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S. Johnston

Publications and source records attributed to S. Johnston.

At least 289 records · Page 16Linked to original sources

Pulsar Timing with the Parkes Radio Telescope for the Fermi Mission

We report here on two years of timing of 168 pulsars using the Parkes radio telescope. The vast majority of these pulsars have spin-down luminosities in excess of 10^34 erg/s and are prime target candidates to be detected in gamma-rays by the Fermi Gamma-Ray Space Telescope. We provide the ephemerides for the ten pulsars being timed at Parkes which have been detected by Fermi in its first year of operation. These ephemerides, in conjunction with the publicly available photon list, can be used to generate gamma-ray profiles from the Fermi archive. We will make the ephemerides of any pulsars of interest available to the community upon request. In addition to the timing ephemerides, we present the parameters for 14 glitches which have occurred in 13 pulsars, seven of which have no previously known glitch history. The Parkes timing programme, in conjunction with Fermi observations, is expected to continue for at least the next four years.

astro-ph.GA↗

Effect of strong correlations on the high energy anomaly in hole- and electron-doped high-Tc superconductors

Recently, angle-resolved photoemission spectroscopy (ARPES) has been used to highlight an anomalously large band renormalization at high binding energies in cuprate superconductors: the high energy 'waterfall' or high energy anomaly (HEA). This paper demonstrates, using a combination of new ARPES measurements and quantum Monte Carlo simulations, that the HEA is not simply the by-product of matrix element effects, but rather represents a cross-over from a quasiparticle band at low binding energies near the Fermi level to valence bands at higher binding energy, assumed to be of strong oxygen character, in both hole- and electron-doped cuprates. While photoemission matrix elements clearly play a role in changing the aesthetic appearance of the band dispersion, i.e. the 'waterfall'-like behavior, they provide an inadequate description for the physics that underlies the strong band renormalization giving rise to the HEA. Model calculations of the single-band Hubbard Hamiltonian showcase the role played by correlations in the formation of the HEA and uncover significant differences in the HEA energy scale for hole- and electron-doped cuprates. In addition, this approach properly captures the transfer of spectral weight accompanying both hole and electron doping in a correlated material and provides a unifying description of the HEA across both sides of the cuprate phase diagram.

cond-mat.str-el↗

Radio detection of LAT PSRs J1741-2054 and J2032+4127: no longer just gamma-ray pulsars

Sixteen pulsars have been discovered so far in blind searches of photons collected with the Large Area Telescope on the Fermi Gamma-ray Space Telescope. We here report the discovery of radio pulsations from two of them. PSR J1741-2054, with period P=413ms, was detected in archival Parkes telescope data and subsequently has been detected at the Green Bank Telescope (GBT). Its received flux varies greatly due to interstellar scintillation and it has a very small dispersion measure of DM=4.7pc/cc, implying a distance of ~0.4kpc and possibly the smallest luminosity of any known radio pulsar. At this distance, for isotropic emission, its gamma-ray luminosity above 0.1GeV corresponds to 25% of the spin-down luminosity of dE/dt=9.4e33erg/s. The gamma-ray profile occupies 1/3 of pulse phase and has three closely-spaced peaks with the first peak lagging the radio pulse by delta=0.29P. We have also identified a soft Swift source that is the likely X-ray counterpart. In many respects PSR J1741-2054 resembles the Geminga pulsar. The second source, PSR J2032+4127, was detected at the GBT. It has P=143ms, and its DM=115pc/cc suggests a distance of ~3.6kpc, but we consider it likely that it is located within the Cyg OB2 stellar association at half that distance. The radio emission is nearly 100% linearly polarized, and the main radio peak precedes by delta=0.15P the first of two narrow gamma-ray peaks that are separated by Delta=0.50P. Faint, diffuse X-ray emission in a Chandra image is possibly its pulsar wind nebula. PSR J2032+4127 likely accounts for the EGRET source 3EG J2033+4118, while its pulsar wind is responsible for the formerly unidentified HEGRA source TeV J2032+4130.

astro-ph.GA↗

Phase-resolved Faraday rotation in pulsars

We have detected significant Rotation Measure variations for 9 bright pulsars, as a function of pulse longitude. An additional sample of 10 pulsars showed a rather constant RM with phase, yet a small degree of RM fluctuation is visible in at least 3 of those cases. In all cases, we have found that the rotation of the polarization position angle across our 1.4 GHz observing band is consistent with the wavelength-squared law of interstellar Faraday Rotation. We provide for the first time convincing evidence that RM variations across the pulse are largely due to interstellar scattering, although we cannot exclude that magnetospheric Faraday Rotation may still have a minor contribution; alternative explanations of this phenomenon, like erroneous de-dispersion and the presence of non-orthogonal polarization modes, are excluded. If the observed, phase-resolved RM variations are common amongst pulsars, then many of the previously measured pulsar RMs may be in error by as much as a few tens of rad m-2.

