Results for "C. Martin Gaskell"

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Off-Axis Energy Variability of AGNs: a New Paradigm for Broad-Line- and Continuum-Emitting RegionsJul 27 2011The general picture of how thermal AGNs work has become clearer in recent years but major observational puzzles threaten to undermine this picture. These puzzles include AGNs with extremely asymmetric emission line profiles, inconsistent multi-wavelength ... More
Off-Axis Energy Generation in Active Galactic Nuclei: Explaining Broad-Line Profiles, Spectropolarimetric Observations, and Velocity-Resolved Reverberation MappingAug 05 2010It is shown that broad-line region (BLR) line profiles ranging from the classic "logarithmic" profile to double-peaked, disk-like profiles are readily explained by the distribution of BLR gas proposed by Gaskell, Klimek, & Nazarova (2007; GKN) without ... More
The case for cases B and C: intrinsic hydrogen line ratios of the broad-line region of active galactic nuclei, reddenings, and accretion disc sizesDec 31 2015May 10 2016Low-redshift active galactic nuclei (AGNs) with extremely blue optical spectral indices are shown to have a mean, velocity-averaged, broad-line H$\alpha$/H$\beta$ ratio of $\thickapprox 2.72 \pm 0.04$, consistent with a Baker-Menzel Case B value. Comparison ... More
Accurate AGN black hole masses and the scatter in the M_{bh} - L_{bulge} relationshipFeb 26 2010A new empirical formulae is given for estimating the masses of black holes in AGNs from the H beta velocity dispersion and the continuum luminosity at 5100 Angstroms. It is calibrated to reverberation-mapping and stellar-dynamical estimates of black hole ... More
Inflow of the Broad-Line Region and the Fundamental Limitations of Reverberation MappingOct 21 2009The evidence from velocity-resolved reverberation mapping showing a net infall of the broad-line region (BLR) of AGNs is reviewed. Different lines in many objects at different epochs give a consistent picture of BLR motions. The motions are dominated ... More
Optical and X-ray Variability of AGNsDec 30 2006I present new comparisons of AGN optical, UV, and X-ray variations. These reveal complex relationships between the different passbands that can change with time in a given object. While there is evidence from several objects that X-ray and optical activity ... More
What broad emission lines tell us about how active galactic nuclei workAug 04 2009Oct 16 2009I review progress made in understanding the nature of the broad-line region (BLR) of active galactic nuclei (AGNs) and the role BLRs play in the AGN phenomenon. The high equivalent widths of the lines imply a high BLR covering factor, and the absence ... More
An Improved [O III] Line Width to Stellar Velocity Dispersion Calibration: Curvature, Scatter, and Lack of Evolution in the Black-Hole Mass Versus Stellar Velocity Dispersion RelationshipAug 03 2009An improved transformation of the full width at half maxima (FWHM) of the [O III] 5007 line in AGNs to the stellar velocity dispersion, sigma, of the host galaxy is given. This significantly reduces the systematic errors in using the [O III] FWHM as a ... More
Accurate Black Hole Mass Measurements for Thermal AGNs and the Origin of the Correlations Between Black Hole Mass and Bulge PropertiesNov 08 2011A simple refinement is proposed to the Dibai method for determining black hole masses in type-1 thermal AGNs. Comparisons with reverberation mapping black hole masses and host galaxy bulge properties suggest that the method is accurate to +/- 0.15 dex. ... More
Close supermassive binary black holesMar 25 2009Oct 16 2009It has been proposed that when the peaks of the broad emission lines in active galactic nuclei (AGNs) are significantly blueshifted or redshifted from the systemic velocity of the host galaxy, this could be a consequence of orbital motion of a supermassive ... More
Accretion Disks and the Nature and Origin of AGN Continuum VariabilityNov 14 2007Theory and observations of the dominant thermal continuum emission in AGNs are examined. After correction for reddening, the steady state AGN optical--UV spectral energy distributions (SEDs) are very similar. The SEDs are dominated energetically by the ... More
The Origin of Wavelength-Dependent Continuum Delays in AGNs - a New ModelDec 17 2006A model of wavelength-dependent lags in optical continuum variability of AGNs is proposed which avoids the problems of the popular ``lamp-post'' model. Rather than being due to reprocessing of high-energy radiation from a hypothetical source above the ... More
