Attila Abramowski, Felix A. Aharonian, Faical Ait Benkhali, A. G. Akhperjanian, Ekrem Oǧuzhan Angüner, Gisela Anton, Shangkari Balenderan, Arnim Balzer, Anna Barnacka, Yvonne Becherini, J. Becker Tjus, K. Bernlöhr, E. Birsin, E. Bissaldi, Jonathan Biteau, Markus Boettcher, Catherine Boisson, J. Bolmont, Pol Bordas, J. Brucker, Francois Brun, Pierre Brun, Tomasz Bulik, Svenja Carrigan, Sabrina Casanova, M. Cerruti, Paula M. Chadwick, R. Chalme-Calvet, Ryan C. G. Chaves, A. Cheesebrough, M. Chretien, Sergio Colafrancesco, Gabriele Cologna, Jan Conrad, C. Couturier, M. Dalton, M. K. Daniel, I. D. Davids, B. Degrange, C. Deil, P. deWilt, H. J. Dicldnson, A. Djannati-Ataï, W. Domainko, G. Dubus, K. Dutson, J. Dyks, M. Dyrda, T. Edwards, Kathrin Egberts, P. Eger, P. Espigat, C. Farnier, S. Fegan, F. Feinstein, M. V. Fernandes, D. Fernandez, A. Fiasson, G. Fontaine, A. Foerster, Matthias Fuessling, M. Gajdus, Y. A. Gallant, T. Garrigoux, B. Giebels, J. F. Glicenstein, M-H Grondin, M. Grudzinska, S. Haeffner, J. Hahn, J. Harris, G. Heinzelmann, G. Henri, G. Hermann, O. Hervet, A. Hillert, James Anthony Hinton, W. Hofmann, P. Hofverberg, Markus Holler, D. Horns, A. Jacholkowska, C. Jahn, M. Jamrozy, M. Janiak, F. Jankowsky, I. Jung, M. A. Kastendieck, K. Katarzynski, U. Katz, S. Kaufmann, B. Khelifi, M. Kieffer, S. Klepser, D. Klochkov, W. Kluzniak, T. Kneiske, D. Kolitzus, Nu Komin, K. Kosack, S. Krakau, F. Krayzel, P. P. Krueger, H. Laffon, G. Lamanna, J. Lefaucheur, A. Lemiere, M. Lemoine-Goumard, J-P Lenain, D. Lennarz, T. Lohse, A. Lopatin, C-C Lu, V. Marandon, A. Marcowith, R. Marx, G. Maurin, N. Maxted, M. Mayer, T. J. L. McComb, J. Mehault, U. Menzler, M. Meyer, R. Moderski, M. Mohamed, E. Moulin, T. Murach, C. L. Naumann, M. de Naurois, J. Niemiec, S. J. Nolan, L. Oakes, S. Ohm, E. de Ona Wilhelmi, B. Opitz, M. Ostrowski, I. Oya, M. Panter, R. D. Parsons, M. Paz Arribas, N. W. Pekeur, G. Pelletier, J. Perez, P-O Petrucci, B. Peyaud, S. Pita, H. Poon, G. Puehlhofer, M. Punch, A. Quirrenbach, S. Raab, M. Raue, A. Reimer, O. Reimer, M. Renaud, R. de los Reyes, F. Rieger, L. Rob, C. Romoli, S. Rosier-Lees, G. Rowell, B. Rudak, C. B. Rulten, V. Sahakian, David M. Sanchez, A. Santangelo, R. Schlickeiser, F. Schuessler, A. Schulz, U. Schwanke, S. Schwarzburg, S. Schwemmer, H. Sol, G. Spengler, F. Spies, L. Stawarz, R. Steenkamp, Christian Stegmann, F. Stinzing, K. Stycz, Iurii Sushch, A. Szostek, P. H. T. Tam, J-P Tavernet, T. Tavernier, A. M. Taylor, R. Terrier, M. Tluczykont, C. Trichard, K. Valerius, C. van Eldik, G. Vasileiadis, C. Venter, A. Viana, P. Vincent, H. J. Voelk, F. Volpe, M. Vorster, S. J. Wagner, P. Wagner, M. Ward, M. Weidinger, Q. Weitzel, R. White, A. Wierzcholska, P. Willmann, A. Woernlein, D. Wouters, M. Zacharias, A. Zajczyk, A. A. Zdziarski, Alraune Zech, H-S Zechlin

• The long gamma-ray burst (GRB) 100621A, at the time the brightest X-ray transient ever detected by Swift-XRT in the 0.3-10 keV range, has been observed with the H.E.S.S. imaging air Cherenkov telescope array, sensitive to gamma radiation in the very-high-energy (VHE, >100 GeV) regime. Due to its relatively small redshift of z similar to 0.5, the favourable position in the southern sky and the relatively short follow-up time (<700 s after the satellite trigger) of the H.E.S.S. observations, this GRB could be within the sensitivity reach of the HESS. instrument. The analysis of the HESS. data shows no indication of emission and yields an integral flux upper limit above similar to 380 GeV of 4.2 x 10(-12) cm(-2) s(-1) s (95% confidence level), assuming a simple Band function extension model. A comparison to a spectral-temporal model, normalised to the prompt flux at sub-MeV energies, constraints the existence of a temporally extended and strong additional hard power law, as has been observed in the other bright X-ray GRB 130427A. AThe long gamma-ray burst (GRB) 100621A, at the time the brightest X-ray transient ever detected by Swift-XRT in the 0.3-10 keV range, has been observed with the H.E.S.S. imaging air Cherenkov telescope array, sensitive to gamma radiation in the very-high-energy (VHE, >100 GeV) regime. Due to its relatively small redshift of z similar to 0.5, the favourable position in the southern sky and the relatively short follow-up time (<700 s after the satellite trigger) of the H.E.S.S. observations, this GRB could be within the sensitivity reach of the HESS. instrument. The analysis of the HESS. data shows no indication of emission and yields an integral flux upper limit above similar to 380 GeV of 4.2 x 10(-12) cm(-2) s(-1) s (95% confidence level), assuming a simple Band function extension model. A comparison to a spectral-temporal model, normalised to the prompt flux at sub-MeV energies, constraints the existence of a temporally extended and strong additional hard power law, as has been observed in the other bright X-ray GRB 130427A. A comparison between the HESS. upper limit and the contemporaneous energy output in X-rays constrains the ratio between the X-ray and VHE gamma-ray fluxes to be greater than 0.4. This value is an important quantity for modelling the afterglow and can constrain leptonic emission scenarios, where leptons are responsible for the X-ray emission and might produce VHE gamma rays.…