The analysis of $gamma$-ray flux variability along with the parsec-scale jet kinematics suggests that the high-energy radiation in the BL Lac object S5 0716+714 has a significant correlation with the mm-VLBI core flux density and with the local orien
tation of the inner jet flow. For the first time in any blazar, we report a significant correlation between the $gamma$-ray flux variations and the variations in the local orientation of the jet outflow (position angle). We find that the $gamma$-ray flux variations lead the 7~mm VLBI core flux variations by 82$pm$32~days, which suggests that the high-energy emission in S5 0716+714 is coming from a region located 3.8$pm$1.9~parsecs closer to the central black hole than the core seen on the mm-VLBI images. The results imply a strong physical and casual connection between $gamma$-ray emission and the inner jet morphology in the source.
We present a high-frequency very long baseline interferometry (VLBI) kinematical study of the BL Lac object S5 0716+714 over the time period of September 2008 to October 2010. The aim of the study is to investigate the relation of the jet kinematics
to the observed broadband flux variability. We find significant non-radial motions in the jet outflow of the source. In the radial direction, the highest measured apparent speed is sim37 c, which is exceptionally high, especially for a BL Lac object. Patterns in the jet flow reveal a roughly stationary feature sim0.15 mas downstream of the core. The long-term fits to the component trajectories reveal acceleration in the sub-mas region of the jet. The measured brightness temperature, T_{B}, follows a continuous trend of decline with distance, T_B propto r_{jet}^{-(2.36pm0.41)}, which suggests a gradient in Doppler factor along the jet axis. Our analysis suggest that a moving disturbance (or a shock wave) from the base of the jet produces the high-energy (optical to gamma-ray) variations upstream of the 7 mm core, and then later causes an outburst in the core. Repetitive optical/gamma-ray flares and the curved trajectories of the associated components suggest that the shock front propagates along a bent trajectory or helical path. Sharper gamma-ray flares could be related to the passage of moving disturbances through the stationary feature. Our analysis suggests that the gamma-ray and radio emission regions have different Doppler factors.
Using millimeter-very long baseline interferometry (VLBI) observations of the BL Lac object S5 0716+714 from August 2008 to September 2013, we investigate variations in the core flux density and orientation of the sub-parsec scale jet i.e. position a
ngle. The gamma-ray data obtained by the Fermi-LAT (Large Area Telescope) are used to investigate the high-energy flux variations over the same time period. For the first time in any blazar, we report a significant correlation between the gamma-ray flux variations and the position angle (PA) variations in the VLBI jet. The cross-correlation analysis also indicates a positive correlation such that the mm-VLBI core flux density variations are delayed with respect to the gamma-ray flux by 82$pm$32 days. This suggests that the high-energy emission is coming from a region located $geq$(3.8$pm$1.9) parsecs upstream of the mm-VLBI core (closer to the central black hole). These results imply that the observed inner jet morphology has a strong connection with the observed gamma-ray flares.
We present a gamma-ray photon flux and spectral variability study of the flat-spectrum radio quasar 3C 273 over a rapid flaring activity period between September 2009 to April 2010. Five major flares are observed in the source during this period. The
most rapid flare observed in the source has a flux doubling time of 1.1 hr. The rapid gamma-ray flares allow us to constrain the location and size of the gamma-ray emission region in the source. The gamma gamma-opacity constrains the Doppler factor, $delta_{gamma} geq$ 10 for the highest energy (15 GeV) photon observed by the {it Fermi}-Large Area Telescope (LAT). Causality arguments constrain the size of the emission region to 1.6$times 10^{15}$ cm. The gamma-ray spectra measured over this period show clear deviations from a simple power law with a break in 1-2 GeV energy range. We discuss possible explanations for the origin of the gamma-ray spectral breaks. Our study suggests that the gamma-ray emission region in 3C 273 is located within the broad line region ($<$1.6 pc). The spectral behavior and temporal characteristics of the individual flares indicate the presence of multiple shock scenarios at the base of the jet.
Densely time sampled multi-frequency flux measurements of the extreme BL Lac object S5 0716+714 over the past three years allow us to study its broad-band variability, and the detailed underlying physics, with emphasis on the location and size of the
emitting regions and the evolution with time. We study the characteristics of some prominent mm-/gamma-ray flares in the context of the shock-in-jet model and investigate the location of the high energy emission region. The rapid rise and decay of the radio flares is in agreement with the formation of a shock and its evolution, if a geometrical variation is included in addition to intrinsic variations of the source. We find evidence for a correlation between flux variations at gamma-ray and radio frequencies. A two month time-delay between gamma-ray and radio flares indicates a non-cospatial origin of gamma-rays and radio flux variations in S5 0716+714.
The GeV observations by Fermi-LAT give us the opportunity to characterize the high-energy emission (100 MeV - 300 GeV) variability properties of the BL Lac object S5 0716+714. In this study, we performed flux and spectral analysis of more than 3 year
long (August 2008 to April 2012) Fermi-LAT data of the source. During this period, the source exhibits two different modes of flux variability with characteristic timescales of ~75 and ~140 days, respectively. We also notice that the flux variations are characterized by a weak spectral hardening. The GeV spectrum of the source shows a clear deviation from a simple power law, and is better explained by a broken power law. Similar to other bright Fermi blazars, the break energy does not vary with the source flux during the different activity states. We discuss several possible scenarios to explain the observed spectral break.
We present the results of a series of radio, optical, X-ray and gamma-ray observations of the BL Lac object S50716+714 carried out between April 2007 and January 2011. The multi-frequency observations were obtained using several ground and space base
d facilities. The intense optical monitoring of the source reveals faster repetitive variations superimposed on a long-term variability trend at a time scale of ~350 days. Episodes of fast variability recur on time scales of ~ 60-70 days. The intense and simultaneous activity at optical and gamma-ray frequencies favors the SSC mechanism for the production of the high-energy emission. Two major low-peaking radio flares were observed during this high optical/gamma-ray activity period. The radio flares are characterized by a rising and a decaying stage and are in agreement with the formation of a shock and its evolution. We found that the evolution of the radio flares requires a geometrical variation in addition to intrinsic variations of the source. Different estimates yield a robust and self-consistent lower limits of delta > 20 and equipartition magnetic field B_eq > 0.36 G. Causality arguments constrain the size of emission region theta < 0.004 mas. We found a significant correlation between flux variations at radio frequencies with those at optical and gamma-rays. The optical/GeV flux variations lead the radio variability by ~65 days. The longer time delays between low-peaking radio outbursts and optical flares imply that optical flares are the precursors of radio ones. An orphan X-ray flare challenges the simple, one-zone emission models, rendering them too simple. Here we also describe the spectral energy distribution modeling of the source from simultaneous data taken through different activity periods.
The emission from blazars is known to be variable at all wavelengths. The flux variability is often accompanied by spectral changes. Spectral energy distribution (SED) changes must be associated with changes in the spectra of emitting electrons and/o
r the physical parameters of the jet. Meaningful modeling of blazar broadband spectra is required to understand the extreme conditions within the emission region. Not only is the broadband SED crucial, but also information about its variability is needed to understand how the highest states of emission occur and how they differ from the low states. This may help in discriminating between models. Here we present the results of our SED modeling of the blazar S5 0716+714 during various phases of its activity. The SEDs are classified into different bins depending on the optical brightness state of the source.