The origin of the relationship between black hole mass and host galaxy bulge luminosityApr 07 2010There is a strong decrease in scatter in the black hole mass versus bulge luminosity relationship with increasing luminosity and very little scatter for the most luminous galaxies. It is shown that this is a natural consequence of the substantial initial ... More
A Look at What Is (and Isn't) Known About Quasar Broad Line Regions and How Narrow-Line Seyfert 1 Galaxies Fit InMay 08 2000The evidence is reviewed that the Broad Line Region (BLR) probably has two distinct components located at about the same distance from the central black hole. One component, BLR II, is optically-thick, low-ionization emission at least some of which arises ... More
An Improved Black Hole Mass - Bulge Luminosity Relationship for AGNsJul 09 2009Two effects have substantially increased the scatter in the AGN black hole mass - host galaxy bulge luminosity relationship derived from SDSS spectra. The first is that at a fixed black hole mass, the SDSS spectrum depends strongly on redshift because ... More
Setting UBVRI Photometric Zero-Points Using Sloan Digital Sky Survey ugriz MagnitudesOct 31 2007We discuss the use of Sloan Digital Sky Survey (SDSS) ugriz point-spread function (PSF) photometry for setting the zero points of UBVRI CCD images. From a comparison with the Landolt (1992) standards and our own photometry we find that there is a fairly ... More
Modeling optical and UV polarization of AGNs I. Imprints of individual scattering regionsJul 04 2005Jan 06 2007[abridged] We investigate the effects of various AGN scattering region geometries on the polarized flux. We introduce a new, publicly available Monte Carlo radiative transfer code, Stokes, which models polarization induced by scattering off free electrons ... More
AGN Reddening and Ultraviolet Extinction Curves from Hubble Space Telescope SpectraNov 07 2007We present intrinsic extinction curves for 14 AGNs. The AGNs have reddenings, E(B-V), of up to 0.36 mag. The majority (13 out of 14) of the extinction curves are not steep in the UV. Of the seven best determined extinction curves, five have extinction ... More
Line Shifts, Broad-Line Region Inflow, and the Feeding of AGNsMay 28 2008Apr 01 2013Velocity-resolved reverberation mapping suggests that the broad-line regions (BLRs) of AGNs can have significant net inflow. We use the STOKES radiative transfer code to show that electron and Rayleigh scattering off the BLR and torus naturally explains ... More
Variability of Active Galactic Nuclei from the Optical to X-ray RegionsJul 09 2009Some progress in understanding AGN variability is reviewed. Reprocessing of X-ray radiation to produce significant amounts of longer-wavelength continua seems to be ruled out. In some objects where there has been correlated X-ray and optical variability, ... More
The Case for Optically-Thick High Velocity Broad Line Region Gas in Active Galactic NucleiFeb 21 2004A combined analysis of the profiles of the main broad quasar emission lines in both Hubble Space Telescope and optical spectra shows that while the profiles of the strong UV lines are quite similar, there is frequently a strong increase in the Ly-alpha/H-alpha ... More
The case for inflow of the broad-line region of active galactic nucleiDec 30 2015The high-ionization lines of the broad-line region (BLR) of thermal active galactic nuclei (AGNs) show blueshifts of a few hundred km/s to several thousand km/sec with respect to the low-ionization lines. This has long been thought to be due to the high-ionization ... More
The location of the dust causing internal reddening of active galactic nucleiJun 28 2016We use the Balmer decrements of the broad-line regions (BLRs) and narrow-line regions (NLRs) of active galactic nuclei (AGNs) as reddening indicators to investigate the location of the dust for four samples of AGNs with reliable estimates of the NLR contribution ... More
The production of strong broad He II emission after the tidal disruption of a main-sequence star by a supermassive black holeOct 03 2013The tidal disruption event (TDE) PS1-10jh lacked strong Balmer lines but showed strong, broad, He II emission both before maximum light and for at least 8 months thereafter. Gezari et al. interpreted this as evidence for the disruption of a rare hydrogen-deficient ... More
