arXiv subjects

S. Johnston

Publications and source records attributed to S. Johnston.

At least 217 records · Page 12Linked to original sources

arXiv2014

An improved measurement of the 2νββ half-life of Xe-136 with EXO-200

We report on an improved measurement of the 2νββ half-life of Xe-136 performed by EXO-200. The use of a large and homogeneous time projection chamber allows for the precise estimate of the fiducial mass used for the measurement, resulting in a small systematic uncertainty. We also discuss in detail the data analysis methods used for double-beta decay searches with EXO-200, while emphasizing those directly related to the present measurement. The Xe-136 2νββ half-life is found to be 2.165 +- 0.016 (stat) +- 0.059 (sys) x 10^21 years. This is the most precisely measured half-life of any 2νββ decay to date.

nucl-ex↗

arXiv2014

The High Time Resolution Universe Pulsar Survey X: Discovery of four millisecond pulsars and updated timing solutions of a further 12

We report on the discovery of four millisecond pulsars (MSPs) in the High Time Resolution Universe (HTRU) pulsar survey being conducted at the Parkes 64-m radio telescope. All four MSPs are in binary systems and are likely to have white dwarf companions. In addition, we present updated timing solutions for 12 previously published HTRU MSPs, revealing new observational parameters such as five proper motion measurements and significant temporal dispersion measure variations in PSR J1017-7156. We discuss the case of PSR J1801-3210, which shows no significant period derivative after four years of timing data. Our best-fit solution shows a period derivative of the order of $10^{-23}$, an extremely small number compared to that of a typical MSP. However, it is likely that the pulsar lies beyond the Galactic Centre, and an unremarkable intrinsic period derivative is reduced to close to zero by the Galactic potential acceleration. Furthermore, we highlight the potential to employ PSR J1801-3210 in the strong equivalence principle test due to its wide and circular orbit. In a broader comparison with the known MSP population, we suggest a correlation between higher mass functions and the presence of eclipses in `very low-mass binary pulsars', implying that eclipses are observed in systems with high orbital inclinations. We also suggest that the distribution of the total mass of binary systems is inversely-related to the Galactic height distribution. Finally, we report on the first detection of PSRs J1543-5149 and J1811-2404 as gamma-ray pulsars.

astro-ph.HE↗

arXiv2014

Multi-wavelength observations of the binary system PSR B1259-63/LS 2883 around the 2010-2011 periastron passage

We report on broad multi-wavelength observations of the 2010-2011 periastron passage of the gamma-ray loud binary system PSR B1259-63. High resolution interferometric radio observations establish extended radio emission trailing the position of the pulsar. Observations with the Fermi Gamma-ray Space Telescope reveal GeV gamma-ray flaring activity of the system, reaching the spin-down luminosity of the pulsar, around 30 days after periastron. There are no clear signatures of variability at radio, X-ray and TeV energies at the time of the GeV flare. Variability around periastron in the H$α$ emission line, can be interpreted as the gravitational interaction between the pulsar and the circumstellar disk. The equivalent width of the H$α$ grows from a few days before periastron until a few days later, and decreases again between 18 and 46 days after periastron. In near infrared we observe the similar decrease of the equivalent width of Br$γ$ line between the 40th and 117th day after the periastron. For the idealized disk, the variability of the H$α$ line represents the variability of the mass and size of the disk. We discuss possible physical relations between the state of the disk and GeV emission under assumption that GeV flare is directly related to the decrease of the disk size.

astro-ph.HE↗

arXiv2013

Evidence of an asteroid encountering a pulsar

Debris disks and asteroid belts are expected to form around young pulsars due to fallback material from their original supernova explosions. Disk material may migrate inwards and interact with a pulsar's magnetosphere, causing changes in torque and emission. Long term monitoring of PSR J0738-4042 reveals both effects. The pulse shape changes multiple times between 1988 and 2012. The torque, inferred via the derivative of the rotational period, changes abruptly from September 2005. This change is accompanied by an emergent radio component that drifts with respect to the rest of the pulse. No known intrinsic pulsar processes can explain these timing and radio emission signatures. The data lead us to postulate that we are witnessing an encounter with an asteroid or in-falling debris from a disk.

