Variations of the TeV energy spectrum at different flux levels of Mkn 421 
observed with the HEGRA system of Cherenkov telescopes

Aharonian, F.; Akhperjanian, A.; Beilicke, M.; Bernlöhr, K.; Borst, H.; Bojahr, H.; Bolz, O.; Coarasa, T.; Contreras, J.; Cortina, J.; Costamante, L.; Denninghoff, S.; Fonseca, V.; Girma, M.; Gotting, N.; Heinzelmann, G.; Hermann, G.; Heusler, A.; Hofmann, W.; Horns, D.; Jung, I.; Kankanyan, R.; Kestel, M.; Kohnle, A.; Konopelko, A.; Kornmeyer, H.; Kranich, D.; Krawczynski, H.; Lampeitl, H.; Lopez, M.; Lorenz, E.; Lucarelli, F.; Mang, O.; Meyer, H.; Mirzoyan, R.; Milite, M.; Moralejo, A.; Ona, E.; Panter, M.; Plyasheshnikov, A.; Pühlhofer, G.; Rauterberg, G.; Reyes, R.; Rhode, W.; Ripken, J.; Rowell, G.; Sahakian, V.; Samorski, M.; Schilling, M.; Siems, M.; Sobzynska, D.; Stamm, W.; Tluczykont, M.; Völk, H. J.; Wiedner, C. A.; Wittek, W.; Remillard, R. A.

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Variations of the TeV energy spectrum at different flux levels of Mkn 421 observed with the HEGRA system of Cherenkov telescopes

MPS-Authors

/persons/resource/persons30244

Aharonian, F.
Division Prof. Dr. Werner Hofmann, MPI for Nuclear Physics, Max Planck Society;

/persons/resource/persons30247

Akhperjanian, A.
Division Prof. Dr. Werner Hofmann, MPI for Nuclear Physics, Max Planck Society;

/persons/resource/persons30290

Beilicke, M.
Division Prof. Dr. Werner Hofmann, MPI for Nuclear Physics, Max Planck Society;

/persons/resource/persons30300

Bernlöhr, K.
Division Prof. Dr. Werner Hofmann, MPI for Nuclear Physics, Max Planck Society;

/persons/resource/persons30322

Bolz, O.
Division Prof. Dr. Werner Hofmann, MPI for Nuclear Physics, Max Planck Society;

/persons/resource/persons30382

Costamante, L.
Division Prof. Dr. Werner Hofmann, MPI for Nuclear Physics, Max Planck Society;

/persons/resource/persons30517

Girma, M.
Division Prof. Dr. Werner Hofmann, MPI for Nuclear Physics, Max Planck Society;

/persons/resource/persons30584

Hermann, G.
Division Prof. Dr. Werner Hofmann, MPI for Nuclear Physics, Max Planck Society;

/persons/resource/persons30591

Heusler, A.
Division Prof. Dr. Werner Hofmann, MPI for Nuclear Physics, Max Planck Society;

/persons/resource/persons30605

Hofmann, W.
Division Prof. Dr. Werner Hofmann, MPI for Nuclear Physics, Max Planck Society;

/persons/resource/persons30623

Horns, D.
Division Prof. Dr. Werner Hofmann, MPI for Nuclear Physics, Max Planck Society;

/persons/resource/persons30641

Jung, I.
Division Prof. Dr. Werner Hofmann, MPI for Nuclear Physics, Max Planck Society;

/persons/resource/persons30653

Kankanyan, R.
Division Prof. Dr. Werner Hofmann, MPI for Nuclear Physics, Max Planck Society;

/persons/resource/persons30699

Kohnle, A.
Division Prof. Dr. Werner Hofmann, MPI for Nuclear Physics, Max Planck Society;

/persons/resource/persons30703

Konopelko, A.
Prof. Heinrich J. Völk, Emeriti, MPI for Nuclear Physics, Max Planck Society;

