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Aleksic, J. ; Ansoldi, S. ; Antonelli, L. A. ; Antoranz, P. ; Babic, A. ; Bangale, P. ; de Almeida, U. Barres ; Barrio, J. A. ; Gonzalez, J. Becerra ; Bednarek, W. ; Berger, K. ; Bernardini, E. ; Bilandli, A. ; Bianch, 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. ; Garczarczyki, 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. ; Longo, F. ; 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, K. ; Salvati, M. ; Satalecka, K. ; Scalzotto, V. ; Scapin, V. ; Schuliz, 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. ; Archambault, S. ; Behera, B. ; Beilicke, M. ; Benbow, W. ; Bird, R. ; Buckley, J. H. ; Bugaev, V. ; Cerruti, M. ; Chen, X. ; Ciupik, L. ; Collins-Hughes, E. ; Cui, W. ; Dumm, J. ; Eisch, J. D. ; Falcone, A. ; Federici, S. ; Feng, Q. ; Finley, J. P. ; Fleischhack, H. ; Fortin, P. ; Fortson, L. ; Furniss, A. ; Griffin, S. ; Griffiths, S. T. ; Grube, J. ; Gyuk, G. ; Hanna, D. ; Holder, J. ; Hughes, G. ; Humensky, T. B. ; Johnson, C. A. ; Kaaret, P. ; Kertzman, M. ; Khassen, Y. ; Kieda, D. ; Krawczynski, H. ; Krennrich, F. ; Kumar, S. ; Lang, M. J. ; Maier, G. ; McArthur, S. ; Meagher, K. ; Moriarty, P. ; Mukherjee, R. ; Ong, R. A. ; Otte, A. N. ; Park, N. ; Pichel, A. ; Pohl, M. ; Popkow, A. ; Prokoph, H. ; Quinn, M. J. ; Ragan, K. ; Rajotte, J. ; Reynolds, P. T. ; Richards, G. T. ; Roache, E. ; Rovero, A. C. ; Sembroski, G. H. ; Shahinyan, K. ; Staszak, D. ; Telezhinsky, Igor O. ; Theiling, M. ; Tucci, J. V. ; Tyler, J. ; Varlotta, A. ; Wakely, S. P. ; Weekes, T. C. ; Weinstein, A. ; Welsing, R. ; Wilhelm, Alina ; Williams, D. A. ; Zitzer, B. ; Villata, M. ; Raiteri, C. ; Aller, H. D. ; Aller, M. F. ; Chen, W. P. ; Jordan, B. ; Koptelova, E. ; Kurtanidze, O. M. ; Lahteenmaki, A. ; McBreen, B. ; Larionov, V. M. ; Lin, C. S. ; Nikolashvili, M. G. ; Angelakis, E. ; Capalbi, M. ; Carraminana, A. ; Carrasco, L. ; Cassaro, P. ; Cesarini, A. ; Fuhrmann, L. ; Giroletti, M. ; Hovatta, T. ; Krichbaum, T. P. ; Krimm, H. A. ; Max-Moerbeck, W. ; Moody, J. W. ; Maccaferri, G. ; Mori, Y. ; Nestoras, I. ; Orlati, A. ; Pace, C. ; Pearson, R. ; Perri, M. ; Readhead, A. C. S. ; Richards, J. L. ; Sadun, A. C. ; Sakamoto, T. ; Tammi, J. ; Tornikoski, M. ; Yatsu, Y. ; Zook, A.
Aims. We perform an extensive characterization of the broadband emission of Mrk 421, as well as its temporal evolution, during the non-flaring (low) state. The high brightness and nearby location (z = 0.031) of Mrk 421 make it an excellent laboratory to study blazar emission. The goal is to learn about the physical processes responsible for the typical emission of Mrk 421, which might also be extended to other blazars that are located farther away and hence are more difficult to study.
Methods. We performed a 4.5-month multi-instrument campaign on Mrk 421 between January 2009 and June 2009, which included VLBA, F-GAMMA, GASP-WEBT, Swift, RXTE, Fermi-LAT, MAGIC, and Whipple, among other instruments and collaborations. This extensive radio to very-high-energy (VHE; E > 100 GeV) gamma-ray dataset provides excellent temporal and energy coverage, which allows detailed studies of the evolution of the broadband spectral energy distribution.