astro-ph.HE↗

arXiv2013

Specific heat of Ca$_{0.32}$Na$_{0.68}$Fe$_2$As$_2$ single crystals: unconventional s$_\pm$ multi-band superconductivity with intermediate repulsive interband coupling and sizable attractive intraband couplings

We report a low-temperature specific heat study of high-quality single crystals of the heavily hole doped superconductor Ca$_{0.32}$Na$_{0.68}$Fe$_2$As$_2$. This compound exhibits bulk superconductivity with a transition temperature $T_c \approx 34$\,K, which is evident from the magnetization, transport, and specific heat measurements. The zero field data manifests a significant electronic specific heat in the normal state with a Sommerfeld coefficient $γ\approx 53$ mJ/mol K$^{2}$. Using a multi-band Eliashberg analysis, we demonstrate that the dependence of the zero field specific heat in the superconducting state is well described by a three-band model with an unconventional s$_\pm$ pairing symmetry and gap magnitudes $Δ_i$ of approximately 2.35, 7.48, and -7.50 meV. Our analysis indicates a non-negligible attractive intraband coupling,which contributes significantly to the relatively high value of $T_c$. The Fermi surface averaged repulsive and attractive coupling strengths are of comparable size and outside the strong coupling limit frequently adopted for describing high-$T_c$ iron pnictide superconductors. We further infer a total mass renormalization of the order of five, including the effects of correlations and electron-boson interactions.

cond-mat.supr-con↗

arXiv2013

The kinematics and orbital dynamics of the PSR B1259$-$63/LS 2883 system from 23 years of pulsar timing

We present an analysis of $23~$yr of pulse arrival times for PSR B1259$-$63. The pulsar is in a binary orbit about its approximately $20~M_{\odot}$ companion LS$~$2883. Our best-fitting timing solution has none of the pulse-number ambiguities that have plagued previous attempts to model the binary orbit. We measure significant first and second time derivatives of the projected semimajor axis and longitude of the periastron of the orbit. These variations are found to be consistent with the precession of the orbital plane due to classical spin-orbit coupling. The derived moment of inertia and spin of the companion are consistent with the companion rotating at near-breakup velocity. The system configuration is also consistent with the geometry derived from both the polarisation of the radio emission and the eclipse of the pulsar by the equatorial disc of the companion. We find strong evidence for orbital period decay that can be attributed to mass loss from the companion star. We also measure a significant proper motion that locates the birth of the system in the Centaurus OB1 association. By combining proper motion of the pulsar with radial velocity measurements of the companion, we measure the three-dimensional velocity of the system. This velocity is used to constrain the masses of the stars prior to the supernova explosion and the kick the pulsar received at or immediately after the explosion.

astro-ph.SR↗

arXiv2013

Broadband Pulsations from PSR B1821-24: Implications for Emission Models and the Pulsar Population of M28

We report a 5.4σ detection of pulsed gamma rays from PSR B1821-24 in the globular cluster M28 using ~44 months of Fermi Large Area Telescope (LAT) data that have been reprocessed with improved instrument calibration constants. We constructed a phase-coherent ephemeris, with post-fit residual RMS of 3 μs, using radio data spanning ~23.2 years, enabling measurements of the multi-wavelength light curve properties of PSR B1821-24 at the milliperiod level. We fold RXTE observations of PSR B1821-24 from 1996 to 2007 and discuss implications on the emission zones. The gamma-ray light curve consists of two peaks, separated by 0.41$\pm$0.02 in phase, with the first gamma-ray peak lagging the first radio peak by 0.05$\pm$0.02 in phase, consistent with the phase of giant radio pulses. We observe significant emission in the off-peak interval of PSR B1821-24 with a best-fit LAT position inconsistent with the core of M28. We do not detect significant gamma-ray pulsations at the spin or orbital periods from any other known pulsar in M28, and we place limits on the number of energetic pulsars in the cluster. The derived gamma-ray efficiency, ~2%, is typical of other gamma-ray pulsars with comparable spin-down power, suggesting that the measured spin-down rate ($2.2\times10^{36}$ erg s$^{-1}$) is not appreciably distorted by acceleration in the cluster potential. This confirms PSR B1821-24 as the second very energetic millisecond pulsar in a globular cluster and raises the question of whether these represent a separate class of objects that only form in regions of very high stellar density

astro-ph.HE↗

arXiv2013

The High Time Resolution Universe Survey - IX: Polarimetry of long-period pulsars

