Evidence for the presence of quasi-periodic oscillations (QPOs) in the optical polarization of the blazar PKS 2155–304, during a period of enhanced gamma-ray brightness, is presented. The periodogram ...of the polarized flux revealed the existence of a prominent peak at T ∼ 13 min, detected at >99.7 per cent significance, and T ∼ 30 min, which was nominally significant at >99 per cent. This is the first evidence of QPOs in the polarization of an active galactic nucleus, potentially opening up a new avenue of studying this phenomenon.
The inner region of the Milky Way halo harbors a large amount of dark matter (DM). Given its proximity, it is one of the most promising targets to look for DM. We report on a search for the ...annihilations of DM particles using γ-ray observations towards the inner 300 pc of the Milky Way, with the H.E.S.S. array of ground-based Cherenkov telescopes. The analysis is based on a 2D maximum likelihood method using Galactic Center (GC) data accumulated by H.E.S.S. over the last 10 years (2004-2014), and does not show any significant γ-ray signal above background. Assuming Einasto and Navarro-Frenk-White DM density profiles at the GC, we derive upper limits on the annihilation cross section ⟨σv⟩. These constraints are the strongest obtained so far in the TeV DM mass range and improve upon previous limits by a factor 5. For the Einasto profile, the constraints reach ⟨σv⟩ values of 6×10^{-26} cm^{3} s^{-1} in the W^{+}W^{-} channel for a DM particle mass of 1.5 TeV, and 2×10^{-26} cm^{3} s^{-1} in the τ^{+}τ^{-} channel for a 1 TeV mass. For the first time, ground-based γ-ray observations have reached sufficient sensitivity to probe ⟨σv⟩ values expected from the thermal relic density for TeV DM particles.
Spectral lines are among the most powerful signatures for dark matter (DM) annihilation searches in very-high-energy γ rays. The central region of the Milky Way halo is one of the most promising ...targets given its large amount of DM and proximity to Earth. We report on a search for a monoenergetic spectral line from self-annihilations of DM particles in the energy range from 300 GeV to 70 TeV using a two-dimensional maximum likelihood method taking advantage of both the spectral and spatial features of the signal versus background. The analysis makes use of Galactic center observations accumulated over ten years (2004-2014) with the H.E.S.S. array of ground-based Cherenkov telescopes. No significant γ-ray excess above the background is found. We derive upper limits on the annihilation cross section ⟨σv⟩ for monoenergetic DM lines at the level of 4×10^{-28} cm^{3} s^{-1} at 1 TeV, assuming an Einasto DM profile for the Milky Way halo. For a DM mass of 1 TeV, they improve over the previous ones by a factor of 6. The present constraints are the strongest obtained so far for DM particles in the mass range 300 GeV-70 TeV. Ground-based γ-ray observations have reached sufficient sensitivity to explore relevant velocity-averaged cross sections for DM annihilation into two γ-ray photons at the level expected from the thermal relic density for TeV DM particles.
An annihilation signal of dark matter is searched for from the central region of the Milky Way. Data acquired in dedicated on-off observations of the Galactic center region with H.E.S.S. are analyzed ...for this purpose. No significant signal is found in a total of ∼9 h of on-off observations. Upper limits on the velocity averaged cross section, ⟨σv⟩, for the annihilation of dark matter particles with masses in the range of ∼300 GeV to ∼10 TeV are derived. In contrast to previous constraints derived from observations of the Galactic center region, the constraints that are derived here apply also under the assumption of a central core of constant dark matter density around the center of the Galaxy. Values of ⟨σv⟩ that are larger than 3×10^{-24} cm^{3}/s are excluded for dark matter particles with masses between ∼1 and ∼4 TeV at 95% C.L. if the radius of the central dark matter density core does not exceed 500 pc. This is the strongest constraint that is derived on ⟨σv⟩ for annihilating TeV mass dark matter without the assumption of a centrally cusped dark matter density distribution in the search region.