Results. Mrk421 was found in its typical (non-flaring) activity state, with a VHE flux of about half that of the Crab Nebula, yet the light curves show significant variability at all wavelengths, the highest variability being in the X-rays. We determined the power spectral densities (PSD) at most wavelengths and found that all PSDs can be described by power-laws without a break, and with indices consistent with pink/red-noise behavior. We observed a harder-when-brighter behavior in the X-ray spectra and measured a positive correlation between VHE and X-ray fluxes with zero time lag. Such characteristics have been reported many times during flaring activity, but here they are reported for the first time in the non-flaring state. We also observed an overall anti-correlation between optical /UV and X-rays extending over the duration of the campaign.
Conclusions. The harder-when-brighter behavior in the X-ray spectra and the measured positive X-ray/VHE correlation during the 2009 multiwavelength campaign suggests that the physical processes dominating the emission during non-flaring states have similarities with those occurring during flaring activity. In particular, this observation supports leptonic scenarios as being responsible for the emission of Mrk 421 during non-flaring activity. Such a temporally extended X-ray /VHE correlation is not driven by any single flaring event, and hence is difficult to explain within the standard hadronic scenarios. The highest variability is observed in the X-ray band, which, within the one-zone synchrotron self-Compton scenario, indicates that the electron energy distribution is most variable at the highest energies.
Aliu, E. ; Archambault, S. ; Arlen, T. ; Aune, T. ; Behera, B. ; Beilicke, M. ; Benbow, W. ; Berger, K. ; Bird, R. ; Bouvier, A. ; Buckley, J. H. ; Bugaev, V. ; Byrum, K. ; Cerruti, M. ; Chen, Xuhui ; Ciupik, L. ; Connolly, M. P. ; Cui, W. ; Duke, C. ; Dumm, J. ; Errando, M. ; Falcone, A. ; Federici, S. ; Feng, Q. ; Finley, J. P. ; Fleischhack, H. ; Fortin, P. ; Fortson, L. ; Furniss, A. ; Galante, N. ; Gillanders, G. H. ; Griffin, S. ; Griffiths, S. T. ; Grube, J. ; Gyuk, G. ; Hanna, D. ; Holder, J. ; Hughes, G. ; Humensky, T. B. ; Johnson, C. A. ; Kaaret, P. ; Kertzman, M. ; Khassen, Y. ; Kieda, D. ; Krawczynski, H. ; Krennrich, F. ; Lang, M. J. ; Madhavan, A. S. ; Maier, G. ; Majumdar, P. ; McArthur, S. ; McCann, A. ; Meagher, K. ; Millis, J. ; Moriarty, P. ; Mukherjee, R. ; Nieto, D. ; Ong, R. A. ; Otte, A. N. ; Park, N. ; Perkins, J. S. ; Pohl, M. ; Popkow, A. ; Prokoph, H. ; Quinn, J. ; Ragan, K. ; Reyes, L. C. ; Reynolds, P. T. ; Richards, G. T. ; Roache, E. ; Sembroski, G. H. ; Smith, A. W. ; Staszak, D. ; Telezhinsky, Igor O. ; Theiling, M. ; Varlotta, A. ; Vassiliev, V. V. ; Vincent, S. ; Wakely, S. P. ; Weekes, T. C. ; Weinstein, A. ; Welsing, R. ; Williams, D. A. ; Zajczyk, A. ; Zitzer, B.
Aliu, E. ; Aune, T. ; Behera, B. ; Beilicke, M. ; Benbow, W. ; Berger, K. ; Bird, R. ; Buckley, J. H. ; Bugaev, V. ; Cardenzana, J. V. ; Cerruti, M. ; Chen, X. ; Ciupik, L. ; Connolly, M. P. ; Cui, W. ; Duke, C. ; Dumm, J. ; Errando, M. ; Falcone, A. ; Federici, Simone ; Feng, Q. ; Finley, J. P. ; Fortin, P. ; Fortson, L. ; Furniss, A. ; Galante, N. ; Gillanders, G. H. ; Griffin, S. ; Griffiths, S. T. ; Grube, J. ; Gyuk, G. ; Hanna, D. ; Holder, J. ; Hughes, G. ; Humensky, T. B. ; Kaaret, P. ; Kargaltsev, Oleg ; Kertzman, M. ; Khassen, Y. ; Kieda, D. ; Krawczynski, H. ; Lang, M. J. ; Madhavan, A. S. ; Maier, G. ; Majumdar, P. ; McCann, A. ; Moriarty, P. ; Mukherjee, R. ; Nieto, D. ; Ong, R. A. ; Otte, A. N. ; Pandel, D. ; Perkins, J. S. ; Pohl, Manuela ; Popkow, A. ; Prokoph, H. ; Quinn, J. ; Ragan, K. ; Rajotte, J. ; Reyes, L. C. ; Reynolds, P. T. ; Richards, G. T. ; Roache, E. ; Sembroski, G. H. ; Skole, C. ; Staszak, D. ; Telezhinsky, Igor O. ; Theiling, M. ; Tucci, J. V. ; Tyler, J. ; Varlotta, A. ; Vincent, S. ; Wakely, S. P. ; Weekes, T. C. ; Weinstein, A. ; Welsing, R. ; Williams, D. A. ; Zitzer, B.
