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energy spectra and colour - colour diagrams ( cd ) are often used to study neutron - star low - mass x - ray binaries ( ns - lmxbs ; e.g. , * ? ? ? the evolution of the energy spectrum and the tracks on the cd are thought to be driven by variations of mass accretion rate , and reflect changes in the configuration of the accretion flow ( e.g. , * ? ? ? * ; * ? ? ? * ; * ? ? ? * ) . in the low - hard state the accretion rate is low , the disc is truncated at large radii ( * ? ? ? * ; * ? ? ? * ; * ? ? ? * see however lin et al . 2007 ) and the energy spectrum is dominated by a hard / comptonised power - law component . when the source accretion rate increases , the disc truncation radius decreases and eventually reaches the last stable orbit . in the high - soft state the accretion rate is high and the energy spectrum is dominated by a soft component , possibly a combination of the accretion disc and the neutron star . the characteristic frequencies ( e.g. , quasi - periodic oscillations , qpos ) in the power density spectra ( pds ) of these systems also change with the source luminosity and inferred mass accretion rate ( e.g. , * ? ? ? * ; * ? ? ? kilohertz ( khz ) qpos have been detected in many ns - lmxbs ( for a review see * ? ? ? * and references therein ) . the upper khz qpo ( from the pair of qpos the one at the highest frequency ) in these systems has been interpreted in terms of characteristic frequencies ( e.g. , the keplerian frequency ) in a geometrically thin accretion disc @xcite . in this scenario , changes of the upper khz qpo frequency reflect changes of the inner disc radius , driven by mass accretion rate . indeed , the frequency of the upper khz qpos is strongly correlated with the hard colour of the source @xcite . several models have been proposed to explain the lower khz qpo in these systems . @xcite suggested a lense - thirring precession model , in which the frequencies of the qpos are associated with the fundamental frequencies of geodesic motion of clumps of gas around the compact object . in the relativistic resonance model @xcite , the khz qpos appear at frequencies that correspond to a coupling between two oscillations modes of the accretion disc . in the beat - frequency model @xcite , the lower khz qpo originates from the interaction between the spin frequency of the ns and material orbiting at the inner edge of the accretion disc . none of these models , however , have so far been able to fully explain all the properties of khz qpos ( e.g. , * ? ? ? * ; * ? ? ? * ; * ? ? ? 4u 163653 is a ns - lmxb that shows regular state transitions with a cycle of @xmath340 days ( e.g. , * ? ? ? * ) , making it an excellent source to study correlations between its spectral and timing properties . the full range of spectral states ( low / hard state , high / soft state , transitional state ) has been observed in this source @xcite . a pair of khz qpos were discovered by @xcite and @xcite . the upper khz qpo has been observed in different states . its central frequency shows a clear correlation with the hard colour of the source @xcite . the lower khz - qpo in 4u 163652 is only detected over a narrow range of hard colour values @xcite . the emission mechanism of the lower khz - qpo is still unclear ( e.g. , * ? ? ? * ; * ? ? ? * ; * ? ? ? * )    we analysed the broadband energy spectra of 4u 163653 to investigate the evolution of the different spectral and timing components as a function of the spectral state of the source . a comparison the different continuum components in the energy spectrum with the properties of the khz qpos at the same state may provide an important clue to understand the origin of the khz qpos and the evolution of the accretion flow geometry . in [ data ] we describe the observations , data reduction and analysis methods , and in  [ result ] we present the results on the temporal and spectral analysis of these data . finally , in  [ discussion ] we discuss our findings and summarise our conclusions . we analysed the whole archival data ( 1576 observations ) from the rossi x - ray timing explorer ( _ rxte _ ) proportional counter array ( pca ; * ? ? ? * ) and the high - energy x - ray timing experiment ( hexte ; * ? ? ? * ) of the ns - lmxb 4u 163653 . we reduced the data using the heasoft package version 6.13 . we extracted pca spectra from the proportional counter unit number 2 ( pcu-2 ) only , since this was the best - calibrated detector and the only one which was always on in all the observations . to extract the spectra of the source we first examined the light curves to identify and remove x - ray bursts from the data . for the hexte data we generated the spectra using cluster b only , since after january 2006 cluster a stopped rocking and could no longer measure the background . for each observation we extracted one pca and hexte x - ray spectrum , respectively . the pca and hexte background spectra were extracted using the standard rxte tools pcabackest and hxtback , respectively . we built instrument response files for the pca and hexte data using pcarsp and hxtrsp , respectively . for each observation we computed fourier power density spectra ( pds ) in the @xmath4 kev band every 16 s from event - mode data . for this we binned the light curves to 1/4096 s , corresponding to a nyquist frequency of 2048 hz . before computing the fourier transform we removed detector dropouts , but we did not subtract the background or applied any dead - time correction before calculating the pds . finally we calculated an average pds per observation normalised as in @xcite . we finally used the procedures described in @xcite to detect and fit the qpos in each pds . we detected khz qpos in 581 out of 1576 observations . we detected the lower khz qpo in 403 out of those 583 observations . , width=316 ]    we used the standard-2 data ( 16-s time - resolution and 129 channels covering the full @xmath4 kev pca band ) to calculate x - ray colours of the source ( see * ? ? ? * for details ) . we defined hard and soft colours as the @xmath5 kev and @xmath6 kev count rate ratios , respectively . we show the cd of all observations of 4u 1636@xmath153 in figure [ fig : ccd ] , with one point per rxte observation . in that figure we parameterised the position of the source in the cd by the length of the solid curve @xmath7 ( see , e.g. * ) , fixing the values of @xmath8 and @xmath9 at the top - right and the bottom - left vertex of the cd , respectively .    for the spectral analysis of 4u 163653 , we used the package xspec v12.7 @xcite . we fitted the pca and hexte spectra simultaneously in the 3.0@xmath125.0 and 25.0@xmath1180.0 kev range , respectively . we included the effect of interstellar absorption using the component phabs with cross - sections of @xcite and solar abundances from @xcite , and we fixed the column density to @xmath10 @xmath11 @xcite . we added a multiplicative factor to the model to account for calibration uncertainties between the pca and hexte . we set this factor to unity for the pca and left it free for the hexte spectra . many models have been proposed to fit the spectra of accreting ns - lmxbs in the past ( e.g. * ? ? ? * ; * ? ? ? most models include at least two components : a soft thermal component that represents the emission from the ns surface ( or boundary layer ) and the accretion disc , and a hard comptonised component that represents the emission from a corona of hot electrons .    in the spectral fitting of 4u 163653 we used the same continuum model as in @xcite , who fit _ xmm - newton _ data down to 0.8 kev . we used a multi - colour disc blackbody ( diskbb in xspec ) to fit the thermal emission from the disc , a single - temperature black body ( bb , bbodyrad in xspec ) to fit the thermal emission from the ns surface ( or boundary layer ) , and a comptonisation model , nthcomp , to fit the comptonised component @xcite . the parameters of bbodyrad and diskbb are the blackbody temperature , @xmath12 , the temperature at the inner disc radius , @xmath13 , and their normalisations , respectively . the parameters of nthcomp are the asymptotic power - law photon index , @xmath2 , the electron temperature of the corona , @xmath14 , the seed photon temperature , @xmath15 , and the normalisation . the seed photons can be from either the ns surface and boundary layer or the accretion disc . @xcite analysed the spectra of six observations of 4u 163653 taken with _ xmm - newton _ and _ rxte _ simultaneously . they tried both the bbodyrad or the diskbb component as the source of seed photons for nthcomp , and concluded that diskbb was the best option . therefore , in this work we chose diskbb as the source of the soft seed photons and we linked the seed photons temperature in nthcomp to the temperature of the diskbb component . in the fits , the value