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high mass x - ray binaries ( hmxbs ) are binary systems consisting of a compact object orbiting a massive companion star .
prior to the launch of the international gamma - ray astrophysics laboratory ( _ integral _ ) in 2002 , most of the known hmxbs contained a be companion . in be - type hmxbs ,
the compact object emits strong x - ray flashes when it crosses the equatorial plane of the companion star , where a thick disk of matter originating from the stellar wind is present .
o and b - type stars have more isotropic stellar winds which absorb the x - ray emission of the compact object , rendering them almost undetectable below a few kev . since the launch of _ integral _ , however , many such systems have been found ( see e.g. * ? ? ?
* ) , raising the ratio of be - hmxbs to o , b - hmxbs close to 0.5 .
the study of this new population may have strong implications on the scenarios of binary formation and evolution . in this perspective
, the use of x - ray spectroscopy at different orbital phases makes it possible to probe the stellar wind , providing two - dimensional information on the density and ionization structure of the wind .
for instance , the soft excess that is present in the soft x - ray spectra of many hmxbs , whose origin is still quite mysterious , is linked to the physics of the wind close to the compact object , especially the region where the fast moving stellar wind collides with the slow moving and highly ionized gas surrounding the compact object @xcite .
igr  j19140@xmath00951  was discovered by _ integral _  by the satellite _ integral _  in march 2003 @xcite .
prior to our study , it was identified an a hmxb with an orbital period of 13.552 days @xcite , composed probably of a neutron star @xcite orbiting a b0.5 i type supergiant companion star @xcite .
thus , it is a good candidate to study the properties of the obscured x - ray binaries .
the data from the two satellites were reduced using the latest software packages available . since igr
j19140@xmath00951  is a fairly faint source , we also corrected the spectra for the galactic x - ray background ( gxb ) , using the parameters measured by @xcite .
this correction aimed mostly at minimising the errors on the measure of the photoelectric absorption , by removing the galactic contribution and keeping only the absorption intrinsic to the source .
we identified the emission process as thermal comptonization , which is quite common for this type of source . but
a more interesting characteristic is the detection of a so called `` soft excess '' .    during our study , some spectra exhibited an excess in the soft x - ray part of the spectra , which we modeled by adding a black body component to the model .
this feature has already been observed in many x - ray binaries ( see * ? ? ? * for a review ) .
its origin is mysterious , but can be linked to the immediate environment of the neutron star , and explained qualitatively by the following scenario . due to its high energy emission , the neutron star ionizes the surrounding material . since the stellar wind is accelerated by ionization in the uv lines , the already ionized gas around the neutron star is no longer accelerated by the stellar radiation field .
when the compact object moves along its orbit , the hot gas will gradually be overtaken by the stellar wind .
this will lead to the formation of a  tail " trailing the neutron star .    as the gas in this tail is ionized , its absorption will be lower , but its precise effect on the emission is not clear .
since the soft excess is only visible just prior the superior conjunction ( between phases 0.25 and 0.5 ) , one possibility is that the hard x - ray emission may be scattered around the tail .
the precise study of this effect , however , would require better spectral resolution in the soft x - ray range than what is available on _
in order to constrain several parameters for the system , we used a simple wind model based of the fact that b - type stars emit strong stellar winds , usually taken to be stationary and spherically symmetric .
see e.g. @xcite for a precise description of the method
. using the evolution of the photoelectric absorption , we were able to constrain the inclination of the system and the mass - loss rate of the companion star .
for the most probable stellar radius of 21 @xmath4 , the lower inclination limit is constrained between 37 and 42 degrees ( fig .
[ meilleur ] , left ) , with @xmath5 being at a minimum in the range 5563 degrees .
[ meilleur ] , right , shows the best - fit model against the experimental normalised data , with a good agreement .
@xmath6 is constraint in the range 0.61.0@xmath710@xmath2 m@xmath3/year , which is consistent with what is expected for a b0.5 type supergiant .
symbols ) , for the stellar model with parameters m@xmath8 = 20m@xmath3 , r@xmath8 = 21r@xmath3 and @xmath9 = 0.5 .
phase 0 corresponds to when the compact object is located between the earth and the companion star ( inferior conjunction).,title="fig:",width=302 ]   symbols ) , for the stellar model with parameters m@xmath8 = 20m@xmath3 , r@xmath8 = 21r@xmath3 and @xmath9 = 0.5 .
phase 0 corresponds to when the compact object is located between the earth and the companion star ( inferior conjunction).,title="fig:",width=283 ]
the study of x - ray binaries is challenging since it is often difficult if not impossible to identify their visible and infrared counterparts .
even if an infrared counterpart were observed , the distance to some systems prohibits the measurement of the orbital characteristics .
our study shows that x - ray observations can overcome these limitations and produce very precise inferences .
the _ rxte _  and
_ integral _  observations of igr  j19140@xmath00951  have led to good measurements of the orbital period of the system and constraints on its inclination angle .    moreover
, we can use the compact object to probe the stellar wind of the companion . in the case of igr
j19140@xmath00951 , we diagnosed the type of the companion ( supergiant o or b ) , the wind density and its structure around the neutron star .
more precise observations could lead to constraints on the mass and radius of the companion , and better constraints on the stellar wind .
this allows the study of new _ integral _
sources , either distant or highly absorbed , and ultimately the determination of new useful data for x - ray binary evolution scenarios .
we have used the _ rxte _  and _ integral _  satellites simultaneously to observe the high mass x - ray binary igr  j19140@xmath00951 . 
 the spectra obtained in the 380 kev range have allowed us to perform a precise spectral analysis of the system along its binary orbit . 
 the spectral evolution confirms the supergiant nature of the companion star and the neutron star nature of the compact object . using a simple stellar wind model to describe the evolution of the photoelectric absorption 
 , we were able to restrict the orbital inclination angle in the range 3775 degrees . 
 this analysis leads to a wind mass - loss rate from the companion star of @xmath1 10@xmath2 m@xmath3/year , consistent with its expected spectral type . 
 we have detected a soft excess in at least three observations , for the first time for this source . 
 such soft excesses have been reported in several hmxbs in the past . 
 we discuss the possible origin of this excess , and suggest , based on its spectral properties and occurrences prior to the superior conjunction , that it may be explained as the reprocessing of the x - ray emission originating from the neutron star by the surrounding ionised gas .