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.
Archambault, S. ; Archer, A. ; Barnacka, Anna ; Behera, B. ; Beilicke, M. ; Benbow, W. ; Berger, K. ; Bird, R. ; Böttcher, Markus ; Buckley, J. H. ; Bugaev, V. ; Cardenzana, J. V. ; Cerruti, M. ; Chen, Xuhui ; Christiansen, J. L. ; Ciupik, L. ; Collins-Hughes, E. ; Connolly, M. P. ; Cui, W. ; Dickinson, H. J. ; Dumm, J. ; Eisch, J. D. ; Errando, M. ; Falcone, A. ; Federici, Simone ; Feng, Q. ; Finley, J. P. ; Fleischhack, H. ; Fortson, L. ; Furniss, A. ; Gillanders, G. H. ; Godambe, S. ; Griffin, S. ; Griffiths, S. T. ; Grube, J. ; Gyuk, G. ; Hakansson, Nils ; Hanna, D. ; Holder, J. ; Hughes, G. ; Johnson, C. A. ; Kaaret, P. ; Kar, P. ; Kertzman, M. ; Khassen, Y. ; Kieda, D. ; Krawczynski, H. ; Kumar, S. ; Lang, M. J. ; Madhavan, A. S. ; Maier, G. ; McArthur, S. ; McCann, A. ; Meagher, K. ; Millis, J. ; Moriarty, P. ; Nelson, T. ; Nieto, D. ; Ong, R. A. ; Otte, A. N. ; Park, N. ; Perkins, J. S. ; Pohl, Martin ; Popkow, A. ; Prokoph, H. ; Pueschel, Elisa ; Quinn, J. ; Ragan, K. ; Rajotte, J. ; Reyes, L. C. ; Reynolds, P. T. ; Richards, G. T. ; Roache, E. ; Sembroski, G. H. ; Shahinyan, K. ; Smith, A. W. ; Staszak, D. ; Sweeney, K. ; Telezhinsky, Igor O. ; Tucci, J. V. ; Tyler, J. ; Varlotta, A. ; Vassiliev, V. V. ; Wakely, S. P. ; Welsing, R. ; Wilhelm, Alina ; Williams, D. A. ; Zitzer, B.