of @xmath2 was constrained to be larger or equal to 1.1 @xcite and the value of @xmath14 was restricted to be larger or equal to 2.5 kev @xcite    from the initial fits , there were always relatively broad residuals between 6 and 7 kev in the _ rxte_/pca spectra . these residuals likely reflect the presence of an iron line in the data . the iron line has also been confirmed with _ xmm - newton _ and _ suzaku _ observations @xcite . we then added a gaussian component with a variable width to the spectrum to fit this line . due to the low spectral resolution of the pca ( @xmath31 kev at @xmath16 kev ) , we can not constrain the central value of the line very well . so we fixed the line at 6.5 kev in all our fittings . fitting simultaneously data taken from _ xmm - newton _ and _ rxte _ , @xcite found that the temperature of bbodyrad was @xmath17 kev , and remained more or less constant as the source moved across the cd . they also found that the temperature at the inner edge of the accretion disc ranged from @xmath30.2 kev to @xmath30.8 kev when the source moved from the low / hard to the high / soft state . since the pca only extends down to 3 kev , @xmath12 and @xmath13 are difficult to constrain simultaneously . therefore , we initially left @xmath13 free and , following the results of @xcite , we fixed @xmath18 to 1.7 kev . we then repeated the analysis fixing @xmath12 to each of these values : 1.5 , 1.6 , 1.8 , 1.9 and 2.0 kev .    from the previous analysis we found that @xmath13 , linked to @xmath15 in nthcomp , was not well constrained during the fits . we therefore interpolated the value of @xmath13 along the cd using the _ xmm - newton spectra fitting results in @xcite . we divided the data into 7 groups based on their @xmath7 values : @xmath19 , @xmath20 , @xmath21 , @xmath22 , @xmath23 , @xmath24 and @xmath25 . in each group we fixed @xmath13 to the interpolated temperatures for @xmath7 equal to 1.1 , 1.3 , 1.5 , 1.7 , 1.9 , 2.1 , and 2.35 , respectively . in this case we left @xmath12 free during the fits . in figure [ fig : sa_ga_kt ] we show the asymptotic power - law photon index , @xmath2 , as a function of @xmath7 in 4u 163653 . the value of @xmath12 indicated in each panel was fixed during the fittings . when the source evolves from the hard spectral state to the transitional state in the cd , the value of @xmath7 increases from 1.0 to 2.0 , @xmath2 increases from 1.8 to 2.2 and the source spectrum becomes soft . as the source moves through the vertex in the cd , @xmath26 , @xmath2 drops sharply to @xmath27 , and covers the range from @xmath27 to @xmath28 . when the source moves out of the vertex to the bottom right part of the cd @xmath2 increases to @xmath28 , and finally decreases back to @xmath23 as @xmath7 increases to @xmath32.5 . it is clear from all panels that @xmath2 shows a significant drop at @xmath26 , regardless of the value we choose for @xmath12 . moreover , at low @xmath7 values ( @xmath29 ) , @xmath2 shows a larger spread for low @xmath12 values . this likely indicates that the lower blackbody temperature ( e.g. @xmath30 kev ) yields comparatively less well - constrained fits of the thermal comptonisation model in 4u 1636@xmath153 . the upper left panel of figure [ fig : sa_ga_pha ] shows @xmath2 as a function of @xmath7 when we used the interpolated values of @xmath13 and left @xmath12 free . the trend in this figure is similar to those in figure [ fig : sa_ga_kt ] . in the upper right panel figure [ fig : sa_ga_pha ] we show the electron temperature , @xmath14 , as a function of @xmath7 . as the source evolves from the hard to the transitional , and then to the soft spectral state @xmath14 first decreases from @xmath31 kev to @xmath32 kev and then stays more or less constant .    in order to visualise the evolution of the individual components and the total spectrum in different states , we chose four observations covering the whole range of @xmath7 values . the four observations , obs1 , obs2 , obs3 and obs4 with @xmath331.05 , 1.90 , 2.15 and 2.35 , respectively , are shown with red filled circles in the upper panels of figure [ fig : sa_ga_pha ] . in the middle and lower panels of figure [ fig : sa_ga_pha ] we show the pca / hexte model spectra of these 4 observations . the spectral components in the plots are diskbb ( red / dashed - dotted ) , bbodyrad ( pink / dotted ) , gaussian ( green / dotted ) and nthcomp ( blue dashed - three