628 research outputs found
The detectability of dark matter annihilation with Fermi using the anisotropy energy spectrum of the gamma-ray background
The energy-dependence of the anisotropy (the anisotropy energy spectrum) of
the large-scale diffuse gamma-ray background can reveal the presence of
multiple source populations. Annihilating dark matter in the substructure of
the Milky Way halo could give rise to a modulation in the anisotropy energy
spectrum of the diffuse gamma-ray emission measured by Fermi, enabling the
detection of a dark matter signal. We determine the detectability of a
dark-matter-induced modulation for scenarios in which unresolved blazars are
the primary contributor to the measured emission above ~1 GeV and find that in
some scenarios pair-annihilation cross sections of order the value expected for
thermal relic dark matter can produce a detectable feature. We anticipate that
the sensitivity of this technique to specific dark matter models could be
improved by tailored likelihood analysis methods.Comment: 9 pages, 7 figures; figures updated, other minor revisions, published
in Ap
Speciation Studies of Trace Metals in the Gulf of Elefsis, Greece
A method for differentiating trace metal species on the basis of relative lability has been applied for Pb, Cd, Zn, in microenvironments of the Gulf of Elefsis. Species are classified as »very labile«, »moderately labile«, »slowly labile« and »inert« depending on the characteristic time scale of the measuring technique using differential pulse anodic stripping voltammetry (DPASV) and the retention on Chelex-100 resin in successive column and batch procedures. The results show that in two extremely different microenvironments (with S of about 17%c and ~45%o) the slowly labile and inert fractions prevail particularly at the surface layer, whereas in the main water body of the gulf the very labile fraction is the predominant one. However at the thermocline and below it the »inert« fraction increases considerably paralleling, broadly, the particulate metal distributions
The substructure of the Perseus star forming region:a survey with Gaia DR2
We use photometric and kinematic data from Gaia DR2 to explore the structure of the star-forming region associated with the molecular cloud of Perseus. Apart from the two well-known clusters, IC 348 and NGC 1333, we present five new clustered groups of young stars, which contain between 30 and 300 members, named Autochthe, Alcaeus, Heleus, Electryon, and Mestor. We demonstrate that these are co-moving groups of young stars, based on how the candidate members are distributed in position, proper motion, parallax, and colour-magnitude space. By comparing their colour-magnitude diagrams to isochrones, we show that they have ages between 1 and 5 Myr. Using 2MASS and WISE colours, we find that the fraction of stars with discs in each group ranges from 10 to ∼50 per cent. The youngest of the new groups is also associated with a reservoir of cold dust, according to the Planck map at 353 GHz. We compare the ages and proper motions of the five new groups to those of IC 348 and NGC 1333. Autochthe is clearly linked with NGC 1333 and may have formed in the same star formation event. The seven groups separate roughly into two sets that share proper motion, parallax, and age: Heleus, Electryon, and Mestor as the older set, and NGC 1333 and Autochthe as the younger set. Alcaeus is kinematically related to the younger set, but at a more advanced age, while the properties of IC 348 overlap with both sets. All older groups in this star-forming region are located at higher galactic latitude.</p
Novel Techniques for Decomposing Diffuse Backgrounds
The total anisotropy of a diffuse background composed of two or more sources, such as the Fermi-Large Area Telescope (LAT)-measured gamma-ray background, is set by the anisotropy of each source population and the contribution of each population to the total intensity. The total anisotropy as a function of energy (the anisotropy energy spectrum) will modulate as the relative contributions of the sources change, implying that the anisotropy energy spectrum also encodes the intensity spectrum of each source class. We develop techniques, applicable to any such diffuse background, for unraveling the intensity spectrum of each component source population given a measurement of the total intensity spectrum and the total anisotropy energy spectrum, without introducing a priori assumptions about the spectra of the source classes. We demonstrate the potential of these methods by applying them to example scenarios for the composition of the Fermi-LAT gamma-ray background consistent with current data and feasible within 10 yr of observation
The connection between the 15 GHz radio and gamma-ray emission in blazars
Since mid-2007 we have carried out a dedicated long-term monitoring programme
at 15 GHz using the Owens Valley Radio Observatory 40 meter telescope. One of
the main goals of this programme is to study the relation between the radio and
gamma-ray emission in blazars and to use it as a tool to locate the site of
high energy emission. Using this large sample of objects we are able to
characterize the radio variability, and study the significance of correlations
between the radio and gamma-ray bands. We find that the radio variability of
many sources can be described using a simple power law power spectral density,
and that when taking into account the red-noise characteristics of the light
curves, cases with significant correlation are rare. We note that while
significant correlations are found in few individual objects, radio variations
are most often delayed with respect to the gamma-ray variations. This suggests
that the gamma-ray emission originates upstream of the radio emission. Because
strong flares in most known gamma-ray-loud blazars are infrequent, longer light
curves are required to settle the issue of the strength of radio-gamma
cross-correlations and establish confidently possible delays between the two.