Archambault, S. ; Aune, T. ; Behera, B. ; Beilicke, M. ; Benbow, W. ; Berger, K. ; Bird, R. ; Biteau, Jonathan ; Bugaev, V. ; Byrum, K. ; Cardenzana, J. V. ; Cerruti, M. ; Chen, Xuhui ; Ciupik, L. ; Connolly, M. P. ; Cui, Wei ; Dumm, J. ; Errando, M. ; Falcone, A. ; Federici, Simone ; Feng, Q. ; Finley, J. P. ; Fleischhack, H. ; Fortson, L. ; Furniss, A. ; Galante, N. ; Gillanders, G. H. ; Griffin, S. ; Griffiths, S. T. ; Grube, J. ; Gyuk, G. ; Hanna, D. ; Holder, J. ; Hughes, G. ; Humensky, T. B. ; Johnson, C. A. ; Kaaret, P. ; Kertzman, M. ; Khassen, Y. ; Kieda, D. ; Krawczynski, H. ; Krennrich, F. ; Kumar, S. ; Lang, M. J. ; Madhavan, A. S. ; Maier, G. ; McCann, A. ; Meagher, K. ; Moriarty, P. ; Mukherjee, R. ; Nieto, Daniel ; Ong, R. A. ; Otte, A. N. ; Park, N. ; Pohl, Martin ; Popkow, A. ; Prokoph, H. ; Quinn, J. ; Ragan, K. ; Rajotte, J. ; Reyes, L. C. ; Reynolds, P. T. ; Richards, G. T. ; Roache, E. ; Sembroski, G. H. ; Shahinyan, K. ; Staszak, D. ; Telezhinsky, Igor O. ; Tucci, J. V. ; Tyler, J. ; Varlotta, A. ; Vassiliev, V. V. ; Vincent, S. ; Wakely, S. P. ; Weinstein, A. ; Welsing, R. ; Wilhelm, Alina ; Williams, D. A. ; Ackermann, Margit ; Ajello, M. ; Albert, A. ; Baldini, L. ; Bastieri, D. ; Bellazzini, R. ; Bissaldi, E. ; Bregeon, Johan ; Buehler, R. ; Buson, S. ; Caliandro, G. A. ; Cameron, R. A. ; Caraveo, P. A. ; Cavazzuti, E. ; Charles, E. ; Chiang, J. ; Ciprini, S. ; Claus, R. ; Cutini, S. ; de Angelis, A. ; de Palma, F. ; Dermer, C. D. ; Digel, S. W. ; Di Venere, L. ; Drell, P. S. ; Favuzzi, C. ; Franckowiak, A. ; Fusco, P. ; Gargano, F. ; Gasparrini, D. ; Giglietto, N. ; Giordano, F. ; Giroletti, M. ; Grenier, I. A. ; Guiriec, S. ; Jogler, T. ; Kuss, M. ; Larsson, S. ; Latronico, L. ; Longo, F. ; Loparco, F. ; Lubrano, P. ; Madejski, G. M. ; Mayer, M. ; Mazziotta, Mario Nicola ; Michelson, P. F. ; Mizuno, T. ; Monzani, M. E. ; Morselli, Aldo ; Murgia, S. ; Nuss, E. ; Ohsugi, T. ; Ormes, J. F. ; Paneque, D. ; Perkins, J. S. ; Piron, F. ; Pivato, G. ; Raino, S. ; Razzano, M. ; Reimer, A. ; Reimer, Olaf ; Ritz, S. ; Schaal, M. ; Sgro, C. ; Siskind, E. J. ; Spinelli, P. ; Takahashi, H. ; Tibaldo, L. ; Tinivella, M. ; Troja, E. ; Vianello, G. ; Werner, M. ; Wood, M.
We present deep VERITAS observations of the blazar PKS 1424+240, along with contemporaneous Fermi Large Area Telescope, Swift X-ray Telescope, and Swift UV Optical Telescope data between 2009 February 19 and 2013 June 8. This blazar resides at a redshift of z >= 0.6035, displaying a significantly attenuated gamma-ray flux above 100 GeV due to photon absorption via pair-production with the extragalactic background light. We present more than 100 hr of VERITAS observations over three years, a multiwavelength light curve, and the contemporaneous spectral energy distributions. The source shows a higher flux of (2.1 +/- 0.3) x 10(-7) photons m(-2) s(-1) above 120 GeV in 2009 and 2011 as compared to the flux measured in 2013, corresponding to (1.02 +/- 0.08) x 10-7 photons m(-2) s(-1) above 120 GeV. The measured differential very high energy (VHE; E >= 100 GeV) spectral indices are Gamma = 3.8 +/- 0.3, 4.3 +/- 0.6 and 4.5 +/- 0.2 in 2009, 2011, and 2013, respectively. No significant spectral change across the observation epochs is detected. We find no evidence for variability at gamma-ray opacities of greater than tau = 2, where it is postulated that any variability would be small and occur on timescales longer than a year if hadronic cosmic-ray interactions with extragalactic photon fields provide a secondary VHE photon flux. The data cannot rule out such variability due to low statistics.