astro-ph.GA↗

Discovery of new gamma-ray pulsars with AGILE

Using gamma-ray data collected by the Astrorivelatore Gamma ad Immagini LEggero (AGILE) satellite over a period of almost one year (from 2007 July to 2008 June), we searched for pulsed signals from 35 potentially interesting radio pulsars, ordered according to $F_γ\propto \sqrt{\dot{E}} d^{-2}$ and for which contemporary or recent radio data were available. AGILE detected three new top-ranking nearby and Vela-like pulsars with good confidence both through timing and spatial analysis. Among the newcomers we find pulsars with very high rotational energy losses, such as the remarkable PSR B1509-58 with a magnetic field in excess of 10^13 Gauss, and PSR J2229+6114 providing a reliable identification for the previously unidentified EGRET source 3EG 2227+6122. Moreover, the powerful millisecond pulsar B1821-24, in the globular cluster M28, is detected during a fraction of the observations. Four other promising gamma-ray pulsar candidates, among which is the notable J2043+2740 with an age in excess of 1 million years, show a possible detection in the timing analysis only and deserve confirmation.

astro-ph.HE↗

High resolution angle resolved photoemission studies on quasi-particle dynamics in graphite

We obtained the spectral function of the graphite H point using high resolution angle resolved photoelectron spectroscopy (ARPES). The extracted width of the spectral function (inverse of the photo-hole lifetime) near the H point is approximately proportional to the energy as expected from the linearly increasing density of states (DOS) near the Fermi energy. This is well accounted by our electron-phonon coupling theory considering the peculiar electronic DOS near the Fermi level. And we also investigated the temperature dependence of the peak widths both experimentally and theoretically. The upper bound for the electron-phonon coupling parameter is ~0.23, nearly the same value as previously reported at the K point. Our analysis of temperature dependent ARPES data at K shows that the energy of phonon mode of graphite has much higher energy scale than 125K which is dominant in electron-phonon coupling.

cond-mat.mtrl-sci↗

Pulsar Timing for the Fermi Gamma-ray Space Telescope

We describe a comprehensive pulsar monitoring campaign for the Large Area Telescope (LAT) on the {\em Fermi Gamma-ray Space Telescope} (formerly GLAST). The detection and study of pulsars in gamma rays give insights into the populations of neutron stars and supernova rates in the Galaxy, into particle acceleration mechanisms in neutron star magnetospheres, and into the "engines" driving pulsar wind nebulae. LAT's unprecedented sensitivity between 20 MeV and 300 GeV together with its 2.4 sr field-of-view makes detection of many gamma-ray pulsars likely, justifying the monitoring of over two hundred pulsars with large spin-down powers. To search for gamma-ray pulsations from most of these pulsars requires a set of phase-connected timing solutions spanning a year or more to properly align the sparse photon arrival times. We describe the choice of pulsars and the instruments involved in the campaign. Attention is paid to verifications of the LAT pulsar software, using for example giant radio pulses from the Crab and from PSR B1937+21 recorded at Nançay, and using X-ray data on PSR J0218+4232 from XMM-Newton. We demonstrate accuracy of the pulsar phase calculations at the microsecond level. Data Table 1 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/ .

astro-ph↗

The Full Spectrum Galactic Terrarium: MHz to TeV Observations of Various Critters

Multi-wavelength studies at radio, infrared, optical, X-ray, and TeV wavelengths have discovered probable counterparts to many Galactic sources of GeV emission detected by EGRET. These include pulsar wind nebulae, high mass X-ray binaries, and mixed morphology supernova remnants. Here we provide an overview of the observational properties of Galactic sources which emit across 19 orders of magnitude in energy. We also present new observations of several sources.

astro-ph↗

Science with ASKAP - the Australian Square Kilometre Array Pathfinder

[ABRIDGED VERSION] The future of cm and m-wave astronomy lies with the Square Kilometre Array (SKA), a telescope under development by a consortium of 17 countries. The SKA will be 50 times more sensitive than any existing radio facility. A majority of the key science for the SKA will be addressed through large-area imaging of the Universe at frequencies from 300 MHz to a few GHz. The Australian SKA Pathfinder (ASKAP) is aimed squarely in this frequency range, and achieves instantaneous wide-area imaging through the development and deployment of phase-array feed systems on parabolic reflectors. This large field-of-view makes ASKAP an unprecedented synoptic telescope poised to achieve substantial advances in SKA key science. The central core of ASKAP will be located at the Murchison Radio Observatory in inland Western Australia, one of the most radio-quiet locations on the Earth and one of the sites selected by the international community as a potential location for the SKA. Following an introductory description of ASKAP, this document contains 7 chapters describing specific science programmes for ASKAP. The combination of location, technological innovation and scientific program will ensure that ASKAP will be a world-leading radio astronomy facility, closely aligned with the scientific and technical direction of the SKA. A brief summary chapter emphasizes the point, and considers discovery space.

