High Mass X-ray Binary Pulsars (HMXBP), in which the companion star is a <br /><br />
source of supersonic stellar wind, provide a laboratory to probe the <br /><br />
velocity profile of such winds. Here, my talk will consist of three parts. <br /><br />
In the first part, I will present our measurements of the X-ray spectral <br /><br />
evolution over the binary orbit for four HMXBP observed with the Rossi <br /><br />
X-ray Timing Explorer and the BeppoSAX satellites. <br /><br />
In one pulsar 4U 1538-52, we find the spherically symmetric stellar wind <br /><br />
model corroborating the observations, whereas in the other three, the <br /><br />
stellar wind seems to be very clumpy. Moreover, in GX 301-2, neither the <br /><br />
presence of a disk nor a gas stream from the companion was validated. <br /><br />
<br /><br />
In the second part, I will present the timing and spectral properties of <br /><br />
the transient Be/X-ray binary pulsar 3A 0535+262 during quiescence using <br /><br />
three observations with BeppoSAX. We report the detection of pulsations at <br /><br />
a very low luminosity of 2 $\\\\times$ 10$^{33}$ erg s$^{-1}$ during one of <br /><br />
the three observations, though at this accretion rate the system is <br /><br />
expected to be in the centrifugally inhibited regime. The X-ray spectra <br /><br />
for the unpulsed observations are best modeled as power law type while a <br /><br />
combined model of power law and black-body is required to fit the pulsed <br /><br />
spectrum. <br /><br />
<br /><br />
Lastly, I will report our results on the study of the spectral properties <br /><br />
of the HMXBP Cen X-3 and 4U 0114+650 in their high and low states. <br /><br />
Thereafter, I will conclude by stating my future research plans.<br /><br />