We present a polarimetric analysis of 49 long-period pulsars discovered as part of the High Time Resolution Universe (HTRU) southern survey. The sources exhibit the typical characteristics of "old" pulsars, with low fractional linear and circular polarisation and narrow, multicomponent profiles. Although the position angle swings are generally complex, for two of the analysed pulsars (J1622-3751 and J1710-2616) we obtained an indication of the geometry via the rotating vector model. We were able to determine a value of the rotation measure (RM) for 34 of the sources which, when combined with their dispersion measures (DM), yields an integrated magnetic field strength along the line of sight. With the data presented here, the total number of values of RM associated to pulsars discovered during the HTRU southern survey sums to 51. The RMs are not consistent with the hypothesis of a counter-clockwise direction of the Galactic magnetic field within an annulus included between 4 and 6 kpc from the Galactic centre. A partial agreement with a counter-clockwise sense of the Galactic magnetic field within the spiral arms is however found in the area of the Carina-Sagittarius arm.

astro-ph.SR↗

arXiv2013

The High Time Resolution Universe Pulsar Survey - VII: discovery of five millisecond pulsars and the different luminosity properties of binary and isolated recycled pulsars

This paper presents the discovery and timing parameters for five millisecond pulsars (MSPs), four in binary systems with probable white dwarf companions and one isolated, found in ongoing processing of the High Time Resolution Universe Pulsar Survey (HTRU). We also present high quality polarimetric data on four of them. These further discoveries confirm the high potential of our survey in finding pulsars with very short spin periods. At least two of these five MSPs are excellent candidates to be included in the Pulsar Timing Array projects. Thanks to the wealth of MSP discoveries in the HTRU survey, we revisit the question of whether the luminosity distributions of isolated and binary MSPs are different. Using the Cordes and Lazio distance model and our new and catalogue flux density measurements, we find that 41 of the 42 most luminous MSPs in the Galactic disk are in binaries and a statistical analysis suggests that the luminosity functions differ with 99.9% significance. We conclude that the formation process that leads to solitary MSPs affects their luminosities, despite their period and period derivatives being similar to those of pulsars in binary systems.

astro-ph.HE↗

arXiv2013

Dispersion measure variations in a sample of 168 pulsars

We analyse dispersion measure (DM) variations in six years of radio observations of more than 160 young pulsars, all gamma ray candidates for the Fermi gamma ray telescope mostly located close to the Galactic plane. DMs were fit across 256 MHz of bandwidth for observations centred at 1.4 GHz and across three frequencies -- 0.7 GHz, 1.4 GHz, and 3.1 GHz -- where multifrequency observations were available. Changes in dispersion measure, dDM/dt, were calculated using a weighted linear fit across all epochs of available DMs. DM variations were detected at a 3σlevel in 11 pulsars, four of which were above 5σ: PSRs J0835-4510, J0908-4913, J1824-1945, and J1833-0827. We find that after 28 years of gradual decline, the DM of PSR J0835-4510 is now increasing. The magnitude of variations in three of the four (PSRs J0835-4510, J0908-4913, and J1833-0827) are above what models would predict for an ISM dominated by Kolmogorov turbulence. We attribute this excess as likely due to the pulsar's local environment - the supernova remnants near PSRs J0835-4510 and J1833-0827 and the pulsar wind nebula around PSR J0908-4913. Upper limits were determined for all pulsars without detectable values of dDM/dt, most limits were found to lie above the levels of variations predicted by ISM theory. We find our results to be consistent with scattering estimates from the NE2001 model along these lines of sight.

