Land surface models (LSMs) use the atmospheric grid as their basic spatial decomposition because their main objective is to provide the lower boundary conditions to the atmosphere. Lateral water ...flows at the surface on the other hand require a much higher spatial discretization as they are closely linked to topographic details. We propose here a methodology to automatically tile the atmospheric grid into hydrological coherent units which are connected through a graph. As water is transported on sub-grids of the LSM, land variables can easily be transferred to the routing network and advected if needed. This is demonstrated here for temperature. The quality of the river networks generated, as represented by the connected hydrological transfer units, are compared to the original data in order to quantify the degradation introduced by the discretization method. The conditions the sub-grid elements impose on the time step of the water transport scheme are evaluated, and a methodology is proposed to find an optimal value. Finally the scheme is applied in an off-line version of the ORCHIDEE (Organising Carbon and Hydrology In Dynamic Ecosystems) LSM over Europe to show that realistic river discharge and temperatures are predicted over the major catchments of the region. The simulated solutions are largely independent of the atmospheric grid used thanks to the proposed sub-grid approach.
The blazar Mrk 501 (z = 0.034) was observed at very-high-energy (VHE, E greater than or similar to 100 GeV) gamma-ray wavelengths during a bright flare on the night of 2014 June 23-24 (MJD 56832) ...with the H.E.S.S. phase-II array of Cherenkov telescopes. Data taken that night by H.E.S.S. at large zenith angle reveal an exceptional number of gamma-ray photons at multi-TeV energies, with rapid flux variability and an energy coverage extending significantly up to 20 TeV. This data set is used to constrain Lorentz invariance violation (LIV) using two independent channels: a temporal approach considers the possibility of an energy dependence in the arrival time of gamma-rays, whereas a spectral approach considers the possibility of modifications to the interaction of VHE gamma-rays with extragalactic background light (EBL) photons. The non-detection of energy-dependent time delays and the non-observation of deviations between the measured spectrum and that of a supposed power-law intrinsic spectrum with standard EBL attenuation are used independently to derive strong constraints on the energy scale of LIV (E-QG) in the subluminal scenario for linear and quadratic perturbations in the dispersion relation of photons. For the case of linear perturbations, the 95% confidence level limits obtained are E-QG,E-1 > 3.6 x 10(17) GeV using the temporal approach and E-QG,E-1 > 2.6 x 10(19) GeV using the spectral approach. For the case of quadratic perturbations, the limits obtained are E-QG,E-2 > 8.5 x 10(10) GeV using the temporal approach and E-QG,E-2 > 7.8 x 10(11) GeV using the spectral approach.
Here we report the results of the first ever contemporaneous multi-wavelength observation campaign on the BL Lac object PKS 2155−304 involving Swift, NuSTAR, Fermi-LAT, and H.E.S.S. The use of these ...instruments allows us to cover a broad energy range, which is important for disentangling the different radiative mechanisms. The source, observed from June 2013 to October 2013, was found in a low flux state with respect to previous observations but exhibited highly significant flux variability in the X-rays. The high-energy end of the synchrotron spectrum can be traced up to 40 keV without significant contamination by high-energy emission. A one-zone synchrotron self-Compton model was used to reproduce the broadband flux of the source for all the observations presented here but failed for previous observations made in April 2013. A lepto-hadronic solution was then explored to explain these earlier observational results.
Magnetar hyperflares are the most plausible explanation for fast radio bursts (FRB) -- enigmatic powerful radio pulses with durations of several milliseconds and high brightness temperatures. The ...first observational evidence for this scenario was obtained in 2020 April when a FRB was detected from the direction of the Galactic magnetar and soft gamma-ray repeater SGR\,1935+2154. The FRB was preceded by two gamma-ray outburst alerts by the BAT instrument aboard the Swift satellite, which triggered follow-up observations by the High Energy Stereoscopic System (H.E.S.S.). H.E.S.S. has observed SGR\,1935+2154 for 2 hr on 2020 April 28. The observations are coincident with X-ray bursts from the magnetar detected by INTEGRAL and Fermi-GBM, thus providing the first very high energy (VHE) gamma-ray observations of a magnetar in a flaring state. High-quality data acquired during these follow-up observations allow us to perform a search for short-time transients. No significant signal at energies \(E>0.6\)~TeV is found and upper limits on the persistent and transient emission are derived. We here present the analysis of these observations and discuss the obtained results and prospects of the H.E.S.S. follow-up program for soft gamma-ray repeaters.