Aliu, E. ; Archer, A. ; Aune, T. ; Barnacka, Anna ; Behera, B. ; Beilicke, M. ; Benbow, W. ; Berger, K. ; Bird, R. ; Buckley, J. H. ; Bugaev, V. ; Byrum, K. ; Cardenzana, J. V. ; Cerruti, M. ; Chen, Xuhui ; Ciupik, L. ; Connolly, M. P. ; Cui, Wei ; Dickinson, H. J. ; Dumm, J. ; Eisch, J. D. ; Errando, M. ; Falcone, A. ; Federici, Simone ; Feng, Q. ; Finley, J. P. ; Fortin, P. ; Fortson, L. ; Furniss, A. ; Galante, N. ; Gillanders, G. H. ; Griffin, S. ; Griffiths, S. T. ; Grube, J. ; Gyuk, G. ; Hakansson, Nils ; Hanna, D. ; Holder, J. ; Hughes, G. ; Humensky, T. B. ; Johnson, C. A. ; Kaaret, P. ; Kar, P. ; Kertzman, M. ; Khassen, Y. ; Kieda, D. ; Krawczynski, H. ; Krennrich, F. ; Kumar, S. ; Lang, M. J. ; Madhavan, A. ; McArthur, S. ; McCann, A. ; Meagher, K. ; Millis, J. ; Moriarty, P. ; Nieto, Daniel ; Ong, R. A. ; Orr, M. ; Otte, A. N. ; Park, N. ; Perkins, J. S. ; Pohl, Martin ; Popkow, A. ; Prokoph, H. ; Pueschel, Elisa ; Quinn, J. ; Ragan, K. ; Rajotte, J. ; Reyes, L. C. ; Reynolds, P. T. ; Richards, G. T. ; Roache, E. ; Sembroski, G. H. ; Shahinyan, K. ; Staszak, D. ; Telezhinsky, Igor O. ; Tucci, J. V. ; Tyler, J. ; Varlotta, A. ; Vassiliev, V. V. ; Wakely, S. P. ; Weinstein, A. ; Welsing, R. ; Wilhelm, Alina ; Williams, D. A. ; Zitzer, B.
We present results from VERITAS observations of the BL Lac object PG 1553+113 spanning the years 2010, 2011, and 2012. The time-averaged spectrum, measured between 160 and 560 GeV, is well described by a power law with a spectral index of 4.33 +/- 0.09. The time-averaged integral flux above 200 GeV measured for this period was (1.69 +/- 0.06) x 10(-11) photons cm(-2) s(-1), corresponding to 6.9% of the Crab Nebula flux. We also present the combined gamma-ray spectrum from the Fermi Large Area Telescope and VERITAS covering an energy range from 100 MeV to 560 GeV. The data are well fit by a power law with an exponential cutoff at 101.9 +/- 3.2 GeV. The origin of the cutoff could be intrinsic to PG 1553+113 or be due to the gamma-ray opacity of our universe through pair production off the extragalactic background light (EBL). Given lower limits to the redshift of z > 0.395 based on optical/UV observations of PG 1553+113, the cutoff would be dominated by EBL absorption. Conversely, the small statistical uncertainties of the VERITAS energy spectrum have allowed us to provide a robust upper limit on the redshift of PG 1553+113 of z <= 0.62. A strongly elevated mean flux of (2.50 +/- 0.14) x10(-11) photons cm(-2) s(-1) (10.3% of the Crab Nebula flux) was observed during 2012, with the daily flux reaching as high as (4.44 +/- 0.71) x10(-11) photons cm(-2) s(-1) (18.3% of the Crab Nebula flux) on MJD 56048. The light curve measured during the 2012 observing season is marginally inconsistent with a steady flux, giving a chi(2) probability for a steady flux of 0.03%.
