680 research outputs found
X-Ray Grating Observations of Recurrent Nova T Pyxidis During The 2011 Outburst
The recurrent nova T Pyx was observed with the X-ray gratings of Chandra and
XMM-Newton, 210 and 235 days, respectively, after the discovery of the 2011
April 14 outburst. The X-ray spectra show prominent emission lines of C, N, and
O, with broadening corresponding to a full width at half maximum of ~2000-3000
km/s, and line ratios consistent with high-density plasma in collisional
ionization equilibrium. On day 210 we also measured soft X-ray continuum
emission that appears to be consistent with a white dwarf (WD) atmosphere at a
temperature ~420,000 K, partially obscured by anisotropic, optically thick
ejecta. The X-ray continuum emission is modulated with the photometric and
spectroscopic period observed in quiescence. The continuum at day 235 indicated
a WD atmosphere at a consistent effective temperature of 25 days earlier, but
with a lower flux. The effective temperature indicates a mass of ~1 solar mass.
The conclusion of partial WD obscuration is supported by the complex geometry
of non-spherically-symmetric ejecta confirmed in recent optical spectra
obtained with the Southern African Large Telescope (SALT) in November and
December of 2012. These spectra exhibited prominent [O III] nebular lines with
velocity structures typical of bipolar ejecta.Comment: Accepted to ApJ 2013 October 23, 14 pages, 9 figures, 3 table
Phosphorothioate Anti-sense Oligonucleotides: The Kinetics and Mechanism of the Generation of the Sulfurising Agent from Phenylacetyl Disulfide (PADS)
The synthesis of phosphorothioate oligonucleotides is often accomplished in the pharmaceutical industry by the sulfurisation of the nucleotide-phosphite using phenylacetyl disulfide (PADS) which has an optimal combination of properties. This is best achieved by an initial âageingâ of PADS for 48 hrs in acetonitrile with 3-picoline to generate polysulfides. The initial base-catalysed degradation of PADS occurs by an E1cB-type elimination to generate a ketene and acyldisulfide anion. Proton abstraction to reversibly generate a carbanion is demonstrated by H/D exchange, the rate of which is greatly increased by electron-withdrawing substituents in the aromatic ring of PADS. The ketene can be trapped intramolecularly by an o-allyl group. The disulfide anion generated subsequently attacks unreacted PADS on sulfur to give polysulfides, the active sulfurising agent. The rate of degradation of PADS is decreased by less basic substituted pyridines and is only first order in PADS indicating that the rate-limiting step is formation of the disulfide anion from the carbanion
Kinetic Resolution in Asymmetric Epoxidation using Iminium Salt Catalysis
The first reported examples of kinetic resolution in epoxidation reactions using iminium salt catalysis are described, providing up to 99% ee in the epoxidation of racemic cis-chromenes
Experimental investigation of boundary layer instabilities and transition on sharp and blunt cones in Oxfordâs High-Density Tunnel
Boundary layer instabilities and transition to turbulence on a 7-degree half-angle cone with varying nose-tip radii in Mach 6 and 7 flow are investigated using a combination of surface heat transfer measurements, surface pressure measurements, and high speed schlieren images. The experiments are performed at unit-Reynolds numbers ranging from [22-44]*10^6/m in University of Oxfordâs High-Density Tunnel (HDT). The transition Reynolds number, Re_{X_T}, increases with increasing nose tip Reynolds number, Re_{R_N}, for Re_{R_N}<10^5. In this range, evidence of second-mode wave instabilities are observed in both schlieren images and surface pressure measurements. For 10^5 < Re_{R_N} < 4*10^5, Re_{X_T} remains constant and coherent streaks above the boundary layer are observed with schlieren imaging. Images of the interaction of these features with a boundary layer breaking down to a fully turbulent state are presented. The freestream disturbance environment is also varied through existence of several steady state plateaus created by the natural operation of the facility, and characterised with multi-point focused laser differential interferometry (FLDI). Re_{X_T} increases by 10-60% with increasing plateau number which is independent of Re_{R_N}. Variation in freestream fluctuation amplitude with frequency and Reynolds number are in agreement with previous studies while variation with plateau is not. The discrepancy is explained by receptivity functions which are sensitive to the inclination angle of disturbances. A method for measuring the inclination angle using correlated FLDI signals is presented and reveals a consistent trend with plateau number. The trend is physically explained by changes in the relative contribution of entropic and acoustic modes with time
Effect of hyperon bulk viscosity on neutron-star r-modes
Neutron stars are expected to contain a significant number of hyperons in
addition to protons and neutrons in the highest density portions of their
cores. Following the work of Jones, we calculate the coefficient of bulk
viscosity due to nonleptonic weak interactions involving hyperons in
neutron-star cores, including new relativistic and superfluid effects. We
evaluate the influence of this new bulk viscosity on the gravitational
radiation driven instability in the r-modes. We find that the instability is
completely suppressed in stars with cores cooler than a few times 10^9 K, but
that stars rotating more rapidly than 10-30% of maximum are unstable for
temperatures around 10^10 K. Since neutron-star cores are expected to cool to a
few times 10^9 K within seconds (much shorter than the r-mode instability
growth time) due to direct Urca processes, we conclude that the gravitational
radiation instability will be suppressed in young neutron stars before it can
significantly change the angular momentum of the star.Comment: final PRD version, minor typos etc correcte
Humpy LNRF-velocity profiles in accretion discs orbiting nearly extreme Kerr black holes. A possible relation to QPOs
Change of sign of the LNRF-velocity gradient has been found for accretion
