Results for "P. Coppi"

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The QUEST-La Silla AGN Variability Survey: selection of AGN candidates through optical variabilityApr 09 2019We used data from the QUEST-La Silla Active Galactic Nuclei (AGN) variability survey to construct light curves for 208,583 sources over $\sim 70$ deg$^2$, with a a limiting magnitude $r \sim 21$. Each light curve has at least 40 epochs and a length of ... More
Constraints on the VHE Emissivity of the Universe from the Diffuse GeV Gamma-Ray BackgroundOct 22 1996VHE (Very High Energy, E>100 GeV) radiation emitted at cosmological distances will pair produce on low-energy diffuse extragalactic background radiation before ever reaching us. This prevents us from directly seeing most of the VHE emission in the Universe. ... More
An Infrared Study of the Large-scale Jet in Quasar PKS 1136-135Mar 13 2007Mar 16 2007We present Spitzer IRAC imaging of the large-scale jet in the quasar PKS 1136-135 at wavelengths of 3.6 and 5.8 um, combined with previous VLA, HST, and Chandra observations. We clearly detect infrared emission from the jet, resulting in the most detailed ... More
Towards Population III: The Collapse and Fragmentation of Primordial GasMar 22 2001We briefly review the motivations for studying the formation of the first ``Population III'' stars and present recent results from our numerical simulations in this area. We discuss the new questions raised as a result of the simulations presented by ... More
Gamma-ray bursts from alternating-current jetsSep 17 2003We propose a gamma-ray burst scenario involving relativistic jets dominated by Poynting flux with alternating toroidal magnetic fields. Such a structure may arise naturally if the jet is formed and powered by the accretion flow in the core of a collapsar. ... More
X-ray/TeV-gamma-ray observations of several strong flares of Mkn 501 during 1997 and implicationsNov 12 1999During more than 6 months in 1997, the BL Lac object Mkn 501 was in an exceptionally bright state, both in the X-ray band and in the Very High Energy (VHE) band. In this paper we present a multiwavelength study of Mkn 501 during this extraordinary outburst. ... More
Very High Energy Gamma-Rays from AGN: Cascading on the Cosmic Background Radiation Fields and the Formation of Pair HalosDec 18 1993Recent high energy gamma-ray observations (E>100 MeV) of blazar AGN show emission spectra with no clear upper energy cutoff. AGN, considered to be possible sources for the highest energy cosmic rays, may have emission extending well into the VHE (very ... More
Probing dissipation mechanisms in BL Lac jets through X-ray polarimetryJan 30 2018The dissipation of energy flux in blazar jets plays a key role in the acceleration of relativistic particles. Two possibilities are commonly considered for the dissipation processes, magnetic reconnection -- possibly triggered by instabilities in magnetically-dominated ... More
The Fragmentation of Pre-enriched Primordial ObjectsApr 17 2001Sep 10 2001Recent theoretical investigations have suggested that the formation of the very first stars, forming out of metal-free gas, was fundamentally different from the present-day case. In this paper, we study the effect of metallicity on the evolution of the ... More
Theoretical Resolution of Magnetic Reconnection in High Energy PlasmasJun 14 2007The formation of macroscopic reconnected magnetic structures (islands) have been observed in advanced experiments on weakly collisional, well confined plasmas while established theories of the drift-tearing modes, which depend strongly on the electron ... More
Thermo-Rotational Instability in Plasma Disks Around Compact ObjectsFeb 12 2008Differentially rotating plasma disks, around compact objects, that are imbedded in a ``seed'' magnetic field are shown to develop vertically localized ballooning modes that are driven by the combined radial gradient of the rotation frequency and vertical ... More
Radio-loud AGN contribution to the Extragalactic Gamma-Ray BackgroundDec 05 2011Origin of extragalactic gamma-ray background (EGRB) is still a matter of debate. EGRB can either have truly diffuse or unresolved discrete point sources origin. Majority of the Fermi and EGRET detected identified sources are blazar. So, they are expected ... More
GeV-TeV Blazar Population StudiesNov 02 2011The synergy between the Fermi-LAT and ground-based Cherenkov telescope arrays gives us the opportunity for the first time to characterize the high energy emission (100 MeV - 10 TeV) from more than 30 blazars. In this study we performed a Fermi-LAT spectral ... More
