5,761 research outputs found
Algorithm based comparison between the integral method and harmonic analysis of the timing jitter of diode-based and solid-state pulsed laser sources
AbstractA comparison between two methods of timing jitter calculation is presented. The integral method utilizes spectral area of the single side-band (SSB) phase noise spectrum to calculate root mean square (rms) timing jitter. In contrast the harmonic analysis exploits the uppermost noise power in high harmonics to retrieve timing fluctuation. The results obtained show that a consistent timing jitter of 1.2ps is found by the integral method and harmonic analysis in gain-switched laser diodes with an external cavity scheme. A comparison of the two approaches in noise measurement of a diode-pumped Yb:KY(WO4)2 passively mode-locked laser is also shown in which both techniques give 2ps rms timing jitter
An ultraviolet excess in the superluminous supernova Gaia16apd reveals a powerful central engine
Since the discovery of superluminous supernovae (SLSNe) in the last decade,
it has been known that these events exhibit bluer spectral energy distributions
than other supernova subtypes, with significant output in the ultraviolet.
However, the event Gaia16apd seems to outshine even the other SLSNe at
rest-frame wavelengths below \AA. Yan et al (2016) have recently
presented HST UV spectra and attributed the UV flux to low metallicity and
hence reduced line blanketing. Here we present UV and optical light curves over
a longer baseline in time, revealing a rapid decline at UV wavelengths despite
a typical optical evolution. Combining the published UV spectra with our own
optical data, we demonstrate that Gaia16apd has a much hotter continuum than
virtually any SLSN at maximum light, but it cools rapidly thereafter and is
indistinguishable from the others by -15 days after peak. Comparing
the equivalent widths of UV absorption lines with those of other events, we
show that the excess UV continuum is a result of a more powerful central power
source, rather than a lack of UV absorption relative to other SLSNe or an
additional component from interaction with the surrounding medium. These
findings strongly support the central-engine hypothesis for hydrogen-poor
SLSNe. An explosion ejecting M, where
is the opacity in cmg, and forming a magnetar with spin
period ms, and G (lower than other SLSNe with
comparable rise-times) can consistently explain the light curve evolution and
high temperature at peak. The host metallicity, Z, is
comparable to other SLSNe.Comment: Updated to match accepted version (ApJL
The Production of Epistemic Culture and Agency during a First-Grade Engineering Design Unit in an Urban Emergent School
Primary school practices are often bound by traditions that perpetuate compliance and skills-based, decontextualized, rote memorization activities. These histories of practice, prevalent in schools serving mostly Black and Brown children, make it inordinately difficult for students to author themselves as knowledge builders (i.e., with epistemic agency), which is a form of injustice. Engineering is a potentially fertile context to support the creation of epistemic culture, whereby young studentsâ assets are recognized, named, and leveraged as they create and shape the groupâs disciplinary knowledge. The authors investigated this potential. The primary research question was: How do first-grade students in an urban emergent school author themselves as epistemic agents during an engineering design unit? Using a social practice theory lens and ethnographic methods, the authors studied 29 days of a materials engineering unit focusing on the teacherâs epistemic commitments, implicit meanings of knowledge in classroom discourse, and practices that opened space for studentsâ epistemic agency. Data collection included fieldnotes and video of class activities and teacher and student interviews. Class discussions about the properties and uses of materials yielded a rich context for studying epistemic culture. The teacherâs epistemic commitments included an eschewing of disciplinary silos, recognizing the nonlinear nature of knowledge-building about engineering, and acknowledging childrenâs thinking as an asset for engineering knowledge production. Examples of studentsâ discursive moves demonstrating epistemic agency included: reminding others about the relevance of previous lessons to the current topic, mirroring the teacherâs instructional moves, claiming voice, space, time, and material resources for knowledge-building, translating one anotherâs ideas, and making unsolicited connections to their lives. Young childrenâs intellectual assets can too easily be overlooked in traditional learning contexts. This study demonstrates the affordances of responsive engineering instruction in recognizing and building on youthsâ intellectual curiosity and enthusiasm for substantively contributing to the classroomâs knowledge-generating practices
Leonardo's rule, self-similarity and wind-induced stresses in trees
Examining botanical trees, Leonardo da Vinci noted that the total
cross-section of branches is conserved across branching nodes. In this Letter,
it is proposed that this rule is a consequence of the tree skeleton having a
self-similar structure and the branch diameters being adjusted to resist
wind-induced loads
Spitzer Space Telescope Infrared Observations of the Binary Neutron Star Merger GW170817
We present Spitzer Space Telescope 3.6 and 4.5 micron observations of the
binary neutron star merger GW170817 at 43, 74, and 264 days post-merger. Using
the final observation as a template, we uncover a source at the position of
GW170817 at 4.5 micron with a brightness of 22.9+/-0.3 AB mag at 43 days and
23.8+/-0.3 AB mag at 74 days (the uncertainty is dominated by systematics from
the image subtraction); no obvious source is detected at 3.6 micron to a
3-sigma limit of >23.3 AB mag in both epochs. The measured brightness is dimmer
by a factor of about 2-3 times compared to our previously published kilonova
model, which is based on UV, optical, and near-IR data at <30 days. However,
the observed fading rate and color (m_{3.6}-m_{4.5}> 0 AB mag) are consistent
with our model. We suggest that the discrepancy is likely due to a transition
to the nebular phase, or a reduced thermalization efficiency at such late time.
