7,070 research outputs found
Voltage-Controlled Spin Selection in a Magnetic Resonant Tunnelling Diode
We have fabricated all II-VI semiconductor resonant tunneling diodes based on
the (Zn,Mn,Be)Se material system, containing dilute magnetic material in the
quantum well, and studied their current-voltage characteristics. When subjected
to an external magnetic field the resulting spin splitting of the levels in the
quantum well leads to a splitting of the transmission resonance into two
separate peaks. This is interpreted as evidence of tunneling transport through
spin polarized levels, and could be the first step towards a voltage controlled
spin filter.Comment: To be published in Phys. Rev. Let
Bayesian Sensitivity Analysis of Flight Parameters in a Hard-Landing Analysis Process
A flight parameter sensor simulation model was developed to assess the conservatism of the landing gear component loads calculated using a typical hard-landing analysis process. Conservatism exists due to factors of safety that are incorporated into any hard-landing analysis process to account for uncertainty in the measurement of certain flight parameters. The flight parameter sensor simulation model consists of 1) an aircraft and landing gear dynamic model to determine the “actual” landing gear loads during a hard landing; 2) an aircraft sensor and data acquisition model to represent the aircraft sensors and flight data recorder systems to investigate the effect of signal processing on the flight parameters; and 3) an automated hard-landing analysis process, representative of that used by airframe and equipment manufacturers, to determine the “simulated” landing gear loads. Using a technique of Bayesian sensitivity analysis, a number of flight parameters are varied in the flight parameter sensor simulation model to gain an understanding of the sensitivity of the difference between actual and simulated loads to the individual flight parameters in symmetric and asymmetric two-point landings. This study shows that the error can be reduced by learning the true value of the following flight parameters: longitudinal tire–runway friction coefficient, aircraft vertical acceleration (related to vertical descent velocity), lateral acceleration (related to lateral velocity), Euler roll angle, mass, center of gravity position, and main landing gear tire type. It was also shown that, due to the modeling techniques used, shock absorber servicing state and tire pressure do not contribute significantly to the error
Four New BL Lac Surveys: Sampling New Populations
The advent of large area deep radio and X-ray surveys is leading to the
creation of many new BL Lac samples. In particular, the ROSAT All-Sky, Green
Bank and FIRST surveys are proving to be rich sources of new BL Lacs. We will
discuss the methods used in four independent BL Lac searches based on these
surveys. Comparison of the broadband spectral energy distributions of these BL
Lacs with those of previously known objects clearly points to the existence of
a large previously unrecognized population of objects with characteristics
intermediate between those exhibited by Low and High energy peaked BL Lacs.Comment: 4 pages, 3 postscript figures, To be published in the Proceedings of
the conference "BL Lac Phenomenon" held in Turku, Finland, June 22-26, 199
Implications of Dramatic Broad Absorption Line Variability in the Quasar FBQS J1408+3054
We have observed a dramatic change in the spectrum of the formerly heavily
absorbed `overlapping-trough' iron low-ionization broad absorption line
(FeLoBAL) quasar FBQS J1408+3054. Over a time span of between 0.6 to 5
rest-frame years, the Mg II trough outflowing at 12,000 km/s decreased in
equivalent width by a factor of two and the Fe II troughs at the same velocity
disappeared. The most likely explanation for the variability is that a
structure in the BAL outflow moved out of our line of sight to the ultraviolet
continuum emitting region of the quasar's accretion disk. Given the size of
that region, this structure must have a transverse velocity of between 2600
km/s and 22,000 km/s. In the context of a simple outflow model, we show that
this BAL structure is located between approximately 5800 and 46,000
Schwarzschild radii from the black hole. That distance corresponds to 1.7 to 14
pc, 11 to 88 times farther from the black hole than the H-beta broad-line
region. The high velocities and the parsec-scale distance for at least this one
FeLoBAL outflow mean that not all FeLoBAL outflows can be associated with
galaxy-scale outflows in ultraluminous infrared galaxies transitioning to
unobscured quasars. The change of FBQS J1408+3054 from an FeLoBAL to a LoBAL
quasar also means that if (some) FeLoBAL quasars have multiwavelength
properties which distinguish them from HiBAL quasars, then some LoBAL quasars
will share those properties. Finally, we extend previous work on how
multiple-epoch spectroscopy of BAL and non-BAL quasars can be used to constrain
the average lifetime of BAL episodes (currently >60 rest-frame years at 90%
confidence).Comment: Final version to appear in MNRAS: references added and factor of 2
underestimate of accretion disk size corrected, resulting in absorber
constrained to be somewhat closer to the black hole. For an animated gif
showing the spectral evolution of the broad absorption line troughs in this
quasar, see http://www.yorku.ca/phall/film19952009.gi
A Computation of the Maximal Order Type of the Term Ordering on Finite Multisets
We give a sharpening of a recent result of Aschenbrenner and Pong about the maximal order type of the term ordering on the finite multisets over a wpo. Moreover we discuss an approach to compute maximal order types of well-partial orders which are related to tree embeddings
The triggering probability of radio-loud AGN: A comparison of high and low excitation radio galaxies in hosts of different colors
Low luminosity radio-loud active galactic nuclei (AGN) are generally found in
massive red elliptical galaxies, where they are thought to be powered through
gas accretion from their surrounding hot halos in a radiatively inefficient
manner. These AGN are often referred to as "low-excitation" radio galaxies
(LERGs). When radio-loud AGN are found in galaxies with a young stellar
population and active star formation, they are usually high-power
radiatively-efficient radio AGN ("high-excitation", HERG). Using a sample of
low-redshift radio galaxies identified within the Sloan Digital Sky Survey
(SDSS), we determine the fraction of galaxies that host a radio-loud AGN,
, as a function of host galaxy stellar mass, , star formation
rate, color (defined by the 4000 \angstrom break strength), radio luminosity
and excitation state (HERG/LERG).
