The pulsar phenomenon was discovered about 30 years ago
(Hewish et al. 1968).
It was soon clear that this emission must be produced by high energy particles
streaming in the magnetosphere of a rotating neutron star
(Gold 1968;
Pacini
1968). Several models and calculations were performed afterwards
to explain the observational results.
Despite these theoretical efforts many problems are still open and, in
particular, that of the location of the emitting regions in the
magnetosphere. Phenomenological models, based on
pulse profiles in different bands of the electromagnetic spectrum,
have been proposed (see, for instance Rankin 1991;
Ulmer 1994), but a
fully satisfactory scenario has not been established yet.
Under this respect the Crab pulsar, PSR 0531+21 (formerly indicated as NP
0532), is a unique object because of its high luminosity and proximity.
In fact, it can be observed across the entire electromagnetic spectrum -
from radio frequencies to 
rays - and, furthermore, it is the only
pulsar for which high energy data covering a rather long time interval
are available.
In recent times the problem of long term changes of the 
-ray pulse
profile of PSR 0531+21 has been debated: Kanbach (1990)
suggested a
possible periodicity of the P2/P1 ratio (i.e. the ratio
between the relative intensities of the two major peaks, which are well
evident in all spectral bands) at energies greater than about 50 MeV.
After the Compton-GRO observations, the significance of this finding
has been discussed in some other papers (Ulmer 1994;
Ramanamurthy et al.
1995), but no conclusive result has been reached. Other attempts to search
for P2/P1 changes in the X-ray and low energy 
-ray band have
also been carried out (Ulmer 1994;
Nagase 1994; Massaro et al. 1995),
but these analyses are complicated by the strong dependence of the ratio
P2/P1 on the energy.
We decided, therefore, to collect all the pulse profiles of PSR 0531+21,
from soft X-rays to medium-high energy 
rays, available in the
literature since the early observations. We then prepared an ATLAS
(Massaro et al. 1996) and organized a computer database which
is available on the Mosaic
home page of the Istituto Astronomico of the Università "La Sapienza"
(Roma) at http://astrm2.rm.astro.it.
In the next sections we describe the database and use it to investigate some problems concerning the structure of the pulse profile such as the dependence on energy and time of the ratio P2/P1, the shape of P1, usually considered the most stable feature, and the P1-P2 phase separation.