Astronomy and Astrophysics – Astrophysics
Scientific paper
Apr 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002aps..aprb17095k&link_type=abstract
American Physical Society, April Meeting, Jointly Sponsored with the High Energy Astrophysics Division (HEAD) of the American As
Astronomy and Astrophysics
Astrophysics
Scientific paper
The largest fraction of the observing time available for the HEGRA (High Energy Gamma Ray Astronomy) system of five imaging atmospheric Čerenkov telescopes was dedicated to study the Blazar AGN. The complete target list amounts to 39 AGN, mostly of the BL Lac type at low red-shifts (z <= 0.2). Each of those objects was observed at least for 10 hrs. Given the HEGRA sensitivity, this allowed a systematic search for point-like TeV γ-ray sources at a flux level of about 10-11 erg cm-2 s-1. Besides two well-established γ-ray sources, Mkn 421 (z=0.031) and Mkn 501 (z=0.034), another BL Lac object, H 1426+428, was firmly detected by HEGRA. Preliminary analysis of the 1ES 1959+650 data also gives evidence of TeV γ-ray emission. Since 1997 HEGRA was continuously monitoring Mkn 501, which went in 1997 into a remarkable outburst state of high γ-ray emission (up to 10 Crab) and strong variability at an intra-day time-scale, even though no statistically significant change of the spectral shape was noticed. The time averaged energy spectrum of Mkn 501, measured in the range from 500 GeV up to 20 TeV, is apparently curved and can be well fitted by a power law with an exponential cut-off at ~ 6 TeV. During the 1999-2001 observation campaigns Mkn 501 was in a rather low state, revealing a possibly substantially softer energy spectrum. On the contrary, Mkn 421 has shown only recently a high emission state with unprecedented high flux (up to 4 Crab) in January-February 2001. Large event statistics allowed measurements of the Mkn 421 energy spectrum up to 10 TeV, which is again curved with an exponential cut-off at ~ 4 TeV. The HEGRA Mkn 421 data show evidence for TeV flux variability on a 30 min time-scale. The successful multi-wavelength campaigns undertaken jointly with the Rossi X-Ray Timing Explorer in June 1998 and February 2000 for Mkn 501 and Mkn 421, respectively, ensured simultaneous X-ray and TeV γ-ray observations, which were used for a comprehensive study of the emission mechanisms of relativistic AGN jets. The third HEGRA BL Lac H 1426+428 has a relatively large red-shift (z=0.129) and its TeV γ-ray spectrum should be significantly modulated by the effect of intergalactic absorption. The energy spectrum of H 1426+428, derived from HEGRA data taken in 1999 and 2000 for rather limited events statistics, is consistent with a power-law of a spectral index similar to the Crab Nebula spectrum although a specifically absorption-modified spectrum is a better approximation to the data. A more precise spectral measurements of H 1426+428 is expected, as the result of the impending large-scale HEGRA observational campaign in 2002. The scientific program for studying Blazar AGN with HEGRA recently yielded a variety of important physics results, which substantially widen our understanding of the emission mechanisms and the propagation of TeV γ-rays.
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