Geminga is an enigmatic radio-quiet
γ
-ray pulsar located at a mere 250 pc distance from Earth. Extended very-high-energy
γ
-ray emission around the pulsar was discovered by Milagro and later ...confirmed by HAWC, which are both water Cherenkov detector-based experiments. However, evidence for the Geminga pulsar wind nebula in gamma rays has long evaded detection by imaging atmospheric Cherenkov telescopes (IACTs) despite targeted observations. The detection of
γ
-ray emission on angular scales ≳2º poses a considerable challenge for the background estimation in IACT data analysis. With recent developments in understanding the complementary background estimation techniques of water Cherenkov and atmospheric Cherenkov instruments, the H.E.S.S. IACT array can now confirm the detection of highly extended
γ
-ray emission around the Geminga pulsar with a radius of at least 3º in the energy range 0.5–40 TeV. We find no indications for statistically significant asymmetries or energy-dependent morphology. A flux normalisation of (2.8 ± 0.7) × 10
−12
cm
−2
s
−1
TeV
−1
at 1 TeV is obtained within a 1º radius region around the pulsar. To investigate the particle transport within the halo of energetic leptons around the pulsar, we fitted an electron diffusion model to the data. The normalisation of the diffusion coefficient obtained of
D
0
= 7.6
−1.2
+1.5
× 10
27
cm
2
s
−1
, at an electron energy of 100 TeV, is compatible with values previously reported for the pulsar halo around Geminga, which is considerably below the Galactic average.
Abstract
Magnetic fields in galaxies and galaxy clusters are believed to be the result of the amplification of intergalactic seed fields during the formation of large-scale structures in the ...universe. However, the origin, strength, and morphology of this intergalactic magnetic field (IGMF) remain unknown. Lower limits on (or indirect detection of) the IGMF can be obtained from observations of high-energy gamma rays from distant blazars. Gamma rays interact with the extragalactic background light to produce electron−positron pairs, which can subsequently initiate electromagnetic cascades. The gamma-ray signature of the cascade depends on the IGMF since it deflects the pairs. Here we report on a new search for this cascade emission using a combined data set from the Fermi Large Area Telescope and the High Energy Stereoscopic System. Using state-of-the-art Monte Carlo predictions for the cascade signal, our results place a lower limit on the IGMF of
B
> 7.1 × 10
−16
G for a coherence length of 1 Mpc even when blazar duty cycles as short as 10 yr are assumed. This improves on previous lower limits by a factor of 2. For longer duty cycles of 10
4
(10
7
) yr, IGMF strengths below 1.8 × 10
−14
G (3.9 × 10
−14
G) are excluded, which rules out specific models for IGMF generation in the early universe.
Most γ -ray detected active galactic nuclei are blazars with one of their relativistic jets pointing towards the Earth. Only a few objects belong to the class of radio galaxies or misaligned blazars. ...Here, we investigate the nature of the object PKS 0625−354, its γ -ray flux and spectral variability and its broad-band spectral emission with observations from H.E.S.S., Fermi -LAT, Swift -XRT, and UVOT taken in November 2018. The H.E.S.S. light curve above 200 GeV shows an outburst in the first night of observations followed by a declining flux with a halving time scale of 5.9 h. The γγ -opacity constrains the upper limit of the angle between the jet and the line of sight to ∼10°. The broad-band spectral energy distribution shows two humps and can be well fitted with a single-zone synchrotron self Compton emission model. We conclude that PKS 0625−354, as an object showing clear features of both blazars and radio galaxies, can be classified as an intermediate active galactic nuclei. Multi-wavelength studies of such intermediate objects exhibiting features of both blazars and radio galaxies are sparse but crucial for the understanding of the broad-band emission of γ -ray detected active galactic nuclei in general.
Abstract
We present the SEMLA (Signal Extraction using Machine
Learning for ALTO) analysis method, developed for the detection of
E>200 GeV γ rays in the context of the ALTO
wide-field-of-view ...atmospheric shower array R&D project. The
scientific focus of ALTO is extragalactic γ-ray astronomy, so
primarily the detection of soft-spectrum γ-ray sources such
as Active Galactic Nuclei and Gamma Ray Bursts. The current phase
of the ALTO R&D project is the optimization of sensitivity for such
sources and includes a number of ideas which are tested and
evaluated through the analysis of dedicated Monte Carlo simulations
and hardware testing. In this context, it is important to clarify
how data are analysed and how results are being obtained. SEMLA
takes advantage of machine learning and comprises four stages:
initial event cleaning (stage A), filtering out of poorly
reconstructed γ-ray events (stage B), followed by
γ-ray signal extraction from proton background events
(stage C) and finally reconstructing the energy of the events
(stage D). The performance achieved through SEMLA is evaluated in
terms of the angular, shower core position, and energy resolution,
together with the effective detection area, and background
suppression. Our methodology can be easily generalized to any
experiment, provided that the signal extraction variables for the
specific analysis project are considered.
Abstract
In 2021 July,
PKS 1510−089
exhibited a significant flux drop in the high-energy
γ
-ray (by a factor 10) and optical (by a factor 5) bands and remained in this low state throughout 2022. ...Similarly, the optical polarization in the source vanished, resulting in the optical spectrum being fully explained through the steady flux of the accretion disk and the broad-line region. Unlike the aforementioned bands, the very-high-energy
γ
-ray and X-ray fluxes did not exhibit a significant flux drop from year to year. This suggests that the steady-state very-high-energy
γ
-ray and X-ray fluxes originate from a different emission region than the vanished parts of the high-energy
γ
-ray and optical jet fluxes. The latter component has disappeared through either a swing of the jet away from the line of sight or a significant drop in the photon production efficiency of the jet close to the black hole. Either change could become visible in high-resolution radio images.
