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Tatischeff, V. ; De Angelis, A. ; Tavani, M. ; Grenier, I. ; Oberlack, U. ; Hanlon, L. ; Walter, R. ; Argan, A. ; von Ballmoos, P. ; Bulgarelli, A. ; Donnarumma, I. ; Hernanz, Margarita ; Kuvvetli, I. ; Mallamaci, M. ; Pearce, M. ; Zdziarski, A. ; Aboudan, A. ; Ajello, M. ; Ambrosi, G. ; Bernard, D. ; Bernardini, E. ; Bonvicini, V. ; Brogna, A. ; Branchesi, M. ; Budtz-Jorgensen, C. ; Bykov, A. ; Campana, R. ; Cardillo, M. ; Ciprini, S. ; Coppi, P. ; Cumani, P. ; da Silva, R. M. Curado ; De Martino, D. ; Diehl, R. ; Doro, M. ; Fioretti, V. ; Funk, S. ; Ghisellini, G. ; Giordano, F. ; Grove, J. E. ; Hamadache, C. ; Hartmann, D. H. ; Hayashida, M. ; Isern, J. ; Kanbach, G. ; Kiener, J. ; Knodlseder, J. ; Labanti, C. ; Laurent, P. ; Leising, M. ; Limousin, O. ; Longo, F. ; Mannheim, K. ; Marisaldi, M. ; Martinez, M. ; Mazziotta, M. N. ; McEnery, J. E. ; Mereghetti, S. ; Minervini, G. ; Moiseev, A. ; Morselli, A. ; Nakazawa, K. ; Orleanski, P. ; Paredes, J. M. ; Patricelli, B. ; Peyre, J. ; Piano, G. ; Pohl, Martin ; Rando, R. ; Roncadelli, M. ; Tavecchio, F. ; Thompson, D. J. ; Turolla, R. ; Ulyanov, A. ; Vacchi, A. ; Wu, X. ; Zoglauer, A.
e-ASTROGAM is a concept for a breakthrough observatory space mission carrying a gamma-ray telescope dedicated to the study of the non-thermal Universe in the photon energy range from 0.15 MeV to 3 GeV. The lower energy limit can be pushed down to energies as low as 30 keV for gamma-ray burst detection with the calorimeter. The mission is based on an advanced space-proven detector technology, with unprecedented sensitivity, angular and energy resolution, combined with remarkable polarimetric capability. Thanks to its performance in the MeV-GeV domain, substantially improving its predecessors, e-ASTROGAM will open a new window on the non-thermal Universe, making pioneering observations of the most powerful Galactic and extragalactic sources, elucidating the nature of their relativistic outflows and their effects on the surroundings. With a line sensitivity in the MeV energy range one to two orders of magnitude better than previous and current generation instruments, e-ASTROGAM will determine the origin of key isotopes fundamental for the understanding of supernova explosion and the chemical evolution of our Galaxy. The mission will be a major player of the multiwavelength, multimessenger time-domain astronomy of the 2030s, and provide unique data of significant interest to a broad astronomical community, complementary to powerful observatories such as LISA, LIGO, Virgo, KAGRA, the Einstein Telescope and the Cosmic Explorer, IceCube-Gen2 and KM3NeT, SKA, ALMA, JWST, E-ELT, LSST, Athena, and the Cherenkov Telescope Array.
