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

• Published in: Astronomy and astrophysics (Berlin) Vol. 478; no. 2; pp. 395 - 401
• Main Authors: Massaro, F., Tramacere, A., Cavaliere, A., Perri, M., Giommi, P.
• Format: Journal Article
• Language: English
• Published: Les Ulis EDP Sciences 01.02.2008
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; galaxies: active; galaxies: BL Lacertae objects: general; radiation mechanisms: non-thermal; X-rays: galaxies; radiation mechanisms: non-thermal; X ray observation; X-ray galaxies; Stochastic processes; X-rays: galaxies; Spectral energy distribution; Cosmic x-ray sources; Relativistic electron; X-ray spectra; Spectral analysis; BL Lacertae objects; Extragalactic source; TeV range; Active galaxies; Radiation mechanism; galaxies: active; Correlations; Positions; Cosmic radio sources; galaxies: BL Lacertae objects: general; Curvature; Synchrotron radiation
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361:20078639

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 Sp increasing with the position Ep 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 Ep 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.