We find the following: 1. LERGs are predominantly found in red galaxies. 2.
The radio-loud AGN fraction of LERGs hosted by galaxies of any color follows a
power law. 3. The fraction of red galaxies
hosting a LERG decreases strongly for increasing radio luminosity. For massive
blue galaxies this is not the case. 4. The fraction of green galaxies hosting a
LERG is lower than that of either red or blue galaxies, at all radio
luminosities. 5. The radio-loud AGN fraction of HERGs hosted by galaxies of any
color follows a power law. 6. HERGs have a
strong preference to be hosted by green or blue galaxies. 7. The fraction of
galaxies hosting a HERG shows only a weak dependence on radio luminosity cut.
8. For both HERGs and LERGs, the hosting probability of blue galaxies shows a
strong dependence on star formation rate. This is not observed in galaxies of a
different color.[abridged]Comment: 7 pages, 6 figure
The SuperMACHO Microlensing Survey
We present the first results from our next-generation microlensing survey,
the SuperMACHO project. We are using the CTIO 4m Blanco telescope and the
MOSAIC imager to carry out a search for microlensing toward the Large
Magellanic Cloud (LMC). We plan to ascertain the nature of the population
responsible for the excess microlensing rate seen by the MACHO project. Our
observing strategy is optimized to measure the differential microlensing rate
across the face of the LMC. We find this derivative to be relatively
insensitive to the details of the LMC's internal structure but a strong
discriminant between Galactic halo and LMC self lensing. In December 2003 we
completed our third year of survey operations. 2003 also marked the first year
of real-time microlensing alerts and photometric and spectroscopic followup. We
have extracted several dozen microlensing candidates, and we present some
preliminary light curves and related information. Similar to the MACHO project,
we find SNe behind the LMC to be a significant contaminant - this background
has not been completely removed from our current single-color candidate sample.
Our follow-up strategy is optimized to discriminate between SNe and true
microlensing.Comment: To appear in Proceedings of IAU Symposium 225: Impact of
Gravitational Lensing on Cosmology, 6 page
Experimental transmission of Zika virus by mosquitoes from central Europe
Mosquitoes collected in Germany in 2016, including Culex pipiens pipiens biotype pipiens, Culex torrentium and Aedes albopictus, as well as Culex pipiens pipiens biotype molestus (in colony since 2011) were experimentally infected with Zika virus (ZIKV) at 18 degrees C or 27 degrees C. None of the Culex taxa showed vector competence for ZIKV. In contrast, Aedes albopictus were susceptible for ZIKV but only at 27 degrees C, with transmission rates similar to an Aedes aegypti laboratory colony tested in parallel.Peer reviewe
Multi-wavelength Characterization of Stellar Flares on Low-mass Stars using SDSS and 2MASS Time Domain Surveys
We present the first rates of flares from M dwarf stars in both red optical and near-infrared (NIR) filters. We have studied ~50,000 M dwarfs from the Sloan Digital Sky Survey (SDSS) Stripe 82 area and 1321 M dwarfs from the Two Micron All Sky Survey (2MASS) Calibration Scan Point Source Working Database that overlap SDSS imaging fields. We assign photometric spectral types from M0 to M6 using (r – i) and (i – z) colors for every star in our sample. Stripe 82 stars each have 50-100 epochs of data, while 2MASS Calibration stars have ~1900 epochs. From these data we estimate the observed rates and theoretical detection thresholds for flares in eight photometric bands as a function of spectral type. Optical flare rates are found to be in agreement with previous studies, while the frequency per hour of NIR flare detections is found to be more than two orders of magnitude lower. An excess of small-amplitude flux increases in all bands exhibits a power-law distribution, which we interpret as the result of flares below our detection thresholds. In order to investigate the recovery efficiency for flares in each filter, we extend a two-component flare model into the NIR. Quiescent M0-M6 spectral templates were used with the model to predict the photometric response of flares from u to Ks . We determine that red optical filters are sensitive to flares with u-band amplitudes 2 mag, and NIR filters to flares with Δu 4.5 mag. Our model predicts that M0 stars have the best color contrast for J-band detections, but M4-M6 stars should yield the highest rate of NIR flares with amplitudes of ΔJ ≥ 0.01 mag. Characterizing flare rates and photometric variations at longer wavelengths is important for predicting the signatures of M dwarf variability in next-generation surveys, and we discuss their impact on surveys such as the Large Synoptic Survey Telescope
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