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the class of sources comprises 6 lmxbs (  , , , , , and ) in which the primary is a neutron star with a low magnetic field ( @xmath6@xmath7 g ) accreting at or near the eddington limit [ 1 ] .
they share similar timing properties and are among the most luminous known lmxbs .
the designation source results from the shape described in a x  ray color@xmath2color diagram ( cd ) , with the movement along the interpreted in terms of changes in the mass accretion rate ( , see , e.g. , [ 1 ] ) . apart from  and , the sources are all found near the galactic mid - plane ( i.e. , @xmath8 ) .    hard x 
ray spectra from both and low luminosity atoll sources have already been reported in the literature [ 2 - 8 ] .
the production of the hard x  ray tails in atoll sources has been presented in the context of various thermal emission models [ 9 ] from which the accretion geometry can be inferred .
the situation is less clear for the sources , where non  thermal mechanisms are invoked to explain the production of such a component , and little , or nothing , is known about the details of the accretion geometry .
we are currently analyzing all of the source observations in the public _ rxte _ database which contain long pointings .
the aim is to create an uniform database that will allow us to make direct hard x  ray spectra comparisons . from this
we expect to better understand the behavior of any non  thermal emission in these sources .
we report here the preliminary results of this work , with data from 3 (  , , and ) out of the 6 known sources .
we used data from hexte [ 10 ] to search for hard x  ray tails in the spectrum of  , , and in the @xmath620220kev interval and data from pca [ 11 ] to determine the position of the source in the cd and to study the 220kev spectrum .
we selected , from the public _
database , those subsets of data in which @xmath95000s of hexte total on  source time was available , in order to achieve good sensitivity at high energies .
table[tab1 ] shows the selected subsets for and .
the list of selected observations of  is given in [ 7 ] .
we used xspec to analyze the pca source spectra , using published models for ( a blackbody plus a disk - blackbody and an iron line , see [ 12 ] ) and ( an absorbed cutoff power - law plus an iron line , see [ 13 ] ) .
a complex multicomponent model ( an absorbed blackbody plus a power - law , a comptonization spectrum , and a gaussian line ) was used to heuristically fit the pca  spectra .
low enewrgy ( 2050kev ) hexte spectra were fitted by a simple thermal bremsstrahlung . the hard x - ray component ( i.e. @xmath10kev ) , found only in  , was modeled as a simple power - law ( see [ 6 ] for a more detailed description of the instrument and procedure used for data analysis ) .
we carefully verified our background subtraction procedures , specially for , which is located near the galactic mid - plane , where the diffuse galactic plane background up to @xmath11 800kev [ 14 ] is known to vary in latitude [ 15 ] .
we took advantage of hexte aperture modulation to remove this contribution to the background since hexte cluster a measured the background at the same latitude as the source .
source confusion is also a concern for due to the presence of 4u1700@xmath1237 ( see , e.g. , [ 16 ] ) inside the field of view of one of the regions used by hexte cluster b to measure background ( the b@xmath13 region ) .
this is easily solved using only the b@xmath14 region to measure the background for hexte cluster b. we found no evidence of source confusion / contamination for and  .
.selected _ rxte _ observations of gx349@xmath152 and cygx-2 [ cols="^,^,^,^,^,^,^,^ " , ]
cygnusx-2 and were easily detected by hexte up to 50kev .
nevertheless , the detection level was _ always _ below 3@xmath1 in the 5075kev band .
we show in fig .
[ fig1 ] a typical spectrum for and together with a detection and a non - detection of a hard x  ray tail in  .
all sources show some degree of variability in the 20 - 50kev range . from the results in [ 7 ] , for  , a factor of 2
was detected , while it was a factor of 5 for and 2 for . among the three ,
is the least luminous in the 220kev energy range , with an average luminosity of 0.4@xmath16 ( using @xmath17 and @xmath18 measurements in [ 17 ] ) , while  and emit at or above eddington levels , for @xmath19 ( see [ 18 ] and [ 19 ] for measured distances to  and , respectively ) .
we found no evidence of the presence of a hard x  ray tail in our database for or .
the hexte 3@xmath20 upper limit to 50100kev flux from is and for is .
for these two sources , a hard x  ray tail was , however , reported by _ bepposax _ ( [ 8 ] and [ 3 ] , respectively ) , at a level of for ( using the fit parameters given in [ 8 ] ; for it is not possible to estimate the flux from [ 3 ] ) .
our results , thus , can be interpreted in terms of variability in the appearance of this component , as was observed in  [ 7 ] on a 4 hour time - scale .
scorpiusx-1 remains as a special case among the sources .