dotted ) , respectively . the best - fitting results for the four observations are given in table [ tab - nthcomp ] . +    +      in the middle - left panel of figure [ fig : sa_ga_pha ] ( obs1 ) the source is in the low / hard state . the broad band x - ray spectrum is dominated by the hard / comptonised component , nthcomp . in this observation the power - law index is @xmath34 and the electron temperature is @xmath35 kev    when the source evolves from the low / hard state towards the vertex in the cd , the value of @xmath7 increases from 1.0 to 2.0 , @xmath2 increases from @xmath36 to @xmath37 and the spectrum becomes soft ( obs2 in the right panel of figure [ fig : sa_ga_pha ] ) . the electron temperature decreases from @xmath38 kev to @xmath39 kev . compared to obs1 , it is clear that the contribution of bbodyrad and diskbb increases , and the emission above 80 kev drops , significantly . when the source is at the transitional state , @xmath40 , the value of @xmath2 covers a large range between 1.3 and 2.5 . the emission from the diskbb and nthcomp components is more or less the same below 5 kev ( bottom left panel of figure [ fig : sa_ga_pha ] ) . the nthcomp component is relatively flat but the high - energy cut - off is at much lower energy than in obs1 . our results are consistent with those of @xcite for the combined _ xmm - newton  rxte _ spectra ( see their figure 4 ) .    when the source moves out from the transitional state to the high - soft state ( obs4 ) , similar to what @xcite show in their figure 4 , the normalization of diskbb decreases and the emission from diskbb is lower than that of nthcomp below 5 kev . = 2.0 mm      from figure [ fig : sa_ga_pha ] and the discussion above , it is apparent that the power - law photon index , @xmath2 , in many of the observations shows a significant drop when the source is in the transitional state . due to the large spread of @xmath2 when 2 @xmath41 @xmath7 @xmath41 2.2 the trend of @xmath2 in this area of the plot is not clear . we therefore first sorted the data in figure [ fig : sa_ga_pha ] according to @xmath7 and rebinned them using a step of 0.025 , 0.005 and 0.02 in the range of 1.0@xmath421.8 , 1.8@xmath422.2 and @xmath432.2 , respectively . the weighted average @xmath2 as a function of @xmath7 is shown in figure [ fig : sa_gamma_rebin ] . from this figure it is apparent that , as @xmath7 increases from @xmath44 to @xmath45 , @xmath2 decreases abruptly from @xmath46 to @xmath47 , around @xmath7 @xmath45 @xmath2 increases again with @xmath7 from @xmath47 to @xmath48 and , as @xmath7 increases from @xmath45 to @xmath49 , @xmath2 decreases from @xmath48 to @xmath50 . in the transitional state @xmath2 shows a large spread , but the plot in figure [ fig : sa_gamma_rebin ] shows that @xmath2 changes significantly with @xmath7 and that the variation of @xmath2 is larger than the statistical fluctuations in the data . the large range of @xmath2 at the transitional state might be due to the fact that the model assumptions ( e.g. , a geometrically thin disc , or a spherical corona ) are no longer valid . figure [ fig : sa_qpo ] shows the centroid frequency of all the khz qpos detected in 4u 163653 as a function of @xmath7 . the filled circles ( black ) and the filled squares ( red ) represent the observations with upper and lower khz qpos , respectively ( see also * ? ? ? the upper khz qpo is detected when @xmath51 , and its frequency increases as @xmath7 increases . the lower khz qpo however , is only detected when @xmath52 , and its frequency covers a broad range over a narrow @xmath7 range .    , in nthcomp as a function of @xmath7 in 4u 1636@xmath153 . the data are the same as in the upper left panel of figure [ fig : sa_ga_pha ] , but have been rebinned as described in the text . , width=316 ]     in 4u 163653 . the filled circles ( black ) and filled squares ( red ) represent the observations with upper and lower khz qpos , respectively . , width=316 ]    in the upper panel of figure [ fig : sa_qpo_sigma ] we plot the power - law photon index , @xmath2 , as a function of @xmath7 ( the same data shown in figure [ fig : sa_ga_pha ] ) . the filled circles ( black ) and the filled squares ( red ) represent the observations with upper and lower khz qpos , respectively . interestingly , observations with lower khz qpos correspond with those observations in which @xmath2 shows a large spread .     