) 991216 has important implications for the properties of the radiating material and hence the nature of the immediate burst environment. We argue that the putative strong ... More
A Correlation Between the Ionization State of the Inner Accretion Disk and the Eddington Ratio of Active Galactic NucleiApr 11 2011X-ray reflection features observed from the innermost regions of accretion disks in Active Galactic Nuclei (AGNs) allow important tests of accretion theory. In recent years it has been possible to use the Fe K line and reflection continuum to parametrize ... More
Classification of O Stars in the Yellow-Green: The Exciting Star VES 735Feb 08 1999Acquiring data for spectral classification of heavily reddened stars using traditional criteria in the blue-violet region of the spectrum can be prohibitively time consuming using small to medium sized telescopes. One such star is the Vatican Observatory ... More
On the Location and Composition of the Dust in the MCG-6-30-15 Warm AbsorberAug 01 2003Hubble Space Telescope images of MCG-6-30-15 show a dust lane crossing the galaxy just below the nucleus. In this paper, we argue that this dust lane is responsible for the observed reddening of the nuclear emission and the Fe I edge hinted at in the ... More
X-Raying an Accretion Disk in Realtime: the Evolution of Ionized Reflection during a Superburst from 4U 1636-536Oct 20 2014Dec 02 2014When a thermonuclear X-ray burst ignites on an accreting neutron star, the accretion disk undergoes sudden strong X-ray illumination, which can drive a range of processes in the disk. Observations of superbursts, with durations of several hours, provide ... More
The XMM-Newton View of the Broad-Line Radio Galaxy 3C 120Jul 21 2004We present the results of a 127 ks XMM-Newton observation of the broad-line radio galaxy 3C 120 performed simultaneously with RXTE. The time-averaged spectrum is Seyfert-like, with a reflection amplitude R ~ 0.5, and a neutral Fe K line with equivalent ... More
Ionized Disc Models of MCG-6-30-15Nov 28 2002Ionized reflection is one of the possible mechanisms proposed to explain the small short-timescale variability of the MCG-6-30-15 Fe K line. In this contribution, we show fits of the Ross & Fabian ionized reflection models to the 325 ks XMM-Newton observation ... More
The Primordial Helium Abundance: Towards Understanding and Removing the Cosmic Scatter in the dY/dZ RelationFeb 02 2000We present results from photoionization models of low-metallicity HII regions. These nebulae form the basis for measuring the primordial helium abundance. Our models show that the helium ionization correction factor (ICF) can be non-negligible for nebulae ... More
X-ray Reflection from Inhomogeneous Accretion Disks: II. Emission Line Variability and Implications for Reverberation MappingOct 08 2004One of the principal scientific objectives of the upcoming Constellation-X mission is to attempt to map the inner regions of accretion disks around black holes in Seyfert galaxies by reverberation mapping of the Fe K fluorescence line. This area of the ... More
Characterizing the Evolving X-Ray Spectral Features During a Superburst from 4U 1636-536Mar 19 2014Recent studies have shown that runaway thermonuclear burning of material accreted onto neutron stars, i.e. Type I X-ray bursts, may affect the accretion disk. We investigate this by performing a detailed time-resolved spectral analysis of the superburst ... More
Ionized Reflection Spectra from Accretion Disks Illuminated by X-ray PulsarsFeb 10 2012Feb 14 2012X-ray reflection signatures are observed around multiple classes of accreting compact objects. Modelling these features yield important constraints on the physics of accretion disks, motivating the development of X-ray reflection models appropriate for ... More
Modeling the Vertical Structure of Nuclear Starburst Discs: A Possible Source of AGN Obscuration at $z\sim 1$Mar 28 2017Nuclear starburst discs (NSDs) are star-forming discs that may be residing in the nuclear regions of active galaxies at intermediate redshifts. One dimensional (1D) analytical models developed by Thompson et al. (2005) show that these discs can possess ... More
