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Issue A&A
Volume 473, Number 3, October III 2007
Page(s) L25 - L28
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20078412



A&A 473, L25-L28 (2007)
DOI: 10.1051/0004-6361:20078412

Letter

Discovery of VHE $\gamma$-rays from the distant BL Lacertae 1ES 0347-121

F. Aharonian1, 0, A. G. Akhperjanian0, U. Barres de Almeida0, A. R. Bazer-Bachi0, B. Behera0, M. Beilicke0, W. Benbow1, K. Bernlöhr1, 0, C. Boisson0, O. Bolz1, V. Borrel0, I. Braun1, E. Brion0, A. M. Brown0, R. Bühler1, T. Bulik0, I. Büsching0, T. Boutelier0, S. Carrigan1, P. M. Chadwick0, L.-M. Chounet0, A. C. Clapson1, G. Coignet0, R. Cornils0, L. Costamante1, 0, M. Dalton0, B. Degrange0, H. J. Dickinson0, A. Djannati-Ataï0, W. Domainko1, L. O'C. Drury0, F. Dubois0, G. Dubus0, J. Dyks0, K. Egberts1, D. Emmanoulopoulos0, P. Espigat0, C. Farnier0, F. Feinstein0, A. Fiasson0, A. Förster1, G. Fontaine0, Seb. Funk0, M. Füßling0, Y. A. Gallant0, B. Giebels0, J. F. Glicenstein0, B. Glück0, P. Goret0, C. Hadjichristidis0, D. Hauser1, M. Hauser0, G. Heinzelmann0, G. Henri0, G. Hermann1, J. A. Hinton0, A. Hoffmann0, W. Hofmann1, M. Holleran0, S. Hoppe1, D. Horns0, A. Jacholkowska0, O. C. de Jager0, I. Jung0, K. Katarzynski0, E. Kendziorra0, M. Kerschhaggl0, B. Khélifi0, D. Keogh0, Nu. Komin0, K. Kosack1, G. Lamanna0, I. J. Latham0, A. Lemière0, M. Lemoine-Goumard0, J.-P. Lenain0, T. Lohse0, J. M. Martin0, O. Martineau-Huynh0, A. Marcowith0, C. Masterson0, D. Maurin0, G. Maurin0, T. J. L. McComb0, R. Moderski0, E. Moulin0, M. de Naurois0, D. Nedbal0, S. J. Nolan0, S. Ohm1, J.-P. Olive0, E. de Oña Wilhelmi0, K. J. Orford0, J. L. Osborne0, M. Ostrowski0, M. Panter1, G. Pedaletti0, G. Pelletier0, P.-O. Petrucci0, S. Pita0, G. Pühlhofer0, M. Punch0, S. Ranchon0, B. C. Raubenheimer0, M. Raue0, S. M. Rayner0, M. Renaud1, J. Ripken0, L. Rob0, L. Rolland0, S. Rosier-Lees0, G. Rowell0, B. Rudak0, J. Ruppel0, V. Sahakian0, A. Santangelo0, R. Schlickeiser0, F. Schöck0, R. Schröder0, U. Schwanke0, S. Schwarzburg0, S. Schwemmer0, A. Shalchi0, H. Sol0, D. Spangler0, L. Stawarz0, R. Steenkamp0, C. Stegmann0, G. Superina0, P. H. Tam0, J.-P. Tavernet0, R. Terrier0, C. van Eldik1, G. Vasileiadis0, C. Venter0, J. P. Vialle0, P. Vincent0, M. Vivier0, H. J. Völk1, F. Volpe0, S. J. Wagner0, M. Ward0, A. A. Zdziarski0, and A. Zech0

1 

(Received 3 August 2007 / Accepted 20 August 2007 )

Abstract
Aims.Our aim is to study the production mechanism for very-high-energy (VHE; >100 GeV) $\gamma$-rays in distant active galactic nuclei (AGN) and use the observed VHE spectrum to derive limits on the Extragalactic Background Light (EBL). We also want to determine physical quantities through the modeling of the object's broad-band spectral energy distribution (SED).
Methods.VHE observations (~25 h live time) of the BL Lac 1ES 0347-121 (redshift z = 0.188) were conducted with the High Energy Stereoscopic System (HESS) between August and December 2006. Contemporaneous X-ray and UV/optical observations from the SWIFT satellite are used to interpret the SED of the source in terms of a synchrotron self Compton (SSC) model.
Results.An excess of 327 events, corresponding to a statistical significance of 10.1 standard deviations, is detected from 1ES 0347-121. Its photon spectrum, ranging from ~250 GeV to ~3 TeV, is well described by a power law with a photon index of $\Gamma = 3.10 \pm 0.23_{\mathrm{stat}} \pm 0.10_{\mathrm{sys}}$. The integral flux above 250 GeV corresponds to ~2% of the flux of the Crab Nebula above the same threshold. No VHE flux variability is detected within the data set.
Conclusions.Constraints on the EBL density at optical to near-infrared wavelengths derived from the photon spectrum of 1ES 0347-121 are close to the strongest limits derived previously. The strong EBL limits confirm earlier findings, that the EBL density in the near-infrared is close to the lower limits from source counts. This implies that the universe is more transparent to VHE $\gamma$-rays than previously believed. An SSC model provides a reasonable description of the contemporaneous SED.


Key words: galaxies: BL Lacertae objects: individual: 1ES0347-121 -- gamma rays: observations -- cosmology: diffuse radiation -- galaxies: BL Lacertae objects: general -- galaxies: active



© ESO 2007


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