2,566 research outputs found
From Majorana Fermions to Topological Order
We consider a system consisting of a 2D network of links between Majorana
fermions on superconducting islands. We show that the fermionic Hamiltonian
modeling this system is topologically-ordered in a region of parameter space.
In particular we show that Kitaev's toric code emerges in fourth-order
perturbation theory. By using a Jordan-Wigner transformation we can map the
model onto a family of signed 2D Ising models in a transverse field where the
signs (FM or AFM) are determined by additional gauge bits. Our mapping allows
an understanding of the non-perturbative regime and the phase transition to a
non-topological phase. We discuss the physics behind a possible implementation
of this model and argue how it can be used for topological quantum computation
by adiabatic changes in the Hamiltonian.Comment: 4+4 pages, 5 figures. v2 has a new reference and a few new comments.
In v3: yet another new reference and Supplementary Material is renamed
Appendix. In v4: several typos are corrected, to appear in Phys. Rev. Let
Low-Mass X-ray Binaries and Globular Clusters in Early-Type Galaxies. I. Chandra Observations
We present a Chandra survey of LMXBs in 24 early-type galaxies. Correcting
for detection incompleteness, the X-ray luminosity function (XLF) of each
galaxy is consistent with a powerlaw with negative logarithmic differential
slope, beta~2.0. However, beta strongly correlates with incompleteness,
indicating the XLF flattens at low-Lx. The composite XLF is well-fitted by a
powerlaw with a break at 2.21(+0.65,-0.56)E38 erg/s and beta=1.40(+0.10,-0.13)
and 2.84(+0.39,-0.30) below and above it, respectively. The break is close to
the Eddington limit for a 1.4Msun neutron-star, but the XLF shape rules out its
representing the division between neutron-star and black-hole systems. Although
the XLFs are similar, we find evidence of some variation between galaxies. The
high-Lx XLF slope does not correlate with age, but may correlate with
[alpha/Fe]. Considering only LMXBs with Lx>1E37 erg/s, matching the LMXBs with
globular clusters (GCs) identified in HST observations of 19 of the galaxies,
we find the probability a GC hosts an LMXB is proportional to LGC^alpha
ZFe^gamma} where alpha=1.01+/-0.19 and gamma=0.33+/-0.11. Correcting for GC
luminosity and colour effects, and detection incompleteness, we find no
evidence that the fraction of LMXBs with Lx>1e37 erg/s in GCs (40%), or the
fraction of GCs hosting LMXBs (~6.5%) varies between galaxies. The spatial
distribution of LMXBs resembles that of GCs, and the specific frequency of
LMXBs is proportional to the GC specific luminosity, consistent with the
hypothesis that all LMXBs form in GCs. If the LMXB lifetime is tau and the duty
cycle is Fd, our results imply ~1.5 (tau/1E8 yr)^-1 /Fd LMXBs are formed per
Gyr per GC and we place an upper limit of 1 active LMXB in the field per
3.4E9Lsun of V-band luminosity.Comment: 24 pages, 17 figures and 6 tables. Accepted for publication in the
Astrophysical Journal. Expanded discussion and various minor revisions to
improve robustness of results. Conclusions unchange
Systematic study of X-ray Cavities in the brightest galaxy of the Draco Constellation NGC 6338
We present results based on the systematic analysis of currently available
Chandra archive data on the brightest galaxy in the Draco constellation NGC
6338, in order to investigate the properties of the X-ray cavities. In the
central ~6 kpc, at least a two and possibly three, X-ray cavities are evident.
