6,630 research outputs found
Chord diagrams and BPHZ subtractions
The combinatorics of the BPHZ subtraction scheme for a class of ladder graphs
for the three point vertex in theory is transcribed into certain
connectivity relations for marked chord diagrams (knots with transversal
intersections). The resolution of the singular crossings using the equivalence
relations in these examples provides confirmation of a proposed fundamental
relationship between knot theory and renormalization in perturbative quantum
field theory.Comment: 12 pages, 5 Postscript figures, LaTex 2
Photon acceleration in variable ultra-relativistic outflows and high-energy spectra of Gamma-Ray Bursts
MeV seed photons produced in shocks in a variable ultra-relativistic outflow
gain energy by the Fermi mechanism, because the photons Compton scatter off
relativistically colliding shells. The Fermi-modified high-energy photon
spectrum has a non-universal slope and a universal cutoff. A significant
increase in the total radiative efficiency is possible. In some gamma ray
bursts, most of the power might be emitted at the high-energy cutoff for this
mechanism, which would be close to 100 MeV for outflows with a mean bulk
Lorentz factor of 100.Comment: 8 pages, submitted to ApJ
Robustness of the Thirty Meter Telescope Primary Mirror Control System
The primary mirror control system for the Thirty Meter Telescope (TMT) maintains the alignment of the 492 segments in the presence of both quasi-static (gravity and thermal) and dynamic disturbances due to unsteady wind loads. The latter results in a desired control bandwidth of 1Hz at high spatial frequencies. The achievable bandwidth is limited by robustness to (i) uncertain telescope structural dynamics (control-structure interaction) and (ii) small perturbations in the ill-conditioned influence matrix that relates segment edge sensor response to actuator commands. Both of these effects are considered herein using models of TMT. The former is explored through multivariable sensitivity analysis on a reduced-order Zernike-basis representation of the structural dynamics. The interaction matrix ("A-matrix") uncertainty has been analyzed theoretically elsewhere, and is examined here for realistic amplitude perturbations due to segment and sensor installation errors, and gravity and thermal induced segment motion. The primary influence of A-matrix uncertainty is on the control of "focusmode"; this is the least observable mode, measurable only through the edge-sensor (gap-dependent) sensitivity to the dihedral angle between segments. Accurately estimating focus-mode will require updating the A-matrix as a function of the measured gap. A-matrix uncertainty also results in a higher gain-margin requirement for focus-mode, and hence the A-matrix and CSI robustness need to be understood simultaneously. Based on the robustness analysis, the desired 1 Hz bandwidth is achievable in the presence of uncertainty for all except the lowest spatial-frequency response patterns of the primary mirror
Gamma-ray Burst Afterglow with Continuous Energy Injection: Signature of a Highly-Magnetized Millisecond Pulsar
We investigate the consequences of a continuously injecting central engine on
the gamma-ray burst afterglow emission, focusing more specifically on a
highly-magnetized millisecond pulsar engine. For initial pulsar parameters
within a certain region of the parameter space, the afterglow lightcurves are
predicted to show a distinctive achromatic bump feature, the onset and duration
of which range from minutes to months, depending on the pulsar and the fireball
parameters. The detection of or upper limits on such features would provide
constraints on the burst progenitor and on magnetar-like central engine models.
An achromatic bump such as that in GRB 000301C afterglow may be caused by a
millisecond pulsar with P0=3.4 millisecond and Bp=2.7e14 Gauss.Comment: 5 pages, emulateapj style, to appear in ApJ Letters, updated with the
accepted version, a few corrections are mad
Multi-GeV Neutrino Emission from Magnetized Gamma Ray Bursts
We investigate the expected neutrino emissivity from nuclear collisions in
magnetically dominated collisional models of gamma-ray bursts, motivated by
recent observational and theoretical developments. The results indicate that
significant multi-GeV neutrino fluxes are expected for model parameter values
which are typical of electromagnetically detected bursts. We show that for
detecting at least one muon event in Icecube and its Deep Core sub-array, a
single burst must be near the high end of the luminosity function and at a
redshift . We also calculate the luminosity and distance ranges
that can generate muon events per GRB in the same detectors, which may
be of interest if simultaneously detected electromagnetically, or if measured
with future extensions of Icecube or other neutrino detectors with larger
effective volume and better sensitivity.Comment: 12 pages, 7 figures, accepted version for Phys.Rev.
