108 research outputs found
X-ray Timing Observations of PSR J1930+1852 in the Crab-like SNR G54.1+0.3
We present new X-ray timing and spectral observations of PSR J1930+1852, the
young energetic pulsar at the center of the non-thermal supernova remnant
G54.1+0.3. Using data obtained with the Rossi X-ray Timing Explorer and Chandra
X-ray observatories we have derived an updated timing ephemeris of the 136 ms
pulsar spanning 6 years. During this interval, however, the period evolution
shows significant variability from the best fit constant spin-down rate of
s s, suggesting strong timing noise
and/or glitch activity. The X-ray emission is highly pulsed (
modulation) and is characterized by an asymmetric, broad profile (
duty cycle) which is nearly twice the radio width. The spectrum of the pulsed
emission is well fitted with an absorbed power law of photon index ; this is marginally harder than that of the unpulsed component. The
total 2-10 keV flux of the pulsar is erg cm
s. These results confirm PSR J1930+1852 as a typical Crab-like pulsar.Comment: 14 pages with 7 figures included, accepted to Ap
The observation of quantum fluctuations in a kagome Heisenberg antiferromagnet
The search for the experimental evidence of quantum spin liquid (QSL) states
is critical but extremely challenging, as the quenched interaction randomness
introduced by structural imperfection is usually inevitable in real materials.
YCu(OH)Br (YCOB) is a spin-1/2 kagome Heisenberg
antiferromagnet (KHA) with strong coupling of 51 K but
without conventional magnetic freezing down to 50 mK 0.001. Here, we report a Br nuclear magnetic resonance (NMR) study of the
local spin susceptibility and dynamics on the single crystal of YCOB. The
temperature dependence of NMR main-line shifts and broadening can be well
understood within the frame of the KHA model with randomly distributed hexagons
of alternate exchanges, compatible with the formation of a randomness-induced
QSL state at low temperatures. The in-plane spin fluctuations as measured by
the spin-lattice relaxation rates () exhibit a weak temperature
dependence down to 0.03. Our results demonstrate
that the majority of spins remain highly fluctuating at low temperatures
despite the quenched disorder in YCOB.Comment: NMR work on YCu3(OH)6.5Br2.5, accepted in Communications Physic
The Radio Properties and Magnetic Field Configuration in the Crab-like Pulsar Wind Nebula G54.1+0.3
We present a multifrequency radio investigation of the Crab-like pulsar wind
nebula (PWN) G54.1+0.3 using the Very Large Array. The high resolution of the
observations reveals that G54.1+0.3 has a complex radio structure which
includes filamentary and loop-like structures that are magnetized, a diffuse
extent similar to the associated diffuse X-ray emission. But the radio and
X-ray structures in the central region differ strikingly, indicating that they
trace very different forms of particle injection from the pulsar and/or
particle acceleration in the nebula. No spectral index gradient is detected in
the radio emission across the PWN, whereas the X-ray emission softens outward
in the nebula. The extensive radio polarization allows us to image in detail
the intrinsic magnetic field, which is well-ordered and reveals that a number
of loop-like filaments are strongly magnetized. In addition, we determine that
there are both radial and toroidal components to the magnetic field structure
of the pulsar wind nebula. Strong mid-IR emission detected in Spitzer Space
Telescope data is closely correlated with the radio emission arising from the
southern edge of G54.1+0.3. In particular, the distributions of radio and X-ray
emission compared with the mid-IR emission suggest that the PWN may be
interacting with this interstellar cloud. This may be the first PWN where we
are directly detecting its interplay with an interstellar cloud that has
survived the impact of the supernova explosion associated with the pulsar's
progenitor.Comment: 35 pages, 19 figures, accepted for publication in the Astrophysical
Journa
A Transmissive X-ray Polarimeter Design For Hard X-ray Focusing Telescopes
The X-ray Timing and Polarization (XTP) is a mission concept for a future
space borne X-ray observatory and is currently selected for early phase study.