All these cavities are roughly of ellipsoidal shapes and show a decrement in
the surface brightness of several tens of percent. In addition to these
cavities, a set of X-ray bright filaments are also noticed which are spatially
coincident with the H{\alpha} filaments over an extent of 15 kpc. The H{\alpha}
emission line filaments are perpendicular to the X- ray cavities. Spectroscopic
analysis of the hot gas in the filaments and cavities reveal that the X-ray
filaments are cooler than the gas contained in the cavities. The emission line
ratios and the extended, asymmetric nature of the H{\alpha} emission line
filaments seen in this system require a harder ionizing source than that
produced by star formation and/or young, massive stars. Radio emission maps
derived from the analysis of 1.4 GHz VLA FIRST survey data failed to show any
association of these X-ray cavities with radio jets, however, the cavities are
filled by radio emission. The total power of the cavities is 17\times 1042 erg
s-1 and the ratio of the radio luminosity to cavity power is ~ 10-4, implying
that most of the jet power is mechanical.Comment: The paper contains 12 figures and 3 tables, Accepted 2011 December 7
for publication in MNRA
Particle acceleration in cooling flow clusters of galaxies: the case of Abell 2626
It has recently been proposed a theoretical model which accounts for the
origin of radio mini-halos observed in some cooling flow clusters as related to
electron re-acceleration by MHD turbulence (Gitti, Brunetti & Setti 2002). The
MHD turbulence is assumed to be frozen into the flow of the thermal ICM and
thus amplified in the cooling flow region. Here we present the application of
this model to a new mini-halo candidate, the cluster A2626, and compare the
results with those obtained for the mini-halo in the Perseus cluster. We
present VLA data at 330 MHz and 1.5 GHz of the diffuse radio emission observed
in A2626, and we show that its main properties can be explained by the model.
We find that the power necessary for the re-acceleration of the relic electron
population is only a factor ~ 0.7% of the maximum power that can be extracted
by the cooling flow (as estimated on the basis of the standard model). We also
discuss the observational properties of known mini-halos in connection with
those of host clusters, showing that the radio power of mini--halos increases
with the maximum power of cooling flows. This trend is expected in the
framework of the model. Possible effects of new Chandra and XMM-Newton
estimates of on this trend are considered: we conclude that even if
earlier derived cooling rates were overestimated, cooling flow powers are still
well above the radio powers emitted by mini-halos.Comment: 12 pages, 11 figures, accepted for publication in Astronomy &
Astrophysic
Conduction and cooling flows
Chandra and XMM-Newton observations have confirmed the presence of large
temperature gradients within the cores of many relaxed clusters of galaxies.
Here we investigate whether thermal conduction operating over those gradients
can supply sufficient heat to offset radiative cooling. Narayan & Medvedev
(2001) and Gruzinov (2002) have noted, using published results on cluster
temperatures, that conduction within a factor of a few of the Spitzer rate is
sufficient to balance bremsstrahlung cooling. From a detailed study of the
temperature and emission measure profiles of Abell 2199 and Abell 1835, we find
that the heat flux required by conduction is consistent with or below the rate
predicted by Spitzer in the outer regions of the core. Conduction may therefore
explain the lack of observational evidence for large mass cooling rates
inferred from arguments based simply on radiative cooling, provided that
conductivity is suppressed by no more than a factor of three below the full
Spitzer rate. To stem cooling in the cluster centre, however, would necessitate
conductivity values at least a factor of two larger than the Spitzer values,
which we consider implausible. This may provide an explanation for the observed
star formation and optical nebulosities in cluster cores. The solution is
likely to be time dependent. We briefly discuss the possible origin of the
cooler gas and the implications for massive galaxies.Comment: 5 pages, 4 figures, accepted by MNRAS. Minor changes following
referee's comment
Complementary tuning of Na+ and K+ channel gating underlies fast and energy-efficient action potentials in GABAergic interneuron axons
Fast-spiking, parvalbumin-expressing GABAergic interneurons (PV+-BCs) express a complex machinery of rapid signaling mechanisms, including specialized voltage-gated ion channels to generate brief action potentials (APs). However, short APs are associated with overlapping Na+ and K+ fluxes and are therefore energetically expensive. How the potentially vicious combination of high AP frequency and inefficient spike generation can be reconciled with limited energy supply is presently unclear. To address this question, we performed direct recordings from the PV+-BC axon, the subcellular structure where active conductances for AP initiation and propagation are located. Surprisingly, the energy required for the AP was, on average, only ∼1.6 times the theoretical minimum. High energy efficiency emerged from the combination of fast inactivation of Na+ channels and delayed activation of Kv3-type K+ channels, which minimized ion flux overlap during APs. Thus, the complementary tuning of axonal Na+ and K+ channel gating optimizes both fast signaling properties and metabolic efficiency. Hu et al. demonstrate that action potentials in parvalbumin-expressing GABAergic interneuron axons are energetically efficient, which is highly unexpected given their brief duration. High energy efficiency emerges from the combination of fast inactivation of voltage-gated Na+ channels and delayed activation of Kv3 channels in the axon
Hazardous substances in frequently used professional cleaning products
A growing number of studies have identified cleaners as a group at risk for adverse health effects of the skin and the respiratory tract. Chemical substances present in cleaning products could be responsible for these effects. Currently, only limited information is available about irritant and health hazardous chemical substances found in cleaning products. We hypothesized that chemical substances present in cleaning products are known health hazardous substances that might be involved in adverse health effects of the skin and the respiratory tract.