astro-ph↗

High-Resolution Timing Observations of Spin-Powered Pulsars with the AGILE Gamma-Ray Telescope

AGILE is a small gamma-ray astronomy satellite mission of the Italian Space Agency dedicated to high-energy astrophysics launched in 2007 April. Its 1 microsecond absolute time tagging capability coupled with a good sensitivity in the 30 MeV-30 GeV range, with simultaneous X-ray monitoring in the 18-60 keV band, makes it perfectly suited for the study of gamma-ray pulsars following up on the CGRO/EGRET heritage. In this paper we present the first AGILE timing results on the known gamma-ray pulsars Vela, Crab, Geminga and B 1706-44. The data were collected from 2007 July to 2008 April, exploiting the mission Science Verification Phase, the Instrument Timing Calibration and the early Observing Pointing Program. Thanks to its large field of view, AGILE collected a large number of gamma-ray photons from these pulsars (about 10,000 pulsed counts for Vela) in only few months of observations. The coupling of AGILE timing capabilities, simultaneous radio/X-ray monitoring and new tools aimed at precise photon phasing, exploiting also timing noise correction, unveiled new interesting features at sub-millisecond level in the pulsars' high-energy light-curves.

astro-ph↗

The Impact of an Oxygen Dopant in an ideal Bi-2212 Crystal

Recent scanning tunneling microscopy studies have shown that local nanoscale pairing inhomogenities are correlated with interstitial oxygen dopants in Bi-2212. Combining electrostatic and cluster calculations, in this paper the impact of a dopant on the local Madelung and charge transfer energies, magnetic exchange J, Zhang-Rice mobility, and interactions with the lattice is investigated. It is found that electrostatic modifications locally increases the charge transfer energy and slightly suppresses J. It is further shown that coupling to c-axis phonons is strongly modified near the dopant. The combined effects of electrostatic modifications and coupling to the lattice yield broadened spectral features, reduced charge gap energies, and a sizable local increase of J. This implies a strong local interplay between antiferromagnetism, polarons, and superconducting pairing.

cond-mat.supr-con↗

High-precision geometry of a double-pole pulsar

High time resolution observations of PSR B0906-49 (or PSR J0908-4913) over a wide range of frequencies have enabled us to determine the geometry and beam shape of the pulsar. We have used the position angle traverse to determine highly-constrained solutions to the rotating vector model which show conclusively that PSR B0906-49 is an orthogonal rotator. The accuracy obtained in measuring the geometry is unprecedented. This may allow tests of high-energy emission models, should the pulsar be detected with GLAST. Although the impact parameter, beta, appears to be frequency dependent, we have shown that this is due to the effect of interstellar scattering. As a result, this pulsar provides some of the strongest evidence yet that the position angle swing is indeed related to a geometrical origin, at least for non-recycled pulsars. We show that the beam structures of the main pulse and interpulse in PSR B0906-49 are remarkably similar. The emission comes from a height of ~230 km and is consistent with originating in a patchy cone located about half way to the last open field lines. The rotation axis and direction of motion of the pulsar appear to be aligned.

astro-ph↗

A high frequency search for radio pulsars in EGRET error boxes

We present a new survey for pulsars in the error boxes of the low-latitude EGRET sources 3EG J1027-5817, 3EG J1800-2338 and 3EG J1810-1032. Although all of these sources have been covered by previous pulsar surveys, the recent discovery of the young, energetic pulsar PSR J1410-6132 at 6.7 GHz has shown that pulsars of this type can be hidden from low frequency surveys. Using an observing frequency of 3.1 GHz we discovered a 91-ms pulsar, PSR J1028-5819, which observations made at the Parkes telescope and the Australia Telescope Compact Array have shown to be young and energetic. We believe this pulsar is likely to be powering the unidentified EGRET source 3EG J1027-5817. Like other energetic pulsars, PSR J1028-5819 is highly linearly polarised, but astonishingly has a pulse duty cycle of only 0.4%, one of the smallest in the entire pulsar catalogue.

astro-ph↗

PSR J1410-6132: A young, energetic pulsar associated with EGRET source 3EG J1410-6147

