H.E.S.S. observations of PKS 2155-304

Aharonian, F.; Akhperjanian, A. G.; Aye, K.-M.; Bazer‐Bachi, A. R.; Beilicke, M.; Benbow, W.; Berge, D.; Berghaus, P.; Bernlöhr, K.; Bolz, O.; Boisson, C.; Borgmeier, C.; Breitling, F.; Brown, A. M.; Gordo, J. Bussons; Chadwick, P. M.; Chitnis, V. R.; Chounet, L.‐M.; Cornils, R.; Costamante, L.; Degrange, B.; Djannati‐Ataï, A.; Drury, L. O’c.; Ergin, T.; Espigat, P.; Feinstein, F.; Fleury, P.; Fontaine, G.; Funk, S.; Gallant, Y. A.; Giebels, B.; Gillessen, S.; Goret, P.; Guy, J.; Hadjichristidis, C.; Hauser, M.; Heinzelmann, G.; Henri, G.; Hermann, G.; Hinton, J. A.; Hofmann, W.; Holleran, M.; Horns, D.; Jager, O. C. de; Jung, I.; Khélifi, B.; Komin, Nu.; Konopelko, A.; Latham, I. J.; Gallou, R. Le; Lemoine, Martin; Lemière, A.; Leroy, N.; Löhse, T.; Marcowith, A.; Masterson, C.; McComb, T. J. L.; Naurois, M. de; Nolan, S. J.; Noutsos, A.; Orford, K. J.; Osborne, J. L.; Ouchrif, M.; Panter, M.; Pelletier, G.; Pita, S.; Pohl, M.; Pühlhofer, G.; Punch, M.; Raubenheimer, Britt; Raue, M.; Raux, J.; Rayner, S. M.; Redondo, I.; Reimer, A.; Ripken, J.; Rivoal, M.; Rob, L.; Rolland, L.; Rowell, G.; Sahakian, V.; Saugé, L.; Schlenker, S.; Schlickeiser, R.; Schuster, C.; Schwanke, U.; Siewert, M.; Sol, H.; Steenkamp, R.; Stegmann, C.; Tavernet, J.‐P.; Théoret, C. G.; Tluczykont, M.; Walt, D. J. van der; Vasileiadis, G.; Vincent, P.; Visser, Bart; Völk, H. J.; Wagner, S. J.

doi:10.1051/0004-6361:20041853

Cited by 224 publications

(275 citation statements)

References 23 publications

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“…Both cases allow the EBL to have a wider range of possible fluxes without yielding implausible values in the reconstructed blazar spectrum. This is the reason why the TeV spectra of the two previous most distant objects, 1ES 1426+428 [11] (z=0.129, Γ obs ≃ 3.5 ± 0.4) and PKS 2155−304 [10] (z=0.116, Γ obs ≃ 3.3 − 3.4 ± 0.06), did not provide such strong constraints as these two new sources, even when measured with much higher signal-to-noise ratio (PKS 2155−304). The intrinsic spectra of H 1426+428 and PKS 2155−304 start to become very hard (Γ < 1) for EBL fluxes around P1.0+E NIR and above.…”

Section: Comparison With Earlier Observationsmentioning

confidence: 99%

See 1 more Smart Citation

A low level of extragalactic background light as revealed by γ-rays from blazars

Aharonian¹,

Akhperjanian²,

Bazer‐Bachi³

et al. 2006

Nature

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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 and1 not yet conclusive, owing to the large uncertainties caused by the bright foreground emission associated with zodiacal light 1 . An alternative approach 2-5 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.The observations were carried out with the High Energy Stereoscopic System 10 (H.E.S.S. ), a system of four imaging atmospheric Cherenkov telescopes operating at energies E ≥ 0.1 TeV. These two blazars are at present the most distant sources for which spectra have been measured at these energies (Tab. 1).Intergalactic absorption is caused by the process of photon-photon collision and pair production. The original spectrum emitted by the source (which we call "intrinsic") is modified such that the observed flux E) , where the optical depth τ (E) depends on the Spectral Energy Distribution (SED) of the Extragalactic Background Light (EBL) (Fig. 1). Details are provided in the Supplementary Notes and Figures. For any reasonable range of fluxes at ultraviolet (UV) and optical/near-infrared wavelengths (O-NIR), τ (E) -and thus absorption -is larger at 1 TeV with respect to 0.2 TeV. This difference makes the observed spectrum steeper (that is, Γ obs > Γ int , for a power-law model dN/dE ∝ E −Γ ) The spectral change ∆Γ=Γ obs − Γ int scales linearly with the EBL normalization, and becomes more pronounced at larger redshifts. Thus more distant objects provide a more sensitive diagnostic tool.In general, if the intrinsic spectrum were sufficiently well known, τ (E) -and thus the EBL SEDcould be effectively measured by comparing intrinsic with observed spectra. Blazars, however, are characterized by a wide range of possible spectra, and the present understanding of their radiation processes is not yet complete enough to reliably predict their intrinsic γ-ray spectra. But for these two sources, with O-NIR fluxes at the level of the "direct" estimates, the intrinsic spectra needed to reproduce the H.E.S.S. data become extremely...