We performed a systematic review of cleaning products used in the Swiss cleaning sector. We surveyed Swiss professional cleaning companies (n = 1476) to identify the most used products (n = 105) for inclusion. Safety data sheets (SDSs) were reviewed and hazardous substances present in cleaning products were tabulated with current European and global harmonized system hazard labels.
Professional cleaning products are mixtures of substances (arithmetic mean 3.5 +/- 2.8), and more than 132 different chemical substances were identified in 105 products. The main groups of chemicals were fragrances, glycol ethers, surfactants, solvents; and to a lesser extent, phosphates, salts, detergents, pH-stabilizers, acids, and bases. Up to 75% of products contained irritant (Xi), 64% harmful (Xn) and 28% corrosive (C) labeled substances. Hazards for eyes (59%) and skin (50%), and hazards by ingestion (60%) were the most reported.
Cleaning products potentially give rise to simultaneous exposures to different chemical substances. As professional cleaners represent a large workforce, and cleaning products are widely used, it is a major public health issue to better understand these exposures. The list of substances provided in this study contains important information for future occupational exposure assessment studies
The Origin of B-Type Runaway Stars: Non-LTE Abundances as a Diagnostic
There are two accepted mechanisms to explain the origin of runaway OB-type
stars: the Binary Supernova Scenario (BSS), and the Cluster Ejection Scenario
(CES). In the former, a supernova explosion within a close binary ejects the
secondary star, while in the latter close multi-body interactions in a dense
cluster cause one or more of the stars to be ejected from the region at high
velocity. Both mechanisms have the potential to affect the surface composition
of the runaway star. TLUSTY non-LTE model atmosphere calculations have been
used to determine atmospheric parameters and carbon, nitrogen, magnesium and
silicon abundances for a sample of B-type runaways. These same analytical tools
were used by Hunter et al. (2009) for their analysis of 50 B-type open cluster
Galactic stars (i.e. non-runaways). Effective temperatures were deduced using
the silicon-ionization balance technique, surface gravities from Balmer line
profiles and microturbulent velocities derived using the Si spectrum. The
runaways show no obvious abundance anomalies when compared with stars in the
open clusters. The runaways do show a spread in composition which almost
certainly reflects the Galactic abundance gradient and a range in the
birthplaces of the runaways in the Galactic disk. Since the observed Galactic
abundance gradients of C, N, Mg and Si are of a similar magnitude, the
abundance ratios (e.g., N/Mg) are, as obtained, essentially uniform across the
sample
Erupting Cataclysmic Variable Stars in the Nearest Globular Cluster, NGC 6397: Intermediate Polars?
NGC 6397 is the closest globular cluster, and hence the ideal place to search
for faint stellar populations such as cataclysmic variables (CVs). HST and
Chandra observers have identified nine certain and likely CVs in this nearby
cluster, including several magnetic CV candidates. We have combined our recent
UV imagery with archival HST images of NGC 6397 to search for new CV candidates
and especially to look for dwarf nova-like eruptive events. We find remarkable
and somewhat unexpected dwarf nova-like eruptions of the two well-known
cataclysmic systems CV2 and CV3. These two objects have been claimed to be {\it
magnetic} CVs, as indicated by their helium emission-line spectra. Magnetic
fields in CVs are usually expected to prevent the disk instability that leads
to dwarf nova eruptions. In fact, most field magnetic CVs are observed to not
undergo eruptions. Our observations of the dwarf nova eruptions of CV2 and CV3
can be reconciled with these objects' HeII emission lines if both objects are
infrequently-erupting intermediate polars, similar to EX Hya. If this is the
case for most globular cluster CVs then we can reconcile the many X-ray and UV
bright CV candidates seen by Chandra and HST with the very small numbers of
erupting dwarf novae observed in cluster cores.Comment: 12 pages, 3 figures. Accepted for publication in The Astronomical
Journal. Two additional authors adde
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