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 active at γrays and optical frequencies until May 2011, when the source started a series of strong flares. The exceptional optical sampling achieved by the GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope (WEBT) in collaboration with the Steward Observatory allows us to perform a detailed comparison with the daily γ-ray observations by Fermi. Discrete correlation analysis between the optical and γ-ray emission reveals correlation with a time lag of 0 ± 1 d, which suggests cospatiality of the corresponding jet emitting regions. A better definition of the time lag is hindered by the daily gaps in the sampling of the extremely fast flux variations. In general, optical flares present more structure and develop on longer time scales than corresponding γ-ray flares. Observations at X-rays and at millimetre wavelengths reveal a common trend, which suggests that the region producing the mm and X-ray radiation is located downstream from the optical and γ-ray-emitting zone in the jet. The mean optical degree of polarisation slightly decreases over the considered period and in general it is higher when the flux is lower. The optical electric vector polarisation angle (EVPA) shows a preferred orientation of about 15 • , nearly aligned with the radio core EVPA and mean jet direction. Oscillations around it increase during the 2011-2012 outburst. We investigate the effects of a geometrical interpretation of the long-term flux variability on the polarisation. A helical magnetic field model predicts an evolution of the mean polarisation that is in reasonable agreement with the observations. These can be fully explained by introducing slight variations in the compression factor in a transverse shock waves model.

Erratic Flaring of Bl Lac in 2012–2013: Multiwavelength Observations

“…14 During the 2012 October to 2013 February period, we also observed the quasar as a science target. Data were reduced in the usual manner, as described in Gurwell et al (2007) and Raiteri et al (2013). Data obtained at 225 GHz (1.3 mm band) and 345 GHz (870 μm band) immediately before, during and after the Herschel observing period are listed in Table 1, with historical SMA data since 2002 in Tables 2 and 3.…”

“…Light curves are shown in Figures 4 and 9. Host galaxy contributions were subtracted, where appropriate, in the resulting SEDs using the values calculated by Raiteri et al (2013): 50% of the host galaxy flux was contained within the 5″ aperture. Consequently, we subtracted 0.5 × (2.89, 1.30, 0.36, 0.026, 0.020, 0.017) mJy for the V, B, U, UW1, UM2, and UW2 bands, respectively.…”

“…At γ-ray energies, the source began a several-month episode of highly variable brightness ranging chaotically from levels of 2.6 × 10 −6 ph s −1 cm −2 down to non-detectable levels <2.0 × 10 −7 ph s −1 cm −2 on timescales of 24 hr. The γ-ray light curve is illustrated in Figure 1; see also Figure 2 of Raiteri et al (2013). Following reports of the flare, we obtained multiwavelength observations with the goals of determining answers to two questions: (1) In response to an injection of energy into the jet, is most of the multiwavelength emission from BL Lac produced in the same location or in multiple locations?…”

“…The discovery of even weak broad emission lines in BL Lac means that in some ways, BL Lac itself is more similar to FSRQs than to less luminous BL Lac objects such as Mkn 421 and Mkn 501 (see discussion in Madejski et al 1999). Direct detection of an accretion disk in the UV-optical spectrum is rendered difficult by a substantial amount of Galactic dust absorption (see discussion in Raiteri et al 2013). No parsec-scale dust internal to BL Lac objects that could serve as a source of seed photons for γ-ray production has yet been detected (Malmrose et al 2011;Plotkin et al 2012).…”

“…Abdo et al (2011) completed a pre-planned multiwavelength campaign that caught BL Lac in a low, relatively quiescent state. Most recently, Raiteri et al (2013) reported on Fermi, Swift, Submillimeter Array (SMA) and Whole Earth Blazar Telescope 11 observations beginning in 2008, through an episode of strong activity in [2011][2012], and continuing to 2012 October 31. Raiteri et al also reviewed previous campaigns.…”