Exploring the nature of broadband variability in the FSRQ 3 C 273

Detailed investigation of broadband flux variability in the blazar 3C 273 allows us to probe the location and size of emiss ion regions and their physical conditions. We report the results on corr elation studies of the flaring activity in 3C 273 observed for a period between 2008 and 2012. The observed broadband variations were inves tigated using the structure function and the discrete corre lation function methods. Starting from the common use of power spectral dens ity analysis (PSD) at X-ray frequencies, we extended our inv estigation to characterize the nature of variability at radio, optical , andγ-ray frequencies. The PSD analysis showed that the optical /IR light curve slopes are consistent with the slope of white noise pro cesses; while, the PSD slopes at radio, X-ray and γ-ray energies are consistent with red-noise processes. We found that the esti mated fractional variability amplitudes have a strong depe ndence on the observed frequency. The flux variations at γ-ray and mm-radio bands are found to be significantly correla ted. Using the estimated time lag of (110±27) days betweenγ-ray and radio light curves where γ-ray variations lead the radio bands, we constrained the loc ation of theγ-ray emission region at a de-projected distance of 1 .2± 0.9 pc from the jet apex. Flux variations at X-ray bands were fou nd to have a significant correlation with variations at both rad io andγ-rays energies. The correlation between X-rays and γ-rays light curves provides a hint of two possible time lags, which sugge sts presence of two components responsible for the X-ray emi ssion. A negative time lag of -(50 ±20) days, where the X-rays are leading the emission, suggest s X-rays are emitted closer to the jet apex from a compact region (0.02–0.05 pc in size) i.e. most likely from the corona at a distance of (0.5 ±0.4) pc from the jet apex. A positive time lag of (110±20) days ( γ-rays are leading the emission) 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 fo und to be well correlated with each other such that the variat ions at higher frequencies are leading the lower frequencies, which could be expected in the standard shock-in-jet model.