Vela X is a region of extended radio emission in the western part of the Vela constellation: one of the nearest pulsar wind nebulae, and associated with the energetic Vela pulsar. A deeper ...observation of the entire Vela X nebula region, also including larger offsets from the cocoon, has been performed with HESS. This re-observation was carried out in order to probe the extent of the non-thermal emission from the Vela X region at TeV energies and to investigate its spectral properties. To increase the sensitivity to the faint gamma -ray emission from the very extended Vela X region, a multivariate analysis method combining three complementary reconstruction techniques of Cherenkov-shower images is applied for the selection of gamma -ray events. The Vela X region exhibits almost uniform gamma -ray spectra over its full extent: the differential energy spectrum can be described by a power-law function with a hard spectral index ... and an exponential cutoff at an energy of TeV.(ProQuest: ... denotes formulae/symbols omitted.)
The previously unidentified very high-energy (VHE) gamma -ray source HESS J1303-631 is re-examined including new data from the HESS Cherenkov telescope array in order to identify this object. ...Detailed morphological and spectral studies of VHE gamma -ray emission as well as of the XMM-Newton X-ray data are performed. Significant energy-dependent morphology of this source, as well as the identification of an associated X-ray PWN from XMM-Newton observations enable identification of the VHE source as an evolved PWN associated to the pulsar PSR J1301-6305. This identification is supported by the one zone leptonic model, which suggests that the energetics of the gamma -ray and X-ray radiation are such that they may have a similar origin in the pulsar nebula. This would explain the low level of synchrotron radiation and the difficulty in detecting counterparts at lower energies, the reason this source was originally classified as a "dark' VHE gamma -ray source.
A deep observation campaign carried out by the High Energy Stereoscopic System (HESS) on Centaurus A enabled the discovery of ...-rays from the blazar 1ES 1312...423, 2... away from the radio galaxy. ...With a differential flux at 1 TeV of ...(1 TeV) = (1.9 ± 0.6... ± 0.4...) x 10... cm... s... TeV... corresponding to 0.5 per cent of the Crab nebula differential flux and a spectral index ... = 2.9 ± 0.5... ± 0.2..., 1ES 1312 423 is one of the faintest sources ever detected in the very high energy (E > 100 GeV) extragalactic sky. A careful analysis using three and a half years of Fermi Large Area Telescope (Fermi-LAT) data allows the discovery at high energies (E > 100 MeV) of a hard spectrum (... = 1.4 ± 0.4... ± 0.2...) source coincident with 1ES 1312...423. Radio, optical, UV and X-ray observations complete the spectral energy distribution of this blazar, now covering 16 decades in energy. The emission is successfully fitted with a synchrotron self-Compton model for the non-thermal component, combined with a blackbody spectrum for the optical emission from the host galaxy. (ProQuest: ... denotes formulae/symbols omitted.)
In some galaxy clusters, powerful active galactic nuclei (AGN) have blown bubbles with cluster scale extent into the ambient medium. The main pressure support of these bubbles is not known to date, ...but cosmic rays are a viable possibility. Hydra A, the closest galaxy cluster hosting a cluster scale AGN outburst, located at a redshift of 0.0538, is investigated for being a gamma-ray emitter with the High Energy Stereoscopic System (H.E.S.S.) array and the Fermi Large Area Telescope (Fermi-LAT). Data obtained in 20.2 h of dedicated H.E.S.S. observations and 38 months of Fermi-LAT data, gathered by its usual all-sky scanning mode, have been analyzed to search for a gamma-ray signal. No signal has been found in either data set. Upper limits on the gamma-ray flux are derived and are compared to models. The non-detection of Hydra A in gamma-rays has important implications on the particle populations and physical conditions inside the bubbles in this system.
1RXS J101015.9-311909 is a galaxy located at a redshift of z = 0.14 hosting an active nucleus belonging to the class of bright BL Lac objects. The authors report on results from VHE observations ...performed between 2006 and 2010 with the H.E.S.S. instrument, an array of four imaging atmospheric Cherenkov telescopes. H.E.S.S. data have been analysed with enhanced analysis methods, making the detection of faint sources more significant. VHE emission at a position coincident with 1RXS J101015.9-311909 is detected with H.E.S.S. for the first time. In a total good-quality livetime of about 49 h, the authors measure 263 excess counts, corresponding to a significance of 7.1 standard deviations. The photon spectrum above 0.2 TeV can be described by a power-law with a photon index of ... . The integral flux above 0.2 TeV is about 0.8% of the flux of the Crab nebula and shows no significant variability over the time reported. (ProQuest: ... denotes formulae/symbols omitted.)
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