Jagiellonian University

About: Jagiellonian University is a education organization based out in Krakow, Poland. It is known for research contribution in the topics: Population & Catalysis. The organization has 17438 authors who have published 44092 publications receiving 862633 citations. The organization is also known as: Academia Cracoviensis & Akademia Krakowska.

Broad-band γ-ray and X-ray spectra of NGC 4151 and their implications for physical processes and geometry

Abstract: We study y-ray observations of NGC 4151 by the Gamma Ray ObselVatory (GRO)/ OSSE contemporaneous with X-ray observations by ROSAT and Ginga in 1991 June andASCA in 1993 May. The spectra are well modelled by thermal Comptonization and a dual neutral absorber. We also find, for the first time for NGC 4151, a Compton~reflecti on spectral component in the Ginga/OSSE data. When reflection is taken into account, the intrinsic X-ray energy spectral index is 0(",0.8 and the plasma temperature is '" 60 ke V for both observations, conditions which imply an optical depth of '" 1. The X-ray spectral index is within the range, 0(~0.95 ±0.15, observed from other Seyfert Is. Also, the OSSE spectra of those and other observations of NGC 4151 are statistically indistinguishable from the average OSSE spectrum of radio-quiet Seyfert Is. Thus, NGC 4151 observed in 1991 and 1993 has the intrinsic X-ray/y-ray spectrum typical for Seyfert Is, and the main property distinguishing it from other Seyfert Is is a large absorbing column of '" 1023 cm -2. We find no evidence for a strong, broad and redshifted Fe KO( line component in the ASCA spectrum of 1993 May. Also, the Compton-reflection component in the Ginga/OSSE spectrum is a few times too small to account for the strength of the broad/redshifted line reported elsewhere to be found in this and other ASCA spectra ofNGC 4151. On the other hand, we confirm previous studies in that archival X-ray data do imply strong intrinsic X-ray variability and hardness of the intrinsic spectrum in low X-ray states. An observed softening of the intrinsic X-ray spectrum with the increasing flux implies that variability in y-rays is weaker than in X-rays, which agrees with the 100-ke V flux changing only within a factor of 2 in archival OSSE and GRANAT/SIGMA observations. The relative hardness of the intrinsic X-ray spectrum rules out the homogeneous hot corona/cold disc model for this source. Instead, the hot plasma has to subtend a small solid angle as seen from the source of UV radiation. If the hot plasma is purely thermal, it consists of electrons rather than e± pairs. On the other hand, the plasma can be pair-dominated if a small fraction of the power is non-thermal.

Energy Spectrum of Cosmic-Ray Electrons at TeV Energies

Abstract: The very large collection area of ground-based gamma-ray telescopes gives them a substantial advantage over balloon/satellite based instruments in the detection of very-high-energy (>600 GeV) cosmic-ray electrons. Here we present the electron spectrum derived from data taken with the H.E.S.S. system of imaging atmospheric Cherenkov telescopes. In this measurement, the first of this type, we are able to extend the measurement of the electron spectrum beyond the range accessible to direct measurements. We find evidence for a substantial steepening in the energy spectrum above 600 GeV compared to lower energies.

The Third Catalog of Active Galactic Nuclei Detected by the Fermi Large Area Telescope

Abstract: The third catalog of active galactic nuclei (AGNs) detected by the Fermi-LAT (3LAC) is presented. It is based on the third Fermi-LAT catalog (3FGL) of sources detected between 100 MeV and 300 GeV w ...