Radiation mechanisms and geometry of Cygnus X-1 in the soft stateMay 12 1999May 13 1999We present X-ray/gamma-ray spectra of Cyg X-1 observed during the transition from the hard to the soft state and in the soft state by ASCA, RXTE and OSSE in 1996 May and June. The spectra consist of a dominant soft component below ~2 keV and a power-law-like ... More
Simultaneous X-Ray and TeV Gamma-Ray Observations of the TeV Blazar Markarian 421 during February and May 2000May 18 2001In this paper we present the results of simultaneous observations of the TeV blazar Markarian 421 (Mrk 421) at X-ray and TeV Gamma-ray energies with the Rossi X-Ray Timing Explorer (RXTE) and the stereoscopic Cherenkov Telescope system of the HEGRA (High ... More
The intergalactic magnetic field constrained by Fermi/LAT observations of the TeV blazar 1ES 0229+200Apr 08 2010Jun 08 2010TeV photons from blazars at relatively large distances, interacting with the optical-IR cosmic background, are efficiently converted into electron-positron pairs. The produced pairs are extremely relativistic (Lorentz factors of the order of 1e6 1e7 and ... More
The CIDA-UCM-Yale Shallow Survey for Emission Line GalaxiesFeb 23 2005We present the CIDA-UCM-Yale (Centro de Investigaciones de Astronomia, Universidad Complutense de Madrid and Yale University) survey for Halpha+[NII]6549,6584 emission-line galaxies using objective-prism spectra. The most important properties of a catalogue ... More
Properties of AGN coronae in the NuSTAR era II: hybrid plasmaJan 24 2017The corona, a hot cloud of electrons close to the centre of the accretion disc, produces the hard X-ray power-law continuum commonly seen in luminous Active Galactic Nuclei (AGN). The continuum has a high-energy turnover, typically in the range of one ... More
The QUEST-La Silla AGN variability survey: connection between AGN variability and black hole physical propertiesAug 02 2018Aug 27 2018We present our statistical analysis of the connection between active galactic nuclei (AGN) variability and physical properties of the central supermassive black hole (SMBH). We constructed optical light curves using data from the QUEST-La Silla AGN variability ... More
High-Energy Polarimetry - a new window to probe extreme physics in AGN jetsMar 11 2019The constantly improving sensitivity of ground-based and space-borne observatories has made possible the detection of high-energy emission (X-rays and gamma-rays) from several thousands of extragalactic sources. Enormous progress has been made in measuring ... More
Workshop Summary for Cracow 1997: Relativistic Jets in AGNsMar 10 1999I present a general overview of the results discussed during the Cracow 1997 workshop on ``Relativistic Jets in AGNs.'' My emphasis will be on showing the significant progress made in several areas over the last few years, pointing out what I feel are ... More
The Physics of Hybrid Thermal/Non-Thermal PlasmasMar 10 1999Models of the continuum radiation from accreting hot plasmas typically assume that the plasma heating mechanism produces energetic particles distributed in energy either as a Maxwellian (the ``thermal'' models) or as an extended power law (the ``non-thermal'' ... More
Unification of Spectral States of Accreting Black HolesNov 26 1997Several galactic black holes show transitions between spectral states. In this paper we present a unifying radiation transfer model that can fit the spectral data in both states. We fit individual broad-band (from 1 keV up to 4 MeV) spectral data for ... More
Delayed GeV--TeV Photons from Gamma-Ray Bursts Producing High-Energy Cosmic RaysMar 27 1996A scenario in which cosmic rays (CRs) above $10^{20}{\rm eV}$ are produced by cosmological gamma-ray bursts (GRBs) is consistent with observations provided that deflections by the inter-galactic magnetic field (IGMF) delay and spread the arrival time ... More
SMARTS optical and infrared monitoring of 12 gamma-ray bright blazarsJan 20 2012We present multiwavelength data for twelve blazars observed from 2008-2010 as part of an ongoing optical-infrared photometric monitoring project. Sources were selected to be bright, southern (dec < 20 deg) blazars observed by the Fermi Gamma-Ray Space ... More