We present the discovery of PSR J1410-6132, a 50-ms pulsar found during a high-frequency survey of the Galactic plane, using a 7-beam 6.3-GHz receiver on the 64-m Parkes radio telescope. The pulsar lies within the error box of the unidentified EGRET source 3EG J1410-6147, has a characteristic age of 26 kyr and a spin-down energy of 10^37 erg s^-1. It has a very high dispersion measure of 960+/-10 cm^-3 pc and the largest rotation measure of any pulsar, RM=+2400 +/- 30 rad m^-2. The pulsar is very scatter-broadened at frequencies of 1.4 GHz and below, making pulsed emission almost impossible to detect. Assuming a distance of 15 kpc, the pulsar's spin-down energy and a gamma-ray efficiency factor of ~10 per cent is sufficient to power the gamma-ray source. We therefore believe we have identified the nature of 3EG J1410-6147. This new discovery suggests that deep targeted high-frequency surveys of inner-galaxy EGRET sources could uncover further young, energetic pulsars.

astro-ph↗

Superconductivity-Induced Self-Energy Evolution of the Nodal Electron of Optimally-Doped Bi2212

The temperature dependent evolution of the renormalization effect in optimally-doped Bi2212 along the nodal direction has been studied via angle-resolved photoemission spectroscopy. Fine structure is observed in the real part of the self-energy (Re$Σ$), including a subkink and maximum, suggesting that electrons couple to a spectrum of bosonic modes, instead of just one mode. Upon cooling through the superconducting phase transition, the fine structures of the extracted Re$Σ$ exhibit a two-processes evolution demonstrating an interplay between kink renormalization and superconductivity. We show that this two-process evolution can be qualitatively explained by a simple Holstein model in which a spectrum of bosonic modes is considered.

cond-mat.supr-con↗

The magnetar 1E 1547.0-5408: radio spectrum, polarimetry, and timing

We have investigated the radio emission from the anomalous X-ray pulsar 1E 1547.0-5408 (PSR J1550-5418) using the Parkes telescope and the Australia Telescope Compact Array. The flux density of the pulsar is roughly the same between 1.4 and 45 GHz, but shows time variability. The radiation is nearly 100% linearly polarized between frequencies of 45 and 3.2 GHz, but from 2.3 to 1.4 GHz it becomes increasingly more depolarized. The rotation measure of -1860 rad/m^2 is the largest for any known pulsar, and implies an average magnetic field strength along the line of sight of 2.7 microG. The pulse profiles are circularly polarized at all frequencies observed, more so at lower frequencies, at the ~15% level. The observed swing of the position angle of linear polarization as a function of pulse phase suggests that in this neutron star the rotation and magnetic axes are nearly aligned, and that its radio emission is only detectable within a small solid angle. Timing measurements indicate that the period derivative of this 2 s pulsar has increased by nearly 40% in a 6-month period. The flat spectrum and variability in flux density and pulse profiles are reminiscent of the properties of XTE J1810-197, the only other known radio-emitting magnetar, and are anomalous by comparison with those of ordinary radio pulsars.

astro-ph↗

An empirical model for the beams of radio pulsars

Motivated by recent results on the location of the radio emission in pulsar magnetospheres, we have developed a model which can account for the large diversity found in the average profile shapes of pulsars. At the centre of our model lies the idea that radio emission at a particular frequency arises from a wide range of altitudes above the surface of the star and that it is confined to a region close to the last open field lines. We assert that the radial height range over which emission occurs is responsible for the complex average pulse shapes rather than the transverse (longitudinal) range proposed in most current models. By implementing an abrupt change in the height range to discriminate between young, short-period, highly-energetic pulsars and their older counterparts, we obtain the observed transition between the simple and complex average pulse profiles observed in each group respectively. Monte Carlo simulations are used to demonstrate the match of our model to real observations.

astro-ph↗

The Magnetic Field of the Solar Corona from Pulsar Observations

We present a novel experiment with the capacity to independently measure both the electron density and the magnetic field of the solar corona. We achieve this through measurement of the excess Faraday rotation due to propagation of the polarised emission from a number of pulsars through the magnetic field of the solar corona. This method yields independent measures of the integrated electron density, via dispersion of the pulsed signal and the magnetic field, via the amount of Faraday rotation. In principle this allows the determination of the integrated magnetic field through the solar corona along many lines of sight without any assumptions regarding the electron density distribution. We present a detection of an increase in the rotation measure of the pulsar J1801$-$2304 of approximately 160 \rad at an elongation of 0.95$^\circ$ from the centre of the solar disk. This corresponds to a lower limit of the magnetic field strength along this line of sight of $> 393μ\mathrm{G}$. The lack of precision in the integrated electron density measurement restricts this result to a limit, but application of coronal plasma models can further constrain this to approximately 20mG, along a path passing 2.5 solar radii from the solar limb. Which is consistent with predictions obtained using extensions to the Source Surface models published by Wilcox Solar Observatory

astro-ph↗