A new statistical approach to the optical spectral variability in a sample of Gamma-Bright Blazars

We present a statistical study based on optical spectroscopic observations of a list of gamma-ray bright blazars. We have used the observations obtained as part of the ground-based observational support program to the Fermi mission conducted at Steward Observatory (Univ. of Arizona). Spectra of about 35 targets have been obtained with an almost weekly cadence. We retrieved observations from the end of 2008 to beginning of 2016, i.e. about 7.5 years. We have obtained synthetic photometry and produced colour-magnitude diagrams which show different trends associated to the object classes: generally BL-Lacs tend to become bluer when brighter, FSRQs redder when brighter, although several objects exhibit different trends depending on the brightness. We have also applied a pattern recognition algorithm to obtain the minimum number of physical components which can explain the variability of the optical spectrum. We have used NMF (Non-Negative Matrix Factorization) instead of PCA (Principal Component Analysis) to avoid un-realistic negative components. For most targets we found that two meta-components are enough to explain the observed spectral variability. The spectral shape of these components will be presented and their association to plausible physical emission mechanisms (synchrotron, accretion disk, inverse-Compton, stellar population, etc) will be discussed. The relative contribution of each component is also studied in relation to colour changes and variability observed in other frequency ranges.