We analyzed 252 hours of High Energy Stereoscopic System (H.E.S.S.) observations towards the supernova remnant (SNR) LMC N132D that were accumulated between December 2004 and March 2016 during a deep ...survey of the Large Magellanic Cloud, adding 104 hours of observations to the previously published data set to ensure a > 5 sigma detection. To broaden the gamma-ray spectral coverage required for modeling the spectral energy distribution, an analysis of Fermi-LAT Pass 8 data was also included. We unambiguously detect N132D at very high energies (VHE) with a significance of 5.7 sigma. We report the results of a detailed analysis of its spectrum and localization based on the extended H.E.S.S. data set. The joint analysis of the extended H.E.S.S and Fermi-LAT data results in a spectral energy distribution in the energy range from 1.7 GeV to 14.8 TeV, which suggests a high luminosity of N132D at GeV and TeV energies. We set a lower limit on a gamma-ray cutoff energy of 8 TeV with a confidence level of 95%. The new gamma-ray spectrum as well as multiwavelength observations of N132D when compared to physical models suggests a hadronic origin of the VHE gamma-ray emission. SNR N132D is a VHE gamma-ray source that shows a spectrum extending to the VHE domain without a spectral cutoff at a few TeV, unlike the younger oxygen-rich SNR Cassiopeia A. The gamma-ray properties of N132D may be affected by an interaction with a nearby molecular cloud that partially lies inside the 95% confidence region of the source position. Abridged
The identification of PeVatrons, hadronic particle accelerators reaching the
knee of the cosmic ray spectrum (few $10^{15}$ eV), is crucial to understand
the origin of cosmic rays in the Galaxy. We ...provide an update on the
unidentified source HESS J1702-420, a promising PeVatron candidate. We present
new observations of HESS J1702-420 made with the High Energy Stereoscopic
System (H.E.S.S.), and processed using improved analysis techniques. The
analysis configuration was optimized to enhance the collection area at the
highest energies. We applied a three-dimensional (3D) likelihood analysis to
model the source region and adjust non thermal radiative spectral models to the
$\gamma$-ray data. We also analyzed archival data from the Fermi Large Area
Telescope (LAT) to constrain the source spectrum at $\gamma$-ray energies >10
GeV. We report the detection of a new source component called HESS J1702-420A,
that was separated from the bulk of TeV emission at a $5.4\sigma$ confidence
level. The power law $\gamma$-ray spectrum of HESS J1702-420A extends with an
index of $\Gamma=1.53\pm0.19_\text{stat}\pm0.20_\text{sys}$ and without
curvature up to the energy band 64-113 TeV, in which it was detected by
H.E.S.S. at a $4.0\sigma$ confidence level. This brings evidence for the source
emission up to $100\,\text{TeV}$, which makes HESS J1702-420A a compelling
candidate site for the presence of extremely high energy cosmic rays.
Remarkably, in a hadronic scenario, the cut-off energy of the proton
distribution powering HESS J1702-420A is found to be higher than 0.5 PeV at a
95% confidence level. HESS J1702-420A becomes therefore one of the most solid
PeVatron candidates detected so far in H.E.S.S. data, altough a leptonic origin
of its emission could not be ruled out either.
We search for an indirect signal of dark matter through very high-energy gamma rays from the Wolf-Lundmark-Melotte (WLM) dwarf irregular galaxy. The pair annihilation of dark matter particles would ...produce Standard Model particles in the final state such as gamma rays, which might be detected by ground-based Cherenkov telescopes. Dwarf irregular galaxies represent promising targets as they are dark matter dominated objects with well measured kinematics and small uncertainties on their dark matter distribution profiles. In 2018, the H.E.S.S. five-telescope array observed the dwarf irregular galaxy WLM for 18 hours. We present the first analysis based on data obtained from an imaging atmospheric Cherenkov telescope for this subclass of dwarf galaxy. As we do not observe any significant excess in the direction of WLM, we interpret the result in terms of constraints on the velocity-weighted cross section for dark matter pair annihilation as a function of the dark matter particle mass for various continuum channels as well as the prompt gamma-gamma emission. For the \(\tau^+\tau^-\) channel the limits reach a \(\langle \sigma v \rangle\) value of about \(4\times 10^{-22}\) cm3s-1 for a dark matter particle mass of 1 TeV. For the prompt gamma-gamma channel, the upper limit reaches a \(\langle \sigma v \rangle\) value of about \(5 \times10^{-24}\) cm3s-1 for a mass of 370 GeV. These limits represent an improvement of up to a factor 200 with respect to previous results for the dwarf irregular galaxies for TeV dark matter search.