Different Velocity Dependences of Physical Conditions of High- and Low-Ionization Lines in Broad-Line RegionsMar 08 2004We present results from a study of high- and low-ionization emission line ratios as a function of projected velocity for a sample of eight active galactic nuclei (AGNs). Our results are based on analysis of high signal-to-noise optical and Hubble Space ... More
The Accuracy of Supermassive Black Hole Masses Determined by the Single-Epoch-Spectrum (Dibai) MethodSep 27 2008Apr 02 2009The first set of supermassive black hole mass estimates, published from 1980 to 1984 by E. A. Dibai, are shown to be in excellent agreement with recent reverberation-mapping estimates. Comparison of the masses of 17 AGNs covering a mass range from about ... More
NGC 5548: The AGN Energy Budget Problem and the Geometry of the Broad-Line Region and TorusNov 07 2007We consider in detail the spectral energy distribution (SED) and multi-wavelength variability of NGC5548. Comparison with the SEDs of other AGNs implies that the internal reddening of NGC5548 is E(B-V) = 0.17 mag. The extinction curve is consistent with ... More
Off-axis irradiation and the polarization of broad emission lines in active galactic nucleiNov 10 2013The STOKES Monte Carlo radiative transfer code has been extended to model the velocity dependence of the polarization of emission lines. We use STOKES to present improved modelling of the velocity-dependent polarization of broad emission lines in active ... More
Optical Variability of Narrow-Line Seyfert 1 GalaxiesMar 14 2004We present results of a broad-band photometric study of the optical variability of six Narrow-Line Seyfert 1 (NLS1) galaxies observed at 172 epochs. We searched for microvariability on 33 nights. Strong evidence for microvariability was found only for ... More
AGN polarization modeling with STOKESJan 07 2007We introduce a new, publicly available Monte Carlo radiative transfer code, STOKES, which has been developed to model polarization induced by scattering off free electrons and dust grains. It can be used in a wide range of astrophysical applications. ... More
Modeling the Polarization of Dusty Scattering Cones in Active Galactic NucleiJan 07 2007We have used the STOKES radiative transfer code, to model polarization induced by dust scattering in the polar regions of Active Galactic Nuclei (AGN). We discuss the wavelength-dependence of the spectral intensity and polarization over the optical/UV ... More
Attenuation from the optical to the extreme ultraviolet by dust associated with broad absorption line quasars: the driving force for outflowsNov 11 2016We use mid-IR to UV observations to derive a mean attenuation curve out to the rest-frame extreme ultraviolet (EUV) for "BAL dust" -- the dust causing the additional extinction of active galactic nuclei (AGNs) with broad absorption lines (BALQSOs). In ... More
Structure and Kinematics of the Broad-Line Region and Torus of Active Galactic NucleiJul 17 2008Energetics considerations imply that the broad-line region (BLR) has a high covering factor. The absence of absorption from the BLR means that the BLR has to have a flattened distribution and be seen through a polar hole. The BLR is the inward extension ... More
Changing-look Narrow-Line Seyfert 1s?Oct 20 2018Two major challenges to unification schemes for active galactic nuclei (AGN) are the existence of Narrow-Line Seyfert 1s (NLS1s) and the existence of changing-look (CL) AGNs. AGNs can drastically change their spectral appearance in the optical (changing ... More
Supermassive Binaries and Extragalactic JetsMay 29 1996Some quasars show Doppler shifted broad emission line peaks. I give new statistics of the occurrence of these peaks and show that, while the most spectacular cases are in quasars with strong radio jets inclined to the line of sight, they are also almost ... More
Evidence for Binary Orbital Motion of a Quasar Broad-Line RegionMay 29 1996Analysis of spectra of the quasar 3C 390.3 covering a period of over 20 yr shows that the blueshifted peak of H beta has been changing its radial velocity at an almost constant rate during this time. The radial velocity has increased by over 1500 km/s. ... More