Lifting the Veil on Obscured Accretion: Active Galactic Nuclei Number Counts and Survey Strategies for Imaging Hard X-ray MissionsMay 05 2011Finding and characterizing the population of active galactic nuclei (AGNs) that produces the X-ray background (XRB) is necessary to connect the history of accretion to observations of galaxy evolution at longer wavelengths. The year 2012 will see the ... More
A Submillimeter View of Star Formation Near the HII Region KR 140Mar 22 2001We present the results of 450 and 850 micron continuum mapping of the HII region KR 140 using the SCUBA instrument on the JCMT. KR 140 is a small (5.7 pc diameter) HII region at a distance of 2.3+/-0.3 kpc. Five of the six IRAS point sources near KR 140 ... More
Fe emission and ionized excess absorption in the luminous quasar 3C109 with XMM-NewtonMay 31 2006We report results from an XMM-Newton observation of the broad-line radio galaxy 3C109 (z=0.3056). Confirming previous results, an Fe emission line is detected, possibly comprising a broad component. However, the data cannot distinguish between an untruncated ... More
The Shape of the Cosmic X-ray Background: Nuclear Starburst Discs and the Redshift Evolution of AGN ObscurationNov 14 2017Jan 08 2018A significant number of active galactic nuclei (AGNs) are observed to be hidden behind dust and gas. The distribution of material around AGNs plays an important role in modeling the cosmic X-ray background (CXB), especially the fraction of Type-2 AGNs ... More
Achromatic late-time variability in thermonuclear X-ray bursts - an accretion disk disrupted by a nova-like shell?Sep 27 2010An unusual Eddington-limited thermonuclear X-ray burst was detected from the accreting neutron star in 2S 0918-549 with the Rossi X-ray Timing Explorer. The burst commenced with a brief (40 ms) precursor and maintained near-Eddington fluxes during the ... More
Extremely Weak Reflection Features in the X-ray Spectrum of XTE J1118+480: Possible Evidence for X-ray-Emitting Jets?Nov 01 2001Jun 26 2002We have simultaneously fit Chandra and RXTE spectra of the Galactic black hole XTE J1118+480 with three models for X-ray reflection. We explored a range of accretion disc ionizations (log(xi)=1-4; xi=L_X/nR^{2}) and iron abundances (0.10-1.00). Our fits ... More
Study of X-ray reflection features in a sample of Sy 1s observed by BeppoSAXFeb 11 2002The ionized disc reflection model of Ross & Fabian is applied to a sample of five Seyfert 1 galaxies observed by BeppoSAX. Good fits were obtained for all the objects in the sample. NGC 4051 and Mkn 335 (usually classified as NLSy 1s) show evidence of ... More
An XMM-Newton observation of Ton S180: Constraints on the continuum emission in ultrasoft Seyfert galaxiesJul 30 2002We present an XMM-Newton observation of the bright, narrow-line, ultrasoft Seyfert 1 galaxy Ton S180. The 0.3-10 keV X-ray spectrum is steep and curved, showing a steep slope above 2.5 keV (Gamma ~ 2.3) and a smooth, featureless excess of emission at ... More
On the equivalent width of the Fe K$α$ line produced by a dusty absorber in active galactic nucleiFeb 12 2015Obscured AGNs provide an opportunity to study the material surrounding the central engine. Geometric and physical constraints on the absorber can be deduced from the reprocessed AGN emission. In particular, the obscuring gas may reprocess the nuclear ... More
Does the AGN Unified Model Evolve with Redshift? Utilizing the X-ray Background to Predict the Mid-Infrared Emission of AGNAug 31 2006Deep X-ray surveys by Chandra and XMM-Newton have resolved about 80% of the 2-10 keV cosmic extragalactic X-ray background (CXRB) into point sources, the majority of which are obscured AGN. The obscuration might be connected to processes within the host ... More
The luminous X-ray hotspot in 4C 74.26: synchrotron or inverse-Compton emission?May 09 2007Jul 13 2007We report the discovery of an X-ray counterpart to the southern radio hotspot of the largest-known radio quasar 4C 74.26 (whose redshift is z=0.104). Both XMM-Newton and Chandra images reveal the same significant (10arcsec, i.e. 19kpc) offset between ... More