De Angelis, A. ; Tatischeff, V. ; Grenier, I. A. ; McEnery, J. ; Mallamaci, Manuela ; Tavani, M. ; Oberlack, U. ; Hanlon, L. ; Walter, R. ; Argan, A. ; Von Ballmoos, P. ; Bulgarelli, A. ; Bykov, A. ; Hernanz, M. ; Kanbach, G. ; Kuvvetli, I. ; Pearce, M. ; Zdziarski, A. ; Conrad, J. ; Ghisellini, G. ; Harding, A. ; Isern, J. ; Leising, M. ; Longo, F. ; Madejski, G. ; Martinez, M. ; Mazziotta, Mario Nicola ; Paredes, J. M. ; Pohl, Martin ; Rando, R. ; Razzano, M. ; Aboudan, A. ; Ackermann, M. ; Addazi, A. ; Ajello, M. ; Albertus, C. ; Alvarez, J. M. ; Ambrosi, G. ; Anton, S. ; Antonelli, L. A. ; Babic, A. ; Baibussinov, B. ; Balbom, M. ; Baldini, L. ; Balman, S. ; Bambi, C. ; Barres de Almeida, U. ; Barrio, J. A. ; Bartels, R. ; Bastieri, D. ; Bednarek, W. ; Bernard, D. ; Bernardini, E. ; Bernasconi, T. ; Bertucci, B. ; Biland, A. ; Bissaldi, E. ; Boettcher, M. ; Bonvicini, V. ; Bosch-Ramon, V. ; Bottacini, E. ; Bozhilov, V. ; Bretz, T. ; Branchesi, M. ; Brdar, V. ; Bringmann, T. ; Brogna, A. ; Jorgensen, C. Budtz ; Busetto, G. ; Buson, S. ; Busso, M. ; Caccianiga, A. ; Camera, S. ; Campana, R. ; Caraveo, P. ; Cardillo, M. ; Carlson, P. ; Celestin, S. ; Cermeno, M. ; Chen, A. ; Cheung, C. C. ; Churazov, E. ; Ciprini, S. ; Coc, A. ; Colafrancesco, S. ; Coleiro, A. ; Collmar, W. ; Coppi, P. ; Curado da Silva, R. ; Cutini, S. ; De Lotto, B. ; de Martino, D. ; De Rosa, A. ; Del Santo, M. ; Delgado, L. ; Diehl, R. ; Dietrich, S. ; Dolgov, A. D. ; Dominguez, A. ; Prester, D. Dominis ; Donnarumma, I. ; Dorner, D. ; Doro, M. ; Dutra, M. ; Elsaesser, D. ; Fabrizio, M. ; Fernandez-Barral, A. ; Fioretti, V. ; Foffano, L. ; Formato, V. ; Fornengo, N. ; Foschini, L. ; Franceschini, A. ; Franckowiak, A. ; Funk, S. ; Fuschino, F. ; Gaggero, D. ; Galanti, G. ; Gargano, F. ; Gasparrini, D. ; Gehrz, R. ; Giammaria, P. ; Giglietto, N. ; Giommi, P. ; Giordano, F. ; Giroletti, M. ; Ghirlanda, G. ; Godinovic, N. ; Gouiffes, C. ; Grove, J. E. ; Hamadache, C. ; Hartmann, D. H. ; Hayashida, M. ; Hryczuk, A. ; Jean, P. ; Johnson, T. ; Jose, J. ; Kaufmann, S. ; Khelifi, B. ; Kiener, J. ; Knodlseder, J. ; Kolem, M. ; Kopp, J. ; Kozhuharov, V. ; Labanti, C. ; Lalkovski, S. ; Laurent, P. ; Limousin, O. ; Linares, M. ; Lindfors, E. ; Lindner, M. ; Liu, J. ; Lombardi, S. ; Loparco, F. ; Lopez-Coto, R. ; Lopez Moya, M. ; Lott, B. ; Lubrano, P. ; Malyshev, D. ; Mankuzhiyil, N. ; Mannheim, K. ; Marcha, M. J. ; Marciano, A. ; Marcote, B. ; Mariotti, M. ; Marisaldi, M. ; McBreen, S. ; Mereghetti, S. ; Merle, A. ; Mignani, R. ; Minervini, G. ; Moiseev, A. ; Morselli, A. ; Moura, F. ; Nakazawa, K. ; Nava, L. ; Nieto, D. ; Orienti, M. ; Orio, M. ; Orlando, E. ; Orleanski, P. ; Paiano, S. ; Paoletti, R. ; Papitto, A. ; Pasquato, M. ; Patricelli, B. ; Perez-Garcia, M. A. ; Persic, M. ; Piano, G. ; Pichel, A. ; Pimenta, M. ; Pittori, C. ; Porter, T. ; Poutanen, J. ; Prandini, E. ; Prantzos, N. ; Produit, N. ; Profumo, S. ; Queiroz, F. S. ; Raino, S. ; Raklev, A. ; Regis, M. ; Reichardt, I. ; Rephaeli, Y. ; Rico, J. ; Rodejohann, W. ; Fernandez, G. Rodriguez ; Roncadelli, M. ; Roso, L. ; Rovero, A. ; Ruffini, R. ; Sala, G. ; Sanchez-Conde, M. A. ; Santangelo, Andrea ; Parkinson, P. Saz ; Sbarrato, T. ; Shearer, A. ; Shellard, R. ; Short, K. ; Siegert, T. ; Siqueira, C. ; Spinelli, P. ; Stamerra, A. ; Starrfield, S. ; Strong, A. ; Strumke, I. ; Tavecchio, F. ; Taverna, R. ; Terzic, T. ; Thompson, D. J. ; Tibolla, O. ; Torres, D. F. ; Turolla, R. ; Ulyanov, A. ; Ursi, A. ; Vacchi, A. ; Van den Abeele, J. ; Vankova-Kirilovai, G. ; Venter, C. ; Verrecchia, F. ; Vincent, P. ; Wang, X. ; Weniger, C. ; Wu, X. ; Zaharijas, G. ; Zampieri, L. ; Zane, S. ; Zimmer, S. ; Zoglauer, A.
