A recent strong X-ray flaring activity of 1ES 1959+650 with possibly less efficient stochastic acceleration

Abstract We present an X-ray flaring activity of 1ES 1959+650 in 2015 August–2016 January, which was the most powerful and prolonged during the 10.75 yr period since the start of its monitoring with X-ray Telescope onboard Swift. A new highest historical 0.3–10 keV count rate was recorded three times that makes this object the third BL Lacertae source exceeding the level of 20 counts s−1. Along with the overall variability by a factor of 5.7, this epoch was characterized by fast X-ray flares by a factor of 2.0–3.1, accompanied with an extreme spectral variability. The source also shows a simultaneous flaring activity in the optical – UV and 0.3–100 GeV bands, although a fast γ-ray flare without significant optical – X-ray counterparts is also found. In contrast to the X-ray flares in the previous years, the stochastic acceleration seems be less important for the electrons responsible for producing X-ray emission during this flare that challenges the earlier suggestion that the electrons in the jets of TeV-detected BL Lacertae objects should undergo an efficient stochastic acceleration resulting in a lower X-ray spectral curvature.