3,096 research outputs found
A non-hyponormal operator generating Stieltjes moment sequences
A linear operator in a complex Hilbert space \hh for which the set
\dzn{S} of its -vectors is dense in \hh and is a Stieltjes moment sequence for every f \in \dzn{S}
is said to generate Stieltjes moment sequences. It is shown that there exists a
closed non-hyponormal operator which generates Stieltjes moment sequences.
What is more, \dzn{S} is a core of any power of . This is
established with the help of a weighted shift on a directed tree with one
branching vertex. The main tool in the construction comes from the theory of
indeterminate Stieltjes moment sequences. As a consequence, it is shown that
there exists a non-hyponormal composition operator in an -space (over a
-finite measure space) which is injective, paranormal and which
generates Stieltjes moment sequences. In contrast to the case of abstract
Hilbert space operators, composition operators which are formally normal and
which generate Stieltjes moment sequences are always subnormal (in fact
normal). The independence assertion of Barry Simon's theorem which
parameterizes von Neumann extensions of a closed real symmetric operator with
deficiency indices is shown to be false
Evidence for a 304-day Orbital Period for GX 1+4
In this paper we report strong evidence for a ~304-day periodicity in the
spin history of the accretion-powered pulsar GX 1+4 that is very likely to be a
signature of the orbital period of the system. Using BATSE public-domain data,
we show a highly-significant periodic modulation of the pulsar frequency from
1991 to date which is in excellent agreement with the ephemeris proposed by
Cutler, Dennis & Dolan in 1986, which were based on a few events of enhanced
spin-up that occurred during the pulsar's spin-up era in the 1970s. Our results
indicate that the orbital period of GX 1+4 is 303.8+-1.1 days, making it by far
the widest low-mass X-ray binary system known. A likely scenario for this
system is an elliptical orbit in which the neutron star decreases its spin-down
rate (or even exhibits a momentary spin-up behavior) at periastron passages due
to the higher torque exerted by the accretion disk onto the magnetosphere of
the neutron star.Comment: 5 pages, 2 figures, 1 single PS file, to appear in "Proceedings of
the 5th Compton Symposium on Gamma-Ray Astrophysics", AI
Optical Monitoring of Quasars: I. Variability
We present an analysis of quasar variability from data collected during a
photometric monitoring of 50 objects carried out at CNPq/Laboratorio Nacional
de Astrofisica, Brazil, between March 1993 and July 1996. A distinctive feature
of this survey is its photometric accuracy, ~ 0.02 V mag, achieved through
differential photometry with CCD detectors, what allows the detection of faint
levels of variability. We find that the relative variability, delta = sigma /
L, observed in the V band is anti-correlated with both luminosity and redshift,
although we have no means of discovering the dominant relation, given the
strong coupling between luminosity and redshift for the objects in our
sample.We introduce a model for the dependence of quasar variability on
frequency that is consistent with multi-wavelength observations of the nuclear
variability of the Seyfert galaxy NGC 4151. We show that correcting the
observed variability for this effect slightly increases the significance of the
trends of variability with luminosity and redshift. Assuming that variability
depends only on the luminosity, we show that the corrected variability is
anti-correlated with luminosity and is in good agreement with predictions of a
simple Poissonian model. The energy derived for the hypothetical pulses, ~
10^50 erg, agrees well with those obtained in other studies. We also find that
the radio-loud objects in our sample tend to be more variable than the
radio-quiet ones, for all luminosities and redshifts.Comment: 17 pages, 12 figures, accepted for publication in MNRAS (uses MNRAS
style
Assessment of institutional barriers to the use of natural gas fuel in automotive vehicle fleets
Institutional barriers to the use of natural gas as a fuel for motor vehicle fleets were identified. Recommendations for barrier removal were developed. Eight types of institutional barriers were assessed: (1) lack of a national standard for the safe design and certification of natural gas vehicles and refueling stations; (2) excessively conservative or misapplied state and local regulations, including bridge and tunnel restrictions, restrictions on types of vehicles that may be fueled by natural gas, zoning regulations that prohibit operation of refueling stations, parking restrictions, application of LPG standards to LNG vehicles, and unintentionally unsafe vehicle or refueling station requirements; (3) need for clarification of EPA's tampering enforcement policy; (4) the U.S. hydrocarbon standard; (5) uncertainty concerning state utility commission jurisdiction; (6) sale for resale prohibitions imposed by natural gas utility companies or state utility commissions; (7) uncertainty of the effects of conversions to natural gas on vehicle manufactures warranties; and (8) need for a natural gas to gasoline equivalent units conversion factor for use in calculation of state road use taxes
The Effect of N-nitrosodimethylamine (NDMA) on Bax and Mcl-1 Expression in Human Neutrophils
In the present study we examined a role of pro-apoptotic Bax and anti-apoptotic Mcl-1 proteins, participating in the regulation of intrinsic apoptosis pathway in human neutrophils (PMNs) exposed to N-nitrosodimethylamine (NDMA), the environmental xenobiotic. For the purpose comparison, the same studies were conducted in autologous peripheral blood mononuclear cells (PBMCs). The production of cytochrome c by PMNs was also determined. A deficit of anti-apoptotic Mcl-1 and overexpression of the pro-apoptotic protein Bax suggest that the apoptosis process in human neutrophils exposed to NDMA is dependent on changes in the expression of these proteins. PMNs were more sensitive to NDMA than PBMCs
Correlated radial velocity and X-ray variations in HD 154791/4U 1700+24
We present evidence for approximately 400-d variations in the radial velocity
of HD 154791 (V934 Her), the suggested optical counterpart of 4U 1700+24. The
variations are correlated with the previously reported approximately 400 d
variations in the X-ray flux of 4U 1700+24, which supports the association of
these two objects, as well as the identification of this system as the second
known X-ray binary in which a neutron star accretes from the wind of a red
giant. The HD 154791 radial velocity variations can be fit with an eccentric
orbit with period 404 +/- 3 d, amplitude K=0.75 +/- 0.12 km/s and eccentricity
e=0.26 +/- 0.15. There are also indications of variations on longer time scales
>~ 2000 d. We have re-examined all available ASM data following an unusually
large X-ray outburst in 1997-98, and confirm that the 1-d averaged 2-10 keV
X-ray flux from 4U 1700+24 is modulated with a period of 400 +/- 20 d. The mean
profile of the persistent X-ray variations was approximately sinusoidal, with
an amplitude of 0.108 +/- 0.012 ASM count/s (corresponding to 31% rms). The
epoch of X-ray maximum was approximately 40 d after the time of periastron
according to the eccentric orbital fit. If the 400 d oscillations from HD
154791/4U 1700+24 are due to orbital motion, then the system parameters are
probably close to those of the only other neutron-star symbiotic-like binary,
GX 1+4. We discuss the similarities and differences between these two systems.Comment: 6 pages, 2 figures; accepted by Ap
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