De Angelis, A. ; Tatischeff, V. ; Tavani, M. ; Oberlack, U. ; Grenier, I. ; Hanloni, L. ; Walter, R. ; Argan, A. ; Von Ballmoos, P. ; Bulgarelli, A. ; Donnarumma, I. ; Hernanz, M. ; Kuvvetli, I. ; Pearce, M. ; Zdziarski, A. ; Aboudan, A. ; Ajello, M. ; Ambrosi, G. ; Bernard, D. ; Bernardini, E. ; Bonvicini, V. ; Brogna, A. ; Branchesi, M. ; Budtz-Jorgensen, C. ; Bykov, A. M. ; Campana, R. ; Cardillo, M. ; Coppi, P. ; De Martino, D. ; Diehl, R. ; Doro, M. ; Fioretti, V. ; Funk, S. ; Ghisellini, G. ; Grove, E. ; Hamadache, C. ; Hartmann, D. H. ; Hayashida, M. ; Isern, J. ; Kanbach, G. ; Kiener, J. ; Knodlseder, J. ; Labanti, C. ; Laurent, P. ; Limousin, O. ; Longo, F. ; Mannheim, K. ; Marisaldi, M. ; Martinez, M. ; Mazziotta, Mario Nicola ; McEnery, J. ; Mereghetti, S. ; Minervini, G. ; Moiseev, A. ; Morselli, A. ; Nakazawa, K. ; Orleanski, P. ; Paredes, J. M. ; Patricelli, B. ; Pevre, J. ; Piano, G. ; Pohl, Martin ; Ramarijaona, H. ; Rando, R. ; Reichardt, I. ; Roncadelli, M. ; Silva, R. ; Tavecchio, F. ; Thompson, D. J. ; Turolla, R. ; Ulyanov, A. ; Vacchi, A. ; Wu, X. ; Zoglauer, A.
e-ASTROGAM (‘enhanced ASTROGAM’) is a breakthrough Observatory space mission, with a detector composed by a Silicon tracker, a calorimeter, and an anticoincidence system, dedicated to the study of the non-thermal Universe in the photon energy range from 0.3 MeV to 3 GeV – the lower energy limit can be pushed to energies as low as 150 keV, albeit with rapidly degrading angular resolution, for the tracker, and to 30 keV for calorimetric detection. The mission is based on an advanced space-proven detector technology, with unprecedented sensitivity, angular and energy resolution, combined with polarimetric capability. Thanks to its performance in the MeV-GeV domain, substantially improving its predecessors, e-ASTROGAM will open a new window on the non-thermal Universe, making pioneering observations of the most powerful Galactic and extragalactic sources, elucidating the nature of their relativistic outflows and their effects on the surroundings. With a line sensitivity in the MeV energy range one to two orders of magnitude better than previous generation instruments, e-ASTROGAM will determine the origin of key isotopes fundamental for the understanding of supernova explosion and the chemical evolution of our Galaxy. The mission will provide unique data of significant interest to a broad astronomical community, complementary to powerful observatories such as LIGO-Virgo-GEO600-KAGRA, SKA, ALMA, E-ELT, TMT, LSST, JWST, Athena, CTA, IceCube, KM3NeT, and the promise of eLISA.