Using the Spitzer data as a guide, we briefly discuss the prospects of
observing future binary neutron star mergers with Spitzer (in LIGO/Virgo
Observing Run 3) and the James Webb Space Telescope (in LIGO/Virgo Observing
Run 4 and beyond).Comment: 6 pages, 2 figures, submitted to ApJ
How branching can change the conductance of ballistic semiconductor devices
We demonstrate that branching of the electron flow in semiconductor
nanostructures can strongly affect macroscopic transport quantities and can
significantly change their dependence on external parameters compared to the
ideal ballistic case even when the system size is much smaller than the mean
free path. In a corner-shaped ballistic device based on a GaAs/AlGaAs
two-dimensional electron gas we observe a splitting of the commensurability
peaks in the magnetoresistance curve. We show that a model which includes a
random disorder potential of the two-dimensional electron gas can account for
the random splitting of the peaks that result from the collimation of the
electron beam. The shape of the splitting depends on the particular realization
of the disorder potential. At the same time magnetic focusing peaks are largely
unaffected by the disorder potential.Comment: accepted for publication in Phys. Rev.
On the Conditions for Neutron-Rich Gamma-Ray Burst Outflows
We calculate the structure and neutron content of neutrino-heated MHD winds
driven from the surface of newly-formed magnetars (``proto-magnetars'') and
from the midplane of hyper-accreting disks, two of the possible central engines
for gamma-ray bursts (GRBs) and hyper-energetic supernovae (SNe). Both the
surface of proto-magnetars and the midplane of neutrino-cooled accretion flows
(NDAFs) are electron degenerate and neutron-rich (neutron-to-proton ratio n/p
>> 1). If this substantial free neutron excess is preserved to large radii in
ultra-relativistic outflows, several important observational consequences may
result. Weak interaction processes, however, can drive n/p to ~1 in the
nondegenerate regions that obtain just above the surfaces of NDAFs and
proto-magnetars. Our calculations show that mildly relativistic neutron-rich
outflows from NDAFs are possible in the presence of a strong poloidal magnetic
field. However, we find that neutron-rich winds possess a minimum mass-loss
rate that likely precludes simultaneously neutron-rich and ultra-relativistic
(Lorentz factor > 100) NDAF winds accompanying a substantial accretion power.
In contrast, proto-magnetars are capable of producing neutron-rich
long-duration GRB outflows ~10-30 seconds following core bounce for
sub-millisecond rotation periods; such outflows would, however, accompany only
extremely energetic events, in which the GRB + SN energy budget exceeds ~ 4e52
ergs. Neutron-rich highly relativistic outflows may also be produced during
some short-duration GRBs by geometrically thick accretion disks formed from
compact object mergers. The implications for r-process nucleosynthesis, optical
transients due to non-relativistic neutron-rich winds, and Nickel production in
proto-magnetar and NDAF winds are also briefly discussed.Comment: 24 pages, 7 figures, submitted to Ap
A Decline in the X-ray through Radio Emission from GW170817 Continues to Support an Off-Axis Structured Jet
We present new observations of the binary neutron star merger GW170817 at
days post-merger, at radio (Karl G. Jansky Very Large
Array; VLA), X-ray (Chandra X-ray Observatory) and optical (Hubble Space
Telescope; HST) wavelengths. These observations provide the first evidence for
a turnover in the X-ray light curve, mirroring a decline in the radio emission
at significance. The radio-to-X-ray spectral energy
distribution exhibits no evolution into the declining phase. Our full
multi-wavelength dataset is consistent with the predicted behavior of our
previously published models of a successful structured jet expanding into a
low-density circumbinary medium, but pure cocoon models with a choked jet
cannot be ruled out. If future observations continue to track our predictions,
we expect that the radio and X-ray emission will remain detectable until days post-merger.Comment: Accepted to ApJL. Updated version includes new VLA observations
extending through 2018 June
Bkm sequences from the human X chromosome contain large clusters of GATA/GACA repeats
In order to determine whether the regional localizations of Bkm repeats detected on the human X chromosome consisted of typical GATA/GACA repeats, clones were isolated, mapped, and sequenced. Nine Bkm-hybridizing clones from Kunkel's fluorescent-activated, cell-sorted X-chromosome library were all unique. Five were mapped in detail with restriction enzymes and the Bkm-hybridizing segments were localized. Confirmation of X chromosomal homology was obtained for 2 of the clones and Bkm segments from these 2 clones were sequenced. Seventeen contiguous GATA repeats were found in each clone and the overall repeat arrangement showed relatively few differences from previously sequenced Bkm sequences. These are the first sequences of human Bkm repeats. The results, when compared with previously published results, suggest that there may be significant differences between the organization of Bkm repeats on the human X and on the human Y chromosome.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/66402/1/j.1469-1809.1988.tb01094.x.pd
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