875 research outputs found
Dark matter powered stars: Constraints from the extragalactic background light
The existence of predominantly cold non-baryonic dark matter is unambiguously
demonstrated by several observations (e.g., structure formation, big bang
nucleosynthesis, gravitational lensing, and rotational curves of spiral
galaxies). A candidate well motivated by particle physics is a weakly
interacting massive particle (WIMP). Self-annihilating WIMPs would affect the
stellar evolution especially in the early universe. Stars powered by
self-annihilating WIMP dark matter should possess different properties compared
with standard stars. While a direct detection of such dark matter powered stars
seems very challenging, their cumulative emission might leave an imprint in the
diffuse metagalactic radiation fields, in particular in the mid-infrared part
of the electromagnetic spectrum. In this work the possible contributions of
dark matter powered stars (dark stars; DSs) to the extragalactic background
light (EBL) are calculated. It is shown that existing data and limits of the
EBL intensity can already be used to rule out some DS parameter sets.Comment: Accepted for publication in ApJ; 7 pages, 5 figure
Probing the origin of VHE emission from M 87 with MWL observations in 2010
The large majority of extragalactic very high energy (VHE; E>100 GeV) sources
belongs to the class of active galactic nuclei (AGN), in particular the BL Lac
sub-class. AGNs are characterized by an extremely bright and compact emission
region, powered by a super-massive black hole (SMBH) and an accretion disk, and
relativistic outflows (jets) detected all across the electro-magnetic spectrum.
In BL Lac sources the jet axis is oriented close to the line of sight, giving
rise to a relativistic boosting of the emission. In radio galaxies, on the
other hand, the jet makes a larger angle to the line of sight allowing to
resolve the central core and the jet in great details. The giant radio galaxy M
87 with its proximity (1 6Mpc) and its very massive black hole ((3-6) x 10^9
M_solar) provides a unique laboratory to investigate VHE emission in such
objects and thereby probe particle acceleration to relativistic energies near
SMBH and in jets. M 87 has been established as a VHE emitter since 2005. The
VHE emission displays strong variability on time-scales as short as a day. It
has been subject of a large joint VHE and multi-wavelength (MWL) monitoring
campaign in 2008, where a rise in the 43 GHz VLBA radio emission of the
innermost region (core) was found to coincide with a flaring activity at VHE.
This had been interpreted as a strong indication that the VHE emission is
produced in the direct vicinity of the SMBH black hole. In 2010 again a flare
at VHE was detected triggering further MWL observations with the VLBA, Chandra,
and other instruments. At the same time M 87 was also observed with the
Fermi-LAT telescope at GeV energies and the European VLBI Network (EVN). In
this contribution preliminary results from the campaign will be presented.Comment: 5 pages, 2 figures, in the proceedings of the "International Workshop
on Beamed and Unbeamed Gamma-Rays from Galaxies" 11-15 April 2011, Lapland
Hotel Olos, Muonio, Finland, Journal of Physics: Conference Series Volume
355, 201
Discovery of VHE gamma-rays from Centaurus A
We report the discovery of faint very high energy (VHE, E > 100 GeV)
gamma-ray emission from the radio galaxy Centaurus A in deep observations
performed with the H.E.S.S. experiment. A signal with a statistical
significance of 5.0 sigma is detected from the region including the radio core
and the inner kpc jets. The integral flux above an energy threshold of ~250 GeV
is measured to be 0.8% of the flux of the Crab Nebula and the spectrum can be
described by a power law with a photon index of 2.7 +/- 0.5_stat +/- 0.2_sys.
No significant flux variability is detected in the data set. The discovery of
VHE gamma-ray emission from Centaurus A reveals particle acceleration in the
source to >TeV energies and, together with M 87, establishes radio galaxies as
a class of VHE emitters.Comment: 4 pages, 3 figures, submitted as proceeding to the 44th Recontre de
Moriond "Very High Energy Phenomena in the Universe", contains an updated SED
including the recent Fermi dat
Crystallographic and topological textures of biological materials and the resulting anisotropy of the mechanical properties
Proton G_E/G_M from beam-target asymmetry
The ratio of the proton's electric to magnetic form factor, G_E/G_M, can be
extracted in elastic electron-proton scattering by measuring either cross
sections, beam-target asymmetry or recoil polarization. Separate determinations
of G_E/G_M by cross sections and recoil polarization observables disagree for
Q^2 > 1 (GeV/c)^2. Measurement by a third technique might uncover an unknown
systematic error in either of the previous measurements. The beam-target
asymmetry has been measured for elastic electron-proton scattering at Q^2 =
1.51 (GeV/c)^2 for target spin orientation aligned perpendicular to the beam
momentum direction. This is the largest Q^2 at which G_E/G_M has been
determined by a beam-target asymmetry experiment. The result, \muG_E/G_M =
0.884 +/- 0.027 +/- 0.029, is compared to previous world data.Comment: 8 pages, 6 figures, Updated to be version published in Physical
Review
Target and beam-target spin asymmetries in exclusive pion electroproduction for Q2>1GeV2 . I. epâeÏ+n
Beam-target double-spin asymmetries and target single-spin asymmetries were measured for the exclusive
Ï
+
electroproduction reaction
Îł
â
p
â
n
Ï
+
. The results were obtained from scattering of 6-GeV longitudinally polarized electrons off longitudinally polarized protons using the CEBAF Large Acceptance Spectrometer at Jefferson Laboratory. The kinematic range covered is
1.1
<
W
<
3
GeV and
1
<
Q
2
<
6
GeV
2
. Results were obtained for about 6000 bins in
W
,
Â
Q
2
,
Â
cos
(
Ξ
â
)
, and
Ï
â
. Except at forward angles, very large target-spin asymmetries are observed over the entire
W
region. Reasonable agreement is found with phenomenological fits to previous data for
W
<
1.6
GeV, but very large differences are seen at higher values of
W
. A generalized parton distributions (GPD)-based model is in poor agreement with the data. When combined with cross-sectional measurements, the present results provide powerful constraints on nucleon resonance amplitudes at moderate and large values of
Q
2
, for resonances with masses as high as 2.4 GeV
Measurement of Exclusive Electroproduction Structure Functions and their Relationship to Transversity GPDs
Exclusive electroproduction at a beam energy of 5.75 GeV has been
measured with the Jefferson Lab CLAS spectrometer. Differential cross sections
were measured at more than 1800 kinematic values in , , , and
, in the range from 1.0 to 4.6 GeV,\ up to 2 GeV,
and from 0.1 to 0.58. Structure functions and were extracted as functions of for each of
17 combinations of and . The data were compared directly with two
handbag-based calculations including both longitudinal and transversity GPDs.
Inclusion of only longitudinal GPDs very strongly underestimates and fails to account for and ,
while inclusion of transversity GPDs brings the calculations into substantially
better agreement with the data. There is very strong sensitivity to the
relative contributions of nucleon helicity flip and helicity non-flip
processes. The results confirm that exclusive electroproduction offers
direct experimental access to the transversity GPDs.Comment: 6 pages, 2 figure
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