Discovery of Polarization Reverberation in NGC 4151Nov 07 2007Feb 24 2012Observations of the optical polarization of NGC 4151 in 1997-2003 show variations of an order of magnitude in the polarized flux while the polarization position angle remains constant. The amplitude of variability of the polarized flux is comparable to ... More
The Nuclear Reddening Curve for Active Galactic Nuclei and the Shape of the Infra-Red to X-Ray Spectral Energy DistributionSep 22 2003Jul 14 2004We present extinction curves derived from the broad emission lines and continua of large samples of both radio-loud and radio-quiet AGNs. The curves are significantly flatter in the UV than are curves for the local ISM. The reddening curves for the radio-quiet ... More
The Blueshifting and Baldwin effects for the [OIII] 5007 Emission Line in Type 1 Active Galactic NucleiMay 05 2011Jun 22 2011We use homogeneous samples of radio-quiet Seyfert 1 galaxies and QSOs selected from the Sloan Digital Sky Survey to investigate the connection between the velocity shift and the equivalent width (EW) of the [OIII] 5007 emission line, and their correlations ... More
The Baldwin effect in the narrow emission lines of AGNsNov 06 2012Nov 07 2012The anti-correlations between the equivalent widths of emission lines and the continuum luminosity in AGNs, known as the Baldwin effect are well established for broad lines, but are less well studied for narrow lines. In this paper we explore the Baldwin ... More
Pion-induced Drell-Yan processes and the flavor-dependent EMC effectJul 22 2010Pion-induced Drell-Yan processes are proposed as a potential tool to measure the flavor dependence of the EMC effect, that is, the flavor-dependent modification of quark distributions in the nuclear medium. Existing pionic Drell-Yan data are compared ... More
How Polarization and Scattering can reveal Geometries, Dynamics, and Feeding of Active Galactic NucleiNov 05 2008Gaskell et al. (2007) introduced polarization reverberation mapping as a new technique to explore the structure of active galactic nuclei. We present modeling results for the time-dependent polarization signal expected from scattering inside a centrally ... More
The relative wavelength independence of IR lags in active galactic nuclei: implications for the distribution of the hot dustNov 06 2015Mar 07 2016We show that, contrary to simple predictions, most AGNs show at best only a small increase of lags in the J, H, K, and L bands with increasing wavelength. We suggest that a possible cause of this near simultaneity of the variability from the near-IR to ... More
Modeling optical and UV polarization of AGNs III. From uniform-density to clumpy regionsMar 18 2015A growing body of evidence suggests that part of, if not all, scattering regions of active galactic nuclei (AGNs) are clumpy. Hence. in this paper, we run radiative transfer models in the optical/UV for a variety of AGN reprocessing regions with different ... More
Optical Variability of the Three Brightest Nearby QuasarsDec 30 2006We report on the relative optical variability of the three brightest nearby quasars, 3C 273, PDS 456, and PHL 1811. All three have comparable absolute magnitudes, but PDS 456 and PHL 1811 are radio quiet. PDS 456 is a broad-line object, but PHL 1811 could ... More
Chemical Abundances in AGN Environment: X-Ray/UV Campaign on the MRK 279 OutflowNov 29 2006We present the first reliable determination of chemical abundances in an AGN outflow. The abundances are extracted from the deep and simultaneous FUSE and HST/STIS observations of Mrk 279. This data set is exceptional for its high signal-to-noise, unblended ... More
X-ray/UV Observing Campaign on the Mrk 279 AGN Outflow: A Global Fitting Analysis of the UV AbsorptionJan 25 2005We present an analysis of the intrinsic UV absorption in the Seyfert 1 galaxy Mrk 279 based on simultaneous long observations with the Hubble Space Telescope (41 ks) and the Far Ultraviolet Spectroscopic Explorer (91 ks). To extract the line-of-sight ... More
Modeling optical and UV polarization of AGNs II. Polarization imaging and complex reprocessingSep 13 2012We model the spectropolarimetric signature resulting from the radiative coupling between the innermost parts of active galactic nuclei (AGNs). We use a new public version of STOKES, a Monte Carlo radiative transfer code presented in the first paper of ... More