discs orbiting rapidly rotating Kerr black holes with spin a > 0.9953 for
Keplerian discs and a > 0.99979 for marginally stable thick discs. Aschenbach
(2004) has identified the maximal rate of change of the orbital velocity within
the "humpy" profile with a locally defined critical frequency of disc
oscillations, but it has been done in a coordinate-dependent form. We define
the critical "humpy" frequency H in general relativistic, coordinate
independent form, and relate the frequency defined in the LNRF to distant
observers. At radius of its definition, so-called "humpy" radius r_h, the
"humpy" frequency H is compared to the radial (R) and vertical (V) epicyclic
frequencies and the orbital frequency of the disc. For Keplerian thin discs, we
show that the epicyclic resonance radii r_31 and r_41 (with V:R = 3:1 or 4:1)
are located in vicinity of r_h where efficient triggering of oscillations with
frequencies ~ H could be expected. Asymptotically (for 1-a < 10^(-4)) the ratio
of the epicyclic and Keplerian frequencies and the humpy frequency is nearly
constant, i.e., almost independent of spin, being for the radial epicyclic
frequency R:H ~ 3:2. For thick discs the situation is more complex due to
dependence on distribution of the specific angular momentum l determining the
disc properties. For l = const tori and 1-a < 10^(-6) the frequency ratios of
the humpy frequency and the orbital and epicyclic frequencies are again nearly
constant and independent of both a and l, being for the radial epicyclic
frequency R:H close to 4. In the limiting case of very slender tori (l ~ l_ms)
the epicyclic resonance radius r_41 ~ r_h for spin 1-a < 2x10^(-4).Comment: 11 pages,10 figures, 1 table. Accepted for publication in Astronomy
and Astrophysic
Self-interest And Public Interest: The Motivations Of Political Actors
Self-Interest and Public Interest in Western Politics showed that the public, politicians, and bureaucrats are often public spirited. But this does not invalidate public-choice theory. Public-choice theory is an ideal type, not a claim that self-interest explains all political behavior. Instead, public-choice theory is useful in creating rules and institutions that guard against the worst case, which would be universal self-interestedness in politics. In contrast, the public-interest hypothesis is neither a comprehensive explanation of political behavior nor a sound basis for institutional design
Ferromagnetic dynamics detected via one- and two-magnon NV relaxometry
The NV center in diamond has proven to be a powerful tool for locally
characterizing the magnetic response of microwave excited ferromagnets. To
date, this has been limited by the requirement that the FMR excitation
frequency be less than the NV spin resonance frequency. Here we report NV
relaxometry based on a two-magnon Raman-like process, enabling detection of FMR
at frequencies higher than the NV frequency. For high microwave drive powers,
we observe an unexpected field-shift of the NV response relative to a
simultaneous microwave absorption signal from a low damping ferrite film. We
show that the field-shifted NV response is due to a second order Suhl
instability. The instability creates a large population of non-equilibrium
magnons which relax the NV spin, even when the uniform mode FMR frequency
exceeds that of the NV spin resonance frequency, hence ruling out the
possibility that the NV is relaxed by a single NV-resonant magnon. We argue
that at high frequencies the NV response is due to a two-magnon relaxation
process in which the difference frequency of two magnons matches the NV
frequency, and at low frequencies we evaluate the lineshape of the one-magnon
NV relaxometry response using spinwave instability theory
Optical modeling and polarization calibration for CMB measurements with ACTPol and Advanced ACTPol
The Atacama Cosmology Telescope Polarimeter (ACTPol) is a polarization
sensitive upgrade to the Atacama Cosmology Telescope. Located at an elevation
of 5190 m, ACTPol measures the Cosmic Microwave Background (CMB) temperature
and polarization with arcminute-scale angular resolution. Calibration of the
detector angles is a critical step in producing maps of the CMB polarization.
Polarization angle offsets in the detector calibration can cause leakage in
polarization from E to B modes and induce a spurious signal in the EB and TB
cross correlations, which eliminates our ability to measure potential
cosmological sources of EB and TB signals, such as cosmic birefringence. We
present our optical modeling and measurements associated with calibrating the
detector angles in ACTPol.Comment: 12 pages, 8 figures, conference proceedings submitted to Proceedings
of SPIE; added reference in section 2 and merged repeated referenc
The Atacama Cosmology Telescope: Dusty Star-Forming Galaxies and Active Galactic Nuclei in the Southern Survey
We present a catalog of 191 extragalactic sources detected by the Atacama
Cosmology Telescope (ACT) at 148 GHz and/or 218 GHz in the 2008 Southern
survey. Flux densities span 14-1700 mJy, and we use source spectral indices
derived using ACT-only data to divide our sources into two sub-populations: 167
radio galaxies powered by central active galactic nuclei (AGN), and 24 dusty
star-forming galaxies (DSFGs). We cross-identify 97% of our sources (166 of the
AGN and 19 of the DSFGs) with those in currently available catalogs. When
combined with flux densities from the Australian Telescope 20 GHz survey and
follow-up observations with the Australia Telescope Compact Array, the
synchrotron-dominated population is seen to exhibit a steepening of the slope
of the spectral energy distribution from 20 to 148 GHz, with the trend
continuing to 218 GHz. The ACT dust-dominated source population has a median
spectral index of 3.7+0.62-0.86, and includes both local galaxies and sources
with redshifts as great as 5.6. Dusty sources with no counterpart in existing
catalogs likely belong to a recently discovered subpopulation of DSFGs lensed
by foreground galaxies or galaxy groups.Comment: 13 pages, 8 figures, 4 table
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