4,022 research outputs found
Broken Lefschetz fibrations and smooth structures on 4–manifolds
The broken genera are orientation preserving diffeomorphism invariants of
closed oriented 4-manifolds, defined via broken Lefschetz fibrations. We study
the properties of the broken genera invariants, and calculate them for various
4-manifolds, while showing that the invariants are sensitive to exotic smooth
structures.Comment: 21 pages, 5 figure
Production of Millisecond Dips in Sco X-1 Count Rates by Dead Time Effects
Chang et al. (2006) reported millisecond duration dips in the X-ray intensity
of Sco X-1 and attributed them to occultations of the source by small
trans-Neptunian objects (TNOs). We have found multiple lines of evidence that
these dips are not astronomical in origin, but rather the result of high-energy
charged particle events in the RXTE PCA detectors. Our analysis of the RXTE
data indicates that at most 10% of the observed dips in Sco X-1 could be due to
occultations by TNOs, and, furthermore, we find no positive or supporting
evidence for any of them being due to TNOs. We therefore believe that it is a
mistake to conclude that any TNOs have been detected via occultation of Sco
X-1.Comment: Submitted to ApJ; uses emulateapj.cls, 8 pages with 8 figure
Detection of bacterial spores with lanthanide-macrocycle binary complexes
The detection of bacterial spores via dipicolinate-triggered lanthanide luminescence has been improved in terms of detection limit, stability, and susceptibility to interferents by use of lanthanide−macrocycle binary complexes. Specifically, we compared the effectiveness of Sm, Eu, Tb, and Dy complexes with the macrocycle 1,4,7,10-tetraazacyclododecane-1,7-diacetate (DO2A) to the corresponding lanthanide aquo ions. The Ln(DO2A)^+ binary complexes bind dipicolinic acid (DPA), a major constituent of bacterial spores, with greater affinity and demonstrate significant improvement in bacterial spore detection. Of the four luminescent lanthanides studied, the terbium complex exhibits the greatest dipicolinate binding affinity (100-fold greater than Tb^(3+) alone, and 10-fold greater than other Ln(DO2A)^+ complexes) and highest quantum yield. Moreover, the inclusion of DO2A extends the pH range over which Tb−DPA coordination is stable, reduces the interference of calcium ions nearly 5-fold, and mitigates phosphate interference 1000-fold compared to free terbium alone. In addition, detection of Bacillus atrophaeus bacterial spores was improved by the use of Tb(DO2A)^+, yielding a 3-fold increase in the signal-to-noise ratio over Tb^(3+). Out of the eight cases investigated, the Tb(DO2A)^+ binary complex is best for the detection of bacterial spores
Use of Gas Electron Multiplier (GEM) Detectors for an Advanced X-ray Monitor
We describe a concept for a NASA SMEX Mission in which Gas Electron
Multiplier (GEM) detectors, developed at CERN, are adapted for use in X-ray
astronomy. These detectors can be used to obtain moderately large detector area
and two-dimensional photon positions with sub mm accuracy in the range of 1.5
to 15 keV. We describe an application of GEMs with xenon gas, coded mask
cameras, and simple circuits for measuring event positions and for
anticoincidence rejection of particle events. The cameras are arranged to cover
most of the celestial sphere, providing high sensitivity and throughput for a
wide variety of cosmic explosions. At longer timescales, persistent X-ray
sources would be monitored with unprecedented levels of coverage. The
sensitivity to faint X-ray sources on a one-day timescale would be improved by
a factor of 6 over the capability of the RXTE All Sky Monitor.Comment: 10 pages, 5 figs., in X-Ray and Gamma Ray Instrumentation for
Astronomy XI, SPIE conference, San Diego, Aug. 200
X-ray Nova XTE J1550-564: RXTE Spectral Observations
Excellent coverage of the 1998 outburst of the X-ray Nova XTE J1550-564 was
provided by the Rossi X-ray Timing Explorer. XTE J1550-564 exhibited an intense
(6.8 Crab) flare on 1998 September 19 (UT), making it the brightest new X-ray
source observed with RXTE. We present a spectral analysis utilizing 60
Proportional Counter Array spectra from 2.5-20 keV spanning 71 days, and a
nearly continuous All Sky Monitor light curve. The spectra were fit to a model
including multicolor blackbody disk and power-law components. XTE J1550-564 is
observed in the very high, high/soft, and intermediate canonical outburst
states of Black Hole X-ray Novae.Comment: 14 pages including 1 table and 4 figures, Accepted to ApJ Letter
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