Transformations between 2MASS, SDSS and BVRI photometric systems: bridging the near infrared and opticalNov 27 2007We present colour transformations for the conversion of the {\em 2MASS} photometric system to the Johnson-Cousins $UBVRI$ system and further into the {\em SDSS} $ugriz$ system. We have taken {\em SDSS} $gri$ magnitudes of stars measured with the 2.5-m ... More
Impact of nuclear dependence of R=σ_L/σ_T on antishadowing in nuclear structure functionsJun 30 2012We study the impact of the nuclear dependence of R=\sigma_L/\sigma_T on the extraction of the F_2^A/F_2^D and F_1^A/F_1^D structure function ratios from the data on the \sigma^A/\sigma^D cross section ratios. Guided by indications of the nuclear dependence ... More
Precision Electron-Beam Polarimetry using Compton Scattering at 1 GeVSep 22 2015Feb 17 2016We report on the highest precision yet achieved in the measurement of the polarization of a low energy, $\mathcal{O}$(1 GeV), electron beam, accomplished using a new polarimeter based on electron-photon scattering, in Hall~C at Jefferson Lab. A number ... More
Reverberation Mapping Results from MDM ObservatoryOct 13 2009We present results from a multi-month reverberation mapping campaign undertaken primarily at MDM Observatory with supporting observations from around the world. We measure broad line region (BLR) radii and black hole masses for six objects. A velocity-resolved ... More
Discovery of Highly Blueshifted Broad Balmer and Metastable Helium Absorption Lines in a Tidal Disruption EventMar 13 2019We report the discovery of non-stellar hydrogen Balmer and metastable helium absorption lines accompanying a transient, high-velocity (0.05$c$) broad absorption line (BAL) system in the optical spectra of the tidal disruption event (TDE) AT2018zr ($z=0.071$). ... More
Reverberation measurement of the inner radius of the dust torus in NGC 4151 during 2008-2013Nov 13 2014Nov 15 2014We investigate the correlation between infrared (JHKL) and optical (B) fluxes of the variable nucleus of the Seyfert galaxy NGC 4151 using partially published data for the last 6 years (2008-2013.). Here we are using the same data as in Oknyansky et al. ... More
Long term optical and X-ray variability of the Seyfert galaxy Markarian 79Dec 03 2008We present the results of concurrent X-ray and optical monitoring of the Seyfert 1 galaxy Mrk 79 over a period of more than five years. We find that on short to medium time-scales (days to a few tens of days) the 2-10 keV X-ray and optical u and V band ... More
Hubble Space Telescope Ultraviolet Spectroscopy of Fourteen Low-Redshift QuasarsOct 13 2006We present low-resolution ultraviolet spectra of 14 low redshift (z<0.8) quasars observed with HST/STIS as part of a Snap project to understand the relationship between quasar outflows and luminosity. By design, all observations cover the CIV emission ... More
A novel comparison of Møller and Compton electron-beam polarimetersOct 19 2016Jan 25 2017We have performed a novel comparison between electron-beam polarimeters based on M{\o}ller and Compton scattering. A sequence of electron-beam polarization measurements were performed at low beam currents ($<$ 5 $\mu$A) during the $Q_{\rm weak}$ experiment ... More
Disk-Jet Connection in the Radio Galaxy 3C 120Sep 10 2009We present the results of extensive multi-frequency monitoring of the radio galaxy 3C 120 between 2002 and 2007 at X-ray, optical, and radio wave bands, as well as imaging with the Very Long Baseline Array (VLBA). Over the 5 yr of observation, significant ... More
A novel comparison of Møller and Compton electron-beam polarimetersOct 19 2016We have performed a novel comparison between electron-beam polarimeters based on M{\o}ller and Compton scattering. A sequence of electron-beam polarization measurements were performed at low beam currents ($<$ 5 $\mu$A) during the $Q_{\rm weak}$ experiment ... More
Nernst effect and diamagnetic response in a stripe model of superconducting cupratesFeb 26 2010Sep 21 2010We examine the possibility that the experimentally observed enhancement of superconducting (SC) fluctuations above the SC transition temperature in the underdoped cuprates is caused by stripes -- an intrinsic electronic inhomogeneity, common to hole-doped ... More
The Chandra X-Ray Observatory: Observations of Neutron StarsNov 09 2004We present here an overview of the status of the Chandra X-ray Observatory which has now been operating for five years. The Observatory is running smoothly, and the scientific return continues to be outstanding. We provide some details on the molecular ... More
