693 research outputs found
GRB 000301C: a possible short/intermediate duration burst connected to a DLA system
We discuss two main aspects of the GRB 000301C afterglow (Fynbo et al. 2000,
Jensen et al. 2000); its short duration and its possible connection with a
Damped Ly-alpha Absorber (DLA). GRB 000301C falls in the short class of bursts,
though it is consistent with belonging to the proposed intermediate class or
the extreme short end of the distribution of long-duration GRBs. Based on two
VLT spectra we estimate the HI column density to be Log(N(HI))=21.2+/-0.5. This
is the first direct indication of a connection between GRB host galaxies and
Damped Ly-alpha Absorbers.Comment: 3 pages, 3 postscript figures. To appear in the proceedings of the
October 2000 Rome Workshop on ``Gamma-Ray Bursts in the Afterglow Era'
Multi-Wavelength Studies of the Optically Dark Gamma-Ray Burst 001025A
We identify the fading X-ray afterglow of GRB 001025A from XMM-Newton
observations obtained 1.9-2.3 days, 2 years, and 2.5 years after the burst. The
non-detection of an optical counterpart to an upper limit of R=25.5, 1.20 days
after the burst, makes GRB 001025A a ``dark'' burst. Based on the X-ray
afterglow spectral properties of GRB 001025A, we argue that some bursts appear
optically dark because their afterglow is faint and their cooling frequency is
close to the X-ray band. This interpretation is applicable to several of the
few other dark bursts where the X-ray spectral index has been measured. The
X-ray afterglow flux of GRB 001025A is an order of magnitude lower than for
typical long-duration gamma-ray bursts. The spectrum of the X-ray afterglow can
be fitted with an absorbed synchrotron emission model, an absorbed thermal
plasma model, or a combination thereof. For the latter, an extrapolation to
optical wavelengths can be reconciled with the R-band upper limit on the
afterglow, without invoking any optical circumburst absorption, provided the
cooling frequency is close to the X-ray band. Alternatively, if the X-ray
afterglow is due to synchrotron emission only, seven magnitudes of extinction
in the observed R-band is required to meet the R-band upper limit, making GRB
001025A much more obscured than bursts with detected optical afterglows. Based
on the column density of X-ray absorbing circumburst matter, an SMC gas-to-dust
ratio is insufficient to produce this amount of extinction. The X-ray tail of
the prompt emission enters a steep temporal decay excluding that the tail of
the prompt emission is the onset of the afterglow (abridged).Comment: 32 pages, 8 figures, ApJ in pres
A Numerical Model of an Acoustic Metamaterial Using the Boundary Element Method Including Viscous and Thermal Losses
[EN] In recent years, boundary element method (BEM) and finite element method (FEM) implementations of acoustics in fluids with viscous and thermal losses have been developed. They are based on the linearized NavierÂżStokes equations with no flow. In this paper, such models with acoustic losses are applied to an acoustic metamaterial. Metamaterials are structures formed by smaller, usually periodic, units showing remarkable physical properties when observed as a whole. Acoustic losses are relevant in metamaterials in the millimeter scale. In addition, their geometry is intricate and challenging for numerical implementation. The results are compared with existing measurements.The authors wish to thank Mads J. Herring Jensen, from the company COMSOL, for his support in setting up the FEM model of the metamaterial. J. Sanchez-Dehesa acknowledges the support by the Spanish Ministerio de Economia y Competitividad, and the European Union Fondo Europeo de Desarrollo Regional (FEDER) through Project No. TEC2014-53088-C3-1-R.Cutanda-Henriquez, V.; Andersen, PR.; Jensen, JS.; Juhl, PM.; SĂĄnchez-Dehesa Moreno-Cid, J. (2017). A Numerical Model of an Acoustic Metamaterial Using the Boundary Element Method Including Viscous and Thermal Losses. Journal of Computational Acoustics. 25(4):1750006-1-1750006-11. doi:10.1142/S0218396X17500060S1750006-11750006-11254Craster, R. V., & Guenneau, S. (Eds.). (2013). Acoustic Metamaterials. Springer Series in Materials Science. doi:10.1007/978-94-007-4813-2Cummer, S. A., Christensen, J., & AlĂš, A. (2016). Controlling sound with acoustic metamaterials. Nature Reviews Materials, 1(3). doi:10.1038/natrevmats.2016.1Cutanda-HenrĂquez, V., & Juhl, P. M. (2013). An axisymmetric boundary element formulation of sound wave propagation in fluids including viscous and thermal losses. The Journal of the Acoustical Society of America, 134(5), 3409-3418. doi:10.1121/1.4823840Bruneau, M., Herzog, P., Kergomard, J., & Polack, J. D. (1989). General formulation of the dispersion equation in bounded visco-thermal fluid, and application to some simple geometries. Wave Motion, 11(5), 441-451. doi:10.1016/0165-2125(89)90018-8Kampinga, W. R., Wijnant, Y. H., & de Boer, A. (2010). Performance of Several Viscothermal Acoustic Finite Elements. Acta Acustica united with Acustica, 96(1), 115-124. doi:10.3813/aaa.918262Kampinga, W. R., Wijnant, Y. H., & de Boer, A. (2011). An Efficient Finite Element Model for Viscothermal Acoustics. Acta Acustica united with Acustica, 97(4), 618-631. doi:10.3813/aaa.918442BELTMAN, W. M. (1999). VISCOTHERMAL WAVE PROPAGATION INCLUDING ACOUSTO-ELASTIC INTERACTION, PART I: THEORY. Journal of Sound and Vibration, 227(3), 555-586. doi:10.1006/jsvi.1999.2355GraciĂĄ-Salgado, R., GarcĂa-Chocano, V. M., Torrent, D., & SĂĄnchez-Dehesa, J. (2013). Negative mass density andĎ-near-zero quasi-two-dimensional metamaterials: Design and applications. Physical Review B, 88(22). doi:10.1103/physrevb.88.224305Homentcovschi, D., & Miles, R. N. (2011). An analytical-numerical method for determining the mechanical response of a condenser microphone. The Journal of the Acoustical Society of America, 130(6), 3698-3705. doi:10.1121/1.3652853Geuzaine, C., & Remacle, J.-F. (2009). Gmsh: A 3-D finite element mesh generator with built-in pre- and post-processing facilities. International Journal for Numerical Methods in Engineering, 79(11), 1309-1331. doi:10.1002/nme.257
VLT identification of the optical afterglow of the gamma-ray burst GRB 000131 at z=4.50
We report the discovery of the gamma-ray burst GRB 000131 and its optical
afterglow. The optical identification was made with the VLT 84 hours after the
burst following a BATSE detection and an Inter Planetary Network localization.
