X-ray spectral evolution of TeV BL Lacertae objects: eleven years of observations with BeppoSAX, XMM-Newton and Swift satellites^*

¹ Dipartimento di Fisica, Università di Roma Tor Vergata via della Ricerca scientifica 1, 00133 Roma, Italy e-mail: massaro@roma2.infn.it
² Dipartimento di Fisica, Università di Roma La Sapienza, Piazzale A. Moro 2, 00185 Roma, Italy
³ ASI Science Data Center, ESRIN, 00044 Frascati, Italy

Received: 8 September 2007
Accepted: 9 November 2007

Abstract

Context.Many of the extragalactic sources detected in γ rays at TeV energies are BL Lac objects. In particular, they belong to the subclass of “high frequency peaked BL Lacs” (HBLs), as their spectral energy distributions exhibit a first peak in the X-ray band. At a closer look, their X-ray spectra appear to be generally curved into a log-parabolic shape. In a previous investigation of Mrk 421, two correlations were found between the spectral parameters. One involves the height S_p increasing with the position E_p of the first peak; this was interpreted as a signature of synchrotron emission from relativistic electrons. The other involves the curvature parameter b decreasing as E_p increases; this points toward statistical/stochastic acceleration processes for the emitting electrons.

Aims.We analyse X-ray spectra of several TeV HBLs to pinpoint their behaviours in the and  planes and to compare them with Mrk 421.

Methods.We perfom X-ray spectral analyses of a sample of 15 BL Lacs. We report the whole set of observations obtained with the BeppoSAX, XMM-Newton and Swift satellites between 29/06/96 and 07/04/07. We focus on five sources (PKS 0548-322, 1H 1426+418, Mrk 501, 1ES 1959+650, PKS 2155-304) whose X-ray observations warrant detailed searching of correlations or trends.

Results.Within our database, we find that four out of five sources, namely PKS 0548-322, 1H 1426+418, Mrk 501 and 1ES 1959+650, follow similar trends as Mrk 421 in the  plane, while PKS 2155-304 differs. As for the  plane, all TeV HBLs follow a similar behaviour.

Conclusions.The trends exhibited by Mrk 421 appear to be shared by several TeV HBLs, such as to warrant discussing predictions from the X-ray spectral evolution to that of TeV emissions.