Magnetism, X-rays, and Accretion Rates in WD 1145+017 and other Polluted White Dwarf Systems

This paper reports circular spectropolarimetry and X-ray observations of several polluted white dwarfs including WD 1145+017, with the aim to constrain the behavior of disk material and instantaneous accretion rates in these evolved planetary systems. Two stars with previously observed Zeeman splitting, WD 0322-019 and WD 2105-820, are detected above 5 sigma and > 1 kG, while WD 1145+017, WD 1929+011, and WD 2326+049 yield (null) detections below this minimum level of confidence. For these latter three stars, high-resolution spectra and atmospheric modeling are used to obtain limits on magnetic field strengths via the absence of Zeeman splitting, finding B* < 20 kG based on data with resolving power R near 40 000. An analytical framework is presented for bulk Earth composition material falling onto the magnetic polar regions of white dwarfs, where X-rays and cyclotron radiation may contribute to accretion luminosity. This analysis is applied to X-ray data for WD 1145+017, WD 1729+371, and WD 2326+049, and the upper bound count rates are modeled with spectra for a range of plasma kT = 1 - 10 keV in both the magnetic and non-magnetic accretion regimes. The results for all three stars are consistent with a typical dusty white dwarf in a steady-state at 1e8 - 1e9 g/s. In particular, the non-magnetic limits for WD 1145+017 are found to be well below previous estimates of up to 1e12 g/s, and likely below 1e10 g/s, thus suggesting the star-disk system may be average in its evolutionary state, and only special in viewing geometry.Comment: 14 pages, 7 figures, 3 tables; accepted to MNRA

The long egress of GJ~436b's giant exosphere

The M dwarf GJ 436 hosts a transiting warm Neptune known to experience atmospheric escape. Previous observations revealed the presence of a giant hydrogen exosphere transiting the star for more than 5 h, and absorbing up to 56% of the flux in the blue wing of the stellar Lyman-{\alpha} line of neutral hydrogen (H i Ly{\alpha}). The unexpected size of this comet-like exosphere prevented observing the full transit of its tail. In this Letter, we present new Ly{\alpha} observations of GJ 436 obtained with the Space Telescope Imaging Spectrograph (STIS) instrument onboard the Hubble Space Telescope. The stability of the Ly{\alpha} line over six years allowed us to combine these new observations with archival data sets, substantially expanding the coverage of the exospheric transit. Hydrogen atoms in the tail of the exospheric cloud keep occulting the star for 10-25 h after the transit of the planet, remarkably confirming a previous prediction based on 3D numerical simulations with the EVaporating Exoplanet code (EVE). This result strengthens the interpretation that the exosphere of GJ 436b is shaped by both radiative braking and charge exchanges with the stellar wind. We further report flux decreases of 15 +/- 2% and 47 +/- 10% in the red wing of the Ly{\alpha} line and in the line of ionised silicon (Si iii). Despite some temporal variability possibly linked with stellar activity, these two signals occur during the exospheric transit and could be of planetary origin. Follow-up observations will be required to assess the possibility that the redshifted Ly{\alpha} and Si iii absorption signatures arise from interactions between the exospheric flow and the magnetic field of the star.Comment: 10 pages, 7 figures, published in A&

C-axis resistivity and high Tc superconductivity

Recently we had proposed a mechanism for the normal-state C-axis resistivity of the high-T$_c$ layered cuprates that involved blocking of the single-particle tunneling between the weakly coupled planes by strong intra-planar electron-electron scattering. This gave a C-axis resistivity that tracks the ab-plane T-linear resistivity, as observed in the high-temperature limit. In this work this mechanism is examined further for its implication for the ground-state energy and superconductivity of the layered cuprates. It is now argued that, unlike the single-particle tunneling, the tunneling of a boson-like pair between the planes prepared in the BCS-type coherent trial state remains unblocked inasmuch as the latter is by construction an eigenstate of the pair annihilation operator. The resulting pair-delocalization along the C-axis offers energetically a comparative advantage to the paired-up trial state, and, thus stabilizes superconductivity. In this scheme the strongly correlated nature of the layered system enters only through the blocking effect, namely that a given electron is effectively repeatedly monitored (intra-planarly scattered) by the other electrons acting as an environment, on a time-scale shorter than the inter-planar tunneling time. Possible relationship to other inter-layer pairing mechanisms proposed by several workers in the field is also briefly discussed.Comment: typos in equations corrected, contents unchange

