742 research outputs found
No evidence for dust extinction in GRB 050904 at z ~ 6.3
No full textContext. Gamma-ray burst (GRB) afterglows are excellent and sensitive probes of gas and dust in star-forming galaxies at all epochs. It has been posited that dust in the early Universe must be different from dust at lower redshifts. To date two reports in the literature directly support this contention, one of which is based on the spectral shape of the afterglow spectrum of GRB050904 at z = 6.295.
Aims. Here we reinvestigate the afterglow of GRB050904 to understand cosmic dust at high redshift.We address the claimed evidence for unusual (supernova-origin) dust in its host galaxy by simultaneously examining the X-ray and optical/near-infrared spectrophotometric data of the afterglow.
Methods. We derived the intrinsic spectral energy distribution (SED) of the afterglow at three different epochs, 0.47, 1.25, and 3.4 days after the burst. We reduced again the Swift X-ray data, the 1.25 days FORS2 z-Gunn photometric data, the spectroscopic and z-band
photometric data at ∼3 days from the Subaru telescope, as well as the critical UKIRT Z-band photometry at 0.47 days, upon which the claim of dust detection largely relies.
Results. We find no evidence of dust extinction in the SED at any time.We computed flux densities at λrest = 1250Å directly from the observed counts at all epochs. In the earliest epoch, 0.47 days, where the claim of dust is strongest, the Z-band suppression is found to
be weaker (0.3 ± 0.2 mag) than previously reported and statistically insignificant (<1.5σ). Furthermore, we find that the photometry of this band is unstable and difficult to calibrate.
Conclusions. From the afterglow SED we demonstrate that there is no evidence of dust extinction in GRB 050904 – the SED at all times can be reproduced without dust, and at 1.25 days in particular, significant extinction can be excluded, with A(3000Å) < 0.27 mag at 95% confidence using the supernova-type extinction curve. We conclude that there is no evidence of any extinction in the afterglow
of GRB050904 and that the presence of supernova-origin dust in the host of GRB050904 must be viewed skeptically
New parametric transducer for resonant detectors: Advances and room temperature test
No full textWe are developing a prototype of cryogenic parametric converter transducer operating at 5 GHz, for the upgrade of the ROG Collaboration resonant G. W. antennas. This device is built on the experience of the Niobe detector (D. G. Blair et al.), with substantial modifications that should let us achieve better stability and sensitivity. The prototype uses as parametric converter a superconducting coaxial cavity with a 50 micron gap (Q 0 = 5 × 10 8 at 1.5K and 100μW RF power dissipation), and a contacless RF coupling for thermal insulation between the 2K stage and the ultra cryogenic (100mK) antenna. The coupler features a constant transmission loss of 0.2dB over a range of displacements of 5mm in x, y and z around the nominal operating position with a separation of 8mm between the two halves of the coupler. In this way the large, low frequency swings (0.5 and 17 Hz), of the 2 Tons antenna around its suspension point have no influence on the transducer performance. To test all the components of the transducer and the system performance, a room temperature prototype is installed on the TART (Test Antenna at Room Temperature) facility at the INFN labs. Using critical coupling for the RF cavity input coupler we manage to keep to a minimum the leakage of the drive signal to the first RF amplifier. In this way we avoid degradation of the RF amplifier noise figure (0.6 dB at room temperature) produced by the RF amplifier saturation Experimental results agree with the full analysis of the room temperature detector performances. © 2008 IOP Publishing Ltd
Integrating longitudinal information in hippocampal volume measurements for the early detection of Alzheimer's disease
