Results for "Jeremy Heyl"

total 3540took 0.11s
LMXBs may be important LIGO sources after allJun 11 2002Andersson et al. and Bildsten proposed that the spin of accreting neutron stars is limited by removal of angular momentum by gravitational radiation which increases dramatically with the spin frequency of the star. Both Bildsten and Andersson et al. argued ... More
The impulsive phase of magnetar giant flares: assessing linear tearing as the trigger mechanismDec 07 2015Giant $\gamma$-ray flares comprise the most extreme radiation events observed from magnetars. Developing on (sub)millisecond timescales and generating vast amounts of energy within a fraction of a second, the initial phase of these extraordinary bursts ... More
The Long-Term Future of Space TravelSep 09 2005Nov 08 2005The fact that we apparently live in an accelerating universe places limitations on where humans might visit. If the current energy density of the universe is dominated by a cosmological constant, a rocket could reach a galaxy observed today at a redshift ... More
Diffractive Microlensing II: Substellar Disk and Halo ObjectsFeb 16 2010Oct 04 2010Microlensing is generally studied in the geometric optics limit. However, diffraction may be important when nearby substellar objects lens occult distant stars. In particular the effects of diffraction become more important as the wavelength of the observation ... More
QED can explain the non-thermal emission from SGRs and AXPs : VariabilityAug 25 2006Owing to effects arising from quantum electrodynamics (QED), magnetohydrodynamical fast modes of sufficient strength will break down to form electron-positron pairs while traversing the magnetospheres of strongly magnetised neutron stars. The bulk of ... More
QED effects are negligible for neutron-star spin-downMar 24 2019Mar 27 2019The energy loss of a rotationally powered pulsar is primarily carried away as electromagnetic radiation and a particle wind. Considering that the magnetic field strength of pulsars ranges from about $10^8$ to $10^{15}$ G, one could expect quantum electrodynamics ... More
Quantum Mechanical Fluctuations at the End of InflationFeb 20 2006Oct 12 2007During the inflationary phase of the early universe, quantum fluctuations in the vacuum generate particles as they stretch beyond the Hubble length. These fluctuations are thought to result in the density fluctuations and gravitational radiation that ... More
See a Black Hole on a ShoestringFeb 16 2006Sep 15 2006The modes of vibration of hanging and partially supported strings provide useful analogies to scalar fields travelling through spacetimes that admit conformally flat spatial sections. This wide class of spacetimes includes static, spherically symmetric ... More
Gravitational Radiation from Strongly Magnetized White DwarfsJan 19 2000Apr 04 2000The magnetic fields of white dwarfs distort their shape generating an anisotropic moment of inertia. A magnetized white dwarf which rotates obliquely relative to the symmetry axis has a mass quadrupole moment which varies in time, so it will emit gravitational ... More
The Birthrate of MagnetarsMar 14 2007Sep 07 2007Magnetars, neutron stars with ultra strong magnetic fields ($B\sim 10^{14} - 10^{15}$G), manifest their exotic nature in the form of soft gamma-ray repeaters and anomalous X-ray pulsars. This study estimates the birthrate of magnetars to be $\sim$ 0.22 ... More
Accretion limits the compactness of static starsMar 03 2003General relativity limits the compactness of static stars. If the pressure of the fluid is positive and the density decreases with distance from the center, the value of the circumferential radius of the star must be greater than (9/4) G M/c^2, or equivalently ... More
R-Modes on Rapidly Rotating, Relativistic Stars: I. Do Type-I Bursts Excite Modes in the Neutron-Star Ocean?Aug 28 2001Sep 23 2003During a Type-I burst, the turbulent deflagation front may excite waves in the neutron star ocean and upper atmosphere with frequencies, $\omega \sim 1$ Hz. These waves may be observed as highly coherent flux oscillations during the burst. The frequencies ... More