High Redshift X-Ray Selected Quasars: CXOCY J125304.0-090737 joins the clubJan 29 2003Jan 30 2003We present a new X-ray selected high redshift quasar CXOCY J125304.0-090737 at z=4.179, discovered by the Calan-Yale Deep Extragalactic Research (CYDER) Survey. This quasar is the fifth X-ray selected high redshift radio quiet quasar ($z>4$) found so ... More
Similarity of the Optical-IR and Gamma-Ray Time Variability of Fermi BlazarsJan 20 2011Mar 07 2012We present the time variability properties of a sample of six blazars, AO 0235+164, 3C 273, 3C 279, PKS 1510-089, PKS 2155-304, and 3C 454.3, at optical-IR as well as gamma-ray energies. These observations were carried out as a part of the Yale/SMARTS ... More
Reddening Independent Quasar Selection from a Wide Field Optical and Near-IR Imaging SurveyDec 13 2000We present preliminary results from a wide field near-IR imaging survey that uses the Cambridge InfraRed Survey Instrument (CIRSI) on the 2.5m Isaac Newton Telescope (INT). CIRSI is a JH-band mosaic imager that contains 4 Rockwell 1024$^{2}$ HgCdTe detectors ... More
The CYDER Survey: First ResultsNov 29 2002We present the Calan-Yale Deep Extragalactic Research (CYDER) Survey. The broad goals of the survey are the study of stellar populations, the star formation history of the universe and the formation and evolution of galaxies. The fields studied include ... More
Near Infrared Variability of obscured and unobscured X-ray selected AGN in the COSMOS fieldOct 03 2017We present our statistical study of near infrared (NIR) variability of X-ray selected Active Galactic Nuclei (AGN) in the COSMOS field, using UltraVISTA data. This is the largest sample of AGN light curves in YJHKs bands, making possible to have a global ... More
Gamma-Ray Summary ReportJan 10 2002This paper reviews the field of gamma-ray astronomy and describes future experiments and prospects for advances in fundamental physics and high-energy astrophysics through gamma-ray measurements. We concentrate on recent progress in the understanding ... More
Ballooning Modes in Thin Accretion Disks: Limits for their ExcitationJun 17 2003The conditions that limit the possible excitation of ideal MHD axisymmetric ballooning modes in thin accretion disks are discussed. As shown earlier by Coppi and Coppi (2001), these modes are well-localized in the vertical direction but have characteristic ... More
The SMARTS Multi-epoch Optical Spectroscopy Atlas (SAMOSA): Using Emission Line Variability to Probe the Location of the Blazar Gamma-emitting RegionFeb 03 2015We present multi-epoch optical spectroscopy of seven southern Fermi-monitored blazars from 2008 - 2013 using the Small and Medium Aperture Research Telescope System (SMARTS), with supplemental spectroscopy and polarization data from the Steward Observatory. ... More
Revealing the First Stellar and Supermassive Black Holes to EXISTFeb 25 2010Mar 16 2010The epochs of origin of the first stars and galaxies, and subsequent growth of the first supermassive black holes, are among the most fundamental questions. Observations of the highest redshift Gamma-Ray Bursts (GRBs) will be the most compelling in situ ... More
An Optical-Near-Infrared Outburst with No Accompanying Gamma-Rays in the Blazar PKS 0208-512Dec 11 2012We report the discovery of an anomalous flare in a bright blazar, namely, PKS 0208-512, one of the targets of the Yale/SMARTS optical-near-infrared (OIR) monitoring program of Fermi blazars. We identify three intervals during which PKS 0208-512 undergoes ... More
Shedding New Light on the 3C 273 Jet with the Spitzer Space TelescopeMay 20 2006We have performed infrared imaging of the jet of the quasar 3C 273 at wavelengths 3.6 and 5.8 microns with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. When combined with the radio, optical and X-ray measurements, the IRAC photometry ... More
Dynamical analysis of the nonlinear growth of the m=n=1 resistive internal modeMar 17 2003A dynamical analysis is presented that self-consistently takes into account the motion of the critical layer, in which the magnetic field reconnects, to describe how the m=n=1 resistive internal kink mode develops in the nonlinear regime. The amplitude ... More
The Contrasting Nature of Gamma-Ray/Optical Variability in the Blazar PKS 0208-512 During Successive OutburstsMar 08 2013The Yale/SMARTS optical-near-IR monitoring program has followed the variations in emission of the Fermi-LAT monitored blazars in the southern sky with closely spaced observations since 2008. We report the discovery of an optical-near-IR (OIR) outburst ... More