The Chandra X-Ray Observatory: Progress Report and HighlightsAug 02 2012Over the past 13 years, the Chandra X-ray Observatory's ability to provide high resolution X-ray images and spectra have established it as one of the most versatile and powerful tools for astrophysical research in the 21st century. Chandra explores the ... More
The Chandra X-Ray OpticsOct 18 2011Oct 24 2011Significant advances in science always take place when the state of the art in instrumentation improves dramatically. NASA's Chandra X-Ray Observatory represents such an advance. Launched in July of 1999, Chandra is an observatory designed to study the ... More
The Chandra X-Ray Observatory (CXO): An OverviewDec 06 1999Significant advances in science inevitably occur when the state of the art in instrumentation improves. NASA's newest Great Observatory, the Chandra X-Ray Observatory (CXO) -- formally known as the Advanced X-Ray Astrophysics Facility (AXAF) -- launched ... More
SO(10) GUTs with large tensor representations on Noncommutative Space-timeNov 12 2013We construct a noncommutative version of a general renormalizable SO(10) GUT with Higgses in the 210, $\overline{126}, 45, 10$ and 120 irreps of SO(10) and a Peccei-Quinn symmetry. Thus, we formulate the noncommutative counterpart of a non-supersymmetric ... More
Yukawa terms in noncommutative SO(10) and E6 GUTsAug 11 2010We propose a method for constructing Yukawa terms for noncommutative SO(10) and E6 GUTs, when these GUTs are formulated within the enveloping-algebra formalism. The most general noncommutative Yukawa term that we propose contains, at first order in thetamunu, ... More
Ground State Properties and Glueball Spectrum in SU(2) Yang-Mills Theory using Gauge Invariant VariablesJan 15 1994We describe a nonperturbative calculation of the spectrum of SU(2) Yang-Mills theory based on a Hamiltonian formulation. Our approach exploits gauge invariant variables similar to those used in nuclear physics to describe collective motion in nuclei.
Don't choose theories: Normative inductive reasoning and the status of physical theoriesJan 01 2017Evaluating theories in physics used to be easy. Our theories provided very distinct predictions. Experimental accuracy was so small that worrying about epistemological problems was not necessary. That is no longer the case. The underdeterminacy problem ... More
Thou shalt not take sides: Cognition, Logic and the need for changing how we believeAug 21 2015We believe, in the sense of supporting ideas and considering them correct while dismissing doubts about them. We take sides about ideas and theories as if that was the right thing to do. And yet, from a rational point of view, this type of support and ... More
Modelling Epistemic SystemsJul 22 2012In this Chapter, I will explore the use of modeling in order to understand how Science works. I will discuss the modeling of scientific communities, providing a general, non-comprehensive overview of existing models, with a focus on the use of the tools ... More
Non-Stationary Covariance Matrices And NoiseOct 21 2006Dec 08 2006The exact meaning of the noise spectrum of eigenvalues of the covariance matrix is discussed. In order to better understand the possible phenomena behind the observed noise, the spectrum of eigenvalues of the covariance matrix is studied under a model ... More
Synchrotron Radiation from Outer Space and the Chandra X-Ray ObservatoryAug 24 2006The universe provides numerous extremely interesting astrophysical sources of synchrotron X radiation. The Chandra X-ray Observatory and other X-ray missions provide powerful probes of these and other cosmic X-ray sources. Chandra is the X-ray component ... More
Social Effects in Science: Modelling Agents for a Better Scientific PracticeMar 30 2009Science is a fundamental human activity and we trust its results because it has several error-correcting mechanisms. Its is subject to experimental tests that are replicated by independent parts. Given the huge amount of information available, scientists ... More
Continuous Opinions and Discrete Actions in Opinion Dynamics ProblemsNov 08 2007Aug 11 2008A model where agents show discrete behavior regarding their actions, but have continuous opinions that are updated by interacting with other agents is presented. This new updating rule is applied to both the voter and Sznajd models for interaction between ... More