Diffractive Microlensing: A New Probe of the Local UniverseFeb 22 2013Diffraction is important when nearby substellar objects gravitationally lens distant stars. If the wavelength of the observation is comparable to the Schwarzschild radius of lensing object, diffraction leaves an observable imprint on the lensing signature. ... More
Constraining white-dwarf kicks in globular clusters : II. Observational SignificanceSep 19 2007If the winds of an asymptotic-giant-branch stars are sufficiently strong are slightly asymmetric, they can alter the star's trajectory through a globular cluster; therefore, if these winds are asymmetric, one would expect young white dwarfs to be less ... More
Orbital evolution with white-dwarf kicksJun 06 2007Sep 10 2007Recent observations of white dwarfs in globular clusters indicate that these stars may get a velocity kick during their time as giants. This velocity kick could originate naturally if the mass loss while on the asymptotic giant branch is slightly asymmetric. ... More
R-Modes on Rapidly Rotating, Relativistic Stars : II. Blackbody EmissionFeb 24 2005Rossby waves (or r-modes) on the surface of a neutron star have become a leading model for the oscillations observed during the tail of Type-I X-ray bursts. Their frequency evolution matches well with the observed frequency drifts of the oscillations ... More
Interfering with Interference: a Pancharatnam Phase PolarimeterFeb 14 2000Mar 06 2002A simple variation of the traditional Young's double slit experiment can demonstrate several subtleties of interference with polarized light, including Berry and Pancharatnam's phase. Since the position of the fringes depends on the polarization state ... More
Vacuum birefringence and the X-ray polarization from black-hole accretion disksMar 10 2018Apr 04 2018In the next decade, x-ray polarimetry will open a new window on the high-energy Universe, as several missions that include an x-ray polarimeter are currently under development. Observations of the polarization of x-rays coming from the accretion disks ... More
Strongly Magnetized Sources: QED and X-ray PolarizationFeb 01 2018Jul 17 2018Radiative corrections of quantum electrodynamics cause a vacuum threaded by magnetic field to be birefringent. This means that radiation of different polarizations travels at different speeds. Even in the strong magnetic fields of astrophysical sources ... More
k-d Match: A Fast Matching Algorithm for Sheared Stellar SamplesApr 03 2013May 02 2013This paper presents new and efficient algorithms for matching stellar catalogues where the transformation between the coordinate systems of the two catalagoues is unknown and may include shearing. Finding a given object whether a star or asterism from ... More
Constraining white-dwarf kicks in globular clusters : III. Cluster HeatingMar 18 2008Mar 31 2009Recent observations of white dwarfs in globular clusters indicate that these stars may get a velocity kick during their time as giants. This velocity kick could originate naturally if the mass loss while on the asymptotic giant branch is slightly asymmetric. ... More
The Synoptic Swift Synergy -- Catching Gamma-Ray Bursts Before They FlyFeb 12 2003Apr 03 2003The advent of large panoramic photometric surveys of the sky offers the possibly of exploring the association of gamma-ray bursts (GRBs) with supernovae. To date, a few gamma-ray bursts have been connected possibly with supernovae: GRB 980425 -- SN 1998bw, ... More
Probing the Properties of Neutron Stars with Type-I X-Ray BurstsApr 21 2000Apr 23 2000The increase in spin frequency as the burning atmospheres of TypeI X-ray bursts cool provides a strong constraint on the radius of the underlying neutron star. If the change in spin frequency is due to a change in the thickness of the atmosphere, the ... More
Diffractive Microlensing III: Astrometric SignaturesMar 01 2010Oct 04 2010Gravitational lensing is generally treated in the geometric optics limit; however, when the wavelength of the radiation approaches or exceeds the Schwarzschild radius of the lens, diffraction becomes important. Although the magnification generated by ... More
Constraining white-dwarf kicks in globular clustersJun 06 2007Sep 07 2007The wind of an asymptotic-giant-branch stars is sufficiently strong that if it is slightly asymmetric, it can propel the star outside of the open cluster of its birth or significantly alter its trajectory through a globular cluster; therefore, if these ... More
