Browsing by Subject "Galaxies: active"

Now showing items 1-10 of 62

    • The 2010 very high energy γ-ray flare and 10 years of multi-wavelength observations of M 87 

      Abramowski, A.; Büsching, I.; De Jager, O.C.; Sheidaei, F.; Venter, C.; Vorster, M.; H.E.S.S. Collaboration (IOP Publishing, 2012)
      The giant radio galaxy M 87 with its proximity (16 Mpc), famous jet, and very massive black hole ((3 – 6) × 109 M☉) provides a unique opportunity to investigate the origin of very high energy (VHE; E > 100 GeV) γ-ray ...

    • The 2012 flare of PG 1553+113 seen with H.E.S.S. and Fermi-LAT 

      Abramowski, A.; Böttcher, M.; Davids, I.D.; Ivascenko, A.; Krüger, P.P.; Pekeur, N.W.; Spanier, F.; Sushch, I.; Venter, C.; Vorster, M.; H.E.S.S. Collaboration (IOP Publishing, 2015)
      Very high energy (VHE, E > 100 GeV) γ-ray flaring activity of the high-frequency peaked BL Lac object PG 1553+113 has been detected by the H.E.S.S. telescopes. The flux of the source increased by a factor of 3 during the ...

    • The ablation of gas clouds by blazar jets 

      Heil, Jonathan; Zacharias, Michael (EDP Sciences, 2020)
      Context. Flaring activity in blazars can last for vastly different timescales, and it may be the result of density enhancements in the jet flow that result from the intrusion of an interstellar cloud into the jet. Aims. ...

    • Attenuation of TeV γ-rays by the starlight photon field of the host galaxy 

      Zacharias, Michael; Chen, Xuhui; Wagner, Stefan J. (Oxford Univ Press, 2017)
      The absorption of TeV γ-ray photons produced in relativistic jets by surrounding soft photon fields is a long-standing problem of jet physics. In some cases, the most likely emission site close to the central black hole ...

    • The awakening of BL Lacertae: observations by Fermi, Swift and the GASP-WEBT 

      Raiteri, C.M.; Böttcher, M.; Villata, M.; D'Ammando, F.; Larionov, V.M. (Oxford University Press, 2013)
      Since the launch of the Fermi satellite, BL Lacertae has been moderately active at γ-rays and optical frequencies until 2011 May, when the source started a series of strong flares. The exceptional optical sampling achieved ...

    • Carbon-loud SDSS BOSS type II quasars at z > 2: high-density gas or secondary production of carbon? 

      Silva, M.; Lagos, P.; Humphrey, A.; Morais, S.G. (Oxford Univ Press, 2020)
      We study the ultraviolet (UV) emission-line ratios of a sample of 145 type II quasars (QSO2s) from Sloan Digital Sky Survey III Baryon Oscillation Spectroscopic Survey, and compare against a grid of active galactic nucleus ...

    • Cascading constraints from neutrino-emitting blazars: the case of TXS 0506+056 

      Reimer, Anita; Böttcher, Markus; Buson, Sara (IOP Publishing, 2019)
      We present a procedure to generally constrain the environments of neutrino-producing sites in photomeson production models of jetted active galactic nuclei (AGNs) where any origin of the dominant target photon field can ...

    • Characterizing the γ-ray long-term variability of PKS 2155-304 with H.E.S.S. and Fermi-LAT 

      Abdalla, H.; Barnard, M.; Böttcher, M.; Davids, I.D.; Garrigoux, T.; Ivascenko, A.; Krüger, P.P.; Pekeur, N.W.; Seyffert, A.S.; Spanier, F.; Sushch, I.; Van der Walt, D.J.; Van Rensburg, C.; Venter, C.; Wadiasingh, Z.; H.E.S.S. Collaboration (EDP Sciences, 2017)
      Studying the temporal variability of BL Lac objects at the highest energies provides unique insights into the extreme physical processes occurring in relativistic jets and in the vicinity of super-massive black holes. To ...

    • Cloud ablation by a relativistic jet and the extended flare in CTA 102 in 2016 and 2017 

      Zacharias, M.; Böttcher, M.; Jankowsky, F.; Lenain, J.-P.; Wagner, S.J. (IOP, 2017)
      In late 2016 and early 2017, the flat spectrum radio quasar CTA 102 exhibited a very strong and long-lasting outburst. The event can be described by a roughly two-month long increase of the baseline flux in the monitored energy ...

    • Constraints on the emission region of 3C 279 during strong flares in 2014 and 2015 through VHE γ-ray observations with H.E.S.S. 

      Abdalla, H.; Arcaro, C.; Backes, M.; Barnard, M.; Böttcher, M.; Chand, T.; Chandra, S.; Ndiyavala, H.; Schutte, H.; Seyffert, A.S.; Van der Walt, D.J.; Van Rensburg, C.; Venter, C.; Wadiasingh, Z.; Zacharias, M.; Żywucka, N.; H.E.S.S. Collaboration (EDP Sciences, 2019)
      The flat spectrum radio quasar 3C 279 is known to exhibit pronounced variability in the high-energy (100 MeV <  E <  100 GeV) γ-ray band, which is continuously monitored with Fermi-LAT. During two periods of high activity ...