1,877 research outputs found
Cosmic rays studied with a hybrid high school detector array
The LORUN/NAHSA system is a pathfinder for hybrid cosmic ray research
combined with education and outreach in the field of astro-particle physics.
Particle detectors and radio antennae were mainly setup by students and placed
on public buildings. After fully digital data acquisition, coincidence
detections were selected. Three candidate events confirmed a working prototype,
which can be multiplied to extend further particle detector arrays on high
schools.Comment: 10 pages, 6 figures. Nigl, A., Timmermans, C., Schellart, P.,
Kuijpers, J., Falcke, H., Horneffer, A., de Vos, C. M., Koopman, Y., Pepping,
H. J., Schoonderbeek, G., Cosmic rays studied with a hybrid high school
detector array, Europhysics News (EPN), Vol. 38, No. 5, accepted on
22/08/200
Thoracoscopic sympathicotomy in children for the treatment of palmar and axillary primary focal hyperhidrosis:Caution advocated
EUVE J0425.6-5714: A Newly Discovered AM Herculis Star
We detected a new AM Her star serendipitously in a 25 day observation with
the EUVE satellite. A coherent period of 85.82 min is present in the EUVE Deep
Survey imager light curve of this source. A spectroscopic identification is
made with a 19th magnitude blue star that has H and He emission lines, and
broad cyclotron humps typical of a magnetic cataclysmic variable. A lower limit
to the polar magnetic field of 46 MG is estimated from the spacing of the
cyclotron harmonics. EUVE J0425.6-5714 is also detected in archival ROSAT HRI
observations spanning two months, and its stable and highly structured light
curve permits us to fit a coherent ephemeris linking the ROSAT and EUVE data
over a 1.3 yr gap. The derived period is 85.82107 +/- 0.00020 min, and the
ephemeris should be accurate to 0.1 cycles until the year 2005. A narrow but
partial X-ray eclipse suggests that this object belongs to the group of Am Her
stars whose viewing geometry is such that the accretion stream periodically
occults the soft X-ray emitting accretion spot on the surface of the white
dwarf. A non-detection of hard X-rays from ASCA observations that are
contemporaneous with the ROSAT HRI shows that the soft X-rays must dominate by
at least an order of magnitude, which is consistent with a known trend among AM
Her stars with large magnetic field. This object should not be confused with
the Seyfert galaxy 1H 0419-577 (= LB 1727), another X-ray/EUV source which lies
only 4' away, and was the principal target of these monitoring observations.Comment: 13 pages, 5 figures, to appear in PASP, Dec. 1998 issu
A high-order Nystrom discretization scheme for boundary integral equations defined on rotationally symmetric surfaces
A scheme for rapidly and accurately computing solutions to boundary integral
equations (BIEs) on rotationally symmetric surfaces in R^3 is presented. The
scheme uses the Fourier transform to reduce the original BIE defined on a
surface to a sequence of BIEs defined on a generating curve for the surface. It
can handle loads that are not necessarily rotationally symmetric. Nystrom
discretization is used to discretize the BIEs on the generating curve. The
quadrature is a high-order Gaussian rule that is modified near the diagonal to
retain high-order accuracy for singular kernels. The reduction in
dimensionality, along with the use of high-order accurate quadratures, leads to
small linear systems that can be inverted directly via, e.g., Gaussian
elimination. This makes the scheme particularly fast in environments involving
multiple right hand sides. It is demonstrated that for BIEs associated with the
Laplace and Helmholtz equations, the kernel in the reduced equations can be
evaluated very rapidly by exploiting recursion relations for Legendre
functions. Numerical examples illustrate the performance of the scheme; in
particular, it is demonstrated that for a BIE associated with Laplace's
equation on a surface discretized using 320,800 points, the set-up phase of the
algorithm takes 1 minute on a standard laptop, and then solves can be executed
in 0.5 seconds.Comment: arXiv admin note: substantial text overlap with
arXiv:1012.56301002.200
VLBI observations of jupiter with the initial test station of LOFAR and the nancay decametric array
AIMS: To demonstrate and test the capability of the next generation of
low-frequency radio telescopes to perform high resolution observations across
intra-continental baselines. Jupiter's strong burst emission is used to perform
broadband full signal cross-correlations on time intervals of up to hundreds of
milliseconds. METHODS: Broadband VLBI observations at about 20 MHz on a
baseline of ~50000 wavelengths were performed to achieve arcsecond angular
resolution. LOFAR's Initial Test Station (LOFAR/ITS, The Netherlands) and the
Nancay Decametric Array (NDA, France) digitize the measured electric field with
12 bit and 14 bit in a 40 MHz baseband. The fine structure in Jupiter's signal
was used for data synchronization prior to correlation on the time-series data.