Correlated variability in the blazar 3C 454.3Dec 30 2008The blazar 3C 454.3 was revealed by the Fermi Gamma-ray Space Telescope to be in an exceptionally high flux state in July 2008. Accordingly, we performed a multi-wavelength monitoring campaign on this blazar using IR and optical observations from the ... More
Reddening-Independent Quasar Selection from a Wide-Field Optical and Near-IR Imaging SurveySep 19 2002We combine deep, wide-field near-IR and optical imaging to demonstrate a reddening-independent quasar selection technique based on identifying outliers in the (g-z) / (z-H) colour diagram. In three fields covering a total of ~0.7 deg^2 to a depth of m_H~18, ... More
The Low-Redshift Quasar-Quasar Correlation Function from an Extragalactic Halpha Emission-Line Survey to z=0.4Dec 06 1999Dec 13 2000We study the large-scale spatial distribution of low-redshift quasars and Seyfert~1 galaxies using a sample of 106 luminous emission-line objects ($\bar{M}_{B} \approx -23$) selected by their H$\alpha$ emission lines in a far-red objective prism survey ... More
ASTRO-H White Paper - Broad-band Spectroscopy and PolarimetryDec 03 2014The broad energy range spanned by ASTRO-H instruments, from ~0.3 to 600 keV, with its high spectral resolution calorimeter and sensitive hard X-ray imaging, offers unique opportunities to study black holes and their environments. The ability to measure ... More
Spectral Fits to the 1999 Aql X-1 Outburst DataJan 06 2003We present analysis and spectral fits of the RXTE data from the May/June 1999 outburst of Aql X-1. These data include observations in the rising portion of the hard state, in the soft state, and in the falling portion of the hard state. We show that the ... More
Short Timescale Correlations Between Line and Continuum Fluxes in Cygnus X-1Apr 09 2002Jun 29 2002We present the results of 16-s time scale spectral fits for Cygnus X-1 in the soft state and in the transition state, using a Comptonised blackbody plus an iron line. On these timescales, we find that that the continuum source flux can vary by factors ... More
Do Small-Scale Dark Matter Fluctuations Govern the Fragmentation of Primordial Gas?Dec 15 1998In order to constrain the initial mass function of the first generation of stars (Population III), one has to study the fragmentation properties of primordial gas. We present results from 3D simulations, based on Smoothed Particle Hydrodynamics, which ... More
Time Dependent Modeling of the Markarian 501 X-ray and TeV Gamma-Ray Data Taken During March and April, 1997Apr 14 2002Jul 23 2003If the high-energy emission from TeV blazars is produced by the Synchrotron Self-Compton (SSC) mechanism, then simultaneous X-ray and Gamma-ray observations of these objects are a powerful probe of the electron (and/or positron) populations responsible ... More
Simultaneous X-Ray and Gamma-Ray Observations of TeV Blazars: Testing Synchro-Compton Emission Models and Probing the Infrared Extragalactic BackgroundMar 10 1999Aug 12 1999The last years have seen a revolution in ground-based gamma-ray detectors. We can now detect the spectra of nearby TeV blazars like Mrk 421 and 501 out to approximately 20 TeV, and during the strongest flares, we can now follow fluctuations in these spectra ... More
Overview of EXIST mission science and implementationAug 19 2010The Energetic X-ray Imaging Survey Telescope (EXIST) is designed to i) use the birth of stellar mass black holes, as revealed by cosmic Gamma-Ray Bursts (GRBs), as probes of the very first stars and galaxies to exist in the Universe. Both their extreme ... More
The QUEST-La Silla AGN Variability SurveyJul 30 2015We present the characterization and initial results from the QUEST-La Silla AGN variability survey. This is an effort to obtain well sampled optical light curves in extragalactic fields with unique multi-wavelength observations. We present photometry ... More
The Impact of Bound Stellar Orbits and General Relativity on the Temporal Behavior of Tidal Disruption FlaresMar 20 2013Sep 18 2013We have carried out general relativistic particle simulations of stars tidally disrupted by massive black holes. When a star is disrupted in a bound orbit with moderate eccentricity instead of a parabolic orbit, the temporal behavior of the resulting ... More
