Effects of EBL extinction on the VHE spectra of blazars

Active Galactic Nuclei (AGN) are the most powerful steady sources of electromagnetic radiation in the cosmos. Blazars are AGN with their jets pointing straight at us, giving us the opportunity to probe deep into the core which generates this tremendous luminosity. The Very High Energy (VHE; E>100 GeV) photons from blazars while travelling through the intergalactic medium, interact with the low energy photons (in the UV-IR range) constituting the Extragalactic Background Light (EBL), by producing e-e+ pairs, and get absorbed. More than 25 nearby blazars (z<0.3) have been detected in VHE, but due to EBL extinction it is extremely difficult to detect distant sources. This phenomena gives us the means to measure the EBL level which cannot be directly measured. In this thesis the effects of EBL absorption on the spectra of blazars is studied. Two methods to discover distant VHE blazars were explored. The observations with HESS led to the detection of the blazar PKS 1510-089 at z=0.36, at a 4.8 sigma level. A cross check analysis with a more advanced analysis tool confirmed this detection at a 8.5 sigma level. The GeV spectrum obtained by analyzing Fermi GST data, was adopted as the intrinsic source spectrum. Models for the gamma-ray spectrum including the EBL absorption was fitted to the gamma-ray data and it was shown that all the 4 current EBL models considered here, fit the data well. It is concluded that this is due to the sensitivity limits of the current VHE instruments. Stronger constraints on the EBL would only be possible for sources with much harder spectrum. The outlook for future experiments is discussed. EBL extinction causes bias in the sample of detected blazars. MonteCarlo simulations were used to generate parent samples for the VHE blazars. These were compared to the true VHE blazar sample and constraints on the parent sample properties were drawn. The lack of a spectral softening with z in the true VHE sample, is found consistent with a parent sample that clearly shows this softening, making it unnecessary to assume any special dependence of the intrinsic spectral index on z. The dependence of the evolution of the EBL on the cosmological model considered, was explored. The uncertainty in cosmology was found to be negligible compared to the uncertainty on the EBL. The dependence of the extinction on the error in the EBL models as well as the effect of neglecting the EBL evolution with z was illustrated

PKS 1510-089: a rare example of a flat spectrum radio quasar with a very high-energy emission

The blazar PKS 1510-089 is an example of flat spectrum radio quasars. High-energy emissions from this class of objects are believed to have been produced by inverse Compton radiation with seed photons originating from the broad line region. In such a paradigm, a lack of very high-energy emissions is expected because of the Klein-Nishina effect and strong absorption in the broad line region. Recent detection of at least three such blazars by Cherenkov telescopes has forced a revision of our understanding of these objects. We have aimed to model the observed spectral energy distribution of PKS 1510-089 from the high-energy flares in March 2009, during which very high-energy emission were also detected by H.E.S.S. We have applied the single-zone internal shock scenario to reproduce the multiwavelength spectrum of PKS~1510-089. We have followed the evolution of the electrons as they propagate along the jet and emit synchrotron and inverse Compton radiation. We have considered two sources of external photons: the dusty torus and the broad line region. We have also examined the effects of the gamma-gamma absorption of the high-energy photons both in the AGN environment (the broad line region and the dusty torus), as well as while traveling over cosmological distances: the extragalactic background light. We have successfully modeled the observed spectrum of PKS 1510-089. In our model, the highest energy emission is the result of the Comptonization of the infrared photons from the dusty torus, thus avoiding Klein-Nishina regime, while the bulk of the emissions in the GeV range may still be dominated by the Comptonization of radiation coming from the broad line region.Comment: Accepted for publication in A&

Potential of EBL and cosmology studies with the Cherenkov Telescope Array

Very high energy (VHE, E >100 GeV) gamma-rays are absorbed via interaction with low-energy photons from the extragalactic background light (EBL) if the involved photon energies are above the threshold for electron-positron pair creation. The VHE gamma-ray absorption, which is energy dependent and increases strongly with redshift, distorts the VHE spectra observed from distant objects. The observed energy spectra of the AGNs carry, therefore, an imprint of the EBL. The detection of VHE gamma-ray spectra of distant sources (z = 0.11 - 0.54) by current generation Imaging Atmospheric Cherenkov Telescopes (IACTs) enabled to set strong upper limits on the EBL density, using certain basic assumptions about blazar physics. In this paper it is studied how the improved sensitivity of the Cherenkov Telescope Array (CTA) and its enlarged energy coverage will enlarge our knowledge about the EBL and its sources. CTA will deliver a large sample of AGN at different redshifts with detailed measured spectra. In addition, it will provide the exciting opportunity to use gamma ray bursts (GRBs) as probes for the EBL density at high redshifts.Comment: 12 pages, 9 figures, to appear in Astroparticle Physics. arXiv admin note: text overlap with arXiv:1005.119

Short GAmma Ray Front Air Cherenkov Experiment (SGARFACE) analysis and cross-calibration with Whipple 10 m

In this work the Short GAmma Ray Front Air Cherenkov Experiment (SGARFACE) instrument is calibrated against the Whipple TeV system as a reference to get a measure of the sensitivity of the instrument. The electronics gain of the system is quantified by the digital counts to photo-electron (d.c./p.e.) ratio, and is a measure of the system's response to a photo-electron collected by the photo-detectors. This ratio is required to reconstruct the number of Cherenkov photons collected by the telescope. The Cherenkov light density and angular distribution in the focal plane of an Imaging Atmosphere Cherenkov Telescope (IACT) allows us to reconstruct the atmospheric-shower parameters. Thus the d.c./p.e. is an important characteristic measure for the instrument. To calculate this measure, an event analysis software for the SGARFACE instrument is prepared, complete with image parameter calculation and event display. Then cosmic-ray events that simultaneously triggered both Whipple and SGARFACE are used to compare the charges (d.c.) given by both instruments. The known (previously calibrated) d.c. to p.e. ratio of the Whipple, is then used to get an estimate of the d.c./p.e. for SGARFACE. The SGARFACE on the Whipple 10 m telescope, at Mt. Hopkins Arizona, is an instrument designed to detect bursts of low energy [Difference] 250 MeV gamma-rays which could be the result of the last stage of PBH evaporation. It has been taking data, concurrently with the Whipple TeV system for more than two years now. The existence of Primordial Black Holes and their evaporation mechanism can only be ascertained by detecting and identifying radiations that might be their signature. Previous attempts to measure the radiation from PBH have put rough upper limits at various energies. SGARFACE is designed to search for PBH radiation at previously unexplored lower energies. The results of this work would enable the data taken by the instrument to be interpreted and analyzed to search for long duration bursts that might be signature of the final explosive stage of PBH evaporation.</p