An XMM-Newton observation of Ark 120: the X-ray spectrum of a `bare' Seyfert 1 nucleusFeb 27 2004Mar 02 2004We report on a long (100 ks) XMM-Newton observation of the bright Seyfert 1 galaxy Arakelian 120. The source previously showed no signs of intrinsic reddening in its infrared-ultraviolet continuum and previous observations had shown no evidence for ionized ... More
How the X-ray spectrum of a Narrow-Line Seyfert 1 galaxy may be reflection dominatedFeb 15 2002A model for the inner regions of accretion flows is presented where, due to disc instabilities, cold and dense material is clumped into deep sheets or rings. Surrounding these density enhancements is hot, tenuous gas where coronal dissipation processes ... More
A Tale of Two Populations: The Contribution of Merger and Secular Processes to the Evolution of Active Galactic NucleiMar 22 2012Due to the co-evolution of supermassive black holes and their host galaxies, understanding the mechanisms that trigger active galactic nuclei (AGN) are imperative to understanding galaxy evolution and the formation of massive galaxies. It is observationally ... More
The Merger-Triggered Active Galactic Nuclei Contribution to the Ultraluminous Infrared Galaxy PopulationJun 05 2012It has long been thought that there is a connection between ultraluminous infrared galaxies (ULIRGs), quasars, and major mergers. Indeed, simulations show that major mergers are capable of triggering massive starbursts and quasars. However, observations ... More
The Young, the Old, and the Dusty: Stellar Populations of AGN HostsJul 19 2011Studying the average properties of active galactic nuclei (AGN) host stellar populations is an important step in understanding the role of AGN in galaxy evolution and the processes which trigger and fuel AGN activity. Here we calculate model spectral ... More
Properties and Expected Number Counts of Active Galactic Nuclei and their Hosts in the Far InfraredJan 18 2011Telescopes like Herschel and the Atacama Large Millimeter/submillimeter Array (ALMA) are creating new opportunities to study sources in the far infrared (FIR), a wavelength region dominated by cold dust emission. Probing cold dust in active galaxies allows ... More
Iron K-alpha Emission from X-ray Reflection: Predictions for Gamma-Ray Burst ModelsAug 20 2001Recent observations of several gamma-ray burst (GRB) afterglows have shown evidence for a large amount of X-ray line emitting material, possibly arising from ionized iron. A significant detection of an X-ray spectral feature, such as that found in the ... More
Short-term variability and PSD analysis of the radio-loud AGN 3C 390.3Aug 05 2009We investigate the short-term variability properties and the power spectral density (PSD) of the Broad-Line Radio Galaxy (BLRG) 3C 390.3 using observations made by XMM, RXTE, and Suzaku on several occasions between October 2004 and December 2006. The ... More
CO Line Emission from Compact Nuclear Starburst Disks Around Active Galactic NucleiApr 13 2012There is substantial evidence for a connection between star formation in the nuclear region of a galaxy and growth of the central supermassive black hole. Furthermore, starburst activity in the region around an active galactic nucleus (AGN) may provide ... More
X-ray Reflection and An Exceptionally Long Thermonuclear Helium Burst from IGR J17062-6143Oct 24 2016Thermonuclear X-ray bursts from accreting neutron stars power brief but strong irradiation of their surroundings, providing a unique way to study accretion physics. We analyze MAXI/GSC and Swift/XRT spectra of an exceptionally long flash observed from ... More
The imprint of carbon combustion on a superburst from the accreting neutron star 4U 1636-536Aug 26 2015Dec 12 2015Superbursts are hours-long X-ray flares attributed to the thermonuclear runaway burning of carbon-rich material in the envelope of accreting neutron stars. By studying the details of the X-ray light curve, properties of carbon combustion can be determined. ... More
