10,422 research outputs found
The exotic fraction among unassociated Fermi sources
Revealing the nature of unassociated high-energy (> 100 MeV) gamma-ray
sources remains a challenge 35 years after their discovery. Of the 934
gamma-ray sources at high Galactic latitude (|b| > 15 degrees) in the First
Fermi-LAT catalogue (1FGL), 316 have no obvious associations at other
wavelengths. In this paper, we apply the K-means unsupervised classification
algorithm to isolate potential counterparts for 18 unassociated Fermi sources
contained within a 3000 square degree `overlap region' of the sky intensively
covered in radio and optical wavelengths. Combining our results with previous
works, we reach potential associations for 119 out of the 128 Fermi sources
within said region. If these associations are correct, we estimate that less
than 20% of all remaining unassociated 1FGL sources at high Galactic latitude
(|b| > 15 degrees) might host `exotic' counterparts distinct from known classes
of gamma-ray emitters. Potentially even these outliers could be explained by
high-redshift/dust-obscured analogues of the associated sample or by
intrinsically faint radio objects. Although such estimate leaves some room for
novel discoveries, it severely restricts the possibility of detecting dark
matter subhaloes and other unconventional types of gamma-ray emitters in the
1FGL. In closing, we argue that the identification of Fermi sources at the low
end of the flux density distribution will be a complex process that might only
be achieved through a clever combination of refined classification algorithms,
multi-wavelength efforts, and dedicated optical spectroscopy.Comment: 7 pages, 5 figures, 2 table
Score-Informed Source Separation for Musical Audio Recordings [An overview]
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Enhanced thermoelectric response of hole-doped LaNiO by ab initio calculations
Thermoelectric properties of the system LaNiO have been
studied ab initio. Large Seebeck coefficient values are predicted for the
parent compound, and to some extent remain in the hole-doped metallic phase,
accompanied of an increase in the conductivity. This system, due to its layered
structure would be a suitable candidate for an improvement of its
thermoelectric figure of merit by nanostructurization in thin films, that has
already been shown to increase the electrical conductivity (). Our
calculations show that in the region around LaNiO the system has a
large thermopower at high temperatures and also a substantially increased
. Films grown with this low-doping concentration will show an optimal
relationship between thermopower and . This result is obtained for
various exchange-correlation schemes (correlated, uncorrelated and
parameter-free) that we use to analyze the electronic structure of the
hole-doped compound.Comment: 10 pages, 6 figure
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