MagnetarsApr 04 2005Ultramagnetized neutron stars or magnetars are magnetically powered neutron stars. Their strong magnetic fields dominate the physical processes in their crusts and their surroundings. The past few years have seen several advances in our theoretical and ... More
Electron-Positron Jets from a Critically Magnetized Black HoleDec 01 2000The curved spacetime surrounding a rotating black hole dramatically alters the structure of nearby electromagnetic fields. The Wald field which is an asymptotically uniform magnetic field aligned with the angular momentum of the hole provides a convenient ... More
Mechanical Properties of non-accreting Neutron Star CrustsAug 16 2012The mechanical properties of a neutron star crust, such as breaking strain and shear modulus, have implications for the detection of gravitational waves from a neutron star as well as bursts from Soft Gamma-ray Repeaters (SGRs). These properties are calculated ... More
Compositional Freeze-Out of Neutron Star CrustsJan 27 2009Aug 21 2009We have investigated the crustal properties of neutron stars without fallback accretion. We have calculated the chemical evolution of the neutron star crust in three different cases (a modified Urca process without the thermal influence of a crust, a ... More
QED is negligible for neutron-star spin-downMar 24 2019The energy loss of a rotationally powered pulsar is primarily carried away as electromagnetic radiation and a particle wind. Considering that the magnetic field strength of pulsars ranges from about $10^8$ G to $10^{15}$ G, one could expect quantum electrodynamics ... More
Probing Axions with Radiation from Magnetic StarsSep 28 2006Nov 15 2006Recent experiments suggest that polarized photons may couple significantly to pseudoscalar particles such as axions. We study the possible observational signatures of axion-photon coupling for radiation from magnetic stars, with particular focus on neutron ... More
Probing Black Hole Magnetic Fields with QEDMay 25 2018The effect of vacuum birefringence is one of the first predictions of quantum electrodynamics (QED): the presence of a charged Dirac field makes the vacuum birefringent when threaded by magnetic fields. This effect, extremely weak for terrestrial magnetic ... More
Casimir Interactions between Magnetic Flux Tubes in a Dense LatticeJul 28 2014Feb 18 2015We use the worldline numerics technique to study a cylindrically symmetric model of magnetic flux tubes in a dense lattice and the non-local Casimir forces acting between regions of magnetic flux. Within a superconductor the magnetic field is constrained ... More
Statistical ages and the cooling rate of X-ray dim isolated neutron starsMay 04 2013Aug 12 2013The cooling theory of neutron stars is corroborated by its comparison with observations of thermally emitting isolated neutron stars and accreting neutron stars in binary systems. An important ingredient for such an analysis is the age of the object, ... More
What is the nature of RX J0720.4-3125?Jan 06 1998RX J0720.4-3125 has recently been identified as a pulsating soft X-ray source in the ROSAT all-sky survey with a period of 8.391 s. Its spectrum is well characterized by a black-body with a temperature of $8 \times 10^5$ K. We propose that the radiation ... More
The Thermal Evolution of Ultramagnetized Neutron StarsOct 21 1997Using recently calculated analytic and numerical models for the thermal structure of ultramagnetized neutron stars, we estimate the effects that ultrastrong magnetic fields $B \ge 10^{14}$ G have on the thermal evolution of a neutron star. Understanding ... More
Broadband Modeling of GRB 021004Nov 12 2002Feb 05 2003We present a broadband modeling of the afterglow of GRB 021004. The optical transient of this burst has been detected very early and densely sampled in several bands. Its light curve shows significant deviations from a simple power law. We use the data ... More
Evidence for the Black Hole Event HorizonApr 26 2002Roughly a dozen X-ray binaries are presently known in which the compact accreting primary stars are too massive to be neutron stars. These primaries are identified as black holes, though there is as yet no definite proof that any of the candidate black ... More