Aleksic, J. ; Ansoldi, S. ; Antonelli, L. A. ; Antoranz, P. ; Babic, A. ; Bangale, P. ; de Almeida, U. Barres ; Barrio, J. A. ; Becerra Gonzalez, J. ; Bednarek, W. ; Berger, K. ; Bernardini, E. ; Biland, A. ; Blanch Bigas, O. ; Bock, R. K. ; Bonnefoy, S. ; Bonnoli, G. ; Borracci, F. ; Bretz, T. ; Carmona, E. ; Carosi, A. ; Fidalgo, D. Carreto ; Colin, P. ; Colombo, E. ; Contreras, J. L. ; Cortina, J. ; Covino, S. ; Da Vela, P. ; Dazzi, F. ; De Angelis, A. ; De Caneva, G. ; De Lotto, B. ; Delgado Mendez, C. ; Doert, M. ; Dominguez, A. ; Prester, Dijana Dominis ; Dorner, D. ; Doro, M. ; Einecke, S. ; Eisenacher, D. ; Elsaesser, D. ; Farina, E. ; Ferenc, D. ; Fonseca, M. V. ; Font, L. ; Frantzen, K. ; Fruck, C. ; Garcia Lopez, R. J. ; Garczarczyk, M. ; Garrido Terrats, D. ; Gaug, M. ; Giavitto, G. ; Godinovic, N. ; Gonzalez Munoz, A. ; Gozzini, S. R. ; Hadamek, A. ; Hadasch, D. ; Herrero, A. ; Hildebrand, D. ; Hose, J. ; Hrupec, D. ; Idec, W. ; Kadenius, V. ; Kellermann, H. ; Knoetig, M. L. ; Krause, J. ; Kushida, J. ; La Barbera, A. ; Lelas, D. ; Lewandowska, N. ; Lindfors, E. ; Lombardi, S. ; Lopez, M. ; Lopez-Coto, R. ; Lopez-Oramas, A. ; Lorenz, E. ; Lozano, I. ; Makariev, M. ; Mallot, K. ; Maneva, G. ; Mankuzhiyil, N. ; Mannheim, K. ; Maraschi, L. ; Marcote, B. ; Mariotti, M. ; Martinez, M. ; Mazin, D. ; Menzel, U. ; Meucci, M. ; Miranda, J. M. ; Mirzoyan, R. ; Moralejo, A. ; Munar-Adrover, P. ; Nakajima, D. ; Niedzwiecki, A. ; Nilsson, K. ; Nowak, N. ; Orito, R. ; Overkemping, A. ; Paiano, S. ; Palatiello, M. ; Paneque, D. ; Paoletti, R. ; Paredes, J. M. ; Paredes-Fortuny, X. ; Partini, S. ; Persic, M. ; Prada, F. ; Moroni, P. G. Prada ; Prandini, E. ; Preziuso, S. ; Puljak, I. ; Reinthal, R. ; Rhode, W. ; Ribo, M. ; Rico, J. ; Garcia, J. Rodriguez ; Ruegamer, S. ; Saggion, A. ; Saito, T. ; Saito, K. ; Salvati, M. ; Satalecka, K. ; Scalzotto, V. ; Scapin, V. ; Schultz, C. ; Schweizer, T. ; Shore, S. N. ; Sillanpaa, A. ; Sitarek, J. ; Snidaric, I. ; Sobczynska, D. ; Spanier, F. ; Stamatescu, V. ; Stamerra, A. ; Steinbring, T. ; Storz, J. ; Sun, S. ; Suric, T. ; Takalo, L. ; Tavecchio, F. ; Temnikov, P. ; Terzic, T. ; Tescaro, D. ; Teshima, M. ; Thaele, J. ; Tibolla, O. ; Torres, D. F. ; Toyama, T. ; Treves, A. ; Uellenbeck, M. ; Vogler, P. ; Wagner, R. M. ; Zandanel, F. ; Zanin, R. ; Behera, B. ; Beilicke, M. ; Benbow, W. ; Berger, K. ; Bird, R. ; Bouvier, A. ; Bugaev, V. ; Cerruti, M. ; Chen, Xuhui ; Ciupik, L. ; Collins-Hughes, E. ; Cui, W. ; Duke, C. ; Dumm, J. ; Falcone, A. ; Federici, Simone ; Feng, Q. ; Finley, J. P. ; Fortson, L. ; Furniss, A. ; Galante, N. ; Gillanders, G. H. ; Griffin, S. ; Griffiths, S. T. ; Grube, J. ; Gyuk, G. ; Hanna, D. ; Holder, J. ; Johnson, C. A. ; Kaaret, P. ; Kertzman, M. ; Kieda, D. ; Krawczynski, H. ; Lang, M. J. ; Madhavan, A. S. ; Maier, G. ; Majumdar, P. ; Meagher, K. ; Moriarty, P. ; Mukherjee, R. ; Nieto, D. ; Ong, R. A. ; Otte, A. N. ; Pichel, A. ; Pohl, Manula ; Popkow, A. ; Prokoph, H. ; Quinn, J. ; Rajotte, J. ; Ratliff, G. ; Reyes, L. C. ; Reynolds, P. T. ; Richards, G. T. ; Roache, E. ; Sembroski, G. H. ; Shahinyan, K. ; Sheidaei, F. ; Smith, A. W. ; Staszak, D. ; Telezhinsky, Igor