Higher Order Variability Properties of Accreting Black HolesApr 09 2002Jun 29 2002To better constrain the emission mechanism underlying the hard state of galactic black hole candidates, we use high-time resolution RXTE lightcurves for Cyg X-1 and GX 399-4 to compute two higher order variability statistics for these objects, the skewness ... More
Hysteresis in the Light Curves of Soft X-ray TransientsSep 06 2002Sep 09 2002Using PCA data from the Rossi X-Ray Timing Explorer (RXTE), we track the spectral states of the neutron star transient system Aql X-1 through a complete outburst cycle. We find a hard-to-soft state transition during the very early, rising phase of the ... More
RXTE/OSSE Fits to the Hard State Spectrum of Cygnus X-1Apr 15 2002We present three spectra of Cygnus X-1 in the low/hard state from 3-1000 keV produced with RXTE and OSSE. We demonstrate that a pure thermal Comptonization model, with Compton reflection fits the data well, with coronal temperatures of about 90 keV, optical ... More
The interacting late-type host galaxy of the radio-loud narrow-line Seyfert 1 IRAS 20181-2244Jul 24 2018Dec 03 2018Narrow-line Seyfert 1 galaxies (NLS1s) are a class of active galactic nuclei (AGN) which are known to be one of the few sources of $\gamma$ rays, which originate in a relativistic beamed jet. Becuase of their relatively large distance, a poorly investigated ... More
Black hole growth and host galaxy morphologyFeb 07 2010We use data from large surveys of the local Universe (SDSS+Galaxy Zoo) to show that the galaxy-black hole connection is linked to host morphology at a fundamental level. The fraction of early-type galaxies with actively growing black holes, and therefore ... More
Highest Energy Cosmic Rays, Grand Unified Theories, and the Diffuse Gamma-Ray BackgroundApr 16 1996Oct 30 1996We explore scenarios where the highest energy cosmic rays (HECR) are produced by new particle physics near the grand unification scale. Using detailed numerical simulations of extragalactic cosmic and gamma-ray propagation, we show the existence of a ... More
The Origin of Line Emission in Massive z~2.3 Galaxies: Evidence for Cosmic Downsizing of AGN Host GalaxiesNov 24 2006Jul 30 2007Using the Gemini Near-InfraRed Spectrograph (GNIRS), we have assembled a complete sample of 20 K-selected galaxies at 2.0<z<2.7 with high quality near-infrared spectra. As described in a previous paper, 9 of these 20 galaxies have strongly suppressed ... More
The Sudden Death of the Nearest QuasarNov 01 2010Galaxy formation is significantly modulated by energy output from supermassive black holes at the centers of galaxies which grow in highly efficient luminous quasar phases. The timescale on which black holes transition into and out of such phases is, ... More
Polarimetry and the High-Energy Emission Mechanisms in Quasar Jets. The Case of PKS 1136-135May 11 2013Jul 09 2013Since the discovery of kiloparsec-scale X-ray emission from quasar jets, the physical processes responsible for their high-energy emission have been poorly defined. A number of mechanisms are under active debate, including synchrotron radiation, inverse-Comptonized ... More
Particle and light fragment emission in peripheral heavy ion collisions at Fermi energiesMay 16 2006A systematic investigation of the average multiplicities of light charged particles and intermediate mass fragments emitted in peripheral and semiperipheral collisions is presented as a function of the beam energy, violence of the collision and mass of ... More
Energetics of midvelocity emissions in peripheral heavy ion collisions at Fermi energiesMay 27 2004Peripheral and semi-peripheral collisions have been studied in the system 93Nb+93Nb at 38 AMeV. The evaporative and midvelocity components of the light charged particle and intermediate mass fragment emissions have been carefully disentangled. In this ... More
The Role of Gas in the Merging of Massive Black Holes in Galactic Nuclei. II. Black Hole Merging in a Clumpy DiskJun 11 2004Using SPH numerical simulations, we investigate the effects of gas on the inspiral and merger of a massive black hole binary. This study is motivated by the very massive nuclear gas disks observed in the central regions of merging galaxies. Here we present ... More
Forming the First Stars in the Universe: The Fragmentation of Primordial GasOct 13 1999In order to constrain the initial mass function (IMF) of the first generation of stars (Population III), we investigate the fragmentation properties of metal-free gas in the context of a hierarchical model of structure formation. We investigate the evolution ... More