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Section: Comparison With Earlier Observationsmentioning

confidence: 99%

“…table gives the total exposure after selection for good quality data, significance of the detected γ-ray signal, energy range used for the spectral fits and the result of a single power-law fit (dN/dE = N 0 (E/TeV) −Γ obs ). The spectra have been calculated applying the technique described in ref 10 .…”

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confidence: 99%

A low level of extragalactic background light as revealed by γ-rays from blazars

Aharonian¹,

Akhperjanian²,

Bazer‐Bachi³

et al. 2006

Nature

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537

615

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“…After calibration of the raw shower images from PMT signals [20], two independent reconstruction techniques were combined to select γ-ray events and reconstruct their direction and energy. The first one uses the Hillas moment method [21]. The second analysis referred hereafter as "Model Analysis", is based on the pixel-per-pixel comparison of the shower image with a template generated by a semi-analytical shower development model.…”

Section: Datasetmentioning

confidence: 99%

“…The separation between γ candidates and hadrons is done using a combination of the Model goodness-of-fit parameter [23] and the Hillas mean scaled width and length parameters, which results in an improved background rejection [19]. Standard cuts on the width and the length of Hillas ellipses combined with the goodness-of-fit are used to suppress the hadronic background [21]. An additional cut on the primary interaction depth is used to improve background rejection.…”

Section: Datasetmentioning

confidence: 99%

Erratum to “Observations of the Sagittarius dwarf galaxy by the HESS experiment and search for a dark matter signal” [Astropart. Phys. 29(1) (2008) 55–62]

Aharonian¹,

Akhperjanian²,

Bazer‐Bachi³

et al. 2010

Astroparticle Physics

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Observations of the Sagittarius dwarf spheroidal (Sgr dSph) galaxy were carried out with the H.E.S.S. array of four imaging air Cherenkov telescopes in June 2006. A total of 11 hours of high quality data are available after data selection. There is no evidence for a very high energy γ-ray signal above the energy threshold at the target position. A 95% C.L. flux limit of 3.6 × 10 −12 cm −2 s −1 above 250 GeV has been derived. Constraints on the velocity-weighted cross section σv are calculated in the framework of Dark Matter particle annihilation using realistic models for the Dark Matter halo profile of Sagittarius dwarf galaxy. Two different models have been investigated encompassing a large class of halo types. A 95% C.L. exclusion limit on σv of the order of 2 × 10 −25 cm 3 s −1 is obtained for a core profile in the 100 GeV -1 TeV neutralino mass range.

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“…The first detection with the High Energy Stereoscopic System (H.E.S.S.) of the source was made during the July and October 2002 observations (Aharonian et al 2005a). The first multiwavelength (MWL) campaign targeting the blazar and organized by the HESS Collaboration was conducted in 2003 (Aharonian et al 2005b).…”

Section: Introductionmentioning

confidence: 99%

Long-term monitoring of PKS 2155−304 with ATOM and H.E.S.S.: investigation of optical/γ-ray correlations in different spectral states

Abramowski¹,

Aharonian²,

Benkhali³

et al. 2014

A&A

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In this paper we report on the analysis of all the available optical and very high-energy γ-ray (>200 GeV) data for the BL Lac object PKS 2155−304, collected simultaneously with the ATOM and H.E.S.S. telescopes from 2007 until 2009. This study also includes X-ray (RXTE, Swift) and highenergy γ-ray (Fermi-LAT) data. During the period analysed, the source was transitioning from its flaring to quiescent optical states, and was characterized by only moderate flux changes at different wavelengths on the timescales of days and months. A flattening of the optical continuum with an increasing optical flux can be noted in the collected dataset, but only occasionally and only at higher flux levels. We did not find any universal relation between the very high-energy γ-ray and optical flux changes on the timescales from days and weeks up to several years. On the other hand, we noted that at higher flux levels the source can follow two distinct tracks in the optical flux-colour diagrams, which seem to be related to distinct γ-ray states of the blazar. The obtained results therefore indicate a complex scaling between the optical and γ-ray emission of PKS 2155−304, with different correlation patterns holding at different epochs, and a γ-ray flux depending on the combination of an optical flux and colour rather than a flux alone.

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H.E.S.S. observations of PKS 2155-304

Cited by 224 publications

References 23 publications

A low level of extragalactic background light as revealed by γ-rays from blazars

A low level of extragalactic background light as revealed by γ-rays from blazars

Erratum to “Observations of the Sagittarius dwarf galaxy by the HESS experiment and search for a dark matter signal” [Astropart. Phys. 29(1) (2008) 55–62]

Long-term monitoring of PKS 2155−304 with ATOM and H.E.S.S.: investigation of optical/γ-ray correlations in different spectral states

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