O. ; Theiling, M. ; Tyler, J. ; Varlotta, A. ; Vincent, S. ; Wakely, S. P. ; Weekes, T. C. ; Welsing, R. ; Williams, D. A. ; Zajczyk, A. ; Zitzer, B. ; Villata, M. ; Raiteri, C. M. ; Ajello, M. ; Perri, M. ; Aller, H. D. ; Aller, M. F. ; Larionov, V. M. ; Efimova, N. V. ; Konstantinova, T. S. ; Kopatskaya, E. N. ; Chen, W. P. ; Koptelova, E. ; Hsiao, H. Y. ; Kurtanidze, O. M. ; Nikolashvili, M. G. ; Kimeridze, G. N. ; Jordan, B. ; Leto, Paolo ; Buemi, C. S. ; Trigilio, C. ; Umana, G. ; Lahteenmaki, A. ; Nieppola, E. ; Tornikoski, M. ; Sainio, J. ; Kadenius, V. ; Giroletti, M. ; Cesarini, A. ; Fuhrmann, L. ; Kovalev, Yu. A. ; Kovalev, Y. Y.
Context. Blazars are variable sources on various timescales over a broad energy range spanning from radio to very high energy (>100 GeV, hereafter VHE). Mrk 501 is one of the brightest blazars at TeV energies and has been extensively studied since its first VHE detection in 1996. However, most of the gamma-ray studies performed on Mrk 501 during the past years relate to flaring activity, when the source detection and characterization with the available gamma-ray instrumentation was easier to perform.
Aims. Our goal is to characterize the source gamma-ray emission in detail, together with the radio-to-X-ray emission, during the non-flaring (low) activity, which is less often studied than the occasional flaring (high) activity.
Methods. We organized a multiwavelength (MW) campaign on Mrk 501 between March and May 2008. This multi-instrument effort included the most sensitive VHE gamma-ray instruments in the northern hemisphere, namely the imaging atmospheric Cherenkov telescopes MAGIC and VERITAS, as well as Swift, RXTE, the F-GAMMA, GASP-WEBT, and other collaborations and instruments. This provided extensive energy and temporal coverage of Mrk 501 throughout the entire campaign.
Results. Mrk 501 was found to be in a low state of activity during the campaign, with a VHE flux in the range of 10%-20% of the Crab nebula flux. Nevertheless, significant flux variations were detected with various instruments, with a trend of increasing variability with energy and a tentative correlation between the X-ray and VHE fluxes. The broadband spectral energy distribution during the two different emission states of the campaign can be adequately described within the homogeneous one-zone synchrotron self-Compton model, with the (slightly) higher state described by an increase in the electron number density.
Conclusions. The one-zone SSC model can adequately describe the broadband spectral energy distribution of the source during the two months covered by the MW campaign. This agrees with previous studies of the broadband emission of this source during flaring and non-flaring states. We report for the first time a tentative X-ray-to-VHE correlation during such a low VHE activity. Although marginally significant, this positive correlation between X-ray and VHE, which has been reported many times during flaring activity, suggests that the mechanisms that dominate the X-ray/VHE emission during non-flaring-activity are not substantially different from those that are responsible for the emission during flaring activity.