No full text<p>Background: Structural MRI measures for monitoring Alzheimer's Disease (AD) progression are becoming instrumental in the clinical practice, and more so in the context of longitudinal studies. This investigation addresses the impact of four image analysis approaches on the longitudinal performance of the hippocampal volume. Methods: We present a hippocampal segmentation algorithm and validate it on a gold-standard manual tracing database. We segmented 460 subjects from ADNI, each subject having been scanned twice at baseline, 12-month and 24month follow-up scan (1.5T, T1 MRI). We used the bilateral hippocampal volume v and its variation, measured as the annualized volume change Λ=δv/year(mm3/y). Four processing approaches with different complexity are compared to maximize the longitudinal information, and they are tested for cohort discrimination ability. Reference cohorts are Controls vs. Alzheimer's Disease (CTRL/AD) and CTRL vs. Mild Cognitive Impairment who subsequently progressed to AD dementia (CTRL/MCI-co). We discuss the conditions on v and the added value of Λ in discriminating subjects. Results: The age-corrected bilateral annualized atrophy rate (%/year) were: -. 1.6 (0.6) for CTRL, -. 2.2 (1.0) for MCI-. nc, -. 3.2 (1.2) for MCI-. co and -. 4.0 (1.5) for AD. Combined (. v, Λ) discrimination ability gave an Area under the ROC curve (. auc). =. 0.93 for CTRL vs AD and auc=. 0.88 for CTRL vs MCI-. co. Conclusions: Longitudinal volume measurements can provide meaningful clinical insight and added value with respect to the baseline provided the analysis procedure embeds the longitudinal information.</p>
An enigmatic long-lasting γ-ray burst not accompanied by a bright supernova
No full textGamma-ray bursts (GRBs) are short, intense flashes of soft γ-rays coming from the distant Universe. Long-duration GRBs (those lasting more than ~2s) are believed to originate from the deaths of massive stars, mainly on the basis of a handful of solid associations between GRBs and supernovae. GRB 060614, one of the closest GRBs discovered, consisted of a 5-s hard spike followed by softer, brighter emission that lasted for ~100s (refs 8, 9). Here we report deep optical observations of GRB 060614 showing no emerging supernova with absolute visual magnitude brighter than MV=-13.7. Any supernova associated with GRB 060614 was therefore at least 100 times fainter, at optical wavelengths, than the other supernovae associated with GRBs. This demonstrates that some long-lasting GRBs can either be associated with a very faint supernova or produced by different phenomena.Fil: Della Valle, M.. Osservatorio Astrofisico di Arcetri; ItaliaFil: Chincarini, G.. Osservatorio Astronomico di Brera; ItaliaFil: Panagia, N.. Space Telescope Science Institute; Estados UnidosFil: Tagliaferri, G.. Osservatorio Astronomico di Brera; ItaliaFil: Malesani, D.. International School for Advanced Studies; ItaliaFil: Testa, V.. Osservatorio Astronomico di Roma; ItaliaFil: Fugazza, D.. Osservatorio Astronomico di Brera; ItaliaFil: Campana, S.. Osservatorio Astronomico di Brera; ItaliaFil: Covino, S.. Osservatorio Astronomico di Brera; ItaliaFil: Mangano, V.. Istituto di Astrofisica Spaziale e Fisica Cosmica di Palermo; ItaliaFil: Antonelli, L. A.. Osservatorio Astronomico di Roma; ItaliaFil: D'Avanzo, P.. Osservatorio Astronomico di Brera; ItaliaFil: Hurley, K.. University Of California Berkeley; Estados UnidosFil: Mirabel Miquele, Igor Felix. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: Pellizza González, Leonardo Javier. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: Piranomonte, S.. Osservatorio Astronomico di Roma; ItaliaFil: Stella, L.. Osservatorio Astronomico di Roma; Itali
Additional file 3: of Metabolic correlates of reserve and resilience in MCI due to Alzheimer's Disease (AD)
No full textSupplementary Table. (DOCX 22 kb
A kinetics-based approach to amyloid PET semi-quantification
No full textPurpose: To develop and validate a semi-quantification method (time-delayed ratio, TDr) applied to amyloid PET scans, based on tracer kinetics information
Integrating longitudinal information in hippocampal volume measurements for the early detection of Alzheimer's disease
No full textBackground: Structural MRI measures for monitoring Alzheimer's Disease (AD) progression are becoming instrumental
in the clinical practice, and more so in the context of longitudinal studies. This investigation addresses the
impact of four image analysis approaches on the longitudinal performance of the hippocampal volume.
