We have developed analysis tools to search for quasi periodic oscillations in light curves from active galactic nuclei, using the following time series techniques: Wavelets, periodogram, Lomb–Scargle periodogram, structure function and multi-harmonic analysis of variance.

The analysis tools incorporate different noise models with significant levels for all the techniques that is an improvement over the previous work. By looking for consistently high significance, we make the detection of periodicities more robust. We apply this tool to a previously reported QPO (Espaillat et al. 2008) in the X-ray light curve of 3C 273 with a periodicity of ∼ 3300 s and find that the significance is only 74% in the wavelet and fails to show up above 95% significance in the periodogram and multi-harmonic analysis of variance.

It is well established that the blazars show flux variations in the complete electromagnetic (EM) spectrum on all possible time scales ranging from a few tens of minutes to several years. Here we report the review of various UV and X-ray flux variability properties of blazars. Our analysis show that UV variability amplitude is smaller than X-rays, mostly soft X-rays hardness ratio show correlations with blazar luminosity and different modes of variability might be operating for different time scales and epochs. Quasi periodic oscillations are seen on a few occasions in blazars, higher fraction of high energy peaked blazars show intra day and short term variabilities in X-rays but variability duty cycle is much less in optical bands on intra day time scale compared to low energy peaked blazars. But these results are yet to be established.

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/or 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.

Here we report our recent discoveries of Quasi-Periodic Oscillations (QPOs) in blazars time series data in X-ray and optical electromagnetic bands. Any such detection can give important clues of the location and nature of the processes of emission mechanism. In the case of radio-quiet AGN, the detected QPOs are very likely to be associated with the accretion disk. But in the case of blazars, it may be associated with jets in the high and outburst states, and in the low-state, it is probably associated with the accretion disk. In this brief review, I summarize the recent QPO detections in blazars. There is one strong evidence of QPO detection in XMM–Newton time series data of narrow line Seyfert 1 galaxy RE J1034+396 about which we will also discuss briefly.