<?xml version="1.0" encoding="UTF-8"?><xml><records><record><source-app name="Biblio" version="7.x">Drupal-Biblio</source-app><ref-type>17</ref-type><contributors><authors><author><style face="normal" font="default" size="100%">Konopelko, Alexander K.</style></author><author><style face="normal" font="default" size="100%">Kirk, John G.</style></author><author><style face="normal" font="default" size="100%">Stecker, Floyd W.</style></author><author><style face="normal" font="default" size="100%">Mastichiadis, Apostolos</style></author></authors></contributors><titles><title><style face="normal" font="default" size="100%">Evidence for Intergalactic Absorption in the TEV Gamma-Ray Spectrum of Markarian 501</style></title><short-title><style face="normal" font="default" size="100%">The Astrophysical Journal</style></short-title></titles><keywords><keyword><style  face="normal" font="default" size="100%">Astrophysics</style></keyword><keyword><style  face="normal" font="default" size="100%">galaxies: active</style></keyword><keyword><style  face="normal" font="default" size="100%">Galaxies: BL Lacertae Objects: Individual: Name: Markarian 501</style></keyword><keyword><style  face="normal" font="default" size="100%">gamma rays: theory</style></keyword></keywords><dates><year><style  face="normal" font="default" size="100%">1999</style></year><pub-dates><date><style  face="normal" font="default" size="100%">1999/06/1</style></date></pub-dates></dates><urls><web-urls><url><style face="normal" font="default" size="100%">https://ui.adsabs.harvard.edu/abs/1999ApJ...518L..13K</style></url></web-urls></urls><volume><style face="normal" font="default" size="100%">518</style></volume><pages><style face="normal" font="default" size="100%">L13 - L15</style></pages><isbn><style face="normal" font="default" size="100%">0004-637X</style></isbn><language><style face="normal" font="default" size="100%">eng</style></language><abstract><style face="normal" font="default" size="100%">The recent High-Energy Gamma-Ray Array (HEGRA) observations of the blazar Mrk 501 show strong curvature in the very high energy γ-ray spectrum. Applying the γ-ray opacity derived from an empirically based model of the intergalactic infrared background radiation field to these observations, we find that the intrinsic spectrum of this source is consistent with a power law: dN&lt;SUB&gt;γ&lt;/SUB&gt;/dE~E&lt;SUP&gt;-α&lt;/SUP&gt;, with α=2.00+/-0.03 over the range 500 GeV-20 TeV. Within current synchrotron self-Compton scenarios, the fact that the TeV spectral energy distribution of Mrk 501 does not vary with luminosity, combined with the correlated, spectrally variable emission in X-rays as observed by the BeppoSAX and Rossi X-Ray Timing Explorer instruments, also independently implies that the intrinsic spectrum must be close to α=2. Thus, the observed curvature in the spectrum is most easily understood as resulting from intergalactic absorption.</style></abstract></record></records></xml>