Phenotypical Behavior and Evolutionary SlaveryFeb 09 2000A new evolutionary solution to Prisoner Dilemma situations is proposed in this paper. A specific genetic code may have different phenotypes, meaning different strategies for different individuals carrying that gene. This means that, under the right parameters, ... More
NC GUTs: A Status ReportJan 25 2011I review the main results that have been obtained so far on the construction of noncommutative GUTs
The UV and IR Origin of Nonabelian Chiral Gauge Anomalies on Noncommutative Minkowski Space-timeAug 15 2000Aug 16 2000We discuss both the UV and IR origin of the one-loop triangle gauge anomalies for noncommutative nonabelian chiral gauge theories with fundamental, adjoint and bi-fundamental fermions for U(N) groups. We find that gauge anomalies only come from planar ... More
The Masses Of The B-Stars In The High Galactic Latitude Eclipsing Binary IT LibOct 28 2002A number of blue stars which appear to be similar to Population I B-stars in the star forming regions of the galactic disk are found more than 1 kpc from the galactic plane. Uncertainties about the true distances and masses of these high latitude B-stars ... More
Renormalisability of the matter determinants in noncommutative gauge theory in the enveloping-algebra formalismJun 27 2007We consider noncommutative gauge theory defined by means of Seiberg-Witten maps for an arbitrary semisimple gauge group. We compute the one-loop UV divergent matter contributions to the gauge field effective action to all orders in the noncommutative ... More
Probing spin glasses with heuristic optimization algorithmsAug 26 2004A sketch of the chapter appearing under the same heading in the book ``New Optimization Algorithms in Physics'' (A.K. Hartmann and H. Rieger, Eds.) is given. After a general introduction to spin glasses, important aspects of heuristic algorithms for tackling ... More
From a sounding rocket per year to an observatory per lifetimeMar 20 2013I summarize the excitement of my role primarily in the early years of X-ray Astronomy. As a "second-generation" X-ray astronomer, I was privileged to participate in the enormous advance of the field, both technically and astrophysically, that occurred ... More
New Results from an old Friend: The Crab Nebula and its PulsarNov 14 2011We summarize here the results, most of which are preliminary, of a number of recent observations of the Crab nebula system with the Chandra X-Ray Observatory. We discuss four different topics: (1) The motion on long (> 1yr) time scales of the southern ... More
High Energy Polarization and the Crab: Historical RemarksAug 17 2009In this paper we first provide a brief history of the handful of X-ray polarization measurements that have been accomplished to date. We then outline new technological developments that indicate more detailed polarization measurements in this important ... More
X-Ray Polarimetry: Historical Remarks and Other ConsiderationsMay 22 2009We briefly discuss the history of X-ray polarimetry for astronomical applications including a guide to the appropriate statistics. We also provide an introduction to some of the new techniques discussed in more detail elsewhere in these proceedings. We ... More
Computing noncommutative Chern-Simons theory radiative corrections on the back of an envelopeApr 10 2001We show that the renormalized U(N) noncommutative Chern-Simons theory can be defined in perturbation theory so that there are no loop corrections to the 1PI functional of the theory in an arbitrary homogeneous axial (time-like, light-like or space-like) ... More
Opinion Particles: Classical Physics and Opinion DynamicsJul 12 2013A model for Opinion Particles, based on Bayesian-inspired models of Opinion Dynamics such as the CODA model is presented. By extending the discrete time characteristic of those models to continuous time, a theory for the movement of opinion particles ... More
Discrete Opinion models as a limit case of the CODA modelJan 22 2012Opinion Dynamics models can be, for most of them, divided between discrete and continuous. They are used in different circumstances and the relationship between them is not clear. Here we will explore the relationship between a model where choices are ... More
