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SUMMARY:Testing cascade models of extragalactic gamma-ray propagation usin
 g observations of extreme TeV blazars with imaging Cherenkov telescopes
DTSTART;VALUE=DATE-TIME:20161011T101500Z
DTEND;VALUE=DATE-TIME:20161011T103000Z
DTSTAMP;VALUE=DATE-TIME:20260713T092554Z
UID:indico-contribution-487@cern.ch
DESCRIPTION:Speakers: Emil Khalikov (SINP MSU)\nRecent observations made b
 y imaging Cherenkov telescopes have found a pile-up in the TeV region of s
 ome blazar spectra in the optically thick regions (optical depth in this c
 ase is the measure of absorption of primary gamma rays on extragalactic ba
 ckground light (EBL) due to the γγ→e+e- process). This pile-up is not 
 consistent with the predictions of the commonly-used models which include 
 only absorption of primary photons and adiabatic losses. We believe that t
 his “anomaly” may be accounted for if certain mechanisms such as elect
 romagnetic (EM) cascade development are taken into consideration.\n    The
 re are three distinct regimes of cascade development depending on the ener
 gy of the incident particle. If this energy is high enough\, then there ar
 e several generations of cascade particles and the observable spectrum is 
 practically independent of the energy and type (electron/photon) of the pr
 imary particle but it depends on the distance to the source (the so-called
  universal regime). If that energy is comparatively low\, the number of ge
 nerations decreases and the spectrum becomes dependent on both energy and 
 type of the primary particle (one-generation regime). The third possible r
 egime is the “extreme Klein-Nishina regime” in which electrons transfe
 r most of their energy to the background photons via inverse Compton scatt
 ering process (this regime may take place at ultrahigh energies of primary
  photons\, E>1 EeV). In our work we\, for the first time\, consider in det
 ail the “one-generation regime”\, the universal regime and the transit
 ion between them.\n    In this talk we will mainly focus on the so-called 
 hadronic cascade models\, in which primary protons create secondary photon
 s by means of pair-production and photopion processes and these photons\, 
 in turn\, initiate EM cascades. We will compare the results obtained in ha
 dronic and electromagnetic cascade models (in the latter primary particles
  are photons instead of protons) and provide formal best fits for these mo
 dels to the widely available data from the HESS and VERITAS experiments (w
 hich has never been done before). The sample of observations used in our w
 ork includes 6 blazars with hard spectra. In addition to that\, to calcula
 te observable spectra for the case of hadronic models\, we will introduce 
 a fast and reasonably accurate hybrid method using a large pre-computed da
 tabase of cascades from primary electrons and photons.\n    We also\, for 
 the first time\, consider a possibility that the beam of primary protons i
 s affected by an intervening galaxy clusters near the source and on the wa
 y from the source to the observer. For the first case\, we perform a detai
 led statistical analysis for the case of blazar 1ES 0229+200 using the emi
 ssion model of Tavecchio (2013) and show that this model is excluded with 
 significance > 5σ unless the magnetic field strength near the source of p
 rotons is much lower than 100 nG. Finally\, we show that the intermediate 
 cluster may significantly affect both shape and normalization of the obser
 vable spectrum.\n\nhttps://indico.particle.mephi.ru/event/4/contributions/
 487/
LOCATION:Milan Hotel Verdi
URL:https://indico.particle.mephi.ru/event/4/contributions/487/
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