Exploring the nature of broadband variability in the FSRQ 3C 273
  Detailed investigation of broadband flux variability in the blazar 3C 273 allows us to probe the location and size of emission regions and their physical conditions. We report the results on correlation studies of the flaring activity observed between 2008 and 2012. The observed broadband variations were investigated using the structure function and the discrete correlation function, and power spectral density analysis (PSD) methods. The PSD analysis showed that the optical/IR light curve slopes are consistent with the slope of white noise processes, while, the PSD slopes at radio, X-ray and gamma-ray energies are consistent with red-noise processes. The flux variations at gamma-ray and mm-radio bands are found to be significantly correlated. Using the estimated time lag of (110\pm27) days between gamma-ray and radio light curves, we constrained the location of the gamma-ray emission region at a de-projected distance of 1.2\pm0.9 pc from the jet apex. Flux variations at X-ray bands were found to have a significant correlation with variations at both radio and \gamma-rays energies. The correlation between X-rays and gamma-rays light curves suggests presence of two components responsible for the X-ray emission. A negative time lag of -(50\pm20) days, where the X-rays are leading the emission, suggests X-rays are emitted closer to the jet apex from a compact region at a distance of ~(0.5\pm0.4) pc from the jet apex. A positive time lag of (110\pm20) days suggests jet-base origin of the other X-ray component at ~(4--5)~pc from the jet apex. The flux variations at radio frequencies were found to be well correlated with each other such that the variations at higher frequencies are leading the lower frequencies, which could be expected in the standard shock-in-jet model. 
C. Chidiac, B. Rani, T. P. Krichbaum, E. Angelakis, L. Fuhrmann, I.   Nestoras, J. A. Zensus, A. Sievers, H. Ungerechts, R. ...
astro-ph.HE