24 research outputs found
Constraints on axionlike particles with H.E.S.S. from the irregularity of the PKS 2155-304 energy spectrum
Axionlike particles (ALPs) are hypothetical light (sub-eV) bosons predicted in some extensions of the Standard Model of particle physics. In astrophysical environments comprising high-energy gamma rays and turbulent magnetic fields, the existence of ALPs can modify the energy spectrum of the gamma rays for a sufficiently large coupling between ALPs and photons. This modification would take the form of an irregular behavior of the energy spectrum in a limited energy range. Data from the H.E.S.S. observations of the distant BL Lac object PKS 2155-304 (z=0.116) are used to derive upper limits at the 95% C.L. on the strength of the ALP coupling to photons, ggammaa<2.1×10-11GeV-1 for an ALP mass between 15 and 60 neV. The results depend on assumptions on the magnetic field around the source, which are chosen conservatively. The derived constraints apply to both light pseudoscalar and scalar bosons that couple to the electromagnetic fieldFil: Medina, Maria Clementina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico La Plata. Instituto Argentino de Radioastronomia (i); ArgentinaFil: H.E.S. S. collaboration
Improving water use in crop production
Globally, agriculture accounts for 80–90% of all freshwater used by humans, and most of that is in crop production. In many areas, this water use is unsustainable; water supplies are also under pressure from other users and are being affected by climate change. Much effort is being made to reduce water use by crops and produce ‘more crop per drop’. This paper examines water use by crops, taking particularly a physiological viewpoint, examining the underlying relationships between carbon uptake, growth and water loss. Key examples of recent progress in both assessing and improving crop water productivity are described. It is clear that improvements in both agronomic and physiological understanding have led to recent increases in water productivity in some crops. We believe that there is substantial potential for further improvements owing to the progress in understanding the physiological responses of plants to water supply, and there is considerable promise within the latest molecular genetic approaches, if linked to the appropriate environmental physiology. We conclude that the interactions between plant and environment require a team approach looking across the disciplines from genes to plants to crops in their particular environments to deliver improved water productivity and contribute to sustainability
Paleogeography of the South Atlantic: a Route for Primates and Rodents into the New World?
Fluorescence explorer (FLEX): An optimised payload to map vegetation photosynthesis from space
Fluorescence explorer (FLEX): An optimised payload to map vegetation photosynthesis from space
Discovery of VHE -ray emission and multi-wavelength observations of the BL Lacertae object 1RXS J101015.9-311909
1RXS J101015.9-311909 is a galaxy located at a redshift of z=0.14 hosting an
active nucleus belonging to the class of bright BL Lac objects. Observations at
high (HE, E > 100 MeV) and very high (VHE, E > 100 GeV) energies provide
insights into the origin of very energetic particles present in such sources
and the radiation processes at work. We report on results from VHE observations
performed between 2006-10 with H.E.S.S. H.E.S.S. data have been analysed with
enhanced analysis methods, making the detection of faint sources more
significant. VHE emission at a position coincident with 1RXS J101015.9-311909
is detected with H.E.S.S. for the first time. In a total good-quality livetime
of about 49 h, we measure 263 excess counts, corresponding to a significance of
7.1\sigma. The photon spectrum above 0.2 TeV can be described by a power-law
with a photon index of \Gamma\ = 3.08\pm0.42_{stat}\pm0.20_{sys}. The integral
flux above 0.2 TeV is about 0.8% of the flux of the Crab nebula and shows no
significant variability over the time reported. In addition, public Fermi/LAT
data are analysed to search for high energy emission from the source. The
Fermi/LAT HE emission is significant at 8.3\sigma\ in the chosen 25-month
dataset. UV and X-ray contemporaneous observations with the Swift satellite in
May 2007 are also reported, together with optical observations performed with
the ATOM telescope located at the H.E.S.S. site. Swift observations reveal an
absorbed X-ray flux of F_{0.3-7 keV} = 1.04^{+0.04}_{-0.05} \times 10^{-11}
erg.cm^{-2}.s^{-1} in the 0.3-7 keV range. Finally, all the available data are
used to study the source's multi-wavelength properties. The SED can be
reproduced using a simple one-zone SSC model with emission from a region with a
Doppler factor of 30 and a magnetic field between 0.025 and 0.16 G. These
parameters are similar to those obtained for other sources of this type.Comment: 10 pages, 7 figures, accepted for publication in Astronomy &
Astrophysic
Spectral Analysis and Interpretation of the -Ray Emission from the Starburst Galaxy NGC 253
Very-high-energy (VHE; E >100 GeV) and high-energy (HE; 100 MeV < E < 100
GeV) data from \gamma-ray observations performed with the H.E.S.S. telescope
array and the Fermi-LAT instrument, respectively, are analysed in order to
investigate the non-thermal processes in the starburst galaxy NGC 253. The VHE
\gamma-ray data can be described by a power law in energy with differential
photon index \Gamma=2.14 \pm 0.18_stat \pm 0.30_sys and differential flux
normalisation at 1 TeV of F_0 = (9.6 \pm 1.5_stat (+5.7,-2.9)_sys) x 10^{-14}
TeV^{-1} cm^{-2} s^{-1}. A power-law fit to the differential HE \gamma-ray
spectrum reveals a photon index of \Gamma=2.24 \pm 0.14_stat \pm 0.03_sys and
an integral flux between 200 MeV and 200 GeV of F(0.2-200 GeV) = (4.9 \pm
1.0_stat \pm 0.3_sys) x 10^{-9} cm^{-2} s^{-1}. No evidence for a spectral
break or turnover is found over the dynamic range of both the LAT instrument
and the H.E.S.S. experiment: a combined fit of a power law to the HE and VHE
\gamma-ray data results in a differential photon index \Gamma=2.34 \pm 0.03
with a p-value of 30%. The \gamma-ray observations indicate that at least about
20% of the energy of the cosmic rays capable of producing hadronic interactions
is channeled into pion production. The smooth alignment between the spectra in
the HE and VHE \gamma-ray domain suggests that the same transport processes
dominate in the entire energy range. Advection is most likely responsible for
charged particle removal from the starburst nucleus from GeV to multiple TeV
energies. In a hadronic scenario for the \gamma-ray production, the single
overall power-law spectrum observed would therefore correspond to the mean
energy spectrum produced by the ensemble of cosmic-ray sources in the starburst
region.Comment: paper accepted for publication in ApJ, 12 pages, 3 figures, 5 table