  X-ray binaries are cauldrons of fundamental physical processes which appear
along practically the whole electromagnetic spectrum. The sub-class of X-ray
transient sources show multifrequency behaviour which deserve particular
attention in order to understand the causing physics. These binary systems
consist of a compact star and an optical star, therefore there is a mutual
influence between these two stars that drive the low energy (LE) (i.e. radio,
IR, optical) and high energy (HE) (i.e. UV, X-ray, $\gamma$-ray) processes. The
LE processes are produced mostly on the optical star and the HE processes
mostly on the compact star, typically a neutron star. Thus it appears evident
that through the study of LE processes it is possible to understand also the HE
processes and vice versa. In this paper we will discuss this problem starting
from the experimental evidence of a delay between LE and HE processes detected
for the first time in the X-ray/Be system A0535+26/HDE245770 (e.g. Giovannelli
\& Sabau-Graziati, 2011; Giovannelli, Bisnovatyi-Kogan \& Klepnev, 2013 (here
after GBK13); Giovannelli et al., 2015b). This delay is common in cataclysmic
variables (CVs) and other binary systems with either a neutron star or a black
hole.
  Since a delay between LE processes and HE processes has been experimentally
observed in several active galactic nuclei (AGNs), we will discuss also the
tidal disruption of stars by massive BHs, following the original idea of Rees
(1998): stars in galactic nuclei can be captured or tidally disrupted by a
central black hole. Some debris would be ejected at high speed, the remainder
would be swallowed by the hole, causing a bright flare lasting at most a few
years.