The unidentified very-high-energy (VHE; E \(>\) 0.1 TeV) \(\gamma\)-ray source, HESS J1826\(-\)130, was discovered with the High Energy Stereoscopic System (HESS) in the Galactic plane. The analysis ...of 215 h of HESS data has revealed a steady \(\gamma\)-ray flux from HESS J1826\(-\)130, which appears extended with a half-width of 0.21\(^{\circ}\) \(\pm\) 0.02\(^{\circ}_{\text{stat}}\) \(\pm\) 0.05\(^{\circ}_{\text{sys}}\). The source spectrum is best fit with either a power-law function with a spectral index \(\Gamma\) = 1.78 \(\pm\) 0.10\(_{\text{stat}}\) \(\pm\) 0.20\(_{\text{sys}}\) and an exponential cut-off at 15.2\(^{+5.5}_{-3.2}\) TeV, or a broken power-law with \(\Gamma_{1}\) = 1.96 \(\pm\) 0.06\(_{\text{stat}}\) \(\pm\) 0.20\(_{\text{sys}}\), \(\Gamma_{2}\) = 3.59 \(\pm\) 0.69\(_{\text{stat}}\) \(\pm\) 0.20\(_{\text{sys}}\) for energies below and above \(E_{\rm{br}}\) = 11.2 \(\pm\) 2.7 TeV, respectively. The VHE flux from HESS J1826\(-\)130 is contaminated by the extended emission of the bright, nearby pulsar wind nebula (PWN), HESS J1825\(-\)137, particularly at the low end of the energy spectrum. Leptonic scenarios for the origin of HESS J1826\(-\)130 VHE emission related to PSR J1826\(-\)1256 are confronted by our spectral and morphological analysis. In a hadronic framework, taking into account the properties of dense gas regions surrounding HESS J1826\(-\)130, the source spectrum would imply an astrophysical object capable of accelerating the parent particle population up to \(\gtrsim\)200 TeV. Our results are also discussed in a multiwavelength context, accounting for both the presence of nearby supernova remnants (SNRs), molecular clouds, and counterparts detected in radio, X-rays, and TeV energies.
The nearby radio galaxy Centaurus A belongs to a class of Active Galaxies that are very luminous at radio wavelengths. The majority of these galaxies show collimated relativistic outflows known as ...jets, that extend over hundreds of thousands of parsecs for the most powerful sources. Accretion of matter onto the central super-massive black hole is believed to fuel these jets and power their emission, with the radio emission being related to the synchrotron radiation of relativistic electrons in magnetic fields. The origin of the extended X-ray emission seen in the kiloparsec-scale jets from these sources is still a matter of debate, although Cen A's X-ray emission has been suggested to originate in electron synchrotron processes. The other possible explanation is Inverse Compton (IC) scattering with CMB soft photons. Synchrotron radiation needs ultra-relativistic electrons (\(\sim50\) TeV), and given their short cooling times, requires some continuous re-acceleration mechanism to be active. IC scattering, on the other hand, does not require very energetic electrons, but requires jets that stay highly relativistic on large scales (\(\geq\)1 Mpc) and that remain well-aligned with the line of sight. Some recent evidence disfavours inverse Compton-CMB models, although other evidence seems to be compatible with them. In principle, the detection of extended gamma-ray emission, directly probing the presence of ultra-relativistic electrons, could distinguish between these options, but instruments have hitherto been unable to resolve the relevant structures. At GeV energies there is also an unusual spectral hardening in Cen A, whose explanation is unclear. Here we report observations of Cen A at TeV energies that resolve its large-scale jet. We interpret the data as evidence for the acceleration of ultra-relativistic electrons in the jet, and favour the synchrotron explanation for the X-rays.
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