We present a new design of X-ray polarimeter based on the time projection gas
chamber. The polarimeter, placed above the focal plane, has an additional rear
window that allows hard X-rays to penetrate (a transmission of nearly 80% at 6
keV) through it and reach the detector on the focal plane. Such a design is to
compensate the low detection efficiency of gas detectors, at a low cost of
sensitivity, and can maximize the science return of multilayer hard X-ray
telescopes without the risk of moving focal plane instruments. The sensitivity
in terms of minimum detectable polarization, based on current instrument
configuration, is expected to be 3% for a 1mCrab source given an observing time
of 10^5 s. We present preliminary test results, including photoelectron tracks
and modulation curves, using a test chamber and polarized X-ray sources in the
lab
Review of X-ray pulsar spacecraft autonomous navigation
This article provides a review on X-ray pulsar-based navigation (XNAV). The
review starts with the basic concept of XNAV, and briefly introduces the past,
present and future projects concerning XNAV. This paper focuses on the advances
of the key techniques supporting XNAV, including the navigation pulsar
database, the X-ray detection system, and the pulse time of arrival estimation.
Moreover, the methods to improve the estimation performance of XNAV are
reviewed. Finally, some remarks on the future development of XNAV are provided.Comment: has been accepted by Chinese Journal of Aeronautic
X-ray Thread G0.13-0.11: A Pulsar Wind Nebula?
We have examined Chandra observations of the recently discovered X-ray thread
G0.13-0.11 in the Galactic center Radio Arc region. Part of the {\sl Chandra}
data was studied by Yusef-Zadeh, Law, & Wardle (2002), who reported the
detection of 6.4-keV line emission in this region. We find, however, that this
line emission is {\sl not} associated with G0.13-0.11. The X-ray spectra of
both G0.13-0.11 and the point-like source embedded are well characterized by a
power law. The 2 -- 10 keV band luminosities of these two components are (G0.13-0.11) and (point source) at the Galactic center distance of 8 kpc. The
morphological, spectral, and luminosity properties strongly indicate that
G0.13-0.11 represents the leading-edge of a pulsar wind nebula, produced by a
pulsar (point source) moving in a strong magnetic field environment. The main
body of this pulsar wind nebula is likely traced by a bow-shaped radio feature,
which is apparently bordered by G0.13-0.11 and is possibly associated with the
prominent nonthermal radio filaments of the Radio Arc. We speculate that young
pulsars may be responsible for various unique nonthermal filamentary radio and
X-ray features observed in the Galactic center region.Comment: 15 pages, accepted for publication in the Astrophysical Journa
Choosing legumes and perennial grasses
Many letters are received annually by the Iowa Agricultural Experiment Station requesting information on the different legumes and grasses. This publication has been prepared to answer the more important questions pertaining to the choice of legumes and grasses for different uses and conditions.2
Legumes of greatest value for different uses or soil conditions in Iowa are (1) alfalfa, (2) medium red clover, (3) mammoth red clover, (4) alsike clover, (5) white clover, (6) the biennial white and yellow sweet clovers, (7) hubam clover, (the annual white sweet clover) (8) Korean lespedeza, (9) dalea and (10) soybeans. A discussion of soybeans is largely omitted in this publication since the growing of this crop is entirely different from that of the others.
Comparison between a New Optical Biometry Device and an Anterior Segment Optical Coherence Tomographer for Measuring Central Corneal Thickness and Anterior Chamber Depth
Purpose. To compare between a new optical biometer (AL-Scan, Nidek Co., Aichi, Japan) and an anterior segment optical coherence tomographer (Visante AS-OCT, Carl Zeiss Meditec, Dublin, USA) for measuring central corneal thickness (CCT), anterior chamber depth (ACD), and aqueous depth (AD). Methods. Sixty-three eyes of 63 normal subjects were examined with AL-Scan and Visante AS-OCT in this prospective study. One eye per subject was measured three times with both devices to record their CCT, ACD, and AD. All procedures were performed by the same operator. Agreement between the two devices was assessed using paired t-tests, Bland-Altman plots, and 95% limits of agreement (LoA). Results. The mean CCT, ACD, and AD measured by AL-Scan were 538.59±27.37 μm, 3.70±0.30 mm, and 3.16±0.30 mm, respectively. The mean values obtained by the Visante OCT were 536.14±26.61 μm for CCT, 3.71±0.29 mm for ACD, and 3.17±0.29 mm for AD. The mean CCT by the AL-Scan was higher than that obtained by the Visante AS-OCT (difference = 2.45±6.07 μm, P<0.05). The differences in ACD and AD measurements were not statistically significant. The 95% LoA of CCT, ACD, and AD were between −9.44 and 14.35 μm, −0.15 and 0.12 mm, and −0.15 and 0.12 mm, respectively. Conclusions. Since these two devices were comparable for measuring CCT, ACD, and AD, their results can be interchangeably used in the clinic
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