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| Home > Publications database > Search for extremely short transientgamma-ray sources with the VERITAS observatory |
| Dissertation / PhD Thesis | PUBDB-2017-00337 |
2016
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Please use a persistent id in citations: doi:10.3204/PUBDB-2017-00337
Abstract: In astronomy, many of the observed sources show a transient behavior. Examples aregamma-ray bursts (GRBs) and active galactic nuclei (AGN). For those source typesthe variability can be very short, in the order of seconds to minutes. Measuringthe flux variations is necessary to understand the underlying physical processesresponsible for the emission. However, the detection of very short flares can bedifficult in the very high-energy range, in which imaging atmospheric Cherenkovtelescopes like VERITAS are operating. This is due to the large background andthe comparable low signal rates.This thesis discusses the implementation of advanced statistical methods (exp-test and Bayesian-Blocks) into the VERITAS analysis framework, that are optimized forthe detection of significant variations in the event rate. The performance of thesemethods is evaluated and compared by using Monte Carlo simulations of minute-scale flares for two different VERITAS states, pre- and post-hardware-upgrade. Itis shown that the advanced methods can improve the detection sensitivity for shortflares with high fluxes of more than the Crab flux (Crab unit = C.U.). For example,flares at 2 C.U. with short durations down to 100 sec are now detectable, which isnot possible with the standard method. In the next step of this thesis, 6 GRB afterglows and 450 runs of AGN data, observedby VERITAS, are analyzed with the advanced methods. In none of the AGN runsa significant detection of short time variability is made, which is in consistence withthe canonical AGN models. The investigation of the 6 GRB afterglows also didnot reveal any short flares in the GeV-TeV range. However, for two of them it waspossible to estimate an upper flux limit of 1.25 C.U. (≈3,7·10$^{−10}$ erg cm$^{−1}$s$^{−1}$[0,1;10 TeV]).
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