The origin of the gamma-ray emission from M 87 is currently a matter of debate. This work aims to localize the very high-energy (VHE; 100 GeV – 100 TeV) gamma-ray emission from M 87 and probe a ...potential extended hadronic emission component in the inner Virgo Cluster. The search for a steady and extended gamma-ray signal around M 87 can constrain the cosmic-ray energy density and the pressure exerted by the cosmic rays onto the intracluster medium and allow us to investigate the role of cosmic rays in the active galactic nucleus feedback as a heating mechanism in the Virgo Cluster. The High Energy Stereoscopic System (H.E.S.S.) telescopes are sensitive to VHE gamma rays and have been used to observe M 87 since 2004. We utilized a Bayesian block analysis to identify M 87 emission states with H.E.S.S. observations from 2004 to 2021, dividing them into low, intermediate, and high states. Because of the causality argument, an extended (≳1 kpc) signal is allowed only in steady emission states. Hence, we fitted the morphology of the 120 h low-state data and find no significant gamma-ray extension. Therefore, we derive for the low state an upper limit of 58″(corresponding to ≈4.6 kpc) in the extension of a single-component morphological model described by a rotationally symmetric 2D Gaussian model at the 99.7% confidence level. Our results exclude the radio lobes (≈30 kpc) as the principal component of the VHE gamma-ray emission from the low state of M 87. The gamma-ray emission is compatible with a single emission region at the radio core of M 87. These results, with the help of two multiple-component models, constrain the maximum cosmic-ray to thermal pressure ratio to
X
CR, max.
≲ 0.32 and the total energy in cosmic-ray protons to
U
CR
≲ 5 × 10
58
erg in the inner 20 kpc of the Virgo Cluster for an assumed cosmic-ray proton power-law distribution in momentum with spectral index
α
p
= 2.1.
Dwarf spheroidal galaxies are among the most promising targets for detecting signals of Dark Matter (DM) annihilations. The H.E.S.S. experiment has observed five of these systems for a total of about ...130 hours. The data are re-analyzed here, and, in the absence of any detected signals, are interpreted in terms of limits on the DM annihilation cross section. Two scenarios are considered: i) DM annihilation into mono-energetic gamma-rays and ii) DM in the form of pure WIMP multiplets that, annihilating into all electroweak bosons, produce a distinctive gamma-ray spectral shape with a high-energy peak at the DM mass and a lower-energy continuum. For case i), upper limits at 95% confidence level of about <sigma upsilon > less than or similar to 3 x 10(-25) cm(3) s(-1) are obtained in the mass range of 400 GeV to 1TeV. For case ii), the full spectral shape of the models is used and several excluded regions are identified, but the thermal masses of the candidates are not robustly ruled out.
Abstract
Primordial Black Holes (PBHs) are hypothetical black holes predicted to have been formed from density fluctuations in the early Universe.
PBHs with an initial mass around 10
14
–10
15
g are ...expected to end their evaporation at present times in a burst of particles and very-high-energy (VHE) gamma rays.
Those gamma rays may be detectable by the High Energy Stereoscopic System (H.E.S.S.), an array of imaging atmospheric Cherenkov telescopes.
This paper reports on the search for evaporation bursts of VHE gamma rays with H.E.S.S., ranging from 10 to 120 seconds, as expected from the final stage of PBH evaporation and using a total of 4816 hours of observations.
The most constraining upper limit on the burst rate of local PBHs is
2000 pc
-3
yr
-1
for a burst interval of 120 seconds, at the 95% confidence level. The implication of these measurements for PBH dark matter are also discussed.
In the context of atmospheric shower arrays designed for γ-ray astronomy and in the context of the ALTO project, we present: a study of the impact of heavier nuclei in the cosmic-ray background on ...the estimated γ-ray detection performance on the basis of dedicated Monte Carlo simulations, a method to calculate the sensitivity to a point-like source, and finally the required observation times to reach a firm detection on a list of known point-like sources.
Context.
Runaway stars form bow shocks by ploughing through the interstellar medium at supersonic speeds and are promising sources of non-thermal emission of photons. One of these objects has been ...found to emit non-thermal radiation in the radio band. This triggered the development of theoretical models predicting non-thermal photons from radio up to very-high-energy (VHE,
E
≥ 0.1 TeV) gamma rays. Subsequently, one bow shock was also detected in X-ray observations. However, the data did not allow discrimination between a hot thermal and a non-thermal origin. Further observations of different candidates at X-ray energies showed no evidence for emission at the position of the bow shocks either. A systematic search in the
Fermi
-LAT energy regime resulted in flux upper limits for 27 candidates listed in the E-BOSS catalogue.
Aim.
Here we perform the first systematic search for VHE gamma-ray emission from bow shocks of runaway stars.
Methods.
Using all available archival H.E.S.S. data we search for very-high-energy gamma-ray emission at the positions of bow shock candidates listed in the second E-BOSS catalogue release. Out of the 73 bow shock candidates in this catalogue, 32 have been observed with H.E.S.S.
Results.
None of the observed 32 bow shock candidates in this population study show significant emission in the H.E.S.S. energy range. Therefore, flux upper limits are calculated in five energy bins and the fraction of the kinetic wind power that is converted into VHE gamma rays is constrained.
Conclusions.
Emission from stellar bow shocks is not detected in the energy range between 0.14 and 18 TeV.The resulting upper limits constrain the level of VHE gamma-ray emission from these objects down to 0.1–1% of the kinetic wind energy.
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