The swift observations of distant high-energy peaked BL Lacertae objects

In this paper, we present the timing and spectral results for the seven distant and poorly investigated BL Lacertae sources with redshifts $z\gtrsim 0.5$, using the archival data obtained with the Swift and other multifrequency instruments during 2006 August–2015 July. Our timing study has revealed a number of the 0.3–10 keV flux changes from the fluctuations observed within one day to the variabilities on timescales of a few months. Namely, the relatively densely-sampled observations of BZB J1517+6525 showed a strong X-ray flare by a factor of ∼5 during 2014 September-December. While this instance can be explained by the propagation of shock wave though the blazar jet which causes a strong heat-up of the relativistic plasma, the lower-amplitude flux fluctuations observed in higher X-ray states could be related to the interaction between the shock front and jet inhomogeneities. The curved 0.3–10 keV spectra showed the ranges of the photon index at 1 keV $a=1.27$(0.10)–2.30(0.09), curvature parameter $b = 0.12$(0.08)–1.19(0.27), synchrotron SED peak position $E_{p} = 0.49$(0.09)–3.39(0.97) keV, while some spectra do not exhibit spectral curvature and fit well with a simple power law model yielding the 0.3–10 keV photon-index $\Gamma =1.48$(0.15)–2.32(0.08). Large spectral curvatures observed for the majority of the log-parabolic spectra hint at the importance of the second-order Fermi mechanism for our targets. Our targets show very high isotropic luminosity $L_{0.3\text{--}10~\text{keV}}\sim 10^{46}~\mbox{erg}\,\mbox{s}^{-1}$ compared to the nearby bright BL Lacertae objects.