GRB 000131 was a bright, long-duration GRB, with an apparent precursor signal
62 s prior to trigger. The afterglow was detected in ESO VLT, NTT, and DK1.54m
follow-up observations. Broad-band and spectroscopic observations of the
spectral energy distribution reveals a sharp break at optical wavelengths which
is interpreted as a Ly-alpha absorption edge at 6700 A. This places GRB 000131
at a redshift of 4.500 +/- 0.015. The inferred isotropic energy release in
gamma rays alone was approximately 10^54 erg (depending on the assumed
cosmology). The rapid power-law decay of the afterglow (index alpha=2.25,
similar to bursts with a prior break in the lightcurve), however, indicates
collimated outflow, which relaxes the energy requirements by a factor of < 200.
The afterglow of GRB 000131 is the first to be identified with an 8-m class
telescope.Comment: 8 pages, 7 figures, accepted to A&A Letter
The bright Gamma-Ray Burst of February 10, 2000: a case study of an optically dark GRB
The gamma-ray burst GRB000210 had the highest gamma-ray peak flux of any
event localized by BeppoSAX as yet but it did not have a detected optical
afterglow. It is therefore one of the events recently classified as dark GRBs
or GHOST (GRB Hiding Optical Source Transient), whose origin is still unclear.
Chandra observations allowed us to localize this GRB within ~1" and a radio
transient was detected with the VLA. We identify the likely (P=0.01) host
galaxy of this burst at z=0.846. The X-ray spectrum of the afterglow shows
intrinsic absorption N_H=5x10**21 cm-2. The amount of dust needed to absorb the
optical flux of this object is consistent with the above HI column density,
given a dust-to-gas ratio similar to that of our Galaxy. We do not find
evidence for a partially ionized absorber expected if the absorption takes
place in a Giant Molecular Cloud. We therefore conclude that either the gas is
local to the GRB, but is condensed in small-scale high-density (n>~10**9 cm-3)
clouds, or that the GRB is located in a dusty, gas-rich region of the galaxy.
Finally, if GRB000210 lies at z>5, its X-ray absorbing medium would have to be
substantially different from that observed in GRBs with optical afterglows.Comment: 29 pages, 7 fig.s, some revisions, ApJ, in pres
Mirroring everyday clinical practice in clinical trial design: a new concept to improve the external validity of randomized double-blind placebo-controlled trials in the pharmacological treatment of major depression
Background: Randomized, double-blind, placebo-controlled trials constitute the gold standard in clinical research when testing the efficacy of new psychopharmacological interventions in the treatment of major depression. However, the blinded use of placebo has been found to influence clinical trial outcomes and may bias patient
selection.
Discussion: To improve clinical trial design in major depression so as to reflect clinical practice more closely we propose to present patients with a balanced view of the benefits of study participation irrespective of their assignment to placebo or active treatment. In addition every participant should be given the option to finally
receive the active medication. A research agenda is outlined to evaluate the impact of the proposed changes on the efficacy of the drug to be evaluated and on the demographic and clinical characteristics of the enrollment fraction with regard to its representativeness of the eligible population.
Summary: We propose a list of measures to be taken to improve the external validity of double-blind, placebocontrolled trials in major depression. The recommended changes to clinical trial design may also be relevant for other psychiatric as well as medical disorders in which expectations regarding treatment outcome may affect the
outcome itself
The GRB-SN connection: GRB030329 and XRF030723
The attempt to secure conclusive, spectroscopic evidence for the GRB/SN
connection has been a central theme in most GRB observing time proposals since
the discovery of the very unusual GRB980425 associated with the peculiar type
Ib/c SN1998bw. GRB030329 provided this evidence to everybody's satisfaction. In
this contribution we show the results of a spectroscopic campaign of the
supernova associated with GRB030329 carried out at ESOs Very Large Telescope.
We also present preliminary results from a photometric and spectroscopic
campaign targeting the X-ray Flash of July 23.Comment: 6 pages, 3 figures, to appear in the Santa Fe GRB Conference
Proceedings, 200
On the Afterglow of the X-ray Flash of 2003 July 23: Photometric evdence for an Off-Axis Gamma-Ray burst with an Associated Supernova
Peer reviewe
City dynamics: mobility and livelihoods of urban residents
City dynamics: mobility and livelihoods of urban resident
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