The long-term multiwavelength observations of the blazar PKS 2005-489

Abstract

We present the results of the long-term multiwavelength observations of the TeV-detected blazar PKS2005−489. During the 15-yr period of the monitoring with X-ray Telescope (XRT) onboard Neil Gehrels Swift Observatory, the most extreme flaring event was recorded in 2009June when the 2–10keV flux boosted by a factor of ∼50 compared to the previous observation with the same instrument and was comparable to that observed during the exceptional flare in 1998. However, strong flaring epochs were followed by much longer periods with significantly lower X-ray states and lesser activity, as revealed during the monitoring with different X-ray instruments and possibly related to the rare emergence of strong shock waves in the jet. Moreover, the target was passive on intraday timescales: only one 0.3–10keV IDV was shown during 60 XRT observations, incorporating very fast brightness increase by 70% within 160s which was superimposed on the significantly slower variability over the 82-ks time interval. Generally, the XRT-band spectra are well-fit with a simple powerlaw model, yielding a very broad range of the 0.3–10keV photon index Γ=1.95(0.04)–3.25(0.09). The source showed a wide range of the synchrotron peak position in the UV–soft X-ray range (with Epmax∼1.5keV), thus confirming its previous classification as a high-energy-peaking BL Lac source. PKS2005−489 did not show a clear “harder-when-brighter” spectral trend that could be related to very different physical conditions in the jet emission zone in diverse epochs. The source exhibited moderate and strong flaring activities in the MeV–GeV and UV–optical–IR energy ranges, respectively, which were weakly correlated with the X-ray variability and demonstrated a need of more complex emission scenarios than one-zone synchrotron self-Compton model. The lognormal function is well-fit with the distributions of the X-ray and MeV–GeV fluxes, thus hinting at the possible imprint of the accretion disc’s variability onto the jet.