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Aharonian, F., Ait Benkhali, F., Arcaro, C., Aschersleben, J., Backes, M., Barbosa, V. M., . . . Zywucka, N. (2023). Constraining the cosmic-ray pressure in the inner Virgo Cluster using HESS observations of M 87. Astronomy and Astrophysics, 675, Article ID A138.
Open this publication in new window or tab >>Constraining the cosmic-ray pressure in the inner Virgo Cluster using HESS observations of M 87
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2023 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 675, article id A138Article in journal (Refereed) Published
Abstract [en]

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 XCR, max. ≲ 0.32 and the total energy in cosmic-ray protons to UCR  ≲  5  ×  1058 erg in the inner 20 kpc of the Virgo Cluster for an assumed cosmic-ray proton power-law distribution in momentum with spectral index αp = 2.1

Place, publisher, year, edition, pages
EDP Sciences, 2023
Keywords
astroparticle physics, gamma rays, galaxies, clusters, intracluster medium, radio continuum
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-123893 (URN)10.1051/0004-6361/202346056 (DOI)001033560900005 ()2-s2.0-85165532261 (Scopus ID)
Available from: 2023-08-25 Created: 2023-08-25 Last updated: 2023-09-13Bibliographically approved
Aharonian, F., Aschersleben, J., Backes, M., Martins, V. B., Batzofin, R., Becherini, Y., . . . Zywucka, N. (2023). Constraints on the Intergalactic Magnetic Field Using Fermi-LAT and HESS Blazar Observations. Astrophysical Journal Letters, 950(2), Article ID L16.
Open this publication in new window or tab >>Constraints on the Intergalactic Magnetic Field Using Fermi-LAT and HESS Blazar Observations
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2023 (English)In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 950, no 2, article id L16Article in journal (Refereed) Published
Abstract [en]

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 x 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 x 10(-14) G (3.9 x 10(-14) G) are excluded, which rules out specific models for IGMF generation in the early universe.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2023
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-124085 (URN)10.3847/2041-8213/acd777 (DOI)001021638900001 ()2-s2.0-85164482030 (Scopus ID)
Available from: 2023-09-05 Created: 2023-09-05 Last updated: 2023-10-11Bibliographically approved
Aharonian, F., Benkhali, F. A., Aschersleben, J., Ashkar, H., Backes, M., Martins, V. B., . . . Zywucka, N. (2023). Detection of extended gamma-ray emission around the Geminga pulsar with HESS. Astronomy and Astrophysics, 673, Article ID A148.
Open this publication in new window or tab >>Detection of extended gamma-ray emission around the Geminga pulsar with HESS
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2023 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 673, article id A148Article in journal (Refereed) Published
Abstract [en]

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 greater than or similar to 2 degrees 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 degrees 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) x 10-12 cm-2 s-1 TeV-1 at 1 TeV is obtained within a 1 degrees 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 x 1027 cm2 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.

Place, publisher, year, edition, pages
EDP Sciences, 2023
Keywords
gamma rays: general, acceleration of particles, pulsars: general, diffusion
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-123665 (URN)10.1051/0004-6361/202245776 (DOI)001008870900001 ()2-s2.0-85161023158 (Scopus ID)
Available from: 2023-08-14 Created: 2023-08-14 Last updated: 2023-09-13Bibliographically approved
Aharonian, F., Benkhali, F. A., Aschersleben, J., Ashkar., H., Backes., M., Baktash, A., . . . Zywucka, N. (2023). HESS Follow-up Observations of GRB 221009A. Astrophysical Journal Letters, 946(1), Article ID L27.
Open this publication in new window or tab >>HESS Follow-up Observations of GRB 221009A
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2023 (English)In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 946, no 1, article id L27Article in journal (Refereed) Published
Abstract [en]

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 f(UL)(95%)=9.7x10(-12)ergcm(-2)s(-1) E-thr = 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.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2023
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-122888 (URN)10.3847/2041-8213/acc405 (DOI)000982947300001 ()2-s2.0-85151509935 (Scopus ID)
Available from: 2023-06-28 Created: 2023-06-28 Last updated: 2023-08-18Bibliographically approved
Aharonian, F., Ait Benkhali, F., Aschersleben, J., Ashkar, H., Backes, M., Barbosa Martins, V., . . . Zywucka, N. (2023). HESS J1809-193: A halo of escaped electrons around a pulsar wind nebula?. Astronomy and Astrophysics, 672, Article ID A103.
Open this publication in new window or tab >>HESS J1809-193: A halo of escaped electrons around a pulsar wind nebula?
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2023 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 672, article id A103Article in journal (Refereed) Published
Abstract [en]

