Open AccessBulk Comptonization spectra in blazars
Author(s) -
Celotti A.,
Ghisellini G.,
Fabian A. C.
Publication year - 2007
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1111/j.1365-2966.2006.11289.x
Subject(s) - physics , blazar , astrophysics , compton scattering , redshift , photon , spectral line , lorentz factor , quasar , astrophysical jet , active galactic nucleus , astronomy , galaxy , gamma ray , lorentz transformation , optics , classical mechanics
We study the time‐dependent spectra produced via the bulk Compton process by a cold, relativistic shell of plasma moving (and accelerating) along the jet of a blazar, scattering on external photons emitted by the accretion disc and reprocessed in the broad‐line region (BLR). The bulk Comptonization of disc photons is shown to yield a spectral component contributing in the far‐ultraviolet band, and would then be currently unobservable. On the contrary, the bulk Comptonization of broad‐line photons may yield a significant feature in the soft X‐ray band. Such a feature is time‐dependent and transient, and dominates over the non‐thermal continuum only when: (i) the dissipation occurs close to, but within, the BLR; and (ii) other competing processes, like the synchrotron self‐Compton emission, yield a negligible flux in the X‐ray band. The presence of a bulk Compton component may account for the X‐ray properties of high‐redshift blazars that show a flattening (and possibly a hump) in the soft X‐rays, previously interpreted as due to intrinsic absorption. We discuss why the conditions leading to a detectable bulk Compton feature might be met only occasionally in high‐redshift blazars, concluding that the absence of such a feature in the spectra of most blazars should not be taken as evidence against matter‐dominated relativistic jets. The detection of such a component carries key information on the bulk Lorentz factor and kinetic energy associated to (cold) leptons.