Hrupec, Dario
(2008)
Extragalactic sources of rapidly variable high energy gamma radiation.
Doctoral thesis, Sveučilište u Zagrebu, Prirodoslovno-matematički fakultet.
Abstract
The atmosphere is an intrinsic part of any imaging atmospheric Čerenkov telescope and the telescope response is therefore sensitive to unpredictable changes in the atmosphere. A lot of observational data taken during non-ideal atmospheric transparency have not been analyzed because of a lack of an appropriate analysis method that would be able to provide corrections for the imperfect transparency. On the other hand, extragalactic sources of high-energy cosmic gamma-rays (e.g. active galactic nuclei and gamma-ray bursts) are usually highly variable and the temporal characteristics of their light curves are key to the understanding of the physics of their sources. It is therefore very important to extend the observation time for variable sources as much as possible. In order to significantly extend the effective observational time of variable gamma-ray sources, we have developed a new data analysis method, which first determines the actual variable atmospheric transparency from the gamma-ray measurements, and then corrects those measurements according to the estimated atmospheric effect. To learn how the clouds influence gamma-ray measurements, we have implemented the simulation of clouds into the Monte Carlo simulation chain. Simulations starts with program Corsika, which simulates the development of particle showers in the atmosphere, and traces the Čerenkov light emitted by charged particles to the telescope. This method may extend the effective observation time in ground-based gamma-ray astronomy in general. Here we report on the application of the method in the first analysis of a particularly important data set from 2001 that includes a period of very strong activity of the blazar Mkn 421. In the near future, modern lidars will be able to precisely measure the distribution of the cloud density along the line of sight of the telescope, and thus provide significantly more information for our correction method. We also developed a new approach to correlation study based on MC simulations and the Fourier convolution. We applied the new method to analyze our CT1 data from 2003 of Mkn 421. We also reanalyzed correlation of some recent MAGIC data on blazars Mkn 421, Mkn 501 and newly discovered Mkn 180.
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