Relativistic Lines and Reflection from the Inner Accretion Disks Around Neutron StarsAug 07 2009Jul 06 2010A number of neutron star low-mass X-ray binaries have recently been discovered to show broad, asymmetric Fe K emission lines in their X-ray spectra. These lines are generally thought to be the most prominent part of a reflection spectrum, originating ... More
Structure of the Accretion Flow in Broad-Line Radio Galaxies: The Case of 3C390.3Jun 03 2009We present XMM and Suzaku observations of the Broad-Line Radio Galaxy (BLRG) 3C390.3. The Fe Ka line has a width FWHM ~ 8,800 km/s, consistent within a factor two with the width of the double-peaked H_alpha line, suggesting an origin from the Broad Line ... More
XMM-Newton discovery of a sharp spectral feature at ~7 keV in the Narrow-Line Seyfert 1 galaxy 1H 0707-495Oct 16 2001We report the first detection of a sharp spectral feature in a Narrow-Line Seyfert 1 galaxy. Using XMM-Newton we have observed 1H0707-495 and find a drop in flux by a factor of more than 2 at a rest-frame energy of ~7 keV without any detectable narrow ... More
On the contribution of active galactic nuclei to reionization (Research Note)Dec 04 2013The electron scattering optical depth constraints on reionization suggest that there may be other sources that contribute to the ionization of hydrogen aside from observable star forming galaxies. Often the calculated value of the electron scattering ... More
Radio Synchrotron Emission from Secondary Leptons in the Vicinity of Sgr A*Jun 23 2007Aug 07 2007A point-like source of ~TeV gamma-rays has recently been seen towards the Galactic center by HESS and other air Cerenkov telescopes. In recent work (Ballantyne et al. 2007), we demonstrated that these gamma-rays can be attributed to high-energy protons ... More
Are Active Galactic Nuclei the Solution to the Excess Cosmic Radio Background at 1.4 GHz?Oct 03 2011Recently the ARCADE 2 experiment measured the cosmic radio background (CRB) and found the brightness temperature of the CRB at 1.4 GHz to be ~480 mK. Integrating the flux density from the observed 1.4 GHz radio source count produces a brightness temperature ... More
A long hard look at MCG-6-30-15 with XMM-NewtonJun 06 2002We present first results from a 325 ks observation of the Seyfert 1 galaxy MCG-6-30-15 with XMM-Newton and BeppoSAX. The strong, broad, skewed iron line is clearly detected and is well characterised by a steep emissivity profile within 6r_g (i.e. 6GM/c^2) ... More
A growth-rate indicator for Compton-thick active galactic nucleiJun 29 2016Due to their heavily obscured central engines, the growth rate of Compton-thick (CT) active galactic nuclei (AGN) is difficult to measure. A statistically significant correlation between the Eddington ratio, {\lambda}$_{Edd}$, and the X-ray power-law ... More
The NuSTAR X-ray spectrum of the low-luminosity AGN in NGC 7213Jul 07 2015We present an analysis of the 3-79 keV NuSTAR spectrum of the low-luminosity active galactic nucleus NGC 7213. In agreement with past observations, we find a lower limit to the high-energy cut-off of Ec > 140 keV, no evidence for a Compton-reflected continuum, ... More
A self-consistent approach to the hard and soft states of 4U 1705-44Apr 12 2010Apr 13 2010We analyzed two XMM-Newton observations of the bright atoll source 4U 1705-44, which can be considered a prototype of the class of the persistent NS LMXBs showing both hard and soft states. The first observation was performed when the source was in a ... More
A bright thermonuclear X-ray burst simultaneously observed with Chandra and RXTEJan 10 2013Mar 29 2013The prototypical accretion-powered millisecond pulsar SAX J1808.4-3658 was observed simultaneously with Chandra-LETGS and RXTE-PCA near the peak of a transient outburst in November 2011. A single thermonuclear (type-I) burst was detected, the brightest ... More
The hard X-ray spectrum of NGC 5506 as seen by NuSTARDec 15 2014NuSTAR observed the bright Compton-thin, narrow line Seyfert 1 galaxy, NGC 5506, for about 56 ks. In agreement with past observations, the spectrum is well fit by a power law with Gamma~1.9, a distant reflection component and narrow ionized iron lines. ... More