Context. HESS J1809-193 is an unassociated very-high-energy gamma-ray source located on the Galactic plane. While it has been connected to the nebula of the energetic pulsar PSR J1809-1917, supernova remnants and molecular clouds present in the vicinity also constitute possible associations. Recently, the detection of gamma-ray emission up to energies of similar to 100 TeV with the HAWC observatory has led to renewed interest in HESS J1809-193.Aims. We aim to understand the origin of the gamma-ray emission of HESS J1809-193.Methods. We analysed 93.2 h of data taken on HESS J1809-193 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 J1809-193. 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 (modelled as an elongated Gaussian with a 1-sigma semi-major and semi-minor axis of similar to 0.62 degrees and similar to 0.35 degrees, respectively) that exhibits a spectral cutoff at similar to 13 TeV, and a compact component (modelled as a symmetric Gaussian with a 1-sigma radius of similar to 0.1 degrees) that is located close to PSR J1809-1917 and shows no clear spectral cutoff. The Fermi-LAT analysis also revealed extended gamma-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.

Place, publisher, year, edition, pages
EDP Sciences, 2023
Keywords
acceleration of particles, radiation mechanisms, non-thermal, pulsars, individual, PSR J1809-1917, gamma rays, general
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-120926 (URN)10.1051/0004-6361/202245459 (DOI)000964835500007 ()2-s2.0-85153369428 (Scopus ID)
Available from: 2023-05-26 Created: 2023-05-26 Last updated: 2023-07-03Bibliographically approved
Acharyya, A., Adams, C. B., Archer, A., Bangale, P., Bartkoske, J. T., Batista, P., . . . Mori, K. (2023). Multiwavelength Observations of the Blazar PKS 0735+178 in Spatial and Temporal Coincidence with an Astrophysical Neutrino Candidate IceCube-211208A. Astrophysical Journal, 954(1), Article ID 70.
Open this publication in new window or tab >>Multiwavelength Observations of the Blazar PKS 0735+178 in Spatial and Temporal Coincidence with an Astrophysical Neutrino Candidate IceCube-211208A
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2023 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 954, no 1, article id 70Article in journal (Refereed) Published
Abstract [en]

We report on multiwavelength target-of-opportunity observations of the blazar PKS 0735+178, located 2 degrees .2 away from the best -fit position of the IceCube neutrino event IceCube-211208A detected on 2021 December 8. The source was in a high -flux state in the optical, ultraviolet, X-ray, and GeV ?-ray bands around the time of the neutrino event, exhibiting daily variability in the soft X-ray flux. The X-ray data from Swift-XRT and NuSTAR characterize the transition between the low-energy and high-energy components of the broadband spectral energy distribution (SED), and the ?-ray data from Fermi-LAT, VERITAS, and H.E.S.S. require a spectral cutoff near 100 GeV. Both the X-ray and ?-ray measurements provide strong constraints on the leptonic and hadronic models. We analytically explore a synchrotron self-Compton model, an external Compton model, and a lepto-hadronic model. Models that are entirely based on internal photon fields face serious difficulties in matching the observed SED. The existence of an external photon field in the source would instead explain the observed ?-ray spectral cutoff in both the leptonic and lepto-hadronic models and allow a proton jet power that marginally agrees with the Eddington limit in the lepto-hadronic model. We show a numerical lepto-hadronic model with external target photons that reproduces the observed SED and is reasonably consistent with the neutrino event despite requiring a high jet power.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2023
Keywords
Active galactic nuclei, Blazars, Neutrino astronomy, Gamma-ray astronomy
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-126681 (URN)10.3847/1538-4357/ace327 (DOI)001056647600001 ()2-s2.0-85174215580 (Scopus ID)
Available from: 2024-01-16 Created: 2024-01-16 Last updated: 2024-02-05Bibliographically approved
Aharonian, F., Benkhali, F. A., Aschersleben, J., Boettcher, M., Backes, M., Martins, V. B., . . . Zywucka, N. (2023). Search for the evaporation of primordial black holes with HESS. Journal of Cosmology and Astroparticle Physics (4), Article ID 040.
Open this publication in new window or tab >>Search for the evaporation of primordial black holes with HESS
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2023 (English)In: Journal of Cosmology and Astroparticle Physics, E-ISSN 1475-7516, no 4, article id 040Article in journal (Refereed) Published
Abstract [en]

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 1014-1015 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.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2023
Keywords
gamma ray experiments, primordial black holes, dark matter experiments
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-123664 (URN)10.1088/1475-7516/2023/04/040 (DOI)001004513900001 ()2-s2.0-85153538419 (Scopus ID)
Available from: 2023-08-14 Created: 2023-08-14 Last updated: 2023-09-13Bibliographically approved
Punch, M., Senniappan, M., Becherini, Y., Kukec Mezek, G., Thoudam, S., Bylund, T. & Ernenwein, J.-P. -. (2023). Sensitivity to point-like sources of the ALTO atmospheric particle detector array, designed for 200GeV-50TeV gamma-ray astronomy. Journal of High Energy Astrophysics, 39, 1-13
Open this publication in new window or tab >>Sensitivity to point-like sources of the ALTO atmospheric particle detector array, designed for 200GeV-50TeV gamma-ray astronomy
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2023 (English)In: Journal of High Energy Astrophysics, ISSN 2214-4048, E-ISSN 2214-4056, Vol. 39, p. 1-13Article in journal (Refereed) Published
Abstract [en]

In the context of atmospheric shower arrays designed for gamma-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 gamma-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.