astro-ph.GA↗

arXiv2013

On the detectability of extragalactic fast radio transients

Recent discoveries of highly dispersed millisecond radio bursts by Thornton et al. in a survey with the Parkes radio telescope at 1.4 GHz point towards an emerging population of sources at cosmological distances whose origin is currently unclear. Here we demonstrate that the scattering effects at lower radio frequencies are less than previously thought, and that the bursts could be detectable at redshifts out to about $z=0.5$ in surveys below 1 GHz. Using a source model in which the bursts are standard candles with bolometric luminosities $\sim 8 \times 10^{44}$ ergs/s uniformly distributed per unit comoving volume, we derive an expression for the observed peak flux density as a function of redshift and use this, together with the rate estimates found by Thornton et al. to find an empirical relationship between event rate and redshift probed by a given survey. The non-detection of any such events in Arecibo 1.4 GHz survey data by Deneva et al., and the Allen Telescope Array survey by Simeon et al. is consistent with our model. Ongoing surveys in the 1--2 GHz band should result in further discoveries. At lower frequencies, assuming a typical radio spectral index $α=-1.4$, the predicted peak flux densities are 10s of Jy. As a result, surveys of such a population with current facilities would not necessarily be sensitivity limited and could be carried out with small arrays to maximize the sky coverage. We predict that sources may already be present in 350-MHz surveys with the Green Bank Telescope. Surveys at 150 MHz with 30 deg$^2$ fields of view could detect one source per hour above 30 Jy.

astro-ph.HE↗

arXiv2013

Quantifying many-body effects by high-resolution Fourier transform scanning tunneling spectroscopy

Many-body phenomena are ubiquitous in solids, as electrons interact with one another and the many excitations arising from lattice, magnetic, and electronic degrees of freedom. These interactions can subtly influence the electronic properties of materials ranging from metals, exotic materials such as graphene, and topological insulators, or they can induce new phases of matter, as in conventional and unconventional superconductors, heavy fermion systems, and other systems of correlated electrons. As no single theoretical approach describes all such phenomena, the development of versatile methods for measuring many-body effects is key for understanding these systems. To date, angle-resolved photoemission spectroscopy (ARPES) has been the method of choice for accessing this physics by directly imaging momentum resolved electronic structure. Scanning tunneling microscopy/spectroscopy (STM/S), renown for its real-space atomic resolution capability, can also access the electronic structure in momentum space using Fourier transform scanning tunneling spectroscopy (FT-STS). Here, we report a high-resolution FT-STS measurement of the Ag(111) surface state, revealing fine structure in the otherwise parabolic electronic dispersion. This deviation is induced by interactions with lattice vibrations and has not been previously resolved by any technique. This study advances STM/STS as a method for quantitatively probing many-body interactions. Combined with the spatial sensitivity of STM/STS, this technique opens a new avenue for studying such interactions at the nano-scale.

cond-mat.mes-hall↗

arXiv2013

A Population of Fast Radio Bursts at Cosmological Distances

Searches for transient astrophysical sources often reveal unexpected classes of objects that are useful physical laboratories. In a recent survey for pulsars and fast transients we have uncovered four millisecond-duration radio transients all more than 40° from the Galactic plane. The bursts' properties indicate that they are of celestial rather than terrestrial origin. Host galaxy and intergalactic medium models suggest that they have cosmological redshifts of 0.5 to 1, and distances of up to 3 gigaparsecs. No temporally coincident x- or gamma-ray signature was identified in association with the bursts. Characterization of the source population and identification of host galaxies offers an opportunity to determine the baryonic content of the Universe.

astro-ph.HE↗

arXiv2013

GMRT discovery of PSR J1544+4937, an eclipsing black-widow pulsar identified with a Fermi LAT source

Using the Giant Metrewave Radio Telescope (GMRT) we performed deep observations to search for radio pulsations in the directions of unidentified Fermi Large Area Telescope (LAT) gamma-ray sources. We report the discovery of an eclipsing black-widow millisecond pulsar, PSR J1544+4937, identified with the un-cataloged gamma-ray source Fermi J1544.2+4941. This 2.16 ms pulsar is in a 2.9 hours compact circular orbit with a very low-mass companion (Mc > 0.017 Msun). At 322 MHz this pulsar is found to be eclipsing for 13% of its orbit, whereas at 607 MHz the pulsar is detected throughout the low-frequency eclipse phase. Variations in the eclipse ingress phase are observed, indicating a clumpy and variable eclipsing medium. Moreover, additional short-duration absorption events are observed around the eclipse boundaries. Using the radio timing ephemeris we were able to detect gamma-ray pulsations from this pulsar, confirming it as the source powering the gamma-ray emission.

astro-ph.HE↗

arXiv2013

The High Time Resolution Universe Pulsar Survey VIII: The Galactic millisecond pulsar population