The Role of Gas in the Merging of Massive Black Holes in Galactic Nuclei. I. Black Hole Merging in a Spherical Gas CloudOct 29 2003Nov 10 2003Using high-resolution SPH numerical simulations, we investigate the effects of gas on the inspiral and merger of a massive black hole binary. This study is motivated by both observational and theoretical work that indicate the presence of large amounts ... More
The polytropic approximation and X-ray scaling relations: constraints on gas and dark matter profiles for galaxy groups and clustersNov 23 2011Jan 27 2012We constrain gas and dark matter (DM) parameters of galaxy groups and clusters, by comparing X-ray scaling relations to theoretical expectations, obtained assuming that the gas is in hydrostatic equilibrium with the DM and follows a polytropic relation. ... More
Tidal Disruption Events by a Massive Black Hole BinaryOct 15 2015Massive black hole binaries (MBHBs) are a natural byproduct of galaxy mergers. Previous studies have shown that flares from stellar tidal disruption events (TDEs) are modified by the presence of a secondary perturber, causing interruptions in the light ... More
Galactic Models of Gamma-Ray BurstsAug 20 1995We describe observational evidence and theoretical calculations which support the high velocity neutron star model of gamma-ray bursts. We estimate the energetic requirements in this model, and discuss possible energy sources. we also consider radiative ... More
From Darkness To Light: The First Stars in the UniverseOct 20 2001Paramount among the processes that ended the cosmic `dark ages' must have been the formation of the first generation of stars (the so-called Population III). We summarize recent progress in constraining its nature, and we discuss the basic physical reasons ... More
The Formation of the First Stars. I. The Primordial Star Forming CloudFeb 28 2001Aug 28 2001To constrain the nature of the very first stars, we investigate the collapse and fragmentation of primordial, metal-free gas clouds. We explore the physics of primordial star formation by means of three-dimensional simulations of the dark matter and gas ... More
Multiwavelength Observations of Strong Flares From the TeV-Blazar 1ES 1959+650Oct 06 2003Following the detection of strong TeV gamma-ray flares from the BL Lac object 1ES 1959+650 with the Whipple 10 m Cherenkov telescope on May 16 and 17, 2002, we performed intensive Target of Opportunity (ToO) radio, optical, X-ray and TeV gamma-ray observations ... More
Cosmological Neutrino Signatures for Grand Unification Scale PhysicsOct 28 1996Physics beyond the standard model might imply the cosmological production of particles with grand unification scale energies. Nucleons and gamma-rays from such processes are candidates for the cosmic rays observed beyond 100 EeV [10**(20) eV]. Using a ... More
Hydrostatic equilibrium profiles for gas in elliptical galaxiesMar 10 2010May 05 2010We present an analytic formulation for the equilibrium gas density profile of early-type galaxies that explicitly includes the contribution of stars in the gravitational potential. We build a realistic model for an isolated elliptical galaxy and explore ... More
Physical Origins of Gas Motions in Galaxy Cluster Cores: Interpreting Hitomi Observations of the Perseus ClusterMay 17 2017Sep 18 2017The Hitomi X-ray satellite has provided the first direct measurements of the plasma velocity dispersion in a galaxy cluster. It finds a relatively "quiescent" gas with a line-of-sight velocity dispersion ~ 160 km/s, at 30 kpc to 60 kpc from the cluster ... More
What Are Gamma-Ray Bursts -- The Unique Role of Very High Energy Gamma-Ray ObservationsFeb 19 2009Gamma-ray bursts (GRBs) have been an enigma since their discovery forty years ago. However, considerable progress unraveling their mysteries has been made in recent years. Developments in observations, theory, and instrumentation have prepared the way ... More
Gamma Ray Burst Section of the White Paper on the Status and Future of Ground-based TeV Gamma-ray AstronomyOct 02 2008This is a report on the findings of the gamma ray burst working group for the white paper on the status and future of TeV gamma-ray astronomy. The white paper is an APS commissioned document, and the overall version has also been released and can be found ... More