Investigating the Covering Fraction Distribution of Swift/BAT AGN with X-ray and IR ObservationsNov 06 2018Apr 08 2019We present an analysis of a sample of 69 local obscured Swift/Burst Alert Telescope active galactic nuclei (AGNs) with X-ray spectra from NuSTAR and infrared (IR) spectral energy distributions from Herschel and WISE. We combine this X-ray and IR phenomenological ... More
Radio/X-ray monitoring of the broad-line radio galaxy 3C 382. High-energy view with XMM-Newton and NuSTARMay 16 2018We present the analysis of five joint XMM-Newton/NuSTAR observations, 20 ks each and separated by 12 days, of the broad-line radio galaxy 3C 382. The data were obtained as part of a campaign performed in September-October 2016 simultaneously with VLBA. ... More
Investigating the Covering Fraction Distribution of Swift/BAT AGN with X-ray and IR ObservationsNov 06 2018We present an analysis of a sample of 69 local obscured Swift/BAT active galactic nuclei (AGN) with X-ray spectra from NuSTAR and IR spectral energy distributions from Herschel and WISE. We combine this X-ray and IR phenomenological modeling and find ... More
Photoevaporation of Circumstellar Disks around Young StarsFeb 10 2004We examine the ability of photoevaporative disk winds to explain the low-velocity components observed in the forbidden line spectra of low-mass T Tauri stars. Using the analytic model of Shu, Johnstone, & Hollenbach (1993) and Hollenbach et al. (1994) ... More
Accretion disks and coronae in the X-ray flashlightNov 16 2017Plasma accreted onto the surface of a neutron star can ignite due to unstable thermonuclear burning and produce a bright flash of X-ray emission called a Type-I X-ray burst. Such events are very common; thousands have been observed to date from over a ... More
JHK Standard Stars for Large Telescopes: the UKIRT Fundamental and Extended ListsFeb 15 2001Feb 20 2001We present high-precision JHK photometry with the 3.8m UK Infrared Telescope (UKIRT) of 83 standard stars, 28 from the widely used preliminary list known as the "UKIRT Faint Standards" (Casali & Hawarden, 1992), referred to here as the Fundamental List, ... More
Emission-line Helium Abundances in Highly Obscured NebulaeJul 27 1999This paper outlines a way to determine the ICF using only infrared data. We identify four line pairs, [NeIII] 36$\micron$/[NeII] 12.8$\micron$, [NeIII]~15.6$\micron$ /[NeII] 12.8$\micron$, [ArIII] 9$\micron$/[ArII] 6.9$\micron$, and [ArIII] 21$\micron$/[ArII] ... More
NuSTAR Observations of the Compton-thick Active Galactic Nucleus and Ultraluminous X-ray Source Candidate in NGC 5643Sep 10 2015We present two NuSTAR observations of the local Seyfert 2 active galactic nucleus (AGN) and an ultraluminous X-ray source (ULX) candidate in NGC 5643. Together with archival data from Chandra, XMM-Newton and Swift-BAT, we perform a high-quality broadband ... More
The NuSTAR Extragalactic Surveys: source catalog and the Compton-thick fraction in the UDS fieldJan 05 2018We present the results and the source catalog of the NuSTAR survey in the UKIDSS Ultra Deep Survey (UDS) field, bridging the gap in depth and area between NuSTAR's ECDFS and COSMOS surveys. The survey covers a $\sim 0.6$ deg$^2$ area of the field for ... More
X-ray bolometric corrections for Compton-thick active galactic nucleiMay 30 2017We present X-ray bolometric correction factors, $\kappa_{Bol}$ ($\equiv L_{Bol}/L_X$), for Compton-thick (CT) active galactic nuclei (AGN) with the aim of testing AGN torus models, probing orientation effects, and estimating the bolometric output of the ... More
Determining the covering factor of Compton-thick active galactic nuclei with NuSTARFeb 25 2015The covering factor of Compton-thick obscuring material associated with the torus in active galactic nuclei (AGN) is at present best understood through the fraction of sources exhibiting Compton-thick absorption along the line of sight ($N_{H}>1.5\times10^{24}$ ... More
NuSTAR observations of Mrk 766: distinguishing reflection from absorptionJul 31 2018We present two new NuSTAR observations of the narrow line Seyfert 1 (NLS1) galaxy Mrk 766 and give constraints on the two scenarios previously proposed to explain its spectrum and that of other NLS1s: relativistic reflection and partial covering. The ... More