Methods:We present a hippocampal segmentation algorithm and validate it on a gold-standard manual tracing
database.We segmented 460 subjects from ADNI, each subject having been scanned twice at baseline, 12-month
and 24 month follow-up scan (1.5 T, T1 MRI). We used the bilateral hippocampal volume v and its variation,
measured as the annualized volume change Λ = δv/year(mm3/y). Four processing approaches with different
complexity are compared to maximize the longitudinal information, and they are tested for cohort discrimination
ability. Reference cohorts are Controls vs. Alzheimer's Disease (CTRL/AD) and CTRL vs. Mild Cognitive Impairment
who subsequently progressed to AD dementi
Implications for the origin of GRB 070201 from LIGO observations
No full textWe analyzed the available LIGO data coincident with GRB 070201, a short duration hard spectrum γ-ray burst whose electromagnetically determined sky position is coincident with the spiral arms of the Andromeda galaxy (M31). Possible progenitors of such short hard GRBs include mergers of neutron stars or a neutron star and black hole, or soft γ-ray repeater (SGR) flares. These events can be accompanied by gravitational-wave emission. No plausible gravitational wave candidates were found within a 180 s long window around the time of GRB 070201. This result implies that a compact binary progenitor of GRB 070201, with masses in the range 1 M⊙ 99% confidence. Indeed, if GRB 070201 were caused by a binary neutron star merger, we find that D < 3.5 Mpc is excluded, assumingrandom inclination, at 90% confidence. The result also implies that an unmodeled gravitational wave burst from GRB 070201 most probably emitted less than 4.4×10-4M⊙c2 (7.9×1050 ergs) in any 100 ms long period within the signal region if the source was in M31 and radiated isotropically at the same frequency as LIGO's peak sensitivity ( f ≈ 150 Hz). This upper limit does not exclude current models of SGRs at the M31 distance
X-ray luminous clusters of galaxies at z > 0.3
No full textThis thesis presents optical CCD imaging taken as part of the follow-up programme to the Massive Cluster Survey, a ROSAT All-Sky Survey based cluster survey aiming to discover X-ray luminous galaxy clusters at redshifts above 0.3. A data-reduction pipeline is developed to process the V, R and I band imaging of 111 clusters observed with the University of Hawaii 2.2 metre telescope and 57 with the ESQ NTT. The galaxian population is examined and all clusters but one contain enough galaxies to account for the X-ray flux. The cluster red-sequence is used to constrain the star-formation epoch to high {z > 1.7) redshift and as a basis for photometric redshifts. The symmetry properties of the clusters are examined to probe merger activity, the cluster selection exhibiting no bias towards, or against, merging clusters. The BCG population is examined. The degree to which the BCG dominates the cluster galaxy population displays no trend with either redshift or cluster X-ray luminosity, indicative of considerable previous as well as ongoing evolution. A quarter of the BCG population show bluer colours than would be expected for an elliptical galaxy at the appropriate redshift, some with known line emission, proving that BCGs are not purely passively evolving galaxies. Multi-object spectroscopy of two clusters is used to confirm these as being massive. Colour-magnitude diagrams of spectroscopically selected galaxies are used to determine that the major source of error in the photometry to is the flux limit of the imaging not the techniques applied. The Veron-Cetty Veron AGN catalogue is cross-correlated with the Abell catalogue to reveal a sample of AGN in clusters which are found to be distributed within clusters as ordinary galaxies, making contamination to the observed X-ray flux a possibility. A search for gravitationally lensed galaxies reveals such objects in 23% of the clusters imaged
Riding the Yield Curve: Diversification of Strategies
No full textRiding the yield curve, the fixed-income strategy of purchasing a longer-dated security and selling before maturity, has long been a popular means to achieve excess returns compared to buying-and-holding, despite its implicit violations of market efficiency and the pure expectations hypothesis of the term structure. This paper looks at the historic excess returns of different strategies across three countries and proposes several statistical and macro-based trading rules which seem to enhance returns even more. While riding based on the Taylor Rule works well even for longer investment horizons, our empirical results indicate that, using expectations implied by Fed funds futures, excess returns can only be increased over short horizons. Furthermore, we demonstrate that duration-neutral strategies are superior to standard riding on a risk- adjusted basis. Overall, our evidence stands in contrast to the pure expectations hypothesis and points to the existence of risk premia which may be exploited consistently.Term Structure, Interest Rates, Market Efficiency, Taylor Rule
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