48,081 research outputs found
A search for millimetric emission from Gamma Ray Bursts
We have used the 2- year Differential Microwave Radiometer data from the
COsmic Background Explorer (COBE) satellite to systematically search for
millimetric (31 - 90 GHz) emission from the Gamma Ray Bursts (GRBs) in the
Burst And Transient Source Experiment (BATSE) GRB 3B catalog. The large
beamsize of the COBE instrument (7 degs FWHM) allows for an efficient search of
the large GRB positional error boxes, although it also means that fluxes from
(point source) GRB objects will be somewhat diluted. A likelihood analysis has
been used to look for a change in the level of millimetric emission from the
locations of 81 GRB events during the first two years (1990 & 1991) of the COBE
mission. The likelihood analysis determined that we did not find any
significant millimetric signal before or after the occurance of the GRB. We
find 95% confidence level upper limits of 175, 192 and 645 Jy or, in terms of
fluxes, of 9.6, 16.3 and 54.8 10^{-13} erg/cm^2/s, respectively at 31, 53 and
90 GHz. We also look separately at different classes of GRBs, including a study
of the top ten (in peak flux) GRBs, the "short burst" and "long burst" subsets,
finding similar upper limits. While these limits may be somewhat higher than
one would like, we estimate that using this technique with future planned
missions could push these limits down to \sim 1 mJy.Comment: 21 pages, 5 figures, to be published in The Astrophysical Journa
Observation of pseudogap in MgB2
Pseudogap phase in superconductors continues to be an outstanding puzzle that
differentiates unconventional superconductors from the conventional ones
(BCS-superconductors). Employing high resolution photoemission spectroscopy on
a highly dense conventional superconductor, MgB2, we discover an interesting
scenario. While the spectral evolution close to the Fermi energy is
commensurate to BCS descriptions as expected, the spectra in the wider energy
range reveal emergence of a pseudogap much above the superconducting transition
temperature indicating apparent departure from the BCS scenario. The energy
scale of the pseudogap is comparable to the energy of E2g phonon mode
responsible for superconductivity in MgB2 and the pseudogap can be attributed
to the effect of electron-phonon coupling on the electronic structure. These
results reveal a scenario of the emergence of the superconducting gap within an
electron-phonon coupling induced pseudogap.Comment: 4 figure
Modified Landau levels, damped harmonic oscillator and two-dimensional pseudo-bosons
In a series of recent papers one of us has analyzed in some details a class
of elementary excitations called {\em pseudo-bosons}. They arise from a special
deformation of the canonical commutation relation [a,a^\dagger]=\1, which is
replaced by [a,b]=\1, with not necessarily equal to . Here,
after a two-dimensional extension of the general framework, we apply the theory
to a generalized version of the two-dimensional Hamiltonian describing Landau
levels. Moreover, for this system, we discuss coherent states and we deduce a
resolution of the identity. We also consider a different class of examples
arising from a classical system, i.e. a damped harmonic oscillator.Comment: in press in Journal of Mathematical Physic
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