Astronomy & Astrophysics

Issue		A&A Volume 448, Number 3, March IV 2006
Page(s)		861 - 871
Section		Extragalactic astronomy
DOI		10.1051/0004-6361:20053644

A&A 448, 861-871 (2006)
DOI: 10.1051/0004-6361:20053644

Log-parabolic spectra and particle acceleration in blazars

III. SSC emission in the TeV band from Mkn 501

E. Massaro¹, A. Tramacere¹, M. Perri², P. Giommi² and G. Tosti³

¹ Dipartimento di Fisica, Università La Sapienza, Piazzale A. Moro 2, 00185 Roma, Italy
e-mail: enrico.massaro@uniroma1.it
² ASI Science Data Center, ESRIN, 00044 Frascati, Italy
³ Dipartimento di Fisica, Università di Perugia, via A. Pascoli, Perugia, Italy

(Received 16 June 2005 / Accepted 11 October 2005 )

Abstract
Curved broad-band spectral distributions of non-thermal sources like blazars are described well by a log-parabolic law where the second degree term measures the curvature. Log-parabolic energy spectra can be obtained for relativistic electrons by means of a statistical acceleration mechanism whose probability of acceleration depends on energy.

In this paper we compute the spectra radiated by an electron population via synchrotron and Synchro-Self Compton processes to derive the relations between the log-parabolic parameters.

These spectra were obtained by means of an accurate numerical code that takes the proper spectral distributions for single particle emission into account.

We found that the ratio between the curvature parameters of the synchrotron spectrum to that of the electrons is equal to ~0.2 instead of 0.25, the value foreseen in the $\delta$ -approximation. Inverse Compton spectra are also intrinsically curved and can be approximated by a log-parabola only in limited ranges. The curvature parameter, estimated around the SED peak, may vary from a lower value than that of the synchrotron spectrum up to that of emitting electrons depending on whether the scattering is in the Thomson or in the Klein-Nishina regime. We applied this analysis to computing the synchro-self Compton emission from the BL Lac object Mkn 501 during the large flare of April 1997. We fit simultaneous BeppoSAX and CAT data and reproduced intensities and spectral curvatures of both components with good accuracy.

The large curvature observed in the TeV range was found to be mainly intrinsic, and therefore did not require a large pair production absorption against the extragalactic background. We regard this finding as an indication that the Universe is more transparent at these energies than previously assumed by several models found in the literature. This conclusion is supported by recent detection of two relatively high redshift blazars with HESS.

Key words: radiation mechanisms: non-thermal -- galaxies: active -- BL Lacertae objects: general -- BL Lacertae objects: individual: Mkn 501

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