2,572 research outputs found
Gamma-Ray Emission from Two Blazars Behind the Galactic Plane: B2013+370 & B2023+336
B2013+370 and B2023+336 are two blazars at low-galactic latitude that were
previously proposed to be the counterparts for the EGRET unidentified sources,
3EG J2016+3657 and 3EG J2027+3429. Gamma-ray emission associated with the EGRET
sources has been detected by the Fermi Gamma-ray Space Telescope, and the two
sources, 1FGL J2015.7+3708 and 1FGL J2027.6+3335, have been classified as
unidentified in the 1-year catalog. This analysis of the Fermi-LAT data
collected during 31 months reveals that the 1FGL sources are spatially
compatible with the blazars, and are significantly variable, supporting the
hypothesis of extragalactic origin for the gamma-ray emission. The gamma-ray
light curves are compared with 15 GHz radio light curves from the 40-m
telescope at the Owens Valley Radio Observatory (OVRO). Simultaneous
variability is seen in both bands for the two blazar candidates. The study is
completed with the X-ray analysis of 1FGL J2015.7+3708 using Swift observations
that were triggered in August 2010 by a Fermi-detected flare. The resulting
spectral energy distribution shows a two-component structure typical of
blazars. We also identify a second source in the field of view of 1FGL
J2027.6+3335 with similar characteristics to the known LAT pulsars. This study
gives solid evidence favoring blazar counterparts for these two unidentified
EGRET and Fermi sources, supporting the hypothesis that a number of
unidentified gamma-ray sources at low galactic latitudes are indeed of
extragalactic origin.Comment: 10 pages, 7 figures, 6 tables, accepted for publication in The
Astrophysical Journa
Spin ordering quantum transitions of superconductors in a magnetic field
We argue that recent neutron scattering measurements by Lake et. al. (Science
291, 1759 (2001)) of the spin excitation spectrum of LSCO in a magnetic field
can be understood in terms of proximity to a phase with co-existing
superconductivity and spin density wave order. We present a general theory for
such quantum transitions, and argue that their low energy spin fluctuations are
controlled by a singular correction from the superflow kinetic energy, acting
in the region outside the vortex cores. We propose numerous experimental tests
of our theory.Comment: 4 pages, 3 eps fig
On the Nature of Small Planets around the Coolest Kepler Stars
We constrain the densities of Earth- to Neptune-size planets around very cool
(Te =3660-4660K) Kepler stars by comparing 1202 Keck/HIRES radial velocity
measurements of 150 nearby stars to a model based on Kepler candidate planet
radii and a power-law mass-radius relation. Our analysis is based on the
presumption that the planet populations around the two sets of stars are the
same. The model can reproduce the observed distribution of radial velocity
variation over a range of parameter values, but, for the expected level of
Doppler systematic error, the highest Kolmogorov-Smirnov probabilities occur
for a power-law index alpha ~ 4, indicating that rocky-metal planets dominate
the planet population in this size range. A single population of gas-rich,
low-density planets with alpha = 2 is ruled out unless our Doppler errors are
>= 5m/s, i.e., much larger than expected based on observations and stellar
chromospheric emission. If small planets are a mix of gamma rocky planets
(alpha = 3.85) and 1-gamma gas-rich planets (alpha = 2), then gamma > 0.5
unless Doppler errors are >=4 m/s. Our comparison also suggests that Kepler's
detection efficiency relative to ideal calculations is less than unity. One
possible source of incompleteness is target stars that are misclassified
subgiants or giants, for which the transits of small planets would be
impossible to detect. Our results are robust to systematic effects, and
plausible errors in the estimated radii of Kepler stars have only moderate
impact.Comment: Accepted to the Astrophysical Journa
The Search for Young Planetary Systems And the Evolution of Young Stars
The Space Interferometer Mission (SIM) will provide a census of planetary systems by con- ducting a broad survey of 2,000 stars that will be sensitive to the presence of planets with masses as small as approx. 15 Earth masses (1 Uranus mass) and a deep survey of approx. 250 of the nearest, stars with a mass limit of approx.3 Earth masses. The broad survey will include stars spanning a wide range of ages, spectral types, metallicity, and other important parameters. Within this larger context, the Young Stars and Planets Key Project will study approx. 200 stars with ages from 1 Myr to 100 Myr to understand the formation and dynamical evolution of gas giant planets. The SIM Young Stars and Planets Project will investigate both the frequency of giant planet formation and the early dynamical history of planetary systems. We will gain insight into how common the basic architecture of our solar system is compared with recently discovered systems with close-in giant planets by examining 200 of the nearest (less than 150 pc) and youngest (1-100 Myr) solar-type stars for planets. The sensitivity of the survey for stars located 140 pc away is shown in the planet mass-separation plane. We expect to find anywhere from 10 (assuming that only the presently known fraction of stars. 5-7%, has planets) to 200 (all young stars have planets) planetary systems. W-e have set our sensitivity threshold to ensure the detection of Jupiter-mass planets in the critical orbital range of 1 to 5 AU. These observations, when combined with the results of planetary searches of mature stars, will allow us to test theories of planetary formation and early solar system evolution. By searching for planets around pre-main sequence stars carefully selected to span an age range from 1 to 100 Myr, we will learn a t what epoch and with what frequency giant planets are found at the water-ice snowline where they are expected to form. This will provide insight into the physical mechanisms by which planets form and migrate from their place of birth, and about their survival rate. With these data in hand, we will provide data, for the first time, on such important questions as: What processes affect the formation and dynamical evolution of planets? When and where do planets form? What is initial mass distribution of planetary systems around young stars? How might planets be destroyed? What is the origin of the eccentricity of planetary orbits? What is the origin of the apparent dearth of companion objects between planets and brown dwarfs seen in mature stars? The observational strategy is a compromise between the desire to extend the planetary mass function as low as possible and the essential need to build up sufficient statistics on planetary occurrence. About half of the sample will be used to address the "where" and "when" of planet formation. We will study classical T Tauri stars (cTTs) which have massive accretion disks and post- accretion, weak-lined T Tauri stars (wTTs). Preliminary estimates suggest the sample will consist of approx. 