The Magnetised Bellows of BetelgeuseSep 23 2011Feb 06 2012We present calculations for a magnetised hybrid wind model for Betelegeuse ($\alpha - $Orionis). The model is a direct application of our previously derived theory, combining a canonical Weber-Davis (WD) stellar wind with dust grains in the envelope of ... More
Constraining white-dwarf kicks in globular clusters : IV. Retarding Core CollapseJan 13 2009May 29 2009Observations of white dwarfs in the globular clusters NGC 6397 and Omega Centauri indicate that these stars may get a velocity kick during their time as giants. If the mass loss while on the asymptotic giant branch is slightly asymmetric, the resulting ... More
Constraining the photon-axion coupling constant with magnetic white dwarfsMay 11 2011Sep 14 2011The light pseudoscalar particle, dubbed the axion, borne out of the Peccei-Quinn solution to the strong CP problem in QCD remains elusive. One avenue of inferring its existence is through its coupling to electromagnetic radiation. So far, laboratory experiments ... More
A QED Model for Non-thermal Emission from SGRs and AXPsFeb 17 2005Previously, we showed that, owing to effects arising from quantum electrodynamics (QED), magnetohydrodynamic fast modes of sufficient strength will break down to form electron-positron pairs while traversing the magnetospheres of strongly magnetised neutron ... More
Hotspot Emission from a Freely Precessing Neutron StarMar 31 2000Apr 04 2000Recent observations of 1E~161348-5055, the neutron-star candidate at the center of the supernova remnant RCW 103, show that a component of its emission varies sinusoidally with a period of approximately six hours. We argue that this period is what one ... More
Hydrogen and Helium Atoms and Molecules in an Intense Magnetic FieldJun 24 1998We calculate the atomic structure of hydrogen and helium, atoms and molecules in an intense magnetic field, analytically and numerically with a judiciously chosen basis.
Almost Analytic Models of Ultramagnetized Neutron Star EnvelopesMay 13 1998Recent ROSAT measurements show that the x-ray emission from isolated neutron stars is modulated at the stellar rotation period. To interpret these measurements, one needs precise calculations of the heat transfer through the thin insulating envelopes ... More
Magnetically Catalyzed FusionAug 25 1996We calculate the reaction cross-sections for the fusion of hydrogen and deuterium in strong magnetic fields as are believed to exist in the atmospheres of neutron stars. We find that in the presence of a strong magnetic field ($B \gsim 10^{12}$G), the ... More
Nonlinear QED Effects in Strong-Field MagnetohydrodynamicsNov 10 1998We examine wave propagation and the formation of shocks in strongly magnetized plasmas by applying a variational technique and the method of characteristics to the coupled magnetohydrodynamic (MHD) and quantum-electrodynamic (QED) equations of motion. ... More
Do magnetars glitch? : Timing irregularities in anomalous X-ray pulsarsAug 12 1998Apr 09 1999We examine the timing history of several anomalous X-ray pulsars (AXPs), and find that they exhibit spin down irregularities statistically similar to those of radio pulsars. We propose that these irregularities are simply glitches like those of radio ... More
Characterizing Entanglement Entropy Produced by Non-Linear Scalar Interactions During InflationOct 02 2008Jul 14 2009The density fluctuations that we observe in the universe today are thought to originate from quantum fluctuations produced during a phase of the early universe called inflation. By evolving a wavefunction describing two coupled Fourier modes of a scalar ... More
A hybrid steady-state magnetohydrodynamic dust-driven stellar wind model for AGB starsJun 11 2010Feb 07 2012We present calculations for a magnetised hybrid wind model for Asymptotic Giant Branch (AGB) stars. The model incorporates a canonical Weber-Davis (WD) stellar wind with dust grains in the envelope of an AGB star. The resulting hybrid picture preserves ... More
