XIAO Hu-bing

(Department of Physics and Astronomy“G.Galilei”,University of Padova,Padova 35131)

Blazars,an extreme subclass of active galactic nuclei(AGNs),are the most persistent cosmological objects that emit electromagnetic radiation and neutrino radiation.Fermi-LAT(FermiLarge Area Telescope)has made great contributions to the research of blazars,and it has released 4 generations of point source catalogues since it was launched in 2008.This thesis aims to make an extensive study of blazars and its content covers a few aspects,including data analysis,spectral energy distribution(SED)modelling and the concept design of a MeV detector.

Firstly,we generated a new machine learning(ML)method to identify those unclassifiedFermi-LAT third source catalogue(3FGL)uncertain class of blazars(BCUs)as either flat-spectrum radio quasars(FSRQs)or BL Lacertae objects(BL Lacs)based on their spectra information in the 3FGL.Our ML method has an accuracy of 92.13%,we managed to predict 573 BCUs to 326 BL Lacs and 247 FSRQs.Comparisons between the results from our method and those from others’works indicated our method is well-performed and efficient.The identification results can guide further optical observation to confirm their classification.

Then,we notice that the main factor affecting the classification of a blazar is its spectra difference.In order to further study the intrinsic mechanism of the jet dynamic property that determines their spectra difference between FSRQs and BL Lacs,we study blazar broadband SED.We modelled 344 blazar broadband SEDs and provided a series of reference parameters,6 parameters to describe the SED shape and 9 parameters to describe the dissipation region.These results indicate that the BL Lacs has higher average electron energy and also shows a stronger Klein-Nishina effect than FSRQs.Besides,the FSRQs and BL Lacs have different photon environment and show different particle acceleration mechanisms in the jet.

At last,we noticed that lack of telescope operating at MeV band is blamed for the incomplete SED of blazars.MeV band is very important to determine SED shape,in addition,the MeV peaked blazar is a subclass of blazars that are expected to show substantial features in their spectra compared to theFermidetected blazars.We made a study of MeV detector design and performance simulation.The original design of this MeV telescope is 2U(10 cm×10 cm×20 cm)to work as a pathfinder in a low-Earth orbit.The simulation shows that the 2U MeV detector is too small to detect flare state blazar in the MeV band.To detect the MeV band spectrum,the detector must be at least 5 times larger.