Pair cascades in extragalactic jets. II. The beamed X-ray spectrum

Amir Levinson, Roger Blandford

Research output: Contribution to journalArticlepeer-review

Abstract

The X-ray through y-ray emission from a relativistic jet is considered within the framework of an inhomogeneous pair-cascade model. It is shown that the overall spectrum from the model exhibits a spectral break at an energy of order (mec2)2/Eb, where Eb is the cutoff energy of the isotropic spectrum of scattered X-ray photons, above which the γ-rays are subject to significant absorption through pair production on the ambient soft radiation. Such a break has been observed thus far in the γ-ray spectra of several blazars; if blazars possess isotropic X-ray spectra similar to those observed in radio-quiet sources, then the relation between the break energy and the cutoff energy, predicted by our model, is roughly satisfied. It is argued that emission just below the y-ray break originates near the base of the jet and is determined by the distribution function of cascading pairs there. Above the break, the slope of the emergent γ-ray spectrum reflects the radial variations of pair acceleration and soft photon intensity and should be steeper than the hard X-ray spectrum. For plausible particle acceleration prescriptions and soft radiation spectra, the characteristic hard X-ray spectra observed in some blazars are reproduced by our model. The X-ray cutoff energy is also related to the dynamics of the jet. It is argued that both the inertia and the radiative drag acting upon a jet increase sharply when the bulk Lorentz factor exceeds re ∼ (mec2/Eb) ∼ 10 and that jets will be accelerated up to this critical Lorentz factor which is of order that inferred from VLBI observations of superluminal radio sources. The utility of multiwaveband observations for advancing our understanding of blazars is stressed.

Original languageEnglish
Pages (from-to)86-92
Number of pages7
JournalAstrophysical Journal
Volume449
Issue number1
DOIs
StatePublished - 10 Aug 1995
Externally publishedYes

Keywords

  • Galaxies: jets
  • Gamma rays: theory
  • X-rays: galaxies

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