We have used millisecond pulsars (MSPs) from the southern High Time Resolution Universe (HTRU) intermediate latitude survey area to simulate the distribution and total population of MSPs in the Galaxy. Our model makes use of the scale factor method, which estimates the ratio of the total number of MSPs in the Galaxy to the known sample. Using our best fit value for the z-height, z=500 pc, we find an underlying population of MSPs of 8.3(\pm 4.2)*10^4 sources down to a limiting luminosity of L_min=0.1 mJy kpc^2 and a luminosity distribution with a steep slope of d\log N/d\log L = -1.45(\pm 0.14). However, at the low end of the luminosity distribution, the uncertainties introduced by small number statistics are large. By omitting very low luminosity pulsars, we find a Galactic population above L_min=0.2 mJy kpc^2 of only 3.0(\pm 0.7)*10^4 MSPs. We have also simulated pulsars with periods shorter than any known MSP, and estimate the maximum number of sub-MSPs in the Galaxy to be 7.8(\pm 5.0)*10^4 pulsars at L=0.1 mJy kpc^2. In addition, we estimate that the high and low latitude parts of the southern HTRU survey will detect 68 and 42 MSPs respectively, including 78 new discoveries. Pulsar luminosity, and hence flux density, is an important input parameter in the model. Some of the published flux densities for the pulsars in our sample do not agree with the observed flux densities from our data set, and we have instead calculated average luminosities from archival data from the Parkes Telescope. We found many luminosities to be very different than their catalogue values, leading to very different population estimates. Large variations in flux density highlight the importance of including scintillation effects in MSP population studies.

astro-ph.SR↗

arXiv2013

Determinant quantum Monte Carlo study of the two-dimensional single-band Hubbard-Holstein model

We have performed numerical studies of the Hubbard-Holstein model in two dimensions using determinant quantum Monte Carlo (DQMC). Here we present details of the method, emphasizing the treatment of the lattice degrees of freedom, and then study the filling and behavior of the fermion sign as a function of model parameters. We find a region of parameter space with large Holstein coupling where the fermion sign recovers despite large values of the Hubbard interaction. This indicates that studies of correlated polarons at finite carrier concentrations are likely accessible to DQMC simulations. We then restrict ourselves to the half-filled model and examine the evolution of the antiferromagnetic structure factor, other metrics for antiferromagnetic and charge-density-wave order, and energetics of the electronic and lattice degrees of freedom as a function of electron-phonon coupling. From this we find further evidence for a competition between charge-density-wave and antiferromagnetic order at half-filling.

cond-mat.str-el↗

arXiv2013

Evidence of d-wave Superconductivity in K_(1-x)Na_xFe_2As_2 (x = 0, 0.1) Single Crystals from Low-Temperature Specific Heat Measurements

From the measurement and analysis of the specific heat of high-quality K_(1-x)Na_xFe_2As_2 single crystals we establish the presence of large T^2 contributions with coefficients alpha_sc ~ 30 mJ/mol K^3 at low-T for both x=0 and 0.1. Together with the observed square root field behavior of the specific heat in the superconducting state both findings evidence d-wave superconductivity on almost all Fermi surface sheets with an average gap amplitude of Delta_0 in the range of 0.4 - 0.8 meV. The derived Delta_0 and the observed T_c agree well with the values calculated within the Eliashberg theory, adopting a spin-fluctuation mediated pairing in the intermediate coupling regime.

cond-mat.supr-con↗

arXiv2013

A connection between radio state changing and glitch activity in PSR J0742-2822

PSR J0742-2822 exhibits two distinct emission states that are identified by discrete changes in the observed pulse profile. These changes have previously been shown to correlate with changes in the derivative of the pulse frequency. In this paper, we use observations with the Parkes radio telescope at a centre frequency of 1369 and 3100 MHz to produce high phase resolution polarisation profiles for the two modes and perform a detailed study of the correlation between observed pulse shape and spin-down rate. We find no correlation for at least 200 days prior to a glitch in the pulsar at MJD 55022, following which the correlation becomes strong. This suggests a link between the emission state switching phenomenon and glitch events. We discuss the possibility that emission state switching is driven by the interaction between the magnetosphere and the interior of the neutron star.

astro-ph.SR↗