Observation of high-energy gamma-radiation from the direction of the active galaxies Mkn 421 and Mkn 501

Details Observation of high-energy gamma-radiation from the direction of the active galaxies Mkn 421 and Mkn 501 Observation of high-energy gamma-radiation from the direction of the active galaxies Mkn 421 and Mkn 501

Jun 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997phdt........21p&link_type=abstract

Ph.D. Thesis, Max-Planck-Institute for Physics, Munich, report MPI-PhE/97- 27, (1997)

Astronomy and Astrophysics

Astronomy

4

Gamma-Ray Astronomy, Cherenkov Telescopes, Mkn 421, Mkn 501, Crab Nebula

Scientific paper

During the last few years, very high energy gamma-ray astronomy with Cherenkov telescopes has become a widely accepted tool of astrophysics. This work describes the second confirmatory detections of the only two extragalactic sources of gamma-rays above 1 TeV (1012 eV) so far established.
In addition to describing the analysis methods specially used in this thesis, the first two chapters give a brief, general introduction to the physics of cosmic rays, active galaxies and the atmospheric Cherenkov technique.
Chapter 3 describes the first two Cherenkov telescopes (CT1 and CT2) of the HEGRA collaboration. Extensive Monte-Carlo studies were carried out to determine the characteristics of these telescopes.
In chapters 4 and 5, observations of three astronomical objects - the active galactic nuclei Mkn 421 and Mkn 501 and a supernova remnant, the Crab Nebula - from the years 1995 and 1996 are analysed with regard to the integral photon flux, the photon energy spectrum and the time-variability of the flux. The observations were made with CT1 and CT2. The Crab Nebula is a well known source and serves as a ``standard candle'' for the calibration of the telescopes. In contrast, Mkn 421 and Mkn 501 had until the end of 1996 been detected by only one other experiment at energies above 1 TeV. Chapter 6 discusses the results obtained in chapters 4 and 5; chapter 7 finally summarizes.
The results of this work can be divided into methodology, determination of the properties of the telescopes used, flux measurements of the three objects observed, flux variability studies, and the calculation of an upper limit to the increase of the optical depth due to gamma-gamma-absorption at the distance of Mkn 501 between energies of 1.2 and 10 TeV.
The technical developments presented comprise two methods to determine the energy spectrum of a gamma-ray source, a procedure for testing the pointing accuracy of a Cherenkov telescope and an investigation of the effective field of view of such a telescope for hadronic showers.
The flux measurements are summarized in table 6.1 on page 183. The Crab Nebula flux - as determined here - agrees well with other independent measurements (see figure 6.1 on page 184). The observed spectrum above 1 TeV is described by a power law of differential spectral index alpha = 2.54 +- 0.24.
For Mkn 421, the observed spectrum above 1 TeV is described by a power law of differential spectral index alpha = 3.12 +- 0.45. The spectrum here is clearly steeper than that of index alpha = 1.7 +- 0.2 observed by the EGRET experiment between 100 MeV and 10 GeV (see figure 6.2 on page 186). In the 1995 and 1996 observing seasons, this source was found to be in a quiescent state, the measurements being consistent with a constant level of emission.
Mkn 501 was observed in 1996 only. Also here no significant deviation from a constant, relatively low flux (compared to that of the Crab Nebula) was found. The spectrum is, however, with an index of alpha = 2.5 +- 0.4, slightly flatter than that of Mkn 421 and extends to at least 10 TeV (see figure 4.22 on page 167 and figure 6.3 on page 188).
With this information, an upper limit on the increase of the optical depth due to gamma-gamma-absorption in the intergalactic infrared background between 1.2 and 10 TeV is derived and found to be just consistent with the predictions.

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