A two-dimensional pseudospectral Hartree-Fock method for low-Z atoms in intense magnetic fieldsMar 15 2012Apr 24 2014The energy levels of the first few low-lying states of helium and lithium atoms in intense magnetic fields up to $\approx 10^8-10^9$~T are calculated in this study. A pseudospectral method is employed for the computational procedure. The methodology involves ... More
Birefringence and Dichroism of the QED VacuumMay 20 1997We use an analytic form for the Heisenberg-Euler Lagrangian to calculate the birefringent and dichroic properties of the vacuum for arbitrarily strong wrenchless fields. PACS : 12.20.Ds, 42.25.Lc 97.60.Jd, 98.70.Rz
On the trigger mechanisms for SGR giant flaresFeb 19 2010May 18 2010We examine two trigger mechanisms, one internal and the other external to the neutron star, that give rise to the intense soft gamma-ray repeater (SGR) giant flares. So far, three giant flares have been observed from the three out of the seven confirmed ... More
Rotational Evolution of Protoneutron StarsMay 06 2003We study the rotational evolution of a protoneutron star with hyperons and nucleons or solely nucleons in its core due to the escape of the trapped neutrinos. It is found that at the early stage of its evolution, the stellar crust contracts significantly, ... More
Using the 2-MASS Photometric Redshift Survey to optimize LIGO Follow-Up ObservationsFeb 24 2016Jul 14 2016The initial discovery of LIGO on 14 September 2015 was the inspiral merger and ring-down of the black hole binary at a distance of about 500~Mpc or a redshift of about 0.1. The search for electromagnetic counterparts for the inspiral of binary black holes ... More
Thermonuclear Stability of Material Accreting onto a Neutron StarMar 19 2003Aug 22 2003We present a global linear stability analysis of nuclear fuel accumulating on the surface of an accreting neutron star and we identify the conditions under which thermonuclear bursts are triggered. The analysis reproduces all the recognized regimes of ... More
Is Mira a magneto-dusty rotator?May 31 2012We investigate the possibility that a magnetic field may be present in the star $o-$Ceti (hereafter, Mira) and that the field plays a role in the star's mass loss. The model presented here is an application of an earlier derived theory that has been successfully ... More
An Analytic Form for the Effective Lagrangian of QED and its Application to Pair Production and Photon SplittingJul 15 1996We derive an analytic form for the Heisenberg-Euler Lagrangian in the limit where the component of the electric field parallel to the magnetic field is small. We expand these analytic functions to all orders in the field strength ($F_{\mu\nu}F^{\mu\nu}$) ... More
A QED Model for the Origin of Bursts from SGRs and AXPsDec 23 2003Aug 31 2004We propose a model to account for the bursts from soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs) in which quantum electrodynamics (QED) plays a vital role. In our theory, that we term ``fast-mode breakdown,'' magnetohydrodynamic (MHD) ... More
RCW 103 -- Revisiting a cooling neutron starMay 07 1998Recent observations of the compact source embedded within the supernova remnant RCW 103 rekindle interest in the origin of this object's emission. We contrast several models in which neutron-star cooling powers RCW 103. Specifically, either the presence ... More
Nonlinear Electromagnetic Waves in Magnetosphere of a MagnetarFeb 15 2010Nov 10 2010We compute electromagnetic wave propagation through the magnetosphere of a magnetar. The magnetosphere is modeled as the QED vacuum and a cold, strongly magnetized plasma. The background field and electromagnetic waves are treated nonperturbatively and ... More
Polluting White Dwarfs with Perturbed Exo-CometsFeb 24 2017Apr 27 2017We present a model to account for the observed debris disks around young white dwarfs and the presence of metal-lines in their spectra. Stellar evolution models predict that the mass-loss on the AGB will be pulsed; furthermore, observations indicate that ... More
On the Lack of Type I X-ray Bursts in Black Hole X-ray Binaries: Evidence for the Event Horizon?Mar 06 2002Jun 28 2002Type I X-ray bursts are very common in neutron star X-ray binaries, but no Type I burst has been seen in the dozen or so binaries in which the accreting compact star is too massive to be a neutron star and therefore is identified as a black hole candidate. ... More
