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
Cosmological \(N\)-body simulations show that Milky Way-sized galaxies harbor a population of unmerged dark matter subhalos. These subhalos could shine in gamma-rays and be eventually detected in ...gamma-ray surveys as unidentified sources. We performed a thorough selection among unidentified Fermi-LAT Objects (UFOs) to identify them as possible TeV-scale dark matter subhalo candidates. We search for very-high-energy (E \(\gtrsim\) 100 GeV) gamma-ray emissions using H.E.S.S. observations towards four selected UFOs. Since no significant very-high-energy gamma-ray emission is detected in any dataset of the four observed UFOs nor in the combined UFO dataset, strong constraints are derived on the product of the velocity-weighted annihilation cross section \(\langle \sigma v \rangle\) by the \(J\)-factor for the dark matter models. The 95% C.L. observed upper limits derived from combined H.E.S.S. observations reach \(\langle \sigma v \rangle J\) values of 3.7\(\times\)10\(^{-5}\) and 8.1\(\times\)10\(^{-6}\) GeV\(^2\)cm\(^{-2}\)s\(^{-1}\) in the \(W^+W^-\) and \(\tau^+\tau^-\) channels, respectively, for a 1 TeV dark matter mass. Focusing on thermal WIMPs, the H.E.S.S. constraints restrict the \(J\)-factors to lie in the range 6.1\(\times\)10\(^{19}\) - 2.0\(\times\)10\(^{21}\) GeV\(^2\)cm\(^{-5}\), and the masses to lie between 0.2 and 6 TeV in the \(W^+W^-\) channel. For the \(\tau^+\tau^-\) channel, the \(J\)-factors lie in the range 7.0\(\times\)10\(^{19}\) - 7.1\(\times\)10\(^{20}\) GeV\(^2\)cm\(^{-5}\) and the masses lie between 0.2 and 0.5 TeV. Assuming model-dependent predictions from cosmological N-body simulations on the \(J\)-factor distribution for Milky Way-sized galaxies, the dark matter models with masses greater than 0.3 TeV for the UFO emissions can be ruled out at high confidence level.
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.
The KM3NeT research infrastructure is under construction in the Mediterranean Sea. It consists of two water Cherenkov neutrino detectors, ARCA and ORCA, aimed at neutrino astrophysics and oscillation ...research, respectively. Instrumenting a large volume of sea water with \(\sim\) 6,200 optical modules comprising a total of \(\sim\) 200,000 photomultiplier tubes, KM3NeT will achieve sensitivity to \(\sim\) 10 MeV neutrinos from Galactic and near-Galactic core-collapse supernovae through the observation of coincident hits in photomultipliers above the background. In this paper, the sensitivity of KM3NeT to a supernova explosion is estimated from detailed analyses of background data from the first KM3NeT detection units and simulations of the neutrino signal. The KM3NeT observational horizon (for a \(5\,\sigma\) discovery) covers essentially the Milky-Way and for the most optimistic model, extends to the Small Magellanic Cloud (\(\sim\) 60 kpc). Detailed studies of the time profile of the neutrino signal allow assessment of the KM3NeT capability to determine the arrival time of the neutrino burst with a few milliseconds precision for sources up to 5\(-\)8 kpc away, and detecting the peculiar signature of the standing accretion shock instability if the core-collapse supernova explosion happens closer than 3\(-\)5 kpc, depending on the progenitor mass. KM3NeT's capability to measure the neutrino flux spectral parameters is also presented.
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.
PMN J1603\(-\)4904 is a likely member of the rare class of \(\gamma\)-ray emitting young radio galaxies. Only one other source, PKS 1718\(-\)649, has been confirmed so far. These objects, which may ...transition into larger radio galaxies, are a stepping stone to understanding AGN evolution. It is not completely clear how these young galaxies, seen edge-on, can produce high-energy \(\gamma\)-rays. PMN J1603\(-\)4904 has been detected by TANAMI Very Long Baseline Interferometry (VLBI) observations and has been followed-up with multiwavelength observations. A Fermi/LAT \(\gamma\)-ray source has been associated with it in the LAT catalogs. We have obtained Chandra observations of the source in order to consider the possibility of source confusion, due to the relatively large positional uncertainty of Fermi/LAT. The goal was to investigate the possibility of other X-ray bright sources in the vicinity of PMN J1603\(-\)4904 that could be counterparts to the \(\gamma\)-ray emission. With Chandra/ACIS, we find no other sources in the uncertainty ellipse of Fermi/LAT data, which includes an improved localization analysis of 8 years of data. We further study the X-ray fluxes and spectra. We conclude that PMN J1603\(-\)4904 is indeed the second confirmed \(\gamma\)-ray bright young radio galaxy.
Dwarf spheroidal galaxy satellites of the Milky Way are prime targets for indirect detection of dark matter with gamma rays due to their proximity, high dark matter content and absence of non-thermal ...emission processes. Recently, the Dark Energy Survey (DES) revealed the existence of new ultra-faint dwarf spheroidal galaxies in the southern-hemisphere sky, therefore ideally located for ground-based observations with the imaging atmospheric Cherenkov telescope array H.E.S.S. We present a search for very-high-energy (\(E\gtrsim100\) GeV) gamma-ray emission using H.E.S.S. observations carried out recently towards Reticulum II, Tucana II, Tucana III, Tucana IV and Grus II satellites. No significant very-high-energy gamma-ray excess is found from the observations on any individual object nor in the combined analysis of all the datasets. Using the most recent modeling of the dark matter distribution in the dwarf galaxy halo, we compute for the first time on DES satellites individual and combined constraints from Cherenkov telescope observations on the annihilation cross section of dark matter particles in the form of Weakly Interacting Massive Particles. The combined 95% C.L. observed upper limits reach \(\langle \sigma v \rangle \simeq 1 \times 10^{-23}\) cm\(^3\)s\(^{-1}\) in the \(W^+W^-\) channel and \(4 \times 10^{-26}\) cm\(^3\)s\(^{-1}\) in the \(\gamma\gamma\) channels for a dark matter mass of 1.5 TeV. The H.E.S.S. constraints well complement the results from Fermi-LAT, HAWC, MAGIC and VERITAS and are currently the most stringent in the \(\gamma\gamma\) channels in the multi-GeV/multi-TeV mass range.
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.
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