For this reason continuous multiwavelength monitoring over a longer time period
is essential for statistical tests of jet emission models.Comment: To appear in the Proceedings of the IAU Symposium No. 313:
"Extragalactic jets from every angle," Galapagos, Ecuador, 15-19 September
2014, F. Massaro, C. C. Cheung, E. Lopez, and A. Siemiginowska (Eds.),
Cambridge University Pres
Demonstration of magnetic field tomography with starlight polarization towards a diffuse sightline of the ISM
The availability of large datasets with stellar distance and polarization
information will enable a tomographic reconstruction of the
(plane-of-the-sky-projected) interstellar magnetic field in the near future. We
demonstrate the feasibility of such a decomposition within a small region of
the diffuse ISM. We combine measurements of starlight (R-band) linear
polarization obtained using the RoboPol polarimeter with stellar distances from
the second Gaia data release. The stellar sample is brighter than 17 mag in the
R band and reaches out to several kpc from the Sun. HI emission spectra reveal
the existence of two distinct clouds along the line of sight. We decompose the
line-of-sight-integrated stellar polarizations to obtain the mean polarization
properties of the two clouds. The two clouds exhibit significant differences in
terms of column density and polarization properties. Their mean
plane-of-the-sky magnetic field orientation differs by 60 degrees. We show how
our tomographic decomposition can be used to constrain our estimates of the
polarizing efficiency of the clouds as well as the frequency dependence of the
polarization angle of polarized dust emission. We also demonstrate a new method
to constrain cloud distances based on this decomposition. Our results represent
a preview of the wealth of information that can be obtained from a tomographic
map of the ISM magnetic field.Comment: 25 pages, 14 figures, published in ApJ, data appear in journa
A luminosity constraint on the origin of unidentified high energy sources
The identification of point sources poses a great challenge for the high
energy community. We present a new approach to evaluate the likelihood of a set
of sources being a Galactic population based on the simple assumption that
galaxies similar to the Milky Way host comparable populations of gamma-ray
emitters. We propose a luminosity constraint on Galactic source populations
which complements existing approaches by constraining the abundance and spatial
distribution of any objects of Galactic origin, rather than focusing on the
properties of a specific candidate emitter. We use M31 as a proxy for the Milky
Way, and demonstrate this technique by applying it to the unidentified EGRET
sources. We find that it is highly improbable that the majority of the
unidentified EGRET sources are members of a Galactic halo population (e.g.,
dark matter subhalos), but that current observations do not provide any
constraints on all of these sources being Galactic objects if they reside
entirely in the disk and bulge. Applying this method to upcoming observations
by the Fermi Gamma-ray Space Telescope has the potential to exclude association
of an even larger number of unidentified sources with any Galactic source
class.Comment: 18 pages, 4 figures, to appear in JPhys
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