Ionization and dissociation equilibrium in strongly-magnetized helium atmosphereOct 09 2006Feb 23 2007Recent observations and theoretical investigations of neutron stars indicate that their atmospheres consist not of hydrogen or iron but possibly other elements such as helium. We calculate the ionization and dissociation equilibrium of helium in the conditions ... More
Dispersion Relations for Bernstein Waves in a Relativistic Pair PlasmaJun 25 2009Aug 21 2009A fully relativistic treatment of Bernstein waves in an electron-positron pair plasma has remained too formidable a task owing to the very complex nature of the problem. In this article, we perform contour integration of the dielectric response function ... More
Magnetic Reconnection Instabilities in Soft-Gamma RepeatersFeb 22 2013We examine an external trigger mechanism that gives rise to the intense soft gamma-ray repeater (SGR) giant flares. Out of the three giant flares, two showcased the existence of a precursor, which we show to have had initiated the main flare. We develop ... More
Parallel Worldline Numerics: Implementation and Error AnalysisJul 28 2014We give an overview of the worldline numerics technique, and discuss the parallel CUDA implementation of a worldline numerics algorithm. In the worldline numerics technique, we wish to generate an ensemble of representative closed-loop particle trajectories, ... More
Powering Anomalous X-ray Pulsars by Neutron Star CoolingAug 20 1997Aug 30 1997Using recently calculated analytic models for the thermal structure of ultramagnetized neutron stars, we estimate the thermal fluxes from young ($t\sim 1000$ yr) ultramagnetized ($B \sim 10^{15}$ G) cooling neutron stars. We find that the pulsed X-ray ... More
QED One-loop Corrections to a Macroscopic Magnetic DipoleMay 20 1997We consider the field equations of a static magnetic field including one-loop QED corrections, and calculate the corrections to the field of a magnetic dipole. PACS: 12.20.Ds, 97.60.Jd, 97.60.Gb
Multidimensional thermal structure of magnetized neutron star envelopesAug 12 1998Recently launched x-ray telescopes have discovered several candidate isolated neutron stars. The thermal radiation from these objects may potentially constrain our understanding of nuclear physics in a realm inaccessible to terrestrial experiments. To ... More
Electromagnetic Shocks in Strong Magnetic FieldsMay 11 1998Jun 04 1998We examine the propagation of electromagnetic radiation through a strong magnetic field using the method of characteristics. Owing to nonlinear effects associated with vacuum polarization, such waves can develop discontinuities analogous to hydrodynamical ... More
Energy levels of light atoms in strong magnetic fieldsApr 24 2014In this review article we provide an overview of the field of atomic structure of light atoms in strong magnetic fields. There is a very rich history of this field which dates back to the very birth of quantum mechanics. At various points in the past ... More
Pseudospectral methods for atoms in strong magnetic fieldsFeb 27 2009Apr 28 2010We present a new pseudospectral algorithm for the calculation of the structure of atoms in strong magnetic fields. We have verified this technique for one, two and three-electron atoms in zero magnetic fields against laboratory results and find typically ... More
Hydrogen and Helium atoms in strong magnetic fieldsJun 19 2008Feb 07 2012The energy levels of hydrogen and helium atoms in strong magnetic fields are calculated in this study. The current work contains estimates of the binding energies of the first few low-lying states of these systems that are improvements upon previous estimates. ... More
Vacuum Decay Constraints on a Cosmological Scalar FieldNov 29 2001If the potential of a scalar field, Phi, which currently provides the ``dark energy'' of the universe, has a negative minimum -M_0^4, then quantum-mechanical fluctuations could nucleate a bubble of Phi at a negative value of the potential. The bubble ... More