/persons/resource/persons30720

Krawczynski, H.
Division Prof. Dr. Werner Hofmann, MPI for Nuclear Physics, Max Planck Society;

/persons/resource/persons30741

Lampeitl, H.
Division Prof. Dr. Werner Hofmann, MPI for Nuclear Physics, Max Planck Society;

/persons/resource/persons30878

Panter, M.
Division Prof. Dr. Werner Hofmann, MPI for Nuclear Physics, Max Planck Society;

/persons/resource/persons30901

Plyasheshnikov, A.
Division Prof. Dr. Werner Hofmann, MPI for Nuclear Physics, Max Planck Society;

/persons/resource/persons30914

Pühlhofer, G.
Division Prof. Dr. Werner Hofmann, MPI for Nuclear Physics, Max Planck Society;

/persons/resource/persons30959

Rowell, G.
Division Prof. Dr. Werner Hofmann, MPI for Nuclear Physics, Max Planck Society;

/persons/resource/persons30987

Schilling, M.
Division Prof. Dr. Werner Hofmann, MPI for Nuclear Physics, Max Planck Society;

/persons/resource/persons31141

Völk, H. J.
Prof. Heinrich J. Völk, Emeriti, MPI for Nuclear Physics, Max Planck Society;

/persons/resource/persons31174

Wiedner, C. A.
Division Prof. Dr. Werner Hofmann, MPI for Nuclear Physics, Max Planck Society;

/persons/resource/persons31183

Wittek, W.
Division Prof. Dr. Werner Hofmann, MPI for Nuclear Physics, Max Planck Society;

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Citation

Aharonian, F., Akhperjanian, A., Beilicke, M., Bernlöhr, K., Borst, H., Bojahr, H., et al. (2002). Variations of the TeV energy spectrum at different flux levels of Mkn 421 observed with the HEGRA system of Cherenkov telescopes. Astronomy & Astrophysics, 393(1), 89-99.

Cite as: https://hdl.handle.net/11858/00-001M-0000-0011-8293-6

Abstract

The nearby BL Lacertae (BL Lac) object Markarian 421 (Mkn 421) at a red shift z = 0.031 was observed to undergo strong TeV gamma-ray outbursts in the observational periods from December 1999 until May 2001. The time averaged flux level F(E > 1 TeV) in the 1999/2000 season was (1.43 +/- 0.04) x 10(-11) ph cm(-2) s(-1), whereas in the 2000/2001 season the average integral flux increased to (4.19 +/- 0.04) x 10(-11) ph cm(-2) s(-1). Both energy spectra are curved and well fit by a power law with an exponential cut-off energy at 3.6(+0.4 0.3)(stat)(+0.9 0.8)(sys) TeV. The respective energy spectra averaged over each of the two time periods indicate a spectral hardening for the 2000/2001 spectrum. The photon index changes from 2.39 +/- 0.09(stat) for 1999/2000 to 2.19 +/- 0.02(sta)t in 2000/2001. The energy spectra derived for different average flux levels ranging from 0.5 to 10 x 10(-11) ph cm(-2) s(-1) follow a clear correlation of photon index and flux level. Generally, the energy spectra are harder for high flux levels. From January to April 2001 Mkn 421 showed rapid variability (doubling time as short as 20 min), accompanied with a spectral hardening with increasing flux level within individual nights. For two successive nights (MJD 51989-51991, March 21-23, 2001), this correlation of spectral hardness and change in flux has been observed within a few hours. The cut-off energy for the Mkn 421 TeV spectrum remains within the errors constant for the different flux levels and differs by DeltaE = 2.6 +/- 0.6(stat) +/- 0.6(sys) TeV from the value determined for Mkn 501. This indicates that the observed exponential cut-off in the energy spectrum of Mkn 421 is not solely caused by absorption of multi-TeV photons by pair-production processes with photons of the extragalactic near/mid infrared background radiation.