g-on Mean Field Theory of the t-J Model

Implication of our recent proposal [J. Phys. Soc. Jpn. 65 (1996) 687] to treat large-amplitude gauge-field fluctuations around the slave-boson mean-field theory for the t-J model has been explored in detail. By attaching gauge flux to spinons and holons and then treating them as free g-on's which respect the time-reversal symmetry, the optimum exclusion (g) and exchange (\a) statistics have been determined in the plane of doping rate and temperature. Two different relations between \a and g have been investigated, namely g=|\a| (Case1) and g=|\a|(2-|\a|) (Case2). The results indicate that slave fermion is favored at low doping while slave boson at high doping. For two dimension, in Case1 intermediate statistics are found in between, while in Case2 no intermediate statistics are found. The consequences of varying the dimensionality and strength of J have been studied also. The latter has no qualitative effect for both cases, while the former has a profound effect in Case1.Comment: 18 pages, 11 figures two of them are figure 8; submitted to Phys. Rev. B; notes and citations are added, as seen in page 17; E-mails: [email protected], [email protected]

BAT X-ray Survey - III: X-ray Spectra and Statistical Properties

In this concluding part of the series of three papers dedicated to the Swift/BAT hard X-ray survey (BXS), we focus on the X-ray spectral analysis and statistical properties of the source sample. Using a dedicated method to extract time-averaged spectra of BAT sources we show that Galactic sources have, generally, softer spectra than extragalactic objects and that Seyfert 2 galaxies are harder than Seyfert 1s. The averaged spectrum of all Seyfert galaxies is consistent with a power-law with photon index of 2.00 (+/-0.07). The cumulative flux-number relation for the extragalactic sources in the 14-170 keV band is best described by a power-law with a slope alpha=1.55 (+/-0.20) and a normalization of 9.6$\pm1.9 \times 10^{-3}$ AGN deg$^{-2}$ (or 396(+/-80) AGN all-sky) above a flux level of 2$\times 10^{-11}$erg cm$^{-2}$ s$^{-1}$ (~0.85 mCrab). The integration of the cumulative flux per unit area indicates that BAT resolves 1-2% of the X-ray background emission in the 14-170 keV band. A sub-sample of 24 extragalactic sources above the 4.5 sigma detection limit is used to study the statistical properties of AGN. This sample comprises local Seyfert galaxies (z=0.026, median value) and ~10% blazars. We find that 55% of the Seyfert galaxies are absorbed by column densities of Log(N_H)>22, but that none is a bona fide Compton-thick. This study shows the capabilities of BAT to probe the hard X-ray sky to the mCrab level.Comment: Accepted for publication in The Astrophysical Journal; 42 pages, 4 tables, 51 figure

New periodic variable stars coincident with ROSAT sources discovered using SuperWASP

We present optical lightcurves of 428 periodic variable stars coincident with ROSAT X-ray sources, detected using the first run of the SuperWASP photometric survey. Only 68 of these were previously recognised as periodic variables. A further 30 of these objects are previously known pre-main sequence stars, for which we detect a modulation period for the first time. Amongst the newly identified periodic variables, many appear to be close eclipsing binaries, their X-ray emission is presumably the result of RS CVn type behaviour. Others are probably BY Dra stars, pre-main sequence stars and other rapid rotators displaying enhanced coronal activity. A number of previously catalogued pulsating variables (RR Lyr stars and Cepheids) coincident with X-ray sources are also seen, but we show hat these are likely to be misclassifications. We identify four objects which are probable low mass eclipsing binary stars, based on their very red colour and light curve morphology

The Bose Metal: gauge field fluctuations and scaling for field tuned quantum phase transitions

In this paper, we extend our previous discussion of the Bose metal to the field tuned case. We point out that the recent observation of the metallic state as an intermediate phase between the superconductor and the insulator in the field tuned experiments on MoGe films is in perfect consistency with the Bose metal scenario. We establish a connection between general dissipation models and gauge field fluctuations and apply this to a discussion of scaling across the quantum phase boundaries of the Bose metallic state. Interestingly, we find that the Bose metal scenario implies a possible {\em two} parameter scaling for resistivity across the Bose metal-insulator transition, which is remarkably consistent with the MoGe data. Scaling at the superconductor-metal transition is also proposed, and a phenomenolgical model for the metallic state is discussed. The effective action of the Bose metal state is described and its low energy excitation spectrum is found to be $\omega \propto k^{3}$.Comment: 15 pages, 1 figur

Discovery of the Acoustic Faraday Effect in Superfluid 3He-B

We report the discovery of the acoustic Faraday effect in superfluid 3He-B. The observation of this effect provides the first direct evidence for propagating transverse acoustic waves in liquid 3He, a mode first predicted by Landau in 1957. The Faraday rotation is large and observable because of spontaneously broken spin-orbit symmetry in 3He-B. We compare the experimental observations with a simulation of the transverse acoustic impedance that includes the field-induced circular birefringence of transverse waves.Comment: 4 pages in RevTex plus 3 postscript figures; new version includes: minor corrections to the text and an updated of list of reference