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

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This article has an erratum: [erratum]

Issue		A&A Volume 478, Number 2, February I 2008


Page(s)		395 - 401
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20078639

A&A 478, 395-401 (2008)
DOI: 10.1051/0004-6361:20078639

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

F. Massaro¹, A. Tramacere², A. Cavaliere¹, M. Perri³, and P. Giommi³

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 $\gamma$ 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 $S_{\rm p}$ increasing with the position $E_{\rm p}$ 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 $E_{\rm p}$ 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 $E_{\rm p}-S_{\rm p}$ and $E_{\rm p}-b$ 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 $E_{\rm p}-S_{\rm p}$ plane, while PKS 2155-304 differs. As for the $E_{\rm p}-b$ 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.

Key words: galaxies: active -- galaxies: BL Lacertae objects: general -- X-rays: galaxies -- radiation mechanisms: non-thermal

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