Aliu, E. ; Archambault, S. ; Archer, A. ; Arlen, T. ; Aune, T. ; Barnacka, Anna ; Behera, B. ; Beilicke, M. ; Benbow, W. ; Berger, K. ; Bird, R. ; Böttcher, Markus ; Bouvier, A. ; Buchovecky, M. ; Buckley, J. H. ; Bugaev, V. ; Cardenzana, J. V. ; Cerruti, M. ; Cesarini, A. ; Chen, Xuhui ; Ciupik, L. ; Collins-Hughes, E. ; Connolly, M. P. ; Cui, W. ; Dumm, J. ; Eisch, J. D. ; Falcone, A. ; Federici, Simone ; Feng, Q. ; Finley, J. P. ; Fleischhack, H. ; Fortin, P. ; Fortson, L. ; Furniss, A. ; Galante, N. ; Gall, D. ; Gillanders, G. H. ; Griffin, S. ; Griffiths, S. T. ; Grube, J. ; Gyuk, G. ; Hütten, M. ; Hakansson, Nils ; Holder, J. ; Hughes, G. ; Humensky, T. B. ; Johnson, C. A. ; Kaaret, P. ; Kar, P. ; Kelley-Hoskins, N. ; Kertzman, M. ; Khassen, Y. ; Kieda, D. ; Krause, M. ; Krawczynski, H. ; Krennrich, F. ; Lang, M. J. ; Madhavan, A. S. ; Maier, G. ; McArthur, S. ; McCann, A. ; Meagher, K. ; Millis, J. ; Moriarty, P. ; Mukherjee, R. ; Nieto, D. ; Ong, R. A. ; Orr, M. ; Otte, A. N. ; Pandel, D. ; Park, N. ; Pelassa, V. ; Perkins, J. S. ; Pichel, A. ; Pohl, Martin ; Popkow, A. ; Quinn, J. ; Ragan, K. ; Reyes, L. C. ; Reynolds, P. T. ; Roache, E. ; Rousselle, J. ; Rovero, A. C. ; Saxon, D. B. ; Sembroski, G. H. ; Shahinyan, K. ; Sheidaei, F. ; Skole, C. ; Smith, A. W. ; Staszak, D. ; Telezhinsky, Igor O. ; Theiling, M. ; Todd, N. W. ; Tucci, J. V. ; Tyler, J. ; Varlotta, A. ; Vassiliev, V. V. ; Vincent, S. ; Wakely, S. P. ; Weiner, O. M. ; Weinstein, A. ; Welsing, R. ; Wilhelm, Alina ; Williams, D. A. ; Zitzer, B. ; Baring, M. G. ; Gonzalez, J. Becerra ; Cillis, A. N. ; Horan, D. ; Paneque, D.
The very high energy (VHE; E > 100 GeV) blazar Markarian 501 was observed between April 17 and May 5 (MJD 54 938-54 956), 2009, as part of an extensive multiwavelength campaign from radio to VHE. Strong VHE yray activity was detected on May 1st with Whipple and VERITAS, when the flux (E > 400 GeV) increased to 10 times the preflare baseline flux (3.9 x 10(-11) ph cm(-2) s(-1)), reaching five times the flux of the Crab Nebula. This coincided with a decrease in the optical polarization and a rotation of the polarization angle by 15. This VHE flare showed a fast flux variation with an increase of a factor similar to 4 in 25 min, and a falling time of similar to 50 min. We present the observations of the quiescent state previous to the flare and of the high state after the flare, focusing on the flux and spectral variability from Whipple, VERITAS, Fermi-LAT, RXTE, and Swift combined with optical and radio data.
Archambault, S. ; Arlen, T. ; Aune, T. ; Behera, B. ; Beilicke, M. ; Benbow, W. ; Bird, R. ; Bouvier, A. ; Buckley, J. H. ; Bugaev, V. ; Byrum, K. ; Cesarini, A. ; Ciupik, L. ; Connolly, M. P. ; Cui, W. ; Errando, M. ; Falcone, A. ; Federici, Simone ; Feng, Q. ; Finley, J. P. ; Fortson, L. ; Furniss, A. ; Galante, N. ; Gall, D. ; Gillanders, G. H. ; Griffin, S. ; Grube, J. ; Gyuk, G. ; Hanna, D. ; Holder, J. ; Hughes, G. ; Humensky, T. B. ; Kaaret, P. ; Kertzman, M. ; Khassen, Y. ; Kieda, D. ; Krawczynski, H. ; Krennrich, F. ; Kumar, S. ; Lang, M. J. ; Madhavan, A. S. ; Maier, G. ; Majumdar, P. ; McArthur, S. ; McCann, A. ; Millis, J. ; Moriarty, P. ; Mukherjee, R. ; de Bhroithe, A. O'Faolain ; Ong, R. A. ; Otte, A. N. ; Park, N. ; Perkins, J. S. ; Pohl, Martin ; Popkow, A. ; Prokoph, H. ; Quinn, J. ; Ragan, K. ; Reyes, L. C. ; Reynolds, P. T. ; Richards, G. T. ; Roache, E. ; Saxon, D. B. ; Sembroski, G. H. ; Smith, A. W. ; Staszak, D. ; Telezhinsky, Igor O. ; Theiling, M. ; Varlotta, A. ; Vassiliev, V. V. ; Vincent, S. ; Wakely, S. P. ; Weekes, T. C. ; Weinstein, A. ; Welsing, R. ; Williams, D. A. ; Zitzer, B. ; Boettcher, Markus ; Fegan, S. J. ; Fortin, P. ; Halpern, J. P. ; Kovalev, Y. Y. ; Lister, M. L. ; Liu, J. ; Pushkarev, A. B. ; Smith, P. S.