Consequences of Interstellar Absorption for Models of Anomalous X-ray PulsarsJun 20 2000We examine properties of thermal radiation emitted by strongly magnetized neutron stars (NSs). In particular, we show that the pulsation amplitudes of the energy-integrated flux are an increasing function of the absorption column density to the source. ... More
Eötvös Experiments with Supermassive Black HolesJun 25 2015By examining the locations of central black holes in two elliptical galaxies, M32 and M87, we derive constraints on the violation of the strong equivalence principle for purely gravitational objects, i.e. black holes, of less than eight percent, $|\eta_N|<0.08$ ... More
QED and the High Polarization of the Thermal Radiation from Neutron StarsMar 05 2002May 16 2002The thermal emission of strongly magnetized neutron-star atmospheres is thought to be highly polarized. However, because of the different orientations of the magnetic field over the surface of the neutron star (NS), it is commonly assumed that the net ... More
Quantifying mass segregation and new core radii for 54 milky way globular clustersAug 16 2013We present core radii for 54 Milky Way globular clusters determined by fitting King-Michie models to cumulative projected star count distributions. We find that fitting star counts rather than surface brightness profiles produces results that differ significantly ... More
Large Pulsar Kicks from Topological CurrentsDec 18 2009We show that kicks generated by topological currents may be responsible for the large velocities seen in a number of pulsars. The majority of the kick builds up within the first second of the star's birth and generates a force about two orders of magnitude ... More
A Fast Algorithm for Finding Point Sources in the Fermi Data Stream: FermiFASTJun 10 2016This paper presents a new and efficient algorithm for finding point sources in the photon event data stream from the Fermi Gamma-Ray Space Telescope. It can rapidly construct about most significant half of the Fermi Third Point Source catalogue (3FGL) ... More
The Modified Schrodinger Poisson Equation -- Quantum PolytropesSep 27 2017Oct 27 2017Axions and axion-like particles are a leading model for the dark matter in the Universe; therefore, dark matter halos may be boson stars in the process of collapsing. We examine a class of static boson stars with a non-minimal coupling to gravity. We ... More
Using long-term transit timing to detect terrestrial planetsOct 10 2006Mar 07 2007We propose that the presence of additional planets in extrasolar planetary systems can be detected by long-term transit timing studies. If a transiting planet is on an eccentric orbit then the presence of another planet causes a secular advance of the ... More
Neutron Star Crustal Mass FractionsOct 02 2007We are investigating mass fractions on the crust of a neutron star which would remain after one year of cooling. We use cooling curves corresponding with various densities, or depths, of the neutron star just after its formation. We assume the modified ... More
Eötvös Experiments with Supermassive Black HolesJun 25 2015Dec 13 2016By examining the locations of central black holes in two elliptical galaxies, M\,32 and M\,87, we derive constraints on the violation of the strong equivalence principle for purely gravitational objects, i.e. black holes, of less than about two-thirds, ... More
Magnetospheric Birefringence Induces Unique Polarization Signatures in Neutron-Star SpectraOct 22 2004Feb 18 2006We study the propagation of polarization light through the magnetosphere of neutron stars. At intermediate frequencies (the optical through the infrared), both the birefringence induced by the plasma and by quantumelectrodynamics influence the observation ... More
Cosmic Rays from Pulsars and MagnetarsMay 06 2010We compare the expected abundance of cosmic-ray electrons and positrons from pulsars and magnetars. We assume that the distribution of infant pulsars and magnetars follows that of high-mass stars in the Milky Way and that the production rate of cosmic ... More
Polarization Evolution in Strong Magnetic FieldsSep 20 1999Extremely strong magnetic fields change the vacuum index of refraction. Although this polarization dependent effect is small for typical neutron stars, it is large enough to decouple the polarization states of photons traveling within the field. The photon ... More
