Variability in observed emission can be considered a defining characteristic of active galactic nuclei (AGNs), and for the roughly 10% of AGNs that are radio-loud (e.g., Jiang et al. 2007) the majority of this variable emission is understood to arise from the relativistic flows of plasma along two oppositely directed jets (e.g., Urry & Padovani 1995). When viewed at small angles to the jet direction the Doppler boosting makes the emission from the approaching jet appear dramatically brighter and also shortens the observed timescales with respect to those in the emitted frame, thereby explaining many of the properties of blazars (e.g., Blandford & Rees 1978; Lister 2001; Gopal-Krishna et al. 2003). Multiband radio studies and very long baseline interferometry (VLBI), coupled with theoretical models, have provided extremely strong evidence for the presence of both moving and standing shocks in these jets (e.g., Marscher & Gear 1985; Hughes et al. 1991; Lister et al. 2001, 2009; Marscher et al. 2008, 2010), indicating significant changes in the fluid flow and/or the density of material ejected into the jets; it is now accepted that the largest flares arise from the production and relativistic propagation of new components seen as radio knots. Changes in the overall direction of the inner portions of the jet, or at least its brightest portions, have also been demonstrated via VLBI (e.g., Biretta et al. 1986; Ros et al. 2000; Piner et al. 2003; Caproni & Abraham 2004). Even modest changes in direction (e.g., Camenzind & Krockenberger 1992; Gopal-Krishna & Wiita 1992) have long been recognized as one way to produce significant changes in the flux and polarization (e.g., Gopal-Krishna & Wiita 1993; Piner et al. 2008). It is to be expected that turbulence is produced within these jets, at least in the vicinity of shocks, and thus some of the variations should arise from such smaller scale motions as has been suggested theoretically (Marscher & Travis 1991; Marscher 2014, Calafut & Wiita 2015) and strongly supported by observations of blazars (e.g., Marscher et al. 2008, 2010; Bhatta et al. 2013). In addition, there is the possibility that portions of the jet are moving much faster than other portions, and such misaligned mini-jets could also produce some extremely rapid fluctuations (Giannios et al. 2009; Biteau & Giebels 2012). Variability on a wide range of timescales can be produced within the accretion disks feeding the central black holes (e.g., Czerny 2006). This presumably dominates the variations from radio-quiet AGNs though not those of radio-loud ones because the special relativistic boosting of the jet emission is so important in the latter (Urry & Padovani 1995). Some of the variations in the jet emission might be traced to plasma fluctuations in the disk being advected into the base of the jets (e.g., Wiita 2006) but the exact origins of the initial fluctuations are not addressed in this work. Here we address the question of whether variations in the bulk flow of a propagating relativistic hydrodynamic (RHD) jet along with sub-grid mildly relativistic turbulence can produce light curves and power spectra resembling those of radio-loud AGNs.
Citation: Camenzind & Krockenberger 1992
Citation function: Background; Background