skip to main content

Title: On the origin of X-ray spectra in luminous blazars

Gamma-ray luminosities of some quasar-associated blazars imply jet powers reaching values comparable to the accretion power even if assuming very strong Doppler boosting and very high efficiency of gamma-ray production. With much lower radiative efficiencies of protons than of electrons, and the recent reports of very strong coupling of electrons with shock-heated protons indicated by particle-in-cell simulations, the leptonic models seem to be strongly favored over the hadronic ones. However, the electron-proton coupling combined with the external-radiation-Compton (ERC) models of gamma-ray production in leptonic models predict extremely hard X-ray spectra, with energy indices αx ~ 0. This is inconsistent with the observed 2-10 keV slopes of blazars, which cluster around αx ~ 0.6. This problem can be resolved by assuming that electrons can be efficiently cooled down radiatively to non-relativistic energies, or that blazar spectra are entirely dominated by the synchrotron self-Compton (SSC) component up to at least 10 keV. Here, we show that the required cooling can be sufficiently efficient only at distances r < 0.03 pc. SSC spectra, on the other hand, can be produced roughly co-spatially with the observed synchrotron and ERC components, which are most likely located roughly at a parsec scale. We show that themore » dominant SSC component can also be produced much further than the dominant synchrotron and ERC components, at distances of gsim 10 pc. Hence, depending on the spatial distribution of the energy dissipation along the jet, one may expect to see γ-ray/optical events with either correlated or uncorrelated X-rays. In all cases the number of e+e pairs per proton is predicted to be very low. The direct verification of the proposed SSC scenario, and particularly the question of the co-spatiality of the SSC component with other spectral components, requires sensitive observations in the hard X-ray band. Lastly, this is now possible with the deployment of the NuSTAR satellite, providing the required sensitivity to monitor the details of the hard X-ray spectra of blazars in the range where the ERC component is predicted to start dominating over the SSC component.« less
Authors:
 [1] ;  [2] ;  [3] ;  [4] ;  [2]
  1. Nicolaus Copernicus Astronomical Center, Warsaw (Poland); SLAC National Accelerator Lab., Menlo Park, CA (United States)
  2. Nicolaus Copernicus Astronomical Center, Warsaw (Poland)
  3. Univ. of Colorado, and National Institute of Standards and Technology, Boulder, CO (United States)
  4. SLAC National Accelerator Lab., Menlo Park, CA (United States)
Publication Date:
OSTI Identifier:
1105894
Report Number(s):
SLAC-PUB-15834
Journal ID: ISSN 0004-637X; arXiv:1311.0670
Grant/Contract Number:
AC02-76SF00515
Type:
Accepted Manuscript
Journal Name:
Astrophysical Journal
Additional Journal Information:
Journal Volume: 779; Journal Issue: 1; Journal ID: ISSN 0004-637X
Publisher:
Institute of Physics (IOP)
Research Org:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Org:
USDOE Office of Science (SC); National Aeronautics and Space Administration (NASA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; astrophysics; ASTRO; acceleration of particles; galaxies: active; galaxies: jets; radiation mechanisms: non-thermal