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Subdued lighting The diffuse light permeating intergalactic space contains precious information about the early Universe. Exactly what information is a matter of conjecture: light from the first stars, perhaps, or direct starlight from later galaxies? Gamma-ray observations of two distant active galaxies with HESS, the High Energy Stereoscopic System array in Namibia, reveal that the diffuse light intensity is lower than expected from recent claims. This suggests that the Universe is more transparent to [gamma]-rays than was thought, and favours galaxies as the dominant light source, rather than early stars. The diffuse extragalactic background light consists of the sum of the starlight emitted by galaxies through the history of the Universe, and it could also have an important contribution from the 'first stars', which may have formed before galaxy formation began. Direct measurements are difficult and not yet conclusive, owing to the large uncertainties caused by the bright foreground emission associated with zodiacal light.sup.1. An alternative approach.sup.2,3,4,5 is to study the absorption features imprinted on the [gamma]-ray spectra of distant extragalactic objects by interactions of those photons with the background light photons.sup.6. Here we report the discovery of [gamma]-ray emission from the blazars.sup.7 H 2356 - 309 and 1ES 1101 - 232, at redshifts z = 0.165 and z = 0.186, respectively. Their unexpectedly hard spectra provide an upper limit on the background light at optical/near-infrared wavelengths that appears to be very close to the lower limit given by the integrated light of resolved galaxies.sup.8. The background flux at these wavelengths accordingly seems to be strongly dominated by the direct starlight from galaxies, thus excluding a large contribution from other sources--in particular from the first stars formed.sup.9. This result also indicates that intergalactic space is more transparent to [gamma]-rays than previously thought.

Galactic cosmic rays reach energies of at least a few petaelectronvolts1 (of the order of 1015 electronvolts). This implies that our Galaxy contains petaelectronvolt accelerators (‘PeVatrons’), but all proposed models of Galactic cosmic-ray accelerators encounter difficulties at exactly these energies2. Dozens of Galactic accelerators capable of accelerating particles to energies of tens of teraelectronvolts (of the order of 1013 electronvolts) were inferred from recent γ-ray observations3. However, none of the currently known accelerators—not even the handful of shell-type supernova remnants commonly believed to supply most Galactic cosmic rays—has shown the characteristic tracers of petaelectronvolt particles, namely, power-law spectra of γ-rays extending without a cut-off or a spectral break to tens of teraelectronvolts4. Here we report deep γ-ray observations with arcminute angular resolution of the region surrounding the Galactic Centre, which show the expected tracer of the presence of petaelectronvolt protons within the central 10 parsecs of the Galaxy. We propose that the supermassive black hole Sagittarius A* is linked to this PeVatron. Sagittarius A* went through active phases in the past, as demonstrated by X-ray outbursts5and an outflow from the Galactic Centre6. Although its current rate of particle acceleration is not sufficient to provide a substantial contribution to Galactic cosmic rays, Sagittarius A* could have plausibly been more active over the last 106–107 years, and therefore should be considered as a viable alternative to supernova remnants as a source of petaelectronvolt Galactic cosmic rays.

A significant fraction of the energy density of the interstellar medium is in the form of high-energy charged particles (cosmic rays) 1 . The origin of these particles remains uncertain. Although it is generally accepted that the only sources capable of supplying the energy required to accelerate the bulk of Galactic cosmic rays are supernova explosions, and even though the mechanism of particle acceleration in expanding supernova remnant (SNR) shocks is thought to be well understood theoretically 2 , 3 , unequivocal evidence for the production of high-energy particles in supernova shells has proven remarkably hard to find. Here we report on observations of the SNR RX J1713.7 - 3946 (G347.3 - 0.5), which was discovered by ROSAT 4 in the X-ray spectrum and later claimed as a source of high-energy γ-rays 5 , 6 of TeV energies (1 TeV = 10 12  eV). We present a TeV γ-ray image of the SNR: the spatially resolved remnant has a shell morphology similar to that seen in X-rays, which demonstrates that very-high-energy particles are accelerated there. The energy spectrum indicates efficient acceleration of charged particles to energies beyond 100 TeV, consistent with current ideas of particle acceleration in young SNR shocks.

Very high energy γ-rays probe the long-standing mystery of the origin of cosmic rays. Produced in the interactions of accelerated particles in astrophysical objects, they can be used to image cosmic particle accelerators. A first sensitive survey of the inner part of the Milky Way with the High Energy Stereoscopic System (HESS) reveals a population of eight previously unknown firmly detected sources of very high energy γ-rays. At least two have no known radio or x-ray counterpart and may be representative of a new class of \"dark\" nucleonic cosmic ray sources.

Subdued lighting The diffuse light permeating intergalactic space contains precious information about the early Universe. Exactly what information is a matter of conjecture: light from the first stars, perhaps, or direct starlight from later galaxies? Gamma-ray observations of two distant active galaxies with HESS, the High Energy Stereoscopic System array in Namibia, reveal that the diffuse light intensity is lower than expected from recent claims. This suggests that the Universe is more transparent to γ-rays than was thought, and favours galaxies as the dominant light source, rather than early stars. The diffuse extragalactic background light consists of the sum of the starlight emitted by galaxies through the history of the Universe, and it could also have an important contribution from the ‘first stars’, which may have formed before galaxy formation began. Direct measurements are difficult and not yet conclusive, owing to the large uncertainties caused by the bright foreground emission associated with zodiacal light 1 . An alternative approach 2 , 3 , 4 , 5 is to study the absorption features imprinted on the γ-ray spectra of distant extragalactic objects by interactions of those photons with the background light photons 6 . Here we report the discovery of γ-ray emission from the blazars 7 H 2356 - 309 and 1ES 1101 - 232, at redshifts z = 0.165 and z = 0.186, respectively. Their unexpectedly hard spectra provide an upper limit on the background light at optical/near-infrared wavelengths that appears to be very close to the lower limit given by the integrated light of resolved galaxies 8 . The background flux at these wavelengths accordingly seems to be strongly dominated by the direct starlight from galaxies, thus excluding a large contribution from other sources—in particular from the first stars formed 9 . This result also indicates that intergalactic space is more transparent to γ-rays than previously thought.

