Warping and precession in galactic and extragalactic accretion disks
  The Bardeen-Petterson general relativistic effect has been suggested as the mechanism responsible for precession in some accretion disk systems. Here we examine separately four mechanisms (tidally-induced, irradiation-induced, magnetically-induced and Bardeen-Petterson-induced) that can lead to warping and precession. We use a sample of eight X-ray binaries and four Active Galactic Nuclei (AGNs) that present signatures of warping and/or precession in their accretion disks to explore the viability of the different mechanisms. For the X-ray binaries SMC X-1 and 4U 1907+09 all four mechanisms provide precession periods compatible with those observed, while for Cyg X-1 and the active galaxies Arp 102B and NGC 1068, only two mechanisms are in agreement with the observations. The irradiation-driven instability seems incapable of producing the inferred precession of the active galaxies in our sample, and the tidally-induced precession can probably be ruled out in the case of Arp 102B. Perhaps the best case for a Bardeen-Petterson precession can be achieved for NGC 1068. Our results show that given the many observational uncertainties that still exist, it is extremely difficult to confirm unambiguously that the Bardeen-Petterson effect has been observed in any of the other sources of our sample. 
Anderson Caproni (STScI, IAG/USP), Mario Livio (STScI), Zulema Abraham   (IAG/USP), Herman J. Mosquera Cuesta (ICRA-BR/CBPF)