4,101 research outputs found
Determination of demagnetizing factors using first-order reversal curves and ferromagnetic resonance
We present a method to quantitatively analyze magnetizing or demagnetizing interactions in arrayed nano-magnets by combining first-order reversal curve (FORC) and ferromagnetic resonance (FMR) measurement data. We develop a function to predict the resulting FORC distribution given: (1) a Gaussian intrinsic distribution in terms of the internal field and (2) a mean interaction field proportional to the sample\u27s magnetic moment. We then perform least-squares regression of our model on experimental FORC measurements of a nanowire array and of a thin film. Combining the obtained interaction field with an FMR fit result allows us to algebraically solve for the effective axial and transverse demagnetizing factors. Our experimental demagnetizing factors agree with expected values and provide quantitative evidence of the demagnetizing interaction between nanowires in an array
Large X-ray Flares from LMC X-4: Discovery of Milli-hertz Quasi-periodic Oscillations and QPO-modulated Pulsations
We report the discovery of milli-hertz (mHz) quasi-periodic oscillations
(QPOs) and QPO-modulated pulsations during large X-ray flares from the
high-mass X-ray binary pulsar LMC X-4 using data from the Rossi X-Ray Timing
Explorer (RXTE). The lightcurves of flares show that, in addition to ~74 mHz
coherent pulsations, there exist two more time-varying temporal structures at
frequencies of ~0.65-1.35 and ~2-20 mHz. These relatively long-term structures
appear in the power density spectra as mHz QPOs and as well-developed sidebands
around the coherent pulse frequency as well, indicating that the amplitudes of
the coherent pulsation is modulated by those of the mHz QPOs. One interesting
feature is that, while the first flare shows symmetric sidebands around the
coherent pulse frequency, the second flare shows significant excess emission in
the lower-frequency sidebands due to the ~2-20 mHz QPOs. We discuss the origin
of the QPOs using a combination of the beat-frequency model and a modified
version of the Keplerian-frequency model. According to our discussion, it seems
to be possible to attribute the origin of the ~0.65-1.35 and ~2-20 mHz QPOs to
the beating between the rotational frequency of the neutron star and the
Keplerian frequency of large accreting clumps near the corotation radius and to
the orbital motion of clumps at Keplerian radii of 2-10 times 10^9 cm,
respectively.Comment: 12 pages, including 4 figures; accepted by ApJ Letter
Long-Term X-ray Monitoring of 1E 1740.7-2942 and GRS 1758-258
We report on long-term observations of the Galactic-bulge black hole
candidates 1E 1740.7-2942 and GRS 1758-258 with the Rossi X-Ray Timing
Explorer. 1E 1740.7-2942 has been observed 77 times and GRS 1758-258 has been
observed 82 times over the past 1000 days. The flux of each object has varied
by no more than a factor of 2.5 during this period, and the indices of the
energy spectra have varied by no more than 0.4. The power spectra are similar
to other black-hole candidates: flat-topped noise, breaking to a power law.
Each object has exhibited a brightening that lasted for several months, and we
have a found a time lag between the photon power-law index and the count rate.
In both sources, the spectrum is softest during the decline from the
brightening. This behavior can be understood in the context of thin-disk and
advection-dominated accretion flows coexisting over a wide range of radii, with
the implication that both sources have low-mass companions and accrete via
Roche-lobe overflow.Comment: Accepted for publication in The Astrophysical Journa
Discovery of a Cyclotron Resonance Scattering Feature in the X-ray Spectrum of XTE J1946+274
Observations of the transient accreting pulsar XTE J1946+274 made with the
Rossi X-ray Timing Explorer during the course of the 1998 September-November
outburst, reveal a cyclotron resonance scattering feature (or "cyclotron line")
in the hard X-ray spectrum near 35 keV. We determine a centroid energy of 36.2
+0.5/-0.7 keV, which implies a magnetic field strength of 3.1(1+z)x10^12 G,
where z is the gravitational redshift of the scattering region. The optical
depth, Tau = 0.33 +0.07/-0.06, and width, sigma = 3.37 +0.92/-0.75 keV, are
typical of known cyclotron lines in other pulsars. This discovery makes XTE
J1946+274 one of thirteen pulsars with securely detected cyclotron lines
resulting in direct magnetic field measurements.Comment: Five pages including four postscript figures and two tables. Uses
emulateapj5. Published in ApJ Letters:
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2001ApJ...563L..35
Produktivität, Narbendichte und Vegetation einer Kurzrasenweide
Productivity, sward structure and floristic diversity of a continuous grazed pasture on organic permanent grassland were determined in 2015. Daily pasture growth reached
less than 50 kg dry matter per hectare in May and stayed low due to unfavorable weather conditions in 2015 (cold spring, dry and hot summer). Forage quality was quite high (> 6,5 MJ NEL/kg dm) most of the grazing season. Tiller density was moderate during summer but increased after sufficient rain in late summer. Perennial ryegrass, Kentucky bluegrass and white clover were the dominant plant species and yielded more the 85 % of dry matter
The Infrared Counterpart of the Microquasar GRS 1758-258
We present revised infrared (2.2 micron) astrometry of the field containing
the Galactic microquasar GRS 1758-258, using observations at the Keck I 10-m
telescope. We find three candidates for the microquasar within a 3 sigma error
circle, but none within 2 sigma. We show that if the 18.4 day X-ray period of
GRS 1758-258 is due to a binary orbit, then only one of the three candidates,
an early K-type giant, is large enough to power the microquasar via Roche lobe
overflow. We therefore identify this star as the infrared counterpart of GRS
1758-258, which we classify as a low mass X-ray binary. Long term infrared
monitoring of this source should provide further information about the
microquasar system, including a confirmation of the X-ray period and an
estimate of the compact object's mass.Comment: 4 pages, 2 figures; accepted for publication in ApJ Letter
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