Resonance Production in STAR
The recent results from resonance production in central Au+Au and p+p
collisions at 200 GeV from the STAR experiment at RHIC
are presented and discussed.Comment: 7 pages, proceedings 19th Winter Workshop on Nuclear Dynamics,
Breckenridge, Colorado, USA, February 8-15, 200
Hard Burst Emission from the Soft Gamma Repeater SGR 1900+14
We present evidence for burst emission from SGR 1900+14 with a power-law high
energy spectrum extending beyond 500 keV. Unlike previous detections of high
energy photons during bursts from SGRs, these emissions are not associated with
high-luminosity burst intervals. Not only is the emission hard, but the spectra
are better fit by Band's GRB function rather than by the traditional
optically-thin thermal bremsstrahlung model. We find that the spectral
evolution within these hard events obeys a hardness/intensity anti-correlation.
Temporally, these events are distinct from typical SGR burst emissions in that
they are longer (~ 1 s) and have relatively smooth profiles. Despite a
difference in peak luminosity of > 1E+11 between these bursts from SGR 1900+14
and cosmological GRBs, there are striking temporal and spectral similarities
between the two kinds of bursts, aside from spectral evolution. We outline an
interpretation of these events in the context of the magnetar model.Comment: 11 pages (text and figures), submitted to ApJ Letters, corrected
erroneous hardness ratio
A Burst and Simultaneous Short-Term Pulsed Flux Enhancement from the Magnetar Candidate 1E 1048.1-5937
We report on the 2004 June 29 burst detected from the direction of the
Anomalous X-ray Pulsar (AXP) 1E 1048.1-5937 using the Rossi X-ray Timing
Explorer (RXTE). We find a simultaneous increase of ~3.5 times the quiescent
value in the 2-10 keV pulsed flux of 1E 1048.1-5937 during the tail of the
burst which identifies the AXP as the burst's origin. The burst was overall
very similar to the two others reported from the direction of this source in
2001. The unambiguous identification of 1E 1048.1-5937 as the burster here
confirms it was the origin of the 2001 bursts as well. The epoch of the burst
peak was very close to the arrival time of 1E 1048.1-5937's pulse peak. The
burst exhibited significant spectral evolution with the trend going from hard
to soft. During the 11 days following the burst, the AXP was observed further
with RXTE, XMM-Newton and Chandra. Pre- and post-burst observations revealed no
change in the total flux or spectrum of the quiescent emission. Comparing all
three bursts detected thus far from this source we find that this event was the
most fluent (>3.3x10^-8 erg/cm^2 in the 2-20 keV band), had the highest peak
flux (59+/-9x10^-10 erg/s/cm^2 in the 2-20 keV band), and the longest duration
(>699 s). The long duration of the burst differentiates it from Soft Gamma
Repeater (SGR) bursts which have typical durations of ~0.1 s. Bursts that occur
preferentially at pulse maximum, have fast-rises and long X-tails containing
the majority of the total burst energy have been seen uniquely from AXPs. The
marked differences between AXP and SGRs bursts may provide new clues to help
understand the physical differences between these objects.Comment: 24 pages, 4 figures, submitted to the Astrophysical Journa
Marked expansion of exocrine and endocrine pancreas with incretin therapy in humans with increased exocrine pancreas dysplasia and the potential for glucagon-producing neuroendocrine tumors.
Controversy exists regarding the potential regenerative influences of incretin therapy on pancreatic ÎČ-cells versus possible adverse pancreatic proliferative effects. Examination of pancreata from age-matched organ donors with type 2 diabetes mellitus (DM) treated by incretin therapy (n = 8) or other therapy (n = 12) and nondiabetic control subjects (n = 14) reveals an âŒ40% increased pancreatic mass in DM treated with incretin therapy, with both increased exocrine cell proliferation (P < 0.0001) and dysplasia (increased pancreatic intraepithelial neoplasia, P < 0.01). Pancreata in DM treated with incretin therapy were notable for α-cell hyperplasia and glucagon-expressing microadenomas (3 of 8) and a neuroendocrine tumor. ÎČ-Cell mass was reduced by âŒ60% in those with DM, yet a sixfold increase was observed in incretin-treated subjects, although DM persisted. Endocrine cells costaining for insulin and glucagon were increased in DM compared with non-DM control subjects (P < 0.05) and markedly further increased by incretin therapy (P < 0.05). In conclusion, incretin therapy in humans resulted in a marked expansion of the exocrine and endocrine pancreatic compartments, the former being accompanied by increased proliferation and dysplasia and the latter by α-cell hyperplasia with the potential for evolution into neuroendocrine tumors
Ion-mediated conformational switches
Molecular switches are ubiquitous in Nature and provide the basis of many forms of transport and signalling. Single synthetic molecules that change conformation, and thus function, reversibly in a stimulus-dependent manner are of great interest not only to chemists but society in general; myriad applications exist in storage, display, sensing and medicine. Here we describe recent developments in the area of ion-mediated switching
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