Klein-Nishina Effects in the Spectra of Non-Thermal Sources Immersed in External Radiation FieldsApr 18 2005We study Klein-Nishina (KN) effects in the spectrum produced by a steady state, non-thermal source where rapidly accelerated electrons cool by emitting synchrotron radiation and Compton upscattering ambient photons produced outside the source. We focus ... More
Time Domain Analysis of Variability in Cygnus X-1: Constraints on the Emission ModelsJun 02 2000We use time domain analysis techniques to investigate the rapid variability of Cygnus X-1. We show that the cross-correlation functions between hard and soft energy bands reach values very close to unity and peak at a lag of less than 2 millisecond for ... More
EAZY: A Fast, Public Photometric Redshift CodeJul 10 2008We describe a new program for determining photometric redshifts, dubbed EAZY. The program is optimized for cases where spectroscopic redshifts are not available, or only available for a biased subset of the galaxies. The code combines features from various ... More
Galaxy Zoo: The fundamentally different co-evolution of supermassive black holes and their early- and late-type host galaxiesJan 18 2010We use data from the Sloan Digital Sky Survey and visual classifications of morphology from the Galaxy Zoo project to study black hole growth in the nearby Universe (z < 0.05) and to break down the AGN host galaxy population by color, stellar mass and ... More
The Gamma Ray Burst section of the White Paper on the Status and Future of Very High Energy Gamma Ray Astronomy: A Brief Preliminary ReportApr 15 2008This is a short report on the preliminary findings of the gamma ray burst (GRB) working group for the white paper on the status and future of very high energy (VHE; >50 GeV) gamma-ray astronomy. The white paper discusses the status of past and current ... More
Polarimetry and the High-Energy Emission Mechanisms in Quasar JetsSep 01 2009The emission mechanisms in extragalactic jets include synchrotron and various inverse-Compton processes. At low (radio through infrared) energies, it is widely agreed that synchrotron emission dominates in both low-power (FR I) and high-power (FR II and ... More
Gamma-ray observations of Cygnus X-1 above 100 MeV in the hard and soft statesMar 01 2013We present the results of multi-year gamma-ray observations by the AGILE satellite of the black hole binary system Cygnus X-1. In a previous investigation we focused on gamma-ray observations of Cygnus X-1 in the hard state during the period mid-2007/2009. ... More
A Large Area CCD Camera for the Schmidt Telescope at the Venezuelan National Astronomical ObservatoryApr 16 2002We have designed, constructed and put into operation a large area CCD camera that covers a large fraction of the image plane of the 1 meter Schmidt telescope at Llano del Hato in Venezuela. The camera consists of 16 CCD devices arranged in a 4 x 4 mosaic ... More
Complex Spectral Variability from Intensive Multi-wavelength Monitoring of Mrk421 in 1998Aug 31 2000We conducted a multi-frequency campaign for the TeV blazar Mrk~421 in 1998 April. The campaign started from a pronounced high amplitude flare recorded by SAX and Whipple; ASCA observation started three days later. In the X-ray data, we detected multiple ... More
Neutrinos, Cosmic Rays and the MeV BandMar 13 2019The possible association of the blazar TXS 0506+056 with a high-energy neutrino detected by IceCube holds the tantalizing potential to answer three astrophysical questions: 1. Where do high-energy neutrinos originate? 2. Where are cosmic rays produced ... More
The Multiwavelength Survey by Yale-Chile (MUSYC): Deep Near-Infrared Imaging and the Selection of Distant GalaxiesDec 21 2006We present deep near-infrared JHK imaging of four 10'x10' fields. The observations were carried out as part of the Multiwavelength Survey by Yale-Chile (MUSYC) with ISPI on the CTIO 4m telescope. The typical point source limiting depths are J~22.5, H~21.5, ... More
Cold optical design for the Large Aperture Simons Observatory telescopeAug 15 2018The Simons Observatory will consist of a single large (6 m diameter) telescope and a number of smaller (0.5 m diameter) refracting telescopes designed to measure the polarization of the Cosmic Microwave Background to unprecedented accuracy. The large ... More
The Multiwavelength Survey by Yale-Chile (MUSYC): Survey Design and Deep Public UBVRIz' Images and Catalogs of the Extended Hubble Deep Field SouthSep 08 2005We present UBVRIz' optical images of the 0.32 square degree Extended Hubble Deep Field South. This is one of four fields comprising the MUSYC survey, which is optimized for the study of galaxies at z=3, AGN demographics, and Galactic structure. We calculate ... More
