Volume 544, August 2012
|Number of page(s)||9|
|Published online||31 July 2012|
High zenith angle observations of PKS 2155-304 with the MAGIC-I telescope
1 IFAE, Edifici Cn., Campus UAB, 08193 Bellaterra, Spain
2 Universidad Complutense, 28040 Madrid, Spain
3 INAF National Institute for Astrophysics, 00136 Rome, Italy
4 Università di Siena, and INFN Pisa, 53100 Siena, Italy
5 Technische Universität Dortmund, 44221 Dortmund, Germany
6 Max-Planck-Institut für Physik, 80805 München, Germany
7 Università di Padova and INFN, 35131 Padova, Italy
8 Inst. de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
9 Depto. de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Spain
10 University of Łódź, 90236 Lodz, Poland
11 Tuorla Observatory, University of Turku, 21500 Piikkiö, Finland
12 Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
13 ETH Zurich, 8093 Zurich, Switzerland
14 Universität Würzburg, 97074 Würzburg, Germany
15 Universitat de Barcelona (ICC/IEEC), 08028 Barcelona, Spain
16 Università di Udine, and INFN Trieste, 33100 Udine, Italy
17 Institut de Ciències de l’Espai (IEEC-CSIC), 08193 Bellaterra, Spain
18 Inst. de Astrofísica de Andalucía (CSIC), 18080 Granada, Spain
19 Croatian MAGIC Consortium, Rudjer Boskovic Institute, University of Rijeka and University of Split, 10000 Zagreb, Croatia
20 Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
21 Inst. for Nucl. Research and Nucl. Energy, 1784 Sofia, Bulgaria
22 INAF/Osservatorio Astronomico and INFN, 34143 Trieste, Italy
23 Università dell’Insubria, Como, 22100 Como, Italy
24 Università di Pisa, and INFN Pisa, 56126 Pisa, Italy
25 ICREA, 08010 Barcelona, Spain
26 Now at: École polytechnique fédérale de Lausanne (EPFL), Lausanne, Switzerland
27 Supported by INFN Padova, Italia
28 Now at: Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), Madrid, Spain
29 Now at: Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Finland
30 Now at: Institut de Ciències de l’Espai (IEEC-CSIC), 08193 Bellaterra, Spain
Received: 9 January 2012
Accepted: 22 June 2012
Context. The high frequency peaked BL Lac PKS 2155-304 with a redshift of z = 0.116 was discovered in 1997 in the very high energy (VHE, E > 100 GeV) γ-ray range by the University of Durham Mark VI γ-ray Cherenkov telescope in Australia with a flux corresponding to 20% of the Crab Nebula flux. It was later observed and detected with high significance by the southern Cherenkov observatory H.E.S.S. establishing this source as the best studied southern TeV blazar. Detection from the northern hemisphere is difficult due to challenging observation conditions under large zenith angles. In July 2006, the H.E.S.S. collaboration reported an extraordinary outburst of VHE γ-emission. During the outburst, the VHE γ-ray emission was found to be variable on the time scales of minutes and with a mean flux of ~7 times the flux observed from the Crab Nebula. Follow-up observations with the MAGIC-I standalone Cherenkov telescope were triggered by this extraordinary outburst and PKS 2155-304 was observed between 28 July to 2 August 2006 for 15 h at large zenith angles.
Aims. We studied the behavior of the source after its extraordinary flare. Furthermore, we developed an analysis method in order to analyze these data taken under large zenith angles.
Methods. Here we present an enhanced analysis method for data taken at high zenith angles. We developed improved methods for event selection that led to a better background suppression.
Results. The quality of the results presented here is superior to the results presented previously for this data set: detection of the source on a higher significance level and a lower analysis threshold. The averaged energy spectrum we derived has a spectral index of (−3.5 ± 0.2) above 400 GeV, which is in good agreement with the spectral shape measured by H.E.S.S. during the major flare on MJD 53 944. Furthermore, we present the spectral energy distribution modeling of PKS 2155-304. With our observations we increased the duty cycle of the source extending the light curve derived by H.E.S.S. after the outburst. Finally, we find night-by-night variability with a maximal amplitude of a factor three to four and an intranight variability in one of the nights (MJD 53 945) with a similar amplitude.
Key words: BL Lacertae objects: individual: PKS 2155-304 / astroparticle physics / methods: data analysis
© ESO, 2012
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