Network generation and evolution based on spatial and opinion dynamics componentsJan 10 2019In this paper, a model for a spatial network evolution based on a Metropolis simulation is presented. The model uses an energy function that depends both on the distance between the nodes and the stated preferences. The agents influence their network ... More
X-Ray/Ultraviolet Observing Campaign of the Markarian 279 Active Galactic Nucleus Outflow: a close look at the absorbing/emitting gas with Chandra-LETGSSep 14 2006We present a Chandra-LETGS observation of the Seyfert 1 galaxy Mrk 279. This observation was carried out simultaneously with HST-STIS and FUSE, in the context of a multiwavelength study of this source. The Chandra pointings were spread over ten days for ... More
The U(1)A anomaly in noncommutative SU(N) theoriesMar 17 2005May 25 2005We work out the one-loop $U(1)_A$ anomaly for noncommutative SU(N) gauge theories up to second order in the noncommutative parameter $\theta^{\mu\nu}$. We set $\theta^{0i}=0$ and conclude that there is no breaking of the classical $U(1)_A$ symmetry of ... More
Monitoring the Crab Nebula with Chandra:A Search for the Location of the gamma-ray FlaresNov 29 2012Dec 26 2012Subsequent to announcements by the AGILE and by the Fermi-LAT teams of the discovery of gamma-ray flares from the Crab Nebula in the fall of 2010, an international collaboration has been monitoring X-Ray emission from the Crab on a regular basis using ... More
The Making of the Chandra X-ray Observatory: the Project Scientist's PerspectiveMar 09 2010We review the history of the development of the Chandra X-ray Observatory from our personal perspective. This review is necessarily biased and limited by space since it attempts to cover a time span approaching 5 decades.
The covariant form of the gauge anomaly on noncommutative R2nOct 04 2001The covariant form of the non-Abelian gauge anomaly on noncommutative R2n is computed for U(N) groups. Its origin and properties are analyzed. Its connection with the consistent form of the gauge anomaly is established. We show along the way that bi-fundamental ... More
Chiral Gauge Anomalies on Noncommutative Minkowski Space-timeFeb 13 2001Chiral gauge anomalies on noncommutative Minkowski Space-time are computed and have their origin elucidated. The consistent form and the covariant form of the anomaly are obtained. Both Fujikawa's method and Feynman diagram techniques are used to carry ... More
Change and Aging Senescence as an adaptationMar 23 2011Understanding why we age is a long-lived open problem in evolutionary biology. Aging is prejudicial to the individual and evolutionary forces should prevent it, but many species show signs of senescence as individuals age. Here, I will propose a model ... More
Random, but not so much: A parameterization for the returns and correlation matrix of financial time seriesJan 02 2007A parameterization that is a modified version of a previous work is proposed for the returns and correlation matrix of financial time series and its properties are studied. This parameterization allows easy introduction of non-stationarity and it shows ... More
Bayesian Updating Rules in Continuous Opinion Dynamics ModelsJul 31 2008In this article, I investigate the use of Bayesian updating rules applied to modeling social agents in the case of continuos opinions models. Given another agent statement about the continuous value of a variable $x$, we will see that interesting dynamics ... More
The Origins and Evolutionary Status of B Stars Found Far From the Galactic Plane I: Composition and Spectral FeaturesAug 11 2004The existence of faint blue stars far above the galactic plane, which have spectra that are similar to nearby Population I B stars, present several interesting questions. Among them are: Can a Population I B star travel from the disk to a position many ... More
Do the gravitational corrections to the beta functions of the quartic and Yukawa couplings have an intrinsic physical meaning?Jul 20 2017We study the beta functions of the quartic and Yukawa couplings of General Relativity and Unimodular Gravity coupled to the $\lambda\phi^4$ and Yukawa theories with masses. We show that the General Relativity corrections to those beta functions as obtained ... More
The Chandra X-Ray Observatory: Five Years of OperationMar 03 2005The Chandra X-ray Observatory is the X-ray component of NASA's Great Observatory Program and has been operating successfully for over five years. We present here brief overview of the technical performance and some of the remarkable discoveries.