Place, publisher, year, edition, pages
Elsevier, 2023
Keywords
Very high-energy gamma-rays, gamma-ray experiments, Extensive air showers, Particle detector arrays, ALTO project, Analysis and statistical methods
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-123655 (URN)10.1016/j.jheap.2023.03.003 (DOI)001022243300001 ()2-s2.0-85161678012 (Scopus ID)
Available from: 2023-08-14 Created: 2023-08-14 Last updated: 2023-09-13Bibliographically approved
Aharonian, F., Benkhali, F. A., Aschersleben, J., Ashkar, H., Backes, M., Martins, V. B., . . . Groenewald, D. (2023). The Vanishing of the Primary Emission Region in PKS 1510-089. Astrophysical Journal Letters, 952(2), Article ID L38.
Open this publication in new window or tab >>The Vanishing of the Primary Emission Region in PKS 1510-089
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2023 (English)In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 952, no 2, article id L38Article in journal (Refereed) Published
Abstract [en]

In 2021 July, PKS 1510-089 exhibited a significant flux drop in the high-energy & gamma;-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 & gamma;-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 & gamma;-ray and X-ray fluxes originate from a different emission region than the vanished parts of the high-energy & gamma;-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.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2023
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-125445 (URN)10.3847/2041-8213/ace3c0 (DOI)001041614800001 ()2-s2.0-85167703002 (Scopus ID)
Available from: 2023-11-06 Created: 2023-11-06 Last updated: 2023-11-30Bibliographically approved
Aharonian, F., Ashkar, H., Backes, M., Martins, V. B., Becherini, Y., Berge, D., . . . Wong, G. (2022). A deep spectromorphological study of the ϒ-ray emission surrounding the young massive stellar cluster Westerlund 1. Astronomy and Astrophysics, 666, Article ID A124.
Open this publication in new window or tab >>A deep spectromorphological study of the ϒ-ray emission surrounding the young massive stellar cluster Westerlund 1
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2022 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 666, article id A124Article in journal (Refereed) Published
Abstract [en]

Context. Young massive stellar clusters are extreme environments and potentially provide the means for efficient particle acceleration. Indeed, they are increasingly considered as being responsible for a significant fraction of cosmic rays (CRs) that are accelerated within the Milky Way. Westerlund 1, the most massive known young stellar cluster in our Galaxy, is a prime candidate for studying this hypothesis. While the very-high-energy gamma-ray source HESS J1646-458 has been detected in the vicinity of Westerlund 1 in the past, its association could not be firmly identified. Aims. We aim to identify the physical processes responsible for the gamma-ray emission around Westerlund 1 and thus to understand the role of massive stellar clusters in the acceleration of Galactic CRs better. Methods. Using 164 h of data recorded with the High Energy Stereoscopic System (H.E.S.S.), we carried out a deep spectromorphological study of the gamma-ray emission of HESS J1646-458. We furthermore employed H I and CO observations of the region to infer the presence of gas that could serve as target material for interactions of accelerated CRs. Results. We detected large-scale (similar to 2 degrees diameter) gamma-ray emission with a complex morphology, exhibiting a shell-like structure and showing no significant variation with gamma-ray energy. The combined energy spectrum of the emission extends to several tens of TeV, and it is uniform across the entire source region. We did not find a clear correlation of the gamma-ray emission with gas clouds as identified through H I and CO observations. Conclusions. We conclude that, of the known objects within the region, only Westerlund 1 can explain the majority of the gamma-ray emission. Several CR acceleration sites and mechanisms are conceivable and discussed in detail. While it seems clear that Westerlund 1 acts as a powerful particle accelerator, no firm conclusions on the contribution of massive stellar clusters to the flux of Galactic CRs in general can be drawn at this point.

Place, publisher, year, edition, pages
EPD Sciences, 2022
Keywords
acceleration of particles, radiation mechanisms, non-thermal, shock waves, stars, massive, gamma rays, general, galaxies, star clusters, individual, Westerlund 1
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-117501 (URN)10.1051/0004-6361/202244323 (DOI)000868825800001 ()2-s2.0-85141010861 (Scopus ID)
Available from: 2022-11-15 Created: 2022-11-15 Last updated: 2022-12-13Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-2115-2930

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