30% cTTs and approx. 70% wTTs, driven in part by the difficulty of making accurate astrometric measurements toward objects with strong variability or prominent disks
The Spin of Holographic Electrons at Nonzero Density and Temperature
We study the Green's function of a gauge invariant fermionic operator in a
strongly coupled field theory at nonzero temperature and density using a dual
gravity description. The gravity model contains a charged black hole in four
dimensional anti-de Sitter space and probe charged fermions. In particular, we
consider the effects of the spin of these probe fermions on the properties of
the Green's function. There exists a spin-orbit coupling between the spin of an
electron and the electric field of a Reissner-Nordstrom black hole. On the
field theory side, this coupling leads to a Rashba like dispersion relation. We
also study the effects of spin on the damping term in the dispersion relation
by considering how the spin affects the placement of the fermionic quasinormal
modes in the complex frequency plane in a WKB limit. An appendix contains some
exact solutions of the Dirac equation in terms of Heun polynomials.Comment: 27 pages, 11 figures; v2: minor changes, published versio
First Observation of the Rare Decay Mode K-long -> e+ e-
In an experiment designed to search for and study very rare two-body decay
modes of the K-long, we have observed four examples of the decay K-long -> e+
e-, where the expected background is 0.17+-0.10 events. This observation
translates into a branching fraction of 8.7^{+5.7}_{-4.1} X 10^{-12},
consistent with recent theoretical predictions. This result represents by far
the smallest branching fraction yet measured in particle physics.Comment: 9 pages, 3 figure
Nonpropagation of massive mode on AdS2 in topologically massive gravity
Making use of Achucarro-Ortiz (AO) type of dimensional reduction, we study
the topologically massive gravity with a negative cosmological constant on AdS2
spacetimes. For a constant dilaton, this two-dimensional model also admits
three AdS2 vacuum solutions, which are related to two AdS3 and warped AdS3
backgrounds with an identification upon uplifting three dimensions. We carry
out the perturbation analysis around these backgrounds to find what is a
physically propagating field. However, it turns out that there is no
propagating massive mode on AdS2 background, in contrast to the Kaluza-Klein
(KK) type of dimensional reduction. We note that two dimensionally reduced
actions are different and thus, the non-equivalence of their on-shell
amplitudes is obtained.Comment: 19 pages, version to appear in EPJ
Five Kepler target stars that show multiple transiting exoplanet candidates
We present and discuss five candidate exoplanetary systems identified with
the Kepler spacecraft. These five systems show transits from multiple exoplanet
candidates. Should these objects prove to be planetary in nature, then these
five systems open new opportunities for the field of exoplanets and provide new
insights into the formation and dynamical evolution of planetary systems. We
discuss the methods used to identify multiple transiting objects from the
Kepler photometry as well as the false-positive rejection methods that have
been applied to these data. One system shows transits from three distinct
objects while the remaining four systems show transits from two objects. Three
systems have planet candidates that are near mean motion
commensurabilities---two near 2:1 and one just outside 5:2. We discuss the
implications that multitransiting systems have on the distribution of orbital
inclinations in planetary systems, and hence their dynamical histories; as well
as their likely masses and chemical compositions. A Monte Carlo study indicates
that, with additional data, most of these systems should exhibit detectable
transit timing variations (TTV) due to gravitational interactions---though none
are apparent in these data. We also discuss new challenges that arise in TTV
analyses due to the presence of more than two planets in a system.Comment: Accepted to Ap
Preoperative stroke before cardiac surgery does not increase risk of postoperative stroke
The optimal time when surgery can be safely performed after stroke is unknown. The purpose of this study was to investigate how cardiac surgery timing after stroke impacts postoperative outcomes between 2011–2017 were reviewed. Variables were extracted from the institutional Society of Thoracic Surgeons database, statewide patient registry, and medical records. Subjects were classified based upon presence of endocarditis and further grouped by timing of preoperative stroke relative to cardiac surgery: Recent (stroke within two weeks before surgery), Intermediate (between two and six weeks before), and Remote (greater than six weeks before). Postoperative outcomes were compared amongst groups. 157 patients were included: 54 in endocarditis and 103 in non-endocarditis, with 47 in Recent, 26 in Intermediate, and 84 in Remote. 30-day mortality and postoperative stroke rate were similar across the three subgroups for both endocarditis and non-endocarditis. Of patients with postoperative stroke, mortality was 30% (95% CI 4.6–66). Timing of cardiac surgery after stroke occurrence does not seem to affect postoperative stroke or mortality. If postoperative stroke does occur, subsequent stroke-related mortality is high
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