RESULTS: Strong emission from Jupiter was detected during snapshots of a few
seconds and detailed features down to microsecond time-scales were identified
in dynamic spectra. Correlations of Jupiter's burst emission returned strong
fringes on 1 ms time-scales over channels as narrow as a hundred kilohertz
bandwidth. CONCLUSIONS: Long baseline interferometry is confirmed at low
frequencies, in spite of phase shifts introduced by variations in ionospheric
propagation characteristics. Phase coherence was preserved over tens to
hundreds of milliseconds with a baseline of ~700 km. No significant variation
with time was found in the correlations and an estimate for the fringe
visibility of 1, suggested that the source was not resolved. The upper limit on
the source region size of Jupiter Io-B S-bursts corresponds to an angular
resolution of ~3 arcsec. Adding remote stations to the LOFAR network at
baselines up to thousand kilometers will provide 10 times higher resolution
down to an arcsecond.Comment: 6 pages, 4 figures. Nigl, A., Zarka, P., Kuijpers, J., Falcke, H.,
Baehren, L., VLBI observations of Jupiter with the Initial Test Station of
LOFAR and the Nancay Decametric Array, A&A, 471, 1099-1104, accepted on
31/05/200
On the Search for Coherent Radiation from Radio Pulsars
We have examined data from pulsars B0950+08 and B0329+54 for evidence of
temporally coherent radiation using the modified coherence function (MCF)
technique of Jenet et al. (2001). We consider the influence of both
instrumental bandpass and interstellar propagation effects. Even after removal
of the effects due to the instrumental bandpass, we detect a signature in the
MCF of our PSR B0329+54 data which is consistent with the definition of a
coherent signal. However, we model the effects due to interstellar
scintillation for this pulsar and show that it reproduces the observed
signature. In particular, the temporal coherence time is close to the
reciprocal of the decorrelation bandwidth due to diffractive scintillation.
Furthermore, comparison of the coherence times of three pulsars reported by
Jenet et al. (2001) with their expected diffractive decorrelation bandwidths
suggests that the detection of coherence in these pulsars is also likely a
result of interstellar scintillation, and is not intrinsic to the pulsars.Comment: 8 pages, 8 figures. Accepted for publication in Astronomy &
Astrophysics (A&A
Riboswitch RSthiT as a molecular tool in Lactococcus lactis
Previous RNA sequencing has allowed the identification of 129 long 5′ untranslated regions (UTRs) in the Lactococcus lactis MG1363 transcriptome. These sequences potentially harbor cis-acting riboswitches. One of the identified extended 5′ UTRs is a putative thiamine pyrophosphate (TPP) riboswitch. It is located immediately upstream of the thiamine transporter gene thiT (llmg_0334). To confirm this assumption, the 5′-UTR sequence was placed upstream of the gene encoding the superfolder green fluorescent protein (sfGFP), sfgfp, allowing the examination of the expression of sfGFP in the presence or absence of thiamine in the medium. The results show that this sequence indeed represents a thiamine-responsive TPP riboswitch. This RNA-based genetic control device was used to successfully restore the mutant phenotype of an L. lactis strain lacking the major autolysin gene, acmA. The L. lactis thiT TPP riboswitch (RS(thiT)) is a useful molecular genetic tool enabling the gradual downregulation of the expression of genes under its control by adjusting the thiamine concentration. IMPORTANCE The capacity of microbes with biotechnological importance to adapt to and survive under quickly changing industrial conditions depends on their ability to adequately control gene expression. Riboswitches are important RNA-based elements involved in rapid and precise gene regulation. Here, we present the identification of a natural thiamine-responsive riboswitch of Lactococcus lactis, a bacterium used worldwide in the production of dairy products. We used it to restore a genetic defect in an L. lactis mutant and show that it is a valuable addition to the ever-expanding L. lactis genetic toolbox
