Introduction
The study of Gamma-ray bursts
(GRBs) has undergone a revolution since the
the first fading sources (afterglows) at X-ray , optical and
radio wavelengths were discovered, making
them the most powerful photon-emitters known in the Universe. Although
much attention has been devoted to the late afterglow emission since then,
the prompt Gamma-ray emission still has to be understood. Many issues remain
unsolved, regarding the nature of the central engine, the different scenarios
giving rise to this prompt emission, and the radiation mechanisms.
Gamma-ray bursts are so complicated and diverse in the time domain
that, at first sight, their behaviour
obeys no simple rule: many bursts have a highly variable temporal
profile with a variability time scale that is significantly shorter than
the overall duration, while in a minority of them there is only one peak,
with no
substructure.
Results: quiescent times in Gamma-ray bursts
Long GRBs often show multiple episodes of emission, separated by background
intervals of variable duration or quiescent times. In other words,
the emission seems to turn off to a very low level and then turn
on again.
This observed property can provide an interesting clue to the nature
of Gamma-ray bursts. At present, it
is unclear if these separated emission episodes are consequence of
the same physical process (e.g. internal or external shocks), and if the
time separation is due to some intrinsic property of the central
source or
of its environment. We have investigated this issue in a recent paper
(Ramirez-Ruiz & Merloni 2000), and we are
currently involved in the preparation of an accompanying paper (Ramirez-Ruiz,
Merloni & Rees 2000), in which we will present the implications
of quiescent times for different scenarios for the origin of Gamma-ray
bursts.