My current line of research consists of the study of the temporal properties of X--ray binary pulsars. The standard model describes this class of objects as binary systems formed by an early-type star (spectral class O--B--Be) orbiting a compact object (a neutron star in the case of binary pulsars) which is accreting matter from the normal star. According to the geometry, mass and evolution of the system, the accretion can take place mainly along the equatorial plane of the compact object, by means of an accretion disk, or roughly isotropic (in this case we speak about wind accretion).
The X--ray emission is due to the accreted matter which is channeled onto the
magnetic polar caps by the intense magnetic field, and there decelerated (the
surface magnetic field strength of a neutron star is 
--
gauss). The pulsation is due to a ``lighthouse'' effect, because of the
misalignment between the spin axis and the (assumed) dipolar magnetic axis.
Specifically, my field of interest is wind-fed X--ray binary systems and their temporal variability [8]. I am interested in both the coherent and the aperiodic component (the aperiodic component is responsible for the pulse to pulse variability, which makes each pulse different from the others). The region in which the aperiodic variability takes place is not yet known. Possible regions are the magnetospheric limit, the region between the magnetospheric radius and the neutron star surface, or the neutron star surface itself. The physical processes which occur in these different regions are different, and of course the time scales associated to them.
At the magnetospheric limit we have that the physics involved is the magnetohydrodynamical instability (both Rayleigh--Taylor and Kelvin--Helmholtz) which allows matter to penetrate the magnetosphere and therefore accretion to occur. The physical processes involved in this region have time scales of the order of 0.1--10 s (growth time for the instabilities).
At the neutron star surface, on the other hand, we expect time scales that are much shorter, because the physical processes involved are Compton cooling and heating, bremsstrahlung and Coulomb interactions. Time scales as short as a microsecond or less can be achieved for processes which occur at the neutron star surface.
During my PhD thesis I have developed a model which describes the aperiodic component both from a statistical point of view and a physical point of view. In the former case I have shown that some power spectra of the wind-fed X--ray binary pulsar GX 301--2 (I have analysed 10 ME EXOSAT observations) can be statistically described in term of a shot noise process with a special response function [19].
From the physical point of view I have shown that the statistical parameters of the model can be associated to the magnetohydrodynamical instability which takes place at the magnetospheric limit. The magnetospheric limit is assumed as the region at which the aperiodic component is originated, and the physical parameters of the instability are expressed in terms of the parameters of the observed power spectra. It is the first time that the theory of plasma penetration in wind-fed X--ray binaries is successfully applied.
As a further result of the model, I predict the presence of a micro-second structure in the emission of this class of object. A proposal to increase the timing resolution of the Proportional Counter Array on-board the X--ray Timing Explorer mission has been presented and accepted, in order to study this phenomenon [16,17,18].
For my PhD thesis I have furthermore analysed four EXOSAT observations of the
wind-fed X--ray binary system 4U 1538--52. Preliminary results have been
presented at the 23
ESLAB Symposium held in Bologna on 1989
[4], while the final results have been published on Astrophysical
Journal [21]. In one observation an absorbing episode was observed,
which may be attributed to an inhomogeneity in the stellar wind of QV Nor, the
optical counterpart of 4U 1538--52. The scale height of this blob is 
---
cm, assuming a blob velocity of 100--1000 Km/s, of
the same kind of inhomogeneities observed in the Vela X--1 system. Using the eclipse
transition time of 
0.06 days we can confirm the scale height of 
cm for the atmosphere of the optical counterpart. The source
shows prominent aperiodic time variability: the frequency spectra for the two
observations out of eclipse are well fitted by a power law plus a constant. The
rms's are in agreement with those observed in other high mass X--ray pulsars:
26 
5% for the March observation and 36 
6% for the August
observation. The 1.5 -- 12 keV phase-averaged X--ray spectrum is consistent
with a power--law of photon spectral index 
, and shows an Iron line
with an E.W. of 
100 eV.
During my permanence at the Laboratory for High Energy Astrophysics I
have analysed 13 pointing observations of the X--ray pulsar Vela X--1 obtained by the
A2 instrument aboard ´´. The first step for performing the temporal analysis of
wind-fed X--ray binary pulsars observed by ´´ was to convert the data in a
format suitable to be read and analysed by 2pt
ronos , a timing analysis software
package developed by Luigi Stella and Lorella Angelini for EXOSAT data.
After this first phase, the analysis of the data taken by the Medium Energy Detector of the A2 experiment (I used data with 80 ms timing resolution, integrated in the energy band 1.5--20 keV) has given the following results [20]:
, and the peak heights of the
normalized pulse profile (each pulse profile has been normalized in the
interval 0--1): higher the intensity, lower the heights of the peaks. An
anti-correlation is present between 
and the pulsed fraction, too.
. This has been interpreted in terms of an
independence of the pulsation mechanism from the physical quantities that
strongly change from one observation to anther, as 
and the accretion rate.
when the source is
spinning down (of about a factor four).
% to 
%). This, together with the fact that 
increases during the spin-down phase, is in agreement with the hypothesis that
the aperiodic component is generated at the magnetospheric limit.
One goal of my research on wind-fed X--ray binary pulsars is to clarify the origin of the aperiodic component observed in the X--ray flux from this class of objects. One of the most important results of my analysis was the identification of the spinning state of the source as an important property which strongly influences the aperiodic component. This put a strong constraint on the possible region in which the aperiodic component is originated, and confirms the hypothesis that this region could be the magnetospheric limit.
If this is the case, then the ``noise'' (the aperiodic component) is generated at the level of the process of accretion of matter. We expect that the pulsation mechanism should not alter the nature of the ``noise'', because the emission beams merely probe different areas of the matter surrounding the neutron star. Therefore the aperiodic and the coherent components could be treated as independent, and the description of a pulsar power spectrum as the sum ( i.e. independence) of a power law plus harmonics of pulsation finds a natural physical interpretation.