X-ray emission from middle-aged pulsarsApr 30 2001We present a simple, unified model which accounts for properties of the X-ray emission from the three middle-aged pulsars PSR 1055-52, PSR 0656+14 and PSR 0630+18 (Geminga). The X-ray radiation from these objects is pulsed more strongly at energies above ... More
How Common Are Magnetars? The Consequences of Magnetic-Field DecayJul 29 1998Ultramagnetized neutron stars or magnetars have been invoked to explain several astrophysical phenomena. We examine how the magnetic field of a magnetar will decay over time and how this decay affects the cooling of the object. We find that for sufficiently ... More
Tests of Gravity Theories Using Supermassive Black HolesApr 08 2017Jul 24 2017Scalar-tensor theories of gravity generally violate the strong equivalence principle, namely compact objects have a suppressed coupling to the scalar force, causing them to fall slower. A black hole is the extreme example where such a coupling vanishes, ... More
Formation and Evolution of Blue Stragglers in 47 TucanaeSep 07 2016Blue stragglers (BSS) are stars whose position in the Color-Magnitude Diagram (CMD) places them above the main sequence turn-off (TO) point of a star cluster. Using data from the core of 47 Tuc in the ultraviolet (UV), we have identified various stellar ... More
Anomalous X-ray Pulsars and Soft gamma-ray Repeaters: Spectral Fits and the Magnetar ModelMar 17 2001Mar 22 2001The energy source powering the X-ray emission from anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) is still uncertain. In one scenario, the presence of an ultramagnetized neutron star, or ``magnetar'', with B on the order of 10^{14} ... More
The High-Energy Polarization-Limiting Radius of Neutron Star Magnetospheres I -- Slowly Rotating Neutron StarsFeb 06 2003In the presence of strong magnetic fields, the vacuum becomes a birefringent medium. We show that this QED effect decouples the polarization modes of photons leaving the NS surface. Both the total intensity and the intensity in each of the two modes is ... More
Globular cluster absolute ages from cooling brown dwarfsFeb 01 2017Globular clusters are the oldest conglomerates of stars in our Galaxy and can be useful laboratories to test theories from stellar evolution to cosmology. In this paper, we present a new method to estimate the absolute age of a globular cluster from observations ... More
Autofib Redshift Survey: II -- The Evolution of the Galaxy Luminosity Function by Spectral TypeOct 04 1996We determine the evolution of the galaxy luminosity function (LF) as a function of spectral type using the Autofib redshift survey, a compendium of over 1700 galaxy redshifts in various magnitude-limited samples spanning b_J=11.5-24.0. To carry out this ... More
Magnetic Lensing near Ultramagnetized Neutron StarsJan 27 1999Extremely strong magnetic fields change the vacuum index of refraction. This induces a lensing effect that is not unlike the lensing phenomenon in strong gravitational fields. The main difference between the two is the polarization dependency of the magnetic ... More
Using Galaxy Formation Simulations to optimise LIGO Follow-Up ObservationsDec 13 2016The recent discovery of gravitational radiation from merging black holes poses a challenge of how to organize the electromagnetic follow-up of gravitational-wave events as well as observed bursts of neutrinos. We propose a technique to select the galaxies ... More
The High-Energy Polarization-Limiting Radius of Neutron Star Magnetospheres II -- Magnetized Hydrogen AtmospheresFeb 17 2005In the presence of strong magnetic fields, the vacuum becomes a birefringent medium. We show that this QED effect couples the direction of the polarization of photons leaving the NS surface, to the direction of the magnetic field along the ray's path. ... More
Dynamical estimate of post main sequence stellar masses in 47 TucanaeMay 18 2016We use the effects of mass segregation on the radial distribution of different stellar populations in the core of 47 Tucanae to find estimates for the masses of stars at different post main sequence evolutionary stages. We take samples of main sequence ... More