The AGILE monitoring of Cygnus X-3: transient gamma-ray emission and spectral constraintsJul 26 2012Mar 07 2013We present the AGILE-GRID monitoring of Cygnus X-3, during the period between November 2007 and July 2009. We report here the whole AGILE-GRID monitoring of Cygnus X-3 in the AGILE "pointing" mode data-taking, to confirm that the gamma-ray activity coincides ... More
Section on Extragalactic Science Topics of the White Paper on the Status and Future of Ground-Based TeV Gamma-Ray AstronomyDec 03 2008This is a report on the findings of the extragalactic science working group for the white paper on the status and future of TeV gamma-ray astronomy. The white paper was commissioned by the American Physical Society, and the full white paper can be found ... More
The QUEST RR Lyrae Survey: Confirmation of the Clump at 50 kpc and Other Over-Densities in the Outer HaloMay 08 2001We have measured the periods and light curves of 148 RR Lyrae variables from V=13.5 to 19.7 from the first 100 sq. degrees of the QUEST RR Lyrae survey. Approximately 55% of these stars belong to the clump of stars detected earlier by the Sloan Digital ... More
New Quasars Detected via Variability in the QUEST1 SurveyOct 31 2003Jan 27 2004By observing the high galactic latitude equatorial sky in drift scan mode with the QUEST (QUasar Equatorial Survey Team) Phase 1 camera, multi-bandpass photometry on a large strip of sky, resolved over a large range of time scales (from hourly to biennially) ... More
Thermal/non-thermal model of Cyg X-1 in the soft stateDec 18 1997We analyze X-ray/gamma-ray observations of Cyg X-1 in the soft state. We find the hybrid thermal/non-thermal model fits the data very well. The significant contribution from the non-thermal electrons is required to account for the observed power law extending ... More
Blazar Optical Variability in the Palomar-QUEST SurveyMay 09 2009We study the ensemble optical variability of 276 FSRQs and 86 BL Lacs in the Palomar-QUEST Survey with the goal of searching for common fluctuation properties, examining the range of behavior across the sample, and characterizing the appearance of blazars ... More
Discovery and Characterization of an Extremely Deep-Eclipsing Cataclysmic Variable: LSQ172554.8-643839Mar 02 2011Mar 03 2011We report the discovery of an eclipsing cataclysmic variable with eclipse depths > 5.7 mag, orbital period 94.657 min, and peak brightness V~18 at J2000 position 17h 25m 54s.8, -64d 38' 39". Detected by visual inspection of images from Yale University's ... More
Quasar Optical Variability in the Palomar-QUEST SurveyFeb 24 2009The ensemble variability properties of nearly 23,000 quasars are studied using the Palomar-QUEST Survey. The survey has covered 15,000 square degrees multiple times over 3.5 years using 7 optical filters, and has been calibrated specifically for variability ... More
Implications of a Possible Clustering of Highest Energy Cosmic RaysMay 25 1996Very recently, a possible clustering of a subset of observed ultrahigh energy cosmic rays above about 40EeV (4x10^19eV) in pairs near the supergalactic plane was reported. We show that a confirmation of this effect would provide information on origin ... More
The Relation Between Compact, Quiescent High Redshift Galaxies and Massive Nearby Elliptical Galaxies: Evidence for Hierarchical, Inside-Out GrowthMar 12 2009Mar 19 2009Recent studies have shown that massive quiescent galaxies at high redshift are much more compact than present-day galaxies of the same mass. Here we compare the radial stellar density profiles and the number density of a sample of massive galaxies at ... More
The Mass Distribution of Stellar Black HolesAug 04 1997Aug 05 1997We examine the distribution of masses of black holes in transient low mass X-ray binary systems. A Bayesian analysis suggests that it is probable that six of the seven systems with measured mass functions have black hole masses clustered near seven solar ... More
The Hunt for Red Quasars: Luminous Obscured Black Hole Growth Unveiled in the Stripe 82 X-ray SurveySep 05 2017We present results of a ground-based near-infrared campaign with Palomar TripleSpec, Keck NIRSPEC, and Gemini GNIRS to target two samples of reddened active galactic nucleus (AGN) candidates from the 31 deg$^2$ Stripe 82 X-ray survey. One sample, which ... More