it is the only one in which a hard x  ray tail has been observed more than once , and by two different instruments ( [ 5 ] and [ 7 ] ) . for , and hard x  ray tails
were reported by _ bepposax _ ( [ 3 ] , [ 4 ] , and [ 8 ] , respectively ) on one occasion . from our combined hexte database
, we found the presence of a hard x  ray tail in 8 out of 28 occasions for  , and zero out of 10 and 13 observations of and , respectively .
fitting our hexte data for and with a power - law with indices frozen in the range 12 ( within the values found for those three sources : see [ 3 ] , [ 7 - 8 ] ) , we found a 3@xmath1 upper limit on the luminosity of the power - law component , @xmath21ergss@xmath5 and @xmath22ergss@xmath5 for and , respectively .
our hexte result ( for @xmath23 ) for hard x  ray tail detections in  is @xmath24ergss@xmath5 .
it thus appears that our observations were sensitive enough to detect hard x  ray tails in and .
as we pointed out in [ 7 ] the chance of observing a hard x  ray tail ( in  ) is higher when the thermal component of the spectrum is brighter . from our results here
( see table [ tab1 ] ) , we have , for @xmath25 1.23.1@xmath26ergss@xmath5 , while for the results are @xmath27 0.42.1@xmath26ergss@xmath5 . the same component in  , when a hard tail is detected [ 7 ] ,
is in the range @xmath28 = 4.59.0@xmath29ergss@xmath5 . while comparable values were not given by the _ bepposax _
results in [ 3 ] , [ 4 ] , and [ 8 ] ( nor by the osse/_cgro _ results in [ 5 ] ) , it is possible to extrapolate the results presented in [ 8 ] in order to find an estimate of the luminosity of the thermal component .
we estimate that the 2050kev luminosity measured by _
bepposax _ was greater than @xmath30ergss@xmath5 .
thus , one can speculate that the production of a hard x  ray tail in a source is a process triggered when the thermal component is brighter than a level of @xmath6@xmath31ergss@xmath5 .
we have shown _ rxte _ results of broad - band spectral analyses of three sources , with emphasis on the hard x  ray spectrum .
we found no evidence for a detection of a hard x  ray tail in the spectra of and , although one detection of such a component has been reported for each of these sources .
we interpret this in terms of variability , which was shown to be as fast as 4 hours in  .
we found an indication that the production of hard x  ray tails in sources is a process triggered when the thermal component brightness is above a value of @xmath6@xmath32ergss@xmath5 . we are currently creating a uniform hexte database including the other three sources ( , , and ) , from which we hope to be able to better understand the production of hard x  ray in sources .
this research has made use of data obtained through the heasarc , provided by nasa / gsfc .
f.d . gratefully acknowledges fapesp / brazil for financial support under grant 99/02352 - 2 .
this research was supported by nasa contract nas5 - 30720 .
\1 . van der klis , m. , _ x - ray binaries _ , edited by w. h. g. lewin , j. van paradjis , and e. p. j. van den heuvel , cambridge university press , cambridge , 1995 , pp.252307 .
strickman , m. , and barret , d. 2000 , `` detections of multiple hard x - ray flares from sco x-1 with osse '' , in _ proceedings of the fifth compton symposium _ , edited by m. l. mcconnel and j. m. ryan , aip conference proceedings 510 , new york , 2000 , pp .
222 - 226 .
we report _ rxte _ results of spectral analyses of three (  , , and ) out of the 6 known sources . no hard 
 x  ray tails were found for ( @xmath0 , 50100kev , 3@xmath1 ) and for ( @xmath0 , 50100kev , 3@xmath1 ) . for  a variable hard x  ray tail ( with an average flux of , 50100kev ) has already been reported . 
 we compare our results to reported detections of a hard component in the spectrum of and . 
 we argue that , taking into account all the results on detections of hard x  ray tails in  and , the appearance of such a component is correlated with the brightness of the thermal component .    
 = cmbx10 scaled 1 = cmbx10 scaled 2 = cmbx10 scaled 3 = cmbx10 scaled 4 = cmbx10 scaled 5     
 scox-1 # 1#2#1@xmath2@xmath3photons@xmath4s@xmath5     address = center for astrophysics and space sciences , university of california , san diego , and + instituto nacional de pesquisas espaciais - inpe + av . dos astronautas 1758 , 12227 - 010 s. j. dos campos , brazil , email=fdamico@mamacass.ucsd.edu ,     address = center for astrophysics and space sciences , university of california , san diego + 9500 gilman dr . , 
 la jolla , ca 92093 - 0424 , email=wheindl@ucsd.edu ,     address = center for astrophysics and space sciences , university of california , san diego + 9500 gilman dr . 
 , la jolla , ca 92093 - 0424 , email=rrothschild@ucsd.edu ,     address = center for astrophysics and space sciences , university of california , san diego + 9500 gilman dr . 
 , la jolla , ca 92093 - 0424 , email=dgruber@ucsd.edu