Abdo, A. A. ; Ackermann, Margit ; Ajello, M. ; Allafort, A. J. ; Baldini, L. ; Ballet, J. ; Barbiellini, G. ; Baring, M. G. ; Bastieri, D. ; Bechtol, K. C. ; Bellazzini, R. ; Berenji, B. ; Blandford, R. D. ; Bloom, E. D. ; Bonamente, E. ; Borgland, A. W. ; Bouvier, A. ; Brandt, T. J. ; Bregeon, Johan ; Brez, A. ; Brigida, M. ; Bruel, P. ; Buehler, R. ; Buson, S. ; Caliandro, G. A. ; Cameron, R. A. ; Cannon, A. ; Caraveo, P. A. ; Carrigan, Svenja ; Casandjian, J. M. ; Cavazzuti, E. ; Cecchi, C. ; Celik, O. ; Charles, E. ; Chekhtman, A. ; Cheung, C. C. ; Chiang, J. ; Ciprini, S. ; Claus, R. ; Cohen-Tanugi, J. ; Conrad, Jan ; Cutini, S. ; Dermer, C. D. ; de Palma, F. ; do Couto e Silva, E. ; Drell, P. S. ; Dubois, R. ; Dumora, D. ; Favuzzi, C. ; Fegan, S. J. ; Ferrara, E. C. ; Focke, W. B. ; Fortin, P. ; Frailis, M. ; Fuhrmann, L. ; Fukazawa, Y. ; Funk, S. ; Fusco, P. ; Gargano, F. ; Gasparrini, D. ; Gehrels, N. ; Germani, S. ; Giglietto, N. ; Giordano, F. ; Giroletti, M. ; Glanzman, T. ; Godfrey, G. ; Grenier, I. A. ; Guillemot, L. ; Guiriec, S. ; Hayashida, M. ; Hays, E. ; Horan, D. ; Hughes, R. E. ; Johannesson, G. ; Johnson, A. S. ; Johnson, W. N. ; Kadler, M. ; Kamae, T. ; Katagiri, H. ; Kataoka, J. ; Knoedlseder, J. ; Kuss, M. ; Lande, J. ; Latronico, L. ; Lee, S. -H. ; Lemoine-Goumard, M. ; Longo, F. ; Loparco, F. ; Lott, B. ; Lovellette, M. N. ; Lubrano, P. ; Madejski, G. M. ; Makeev, A. ; Max-Moerbeck, W. ; Mazziotta, Mario Nicola ; McEnery, J. E. ; Mehault, J. ; Michelson, P. F. ; Mitthumsiri, W. ; Mizuno, T. ; Moiseev, A. A. ; Monte, C. ; Monzani, M. E. ; Morselli, A. ; Moskalenko, I. V. ; Murgia, S. ; Naumann-Godo, M. ; Nishino, S. ; Nolan, P. L. ; Norris, J. P. ; Nuss, E. ; Ohsugi, T. ; Okumura, A. ; Omodei, N. ; Orlando, E. ; Ormes, J. F. ; Paneque, D. ; Panetta, J. H. ; Parent, D. ; Pavlidou, V. ; Pearson, T. J. ; Pelassa, V. ; Pepe, M. ; Pesce-Rollins, M. ; Piron, F. ; Porter, T. A. ; Raino, S. ; Rando, R. ; Razzano, M. ; Readhead, A. ; Reimer, A. ; Reimer, O. ; Richards, J. L. ; Ripken, J. ; Ritz, S. ; Roth, M. ; Sadrozinski, H. F. -W. ; Sanchez, D. ; Sander, A. ; Scargle, J. D. ; Sgro, C. ; Siskind, E. J. ; Smith, P. D. ; Spandre, G. ; Spinelli, P. ; Stawarz, L. ; Stevenson, M. ; Strickman, M. S. ; Sokolovsky, K. V. ; Suson, D. J. ; Takahashi, H. ; Takahashi, T. ; Tanaka, T. ; Thayer, J. B. ; Thayer, J. G. ; Thompson, D. J. ; Tibaldo, L. ; Torres, F. ; Tosti, G. ; Tramacere, A. ; Uchiyama, Y. ; Usher, T. L. ; Vandenbroucke, J. ; Vasileiou, V. ; Vilchez, N. ; Vitale, V. ; Waite, A. P. ; Wang, P. ; Wehrle, A. E. ; Winer, B. L. ; Wood, K. S. ; Yang, Z. ; Ylinen, T. ; Zensus, J. A. ; Ziegler, M. ; Aleksic, J. ; Antonelli, L. A. ; Antoranz, P. ; Backes, Michael ; Barrio, J. A. ; Gonzalez, J. Becerra ; Bednarek, W. ; Berdyugin, A. ; Berger, K. ; Bernardini, E. ; Biland, A. ; Blanch Bigas, O. ; Bock, R. K. ; Boller, A. ; Bonnoli, G. ; Bordas, Pol ; Tridon, D. Borla ; Bosch-Ramon, Valentin ; Bose, D. ; Braun, I. ; Bretz, T. ; Camara, M. ; Carmona, E. ; Carosi, A. ; Colin, P. ; Colombo, E. ; Contreras, J. L. ; Cortina, J. ; Covino, S. ; Dazzi, F. ; de Angelis, A. ; del Pozo, E. De Cea ; De Lotto, B. ; De Maria, M. ; De Sabata, F. ; Mendez, C. Delgado ; Ortega, A. Diago ; Doert, M. ; Dominguez, A. ; Prester, Dijana Dominis ; Dorner, D. ; Doro, M. ; Elsaesser, D. ; Ferenc, D. ; Fonseca, M. V. ; Font, L. ; Lopen, R. J. Garcia ; Garczarczyk, M. ; Gaug, M. ; Giavitto, G. ; Godinovi, N. ; Hadasch, D. ; Herrero, A. ; Hildebrand, D. ; Hoehne-Moench, D. ; Hose, J. ; Hrupec, D. ; Jogler, T. ; Klepser, S. ; Kraehenbuehl, T. ; Kranich, D. ; Krause, J. ; La Barbera, A. ; Leonardo, E. ; Lindfors, E. ; Lombardi, S. ; Lopez, M. ; Lorenz, E. ; Majumdar, P. ; Makariev, E. ; Maneva, G. ; Mankuzhiyil, N. ; Mannheim, K. ; Maraschi, L. ; Mariotti, M. ; Martinez, M. ; Mazin, D. ; Meucci, M. ; Miranda, J. M. ; Mirzoyan, R. ; Miyamoto, H. ; Moldon, J. ; Moralejo, A. ; Nieto, D. ; Nilsson, K. ; Orito, R. ; Oya, I. ; Paoletti, R. ; Paredes, J. M. ; Partini, S. ; Pasanen, M. ; Pauss, F. ; Pegna, R. G. ; Perez-Torres, M. A. ; Persic, M. ; Peruzzo, J. ; Pochon, J. ; Moroni, P. G. Prada ; Prada, F. ; Prandini, E. ; Puchades, N. ; Puljak, I. ; Reichardt, T. ; Reinthal, R. ; Rhode, W. ; Ribo, M. ; Rico, J. ; Rissi, M. ; Ruegamer, S. ; Saggion, A. ; Saito, K. ; Saito, T. 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We report on the gamma-ray activity of the blazar Mrk 501 during the first 480 days of Fermi operation. We find that the average Large Area Telescope (LAT) gamma-ray spectrum of Mrk 501 can be well described by a single power-law function with a photon index of 1.78 +/- 0.03. While we observe relatively mild flux variations with the Fermi-LAT (within less than a factor of two), we detect remarkable spectral variability where the hardest observed spectral index within the LAT energy range is 1.52 +/- 0.14, and the softest one is 2.51 +/- 0.20. These unexpected spectral changes do not correlate with the measured flux variations above 0.3 GeV. In this paper, we also present the first results from the 4.5 month long multifrequency campaign (2009 March 15-August 1) on Mrk 501, which included the Very Long Baseline Array (VLBA), Swift, RXTE, MAGIC, and VERITAS, the F-GAMMA, GASP-WEBT, and other collaborations and instruments which provided excellent temporal and energy coverage of the source throughout the entire campaign. The extensive radio to TeV data set from this campaign provides us with the most detailed spectral energy distribution yet collected for this source during its relatively low activity. The average spectral energy distribution of Mrk 501 is well described by the standard one-zone synchrotron self-Compton (SSC) model. In the framework of this model, we find that the dominant emission region is characterized by a size less than or similar to 0.1 pc (comparable within a factor of few to the size of the partially resolved VLBA core at 15-43 GHz), and that the total jet power (similar or equal to 10(44) erg s(-1)) constitutes only a small fraction (similar to 10(-3)) of the Eddington luminosity. The energy distribution of the freshly accelerated radiating electrons required to fit the time-averaged data has a broken power-law form in the energy range 0.3 GeV-10 TeV, with spectral indices 2.2 and 2.7 below and above the break energy of 20 GeV. We argue that such a form is consistent with a scenario in which the bulk of the energy dissipation within the dominant emission zone of Mrk 501 is due to relativistic, proton-mediated shocks. We find that the ultrarelativistic electrons and mildly relativistic protons within the blazar zone, if comparable in number, are in approximate energy equipartition, with their energy dominating the jet magnetic field energy by about two orders of magnitude.
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