ORFEUS II Far-UV Spectroscopy of AM Herculis
Six high-resolution (\lambda/\Delta\lambda ~ 3000) far-UV (\lambda\lambda =
910-1210 \AA) spectra of the magnetic cataclysmic variable AM Herculis were
acquired in 1996 November during the flight of the ORFEUS-SPAS II mission. AM
Her was in a high optical state at the time of the observations, and the
spectra reveal emission lines of O VI \lambda\lambda 1032, 1038, C III \lambda
977, \lambda 1176, and He II \lambda 1085 superposed on a nearly flat
continuum. Continuum flux variations can be described as per Gansicke et al. by
a ~ 20 kK white dwarf with a ~ 37 kK hot spot covering a fraction f~0.15 of the
surface of the white dwarf, but we caution that the expected Lyman absorption
lines are not detected. The O VI emission lines have narrow and broad component
structure similar to that of the optical emission lines, with radial velocities
consistent with an origin in the irradiated face of the secondary and the
accretion funnel, respectively. The density of the narrow- and broad-line
regions is n_{nlr} ~ 3\times 10^{10} cm^{-3} and n_{blr} ~ 1\times 10^{12}
cm^{-3}, respectively, yet the narrow-line region is optically thick in the O
VI line and the broad-line region is optically thin; apparently, the velocity
shear in the broad-line region allows the O VI photons to escape, rendering the
gas effectively optically thin. Unexplained are the orbital phase variations of
the emission-line fluxes.Comment: 15 pages, 6 Postscript figures; LaTeX format, uses aaspp4.sty;
table2.tex included separately because it must be printed sideways - see
instructions in the file; accepted on April 17, 1998 for publication in The
Astrophysical Journa
The Far-Ultraviolet Spectrum and Short Timescale Variability of AM Herculis from Observations with the Hopkins Ultraviolet Telescope
Using the Hopkins Ultraviolet Telescope (HUT), we have obtained 850-1850
angstrom spectra of the magnetic cataclysmic variable star AM Her in the high
state. These observations provide high time resolution spectra of AM Her in the
FUV and sample much of the orbital period of the system. The spectra are not
well-modelled in terms of simple white dwarf (WD) atmospheres, especially at
wavelengths shortward of Lyman alpha. The continuum flux changes by a factor of
2 near the Lyman limit as a function of orbital phase; the peak fluxes are
observed near magnetic phase 0.6 when the accreting pole of the WD is most
clearly visible. The spectrum of the hotspot can be modelled in terms of a 100
000 K WD atmosphere covering 2% of the WD surface. The high time resolution of
the HUT data allows an analysis of the short term variability and shows the UV
luminosity to change by as much as 50% on timescales as short as 10 s. This
rapid variability is shown to be inconsistent with the clumpy accretion model
proposed to account for the soft X-ray excess in polars. We see an increase in
narrow line emission during these flares when the heated face of the secondary
is in view. The He II narrow line flux is partially eclipsed at secondary
conjunction, implying that the inclination of the system is greater than 45
degrees. We also present results from models of the heated face of the
secondary. These models show that reprocessing on the face of the secondary
star of X-ray/EUV emission from the accretion region near the WD can account
for the intensities and kinematics of most of the narrow line components
observed.Comment: 19 pp., 12 fig., 3 tbl. To appear in The Astrophysical Journal. Also
available at http://greeley.pha.jhu.edu/papers/amherpp.ps.g
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