The long-term Swift observations of the high-energy peaked BL Lacertae source 1ES 1959+650

• Published in: Monthly notices of the Royal Astronomical Society 2016-03, Vol.457 (1), p.704-722
• Main Authors: Kapanadze, B., Romano, P., Vercellone, S., Kapanadze, S., Mdzinarishvili, T., Kharshiladze, G.
• Format: Article
• Language: English
• Subjects: Acceleration; Astronomy; BL Lacertae objects; Electrons; Energy; Flares; Fluxes; Spectral energy distribution; Spectrum analysis; Stochastic models; Stochasticity; Symbols; Synchrotrons; X-rays
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stv3004

We present the results based on the monitoring of the high-energy peaked BL Lacertae object 1ES 1959+650 by the Swift satellite during 2005–2014. Our timing study shows that the source was highly variable on longer (weeks-to-months) time-scales with the 0.3–10 keV fluxes ranging by a factor of 8. It sometimes showed a significant intra-day variability in the course of ∼1 ks, detected mainly in the epochs of higher brightness states. The flux variability exhibited an erratic character and no signatures of periodic variations are revealed. The X-ray spectra were mainly curved with broad ranges of photon index, curvature parameter, hardness ratio, synchrotron spectral energy distribution (SED) peak location which exhibited a significant variability with the flux at different time-scales. Our study of multi-wavelength cross-correlations shows that the one-zone synchrotron self-Compton scenario was not always valid for 1ES 1959+650. The X-ray flares were sometimes not accompanied with an increasing activity in the γ-ray or lower-energy parts of the spectrum and vice versa. Similar to the prominent ‘orphan’ TeV event in 2002, significant flares in the high-energy and very high energy bands in 2009 May and 2012 May were not accompanied by those in the synchrotron part of the spectrum. Similar to other TeV-detected high-energy peaked BLLs, the stochastic acceleration of the electrons from the magnetic turbulence close to the shock front may be more important for our target compared to other scenarios since it showed mainly broader synchrotron SEDs during the X-ray flares expected when the stochastic mechanism is more efficient.