We report the detection of a new TeV gamma-ray source, VER J0521+211, based on observations made with the VERITAS imaging atmospheric Cherenkov Telescope Array. These observations were motivated by the discovery of a cluster of >30 GeV photons in the first year of Fermi Large Area Telescope observations. VER J0521+211 is relatively bright at TeV energies, with a mean photon flux of (1.93 +/- 0.13(stat) +/- 0.78(sys)) x 10(-11) cm(-2) s(-1) above 0.2 TeV during the period of the VERITAS observations. The source is strongly variable on a daily timescale across all wavebands, from optical to TeV, with a peak flux corresponding to similar to 0.3 times the steady Crab Nebula flux at TeV energies. Follow-up observations in the optical and X-ray bands classify the newly discovered TeV source as a BL Lac-type blazar with uncertain redshift, although recent measurements suggest z = 0.108. VER J0521+211 exhibits all the defining properties of blazars in radio, optical, X-ray, and gamma-ray wavelengths.
Aliu, E. ; Archambault, S. ; Behera, B. ; Berger, K. ; Beilicke, M. ; Benbow, W. ; Bird, R. ; Bouvier, A. ; Bugaev, V. ; Cerruti, M. ; Chen, Xuhui ; Ciupik, L. ; Connolly, M. P. ; Cui, W. ; Dumm, J. ; Falcone, A. ; Federici, Simone ; Feng, Q. ; Finley, J. P. ; Fortin, P. ; Fortson, L. ; Furniss, A. ; Galante, N. ; Gillanders, G. H. ; Griffin, S. ; Griffiths, S. T. ; Grube, J. ; Gyuk, G. ; Hanna, D. ; Holder, J. ; Hughes, G. ; Humensky, T. B. ; Kaaret, P. ; Kertzman, M. ; Khassen, Y. ; Kieda, D. ; Krennrich, F. ; Lang, M. J. ; Maier, G. ; Majumdar, P. ; McArthur, S. ; McCann, A. ; Moriarty, P. ; Mukherjee, R. ; de Bhroithe, A. O'Faolain ; Ong, R. A. ; Otte, A. N. ; Park, N. ; Perkins, J. S. ; Pohl, Martin ; Popkow, A. ; Prokoph, H. ; Quinn, J. ; Ragan, K. ; Rajotte, J. ; Ratliff, G. ; Reynolds, P. T. ; Richards, G. T. ; Roache, E. ; Sembroski, G. H. ; Sheidaei, F. ; Skole, C. ; Smith, A. W. ; Staszak, D. ; Telezhinsky, Igor O. ; Tyler, J. ; Varlotta, A. ; Vincent, S. ; Wakely, S. P. ; Weekes, T. C. ; Weinstein, A. ; Welsing, R. ; Zajczyk, A. ; Zitzer, B.
We present the results of a multiwavelength observational campaign on the TeV binary system LS I +61 degrees 303 with the VERITAS telescope array (>200 GeV), Fermi-LAT (0.3-300 GeV), and Swift/XRT (2-10 keV). The data were taken from 2011 December through 2012 January and show a strong detection in all three wavebands. During this period VERITAS obtained 24.9 hr of quality selected livetime data in which LS I +61 degrees 303 was detected at a statistical significance of 11.9 sigma. These TeV observations show evidence for nightly variability in the TeV regime at a post-trial significance of 3.6 sigma. The combination of the simultaneously obtained TeV and X-ray fluxes do not demonstrate any evidence for a correlation between emission in the two bands. For the first time since the launch of the Fermi satellite in 2008, this TeV detection allows the construction of a detailed MeV-TeV spectral energy distribution from LS I +61 degrees 303. This spectrum shows a distinct cutoff in emission near 4 GeV, with emission seen by the VERITAS observations following a simple power-law above 200 GeV. This feature in the spectrum of LS I +61 degrees 303, obtained from overlapping observations with Fermi-LAT and VERITAS, may indicate that there are two distinct populations of accelerated particles producing the GeV and TeV emission.