56 research outputs found
The Vanishing of the Primary Emission Region in PKS 1510-089
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
H.E.S.S. Follow-up Observations of GRB 221009A
GRB 221009A is the brightest gamma-ray burst (GRB) ever detected. To probe the very-high-energy (VHE; >100 GeV) emission, the High Energy Stereoscopic System (H.E.S.S.) began observations 53 hr after the triggering event, when the brightness of the moonlight no longer precluded observations. We derive differential and integral upper limits using H.E.S.S. data from the third, fourth, and ninth nights after the initial GRB detection, after applying atmospheric corrections. The combined observations yield an integral energy flux upper limit of FUL = 9.7 ´ 10- erg cm- s- 95% 12 2 1 above Ethr = 650 GeV. The constraints derived from the H.E.S.S. observations complement the available multiwavelength data. The radio to X-ray data are consistent with synchrotron emission from a single electron population, with the peak in the spectral energy distribution occurring above the X-ray band. Compared to the VHE-bright GRB 190829A, the upper limits for GRB 221009A imply a smaller gamma-ray to X-ray flux ratio in the afterglow. Even in the absence of a detection, the H.E.S.S. upper limits thus contribute to the multiwavelength picture of GRB 221009A, effectively ruling out an IC-dominated scenario.F. Aharonian ... S. Einecke ... G. Rowell ... et al. (H.E.S.S. Collaboration
The Vanishing of the Primary Emission Region in PKS 1510–089
Published 2023 August 1In 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.F. Aharonian ... S. Einecke ... G. Rowell ... et al. (H.E.S.S. Collaboration
Constraints on the intergalactic magnetic field using Fermi-LAT and H.E.S.S. blazar observations
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 -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 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 () yr, IGMF strengths below G
( G) are excluded, which rules out specific models for IGMF
generation in the early universe.Comment: 20 pages, 7 figures, 4 tables. Accepted for publication in ApJ
Letters. Auxiliary data is provided in electronic format at
https://zenodo.org/record/801431
HESS J1809193: a halo of escaped electrons around a pulsar wind nebula?
Context. HESS J1809193 is an unassociated very-high-energy -ray
source located on the Galactic plane. While it has been connected to the nebula
of the energetic pulsar PSR J18091917, supernova remnants and molecular
clouds present in the vicinity also constitute possible associations. Recently,
the detection of -ray emission up to energies of 100 TeV with the
HAWC observatory has led to renewed interest in HESS J1809193.
Aims. We aim to understand the origin of the -ray emission of HESS
J1809193.
Methods. We analysed 93.2 h of data taken on HESS J1809193 above 0.27 TeV
with the High Energy Stereoscopic System (H.E.S.S.), using a multi-component,
three-dimensional likelihood analysis. In addition, we provide a new analysis
of 12.5 yr of Fermi-LAT data above 1 GeV within the region of HESS J1809193.
The obtained results are interpreted in a time-dependent modelling framework.
Results. For the first time, we were able to resolve the emission detected
with H.E.S.S. into two components: an extended component that exhibits a
spectral cut-off at 13 TeV, and a compact component that is located close
to PSR J18091917 and shows no clear spectral cut-off. The Fermi-LAT analysis
also revealed extended -ray emission, on scales similar to that of the
extended H.E.S.S. component.
Conclusions. Our modelling indicates that based on its spectrum and spatial
extent, the extended H.E.S.S. component is likely caused by inverse Compton
emission from old electrons that form a halo around the pulsar wind nebula. The
compact component could be connected to either the pulsar wind nebula or the
supernova remnant and molecular clouds. Due to its comparatively steep
spectrum, modelling the Fermi-LAT emission together with the H.E.S.S.
components is not straightforward. (abridged)Comment: 14 pages, 10 figures. Accepted for publication in A&A. Corresponding
authors: Vikas Joshi, Lars Mohrman
Detection of extended gamma-ray emission around the Geminga pulsar with H.E.S.S
Geminga is an enigmatic radio-quiet gamma-ray pulsar located at a mere 250 pc
distance from Earth. Extended very-high-energy gamma-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 gamma-ray emission on angular scales > 2 deg 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 gamma-ray
emission around the Geminga pulsar with a radius of at least 3 deg in the
energy range 0.5-40 TeV. We find no indications for statistically significant
asymmetries or energy-dependent morphology. A flux normalisation of
cmsTeV at 1 TeV is obtained
within a 1 deg 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
cms, 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.Comment: 16 pages, 15 figures, 7 tables. Accepted for publication in Astronomy
& Astrophysic
H.E.S.S. follow-up observations of GRB221009A
GRB221009A is the brightest gamma-ray burst ever detected. To probe the
very-high-energy (VHE, \!100 GeV) emission, the High Energy Stereoscopic
System (H.E.S.S.) began observations 53 hours after the triggering event, when
the brightness of the moonlight no longer precluded observations. We derive
differential and integral upper limits using H.E.S.S. data from the third,
fourth, and ninth nights after the initial GRB detection, after applying
atmospheric corrections. The combined observations yield an integral energy
flux upper limit of above GeV. The
constraints derived from the H.E.S.S. observations complement the available
multiwavelength data. The radio to X-ray data are consistent with synchrotron
emission from a single electron population, with the peak in the SED occurring
above the X-ray band. Compared to the VHE-bright GRB190829A, the upper limits
for GRB221009A imply a smaller gamma-ray to X-ray flux ratio in the afterglow.
Even in the absence of a detection, the H.E.S.S. upper limits thus contribute
to the multiwavelength picture of GRB221009A, effectively ruling out an IC
dominated scenario.Comment: 10 pages, 4 figures. Accepted for publication in APJL. Corresponding
authors: J. Damascene Mbarubucyeye, H. Ashkar, S. J. Zhu, B. Reville, F.
Sch\"ussle
Optimal fuzzy control to reduce energy consumption in distillation columns
Distillation columns constitute a signi cant fraction of the capital invested in the re neries around the world; their control requires a major part of the total operating cost of chemical processes, if the used strategy is not adequate. This article presents the application of optimal fuzzy control to reduce the energy consumption of a Benzene-Toluene distillation column. This method is based on the determination of the speci c values of the fuzzy controller parameters such that certain performance criterion is minimised. Results of a simulation study are presented showing the potential improvement o ered by this metho
Non–interacting fuzzy control system design for distillation columns
This paper proposes a fuzzy multiloop control design for a distillation column. The interaction that occurs between the strategic distillation column variables, which represent a major constraint to construct the rules base when the fuzzy multivariable control of the distillation control is considered, are reduced by introducing a compensator in cascade with the distillation column. The introduced compensator is designed using an interaction analysis method. Thus, a complete fuzzy rule base is generated for each loop independently without domain experts. The main advantage of the proposed fuzzy multiloop strategy is that the determination of the rule base is simplified and facilitated; besides a weak level of interaction and the control stability are guaranteed. Consequently, in comparison with the conventional multiloop control based on classical PI controllers, the fuzzy multiloop control achieves better control performanc
Constraints on the Intergalactic Magnetic Field Using Fermi-LAT and H.E.S.S. Blazar Observations
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 10⁴ (10⁷) 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.F. Aharonian ... G. Rowell ... et al. (H.E.S.S. Collaboration, and M. Meyer Fermi-LAT Collaboration
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