The awakening of BL Lacertae: observations by Fermi, Swift and the GASP-WEBT★

Abstract

Since the launch of the Fermi satellite, BL Lacertae has been moderately\nactive at gamma-rays and optical frequencies until May 2011, when the source\nstarted a series of strong flares. The exceptional optical sampling achieved by\nthe GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope\n(WEBT) in collaboration with the Steward Observatory allows us to perform a\ndetailed comparison with the daily gamma-ray observations by Fermi. Discrete\ncorrelation analysis between the optical and gamma-ray emission reveals\ncorrelation with a time lag of 0 +- 1 d, which suggests cospatiality of the\ncorresponding jet emitting regions. A better definition of the time lag is\nhindered by the daily gaps in the sampling of the extremely fast flux\nvariations. In general, optical flares present more structure and develop on\nlonger time scales than corresponding gamma-ray flares. Observations at X-rays\nand at millimetre wavelengths reveal a common trend, which suggests that the\nregion producing the mm and X-ray radiation is located downstream from the\noptical and gamma-ray-emitting zone in the jet. The mean optical degree of\npolarisation slightly decreases over the considered period and in general it is\nhigher when the flux is lower. The optical electric vector polarisation angle\n(EVPA) shows a preferred orientation of about 15 deg, nearly aligned with the\nradio core EVPA and mean jet direction. Oscillations around it increase during\nthe 2011-2012 outburst. We investigate the effects of a geometrical\ninterpretation of the long-term flux variability on the polarisation. A helical\nmagnetic field model predicts an evolution of the mean polarisation that is in\nreasonable agreement with the observations. These can be fully explained by\nintroducing slight variations in the compression factor in a transverse shock\nwaves model.\n