Central issues Events at the centre of our Galaxy are key to our understanding of high-energy processes in the Universe, since it contains examples of virtually every type of exotic object known to astronomers. The very-high-energy γ-ray emission from the Galactic Centre region has now been measured using HESS, the High Energy Stereoscopic System recently constructed in Namibia, South West Africa. HESS operates at energies above the regime accessible to satellite-based detectors, taking γ-ray astronomy into new territory. The results show that these clouds are glowing in very high energy γ-rays. The glow is caused by constant bombardment of the clouds by cosmic rays — probably protons and nuclei — produced close to the central black hole or in the expanding blast waves of supernova explosions. The source of Galactic cosmic rays (with energies up to 10 15  eV) remains unclear, although it is widely believed that they originate in the shock waves of expanding supernova remnants 1 , 2 . At present the best way to investigate their acceleration and propagation is by observing the γ-rays produced when cosmic rays interact with interstellar gas 3 . Here we report observations of an extended region of very-high-energy (> 10 11  eV) γ-ray emission correlated spatially with a complex of giant molecular clouds in the central 200 parsecs of the Milky Way. The hardness of the γ-ray spectrum and the conditions in those molecular clouds indicate that the cosmic rays giving rise to the γ-rays are likely to be protons and nuclei rather than electrons. The energy associated with the cosmic rays could have come from a single supernova explosion around 10 4 years ago.

The extragalactic background light (EBL) is the diffuse radiation with the second highest energy density in the Universe after the cosmic microwave background. The aim of this study is the measurement of the imprint of the EBL opacity to gamma-rays on the spectra of the brightest extragalactic sources detected with the High Energy Stereoscopic System (H.E.S.S.). The originality of the method lies in the joint fit of the EBL optical depth and of the intrinsic spectra of the sources, assuming intrinsic smoothness. Analysis of a total of ~10^5 gamma-ray events enables the detection of an EBL signature at the 8.8 std dev level and constitutes the first measurement of the EBL optical depth using very-high energy (E>100 GeV) gamma-rays. The EBL flux density is constrained over almost two decades of wavelengths (0.30-17 microns) and the peak value at 1.4 micron is derived as 15 +/- 2 (stat) +/- 3 (sys) nW / m^2 sr.

The accretion of matter onto a massive black hole is believed to feed the relativistic plasma jets found in many active galactic nuclei (AGN). Although some AGN accelerate particles to energies exceeding 1012 electron volts and are bright sources of very-high-energy (VHE) γ-ray emission, it is not yet known where the VHE emission originates. Here we report on radio and VHE observations of the radio galaxy Messier 87, revealing a period of extremely strong VHE γ-ray flares accompanied by a strong increase of the radio flux from its nucleus. These results imply that charged particles are accelerated to very high energies in the immediate vicinity of the black hole. [PUBLICATION ABSTRACT]

Vela X is a region of extended radio emission in the western part of the Vela constellation: one of the nearest pulsar wind nebulae (PWNe), and associated with the energetic Vela pulsar (PSR B0833-45). Extended very-high-energy (VHE) \\(\\gamma\\)-ray emission (HESS \\(\\mathrm{J0835\\mhyphen 455}\\)) was discovered using the H.E.S.S. experiment in 2004. The VHE \\(\\gamma\\)-ray emission was found to be coincident with a region of X-ray emission discovered with \\({\\it ROSAT}\\) above 1.5 keV (the so-called \\textit{Vela X cocoon}): a filamentary structure extending southwest from the pulsar to the centre of Vela X. A deeper observation of the entire Vela X nebula region, also including larger offsets from the cocoon, has been performed with H.E.S.S. 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. In order 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 analysis is performed with the On/Off background method, which estimates the background from separate observations pointing away from Vela X; towards regions free of \\(\\gamma\\)-ray sources but with comparable observation conditions. The \\(\\gamma\\)-ray surface brightness over the large Vela X region reveals that the detection of non-thermal VHE \\(\\gamma\\)-ray emission from the PWN HESS \\(\\mathrm{J0835\\mhyphen 455}\\) is statistically significant over a region of radius 1.2\\(^{\\circ}\\) around the position \\(\\alpha\\) = 08\\(^{\\mathrm{h}}\\) 35\\(^{\\mathrm{m}}\\) 00\\(^{\\mathrm{s}}\\), \\(\\delta\\) = -45\\(^{\\circ}\\) 36\\(^{\\mathrm{\\prime}}\\) 00\\(^{\\mathrm{\\prime}\\mathrm{\\prime}}\\) (J2000).

In some galaxy clusters powerful 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. For such a scenario copious gamma-ray emission is expected as a tracer of cosmic rays from these systems. 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 hours 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. These are the first limits on gamma-ray emission ever presented for galaxy clusters hosting cluster scale AGN outbursts. 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. For the case of bubbles mainly supported by hadronic cosmic rays, the most favorable scenario, that involves full mixing between cosmic rays and embedding medium, can be excluded. However, hadronic cosmic rays still remain a viable pressure support agent to sustain the bubbles against the thermal pressure of the ambient medium. The largest population of highly-energetic electrons which are relevant for inverse-Compton gamma-ray production is found in the youngest inner lobes of Hydra A. The limit on the inverse-Compton gamma-ray flux excludes a magnetic field below half of the equipartition value of 16 muG in the inner lobes.