  Extragalactic nuclear activity is best explored with observations at high
energies, where the most extreme flux and spectral variations are expected to
occur, witnessing changes in the accretion flow or in the kinematics of the
plasma. In active galactic nuclei of blazar type, these variations are the most
dramatic. By following blazar outbursts from their onset and by correlating the
observed variations at many different wavelengths we can reconstruct the
behavior of the plasma and map out the development of the flare within the jet.
The advent of the Fermi satellite has allowed the start of a systematic and
intensive monitoring program of blazars. Blazar outbursts are very effectively
detected by the LAT instrument in the MeV-GeV domain, and these can be promptly
followed up with other facilities. Based on a Fermi LAT detection of a high
MeV-GeV state, we have observed the blazar PKS 1502+106 with the INTEGRAL
satellite between 9 and 11 August 2008. Simultaneous Swift observations have
been also accomplished, as well as optical follow-up at the Nordic Optical
Telescope. The IBIS instrument onboard INTEGRAL detected a source at a position
inconsistent with the optical coordinates of PKS 1502+106, but consistent with
those of the Seyfert 1 galaxy Mkn 841, located at 6.8 arcmin south-west from
the blazar, which is therefore responsible for all the hard X-ray flux detected
by IBIS. At the location of the blazar, IBIS sets an upper limit of ~10^{-11}
erg/s/cm2 on the 15-60 keV flux, that turns out to be consistent with a model
of inverse Compton scattering accounting for the soft X-ray and gamma-ray
spectra measured by Swift XRT and Fermi LAT, respectively. The gamma-ray
spectrum during the outburst indicates substantial variability of the
characteristic energy of the inverse Compton component in this
blazar.(abridged).
