Stochastic Acceleration and the Evolution of Spectral Distributions in SynchroSelfCompton Sources: A Selfconsistent Modeling of Blazars' Flares
Abstract
The broadband spectral distributions of nonthermal sources, such as those of several known blazars, are well described by a logparabolic fit. The seconddegree term in these fits measures the curvature in the spectrum. In this paper, we investigate whether the curvature parameter observed in the spectra of the synchrotron emission can be used as a fingerprint of stochastic acceleration. As a first approach, we use the multiplicative central limit theorem to show how fluctuations in the energy gain result in the broadening of the spectral shape, introducing a curvature into the energy distribution. Then, by means of a Monte Carlo description, we investigate how the curvature produced in the electron distribution is linked to the diffusion in momentum space. To get a more generic description of the problem we turn to the diffusion equation in momentum space. We first study some "standard" scenarios, in order to understand the conditions that make the curvature in the spectra significant, and the relevance of cooling during the acceleration process. We try to quantify the correlation between the curvature and the diffusive process in the preequilibrium stage, and investigate how the transition between the KleinNishina and the Thomson regimes, in inverse Compton cooling, determine the curvature in the distribution at equilibrium. We apply these results to some observed trends, such as the anticorrelation between the peak energy and the curvature term observed in the spectra of Mrk 421, and a sample of BL Lac objects whose synchrotron emission peaks at Xray energies.
 Publication:

The Astrophysical Journal
 Pub Date:
 October 2011
 DOI:
 10.1088/0004637X/739/2/66
 arXiv:
 arXiv:1107.1879
 Bibcode:
 2011ApJ...739...66T
 Keywords:

 acceleration of particles;
 BL Lacertae objects: general;
 BL Lacertae objects: individual: Mrk 421 Mrk 501 1H 1426+428 1ES 1959+650 Mrk 180 PKS 0548─32;
 Astrophysics  High Energy Astrophysical Phenomena;
 Astrophysics  Cosmology and Nongalactic Astrophysics
 EPrint:
 16 pages, 13 figures, Accepted for publication in The Astrophysical Journal