and @xmath53 as a function of @xmath7 in 4u 163653 . the filled circles ( black ) and filled squares ( red ) represent the observations with upper and lower khz qpos , respectively . the gray crosses represent observations in which no khz qpo was detected . , title="fig:",width=316 ]   and @xmath53 as a function of @xmath7 in 4u 163653 . the filled circles ( black ) and filled squares ( red ) represent the observations with upper and lower khz qpos , respectively . the gray crosses represent observations in which no khz qpo was detected . , title="fig:",width=316 ]    in the case of comptonisation in spherically symmetric region @xmath2 can be expressed as :    @xmath54^{1/2 } - \frac{1}{2 } ,     \label{eq_gamma}\ ] ]    where @xmath53 is the optical depth of the medium , and @xmath55 is the electron temperature of the comptonising region @xcite . we calculated @xmath53 for each observation and plotted them as a function of @xmath7 in the lower panel of figure [ fig : sa_qpo_sigma ] . from this figure it is apparent that in the low / hard state the optical depth remains more or less constant at @xmath56 as @xmath7 increases from @xmath57 to @xmath58 . when the source enters the transitional state , with @xmath59 , @xmath53 increases very abruptly with @xmath7 . in the observations with a lower khz qpo @xmath53 covers the range from @xmath60 to @xmath61 . a large value of @xmath53 has also been found in the soft state of other ns - lmxbs ( e.g. 4u 1608@xmath152 * ? ? ? the large variation of @xmath2 ( and @xmath53 ) over a small region in the cd indicates that , in the transitional state , either the spectrum changes significantly or the model is no longer physically appropriate . even if the latter was the case , it is still interesting that the lower khz qpo is detected only in this part of the diagram . we fitted the @xmath0 kev x - ray spectra of all _ rxte _ observations of the neutron - star low - mass x - ray binary 4u 163653 with a model that includes a comptonised component . we subsequently studied the relation between the parameters of this component and the presence of khz qpos as a function of the source state , using the parameter @xmath7 to measure the position of the source in the cd . we found that , while the upper khz qpo is present in almost all states of the source , the lower khz qpo appears only during the transition from the hard to the soft state , in observations in which the optical depth of the comptonised component is relatively high and increases very sharply with @xmath7 , from @xmath62 to @xmath63 . in the low - hard state , the power - law index of the comptonised component , @xmath2 , increases gradually from @xmath64 to @xmath65 as the source spectrum softens and the source moves in the cd from the hard to the transitional state . in these observations @xmath2 is well correlated with the source hard colour and @xmath7 . the power density spectra of these observations show only the upper khz qpo , with the qpo frequency being also correlated both with hard colour and @xmath7 . similar correlations have been reported in other ns - lmxbs ( e.g. , 4u 0614 + 09 , 4u 160852 , 4u 172834 , and aql x1 ; * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? these correlations between spectral and timing properties in 4u 163653 are consistent with the scenario in which the truncation radius of the accretion disc decreases as mass accretion increases : as the x - ray luminosity of the source ( and the inferred mass accretion rate through the disc ) increases , the inner truncation radius of the accretion disc decreases ( e.g. * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? * ) and the disc temperature @xcite , and hence the soft flux , in the system increases . this in turn boosts the cooling of the corona through inverse compton scattering , which leads to a drop of the electron temperature and an increase of the optical depth of the corona @xcite , and the energy spectrum of the source softens . in the case of 4u 163653 this picture is further supported by fits to the _ xmm - newton _ and _ rxte _ ( pca@xmath66hexte ) spectra in the @xmath67 kev band over a range of source spectral states @xcite . finally , if the frequency of the upper khz qpo reflects the keplerian frequency at the inner edge of the accretion disc , a smaller truncation radius yields a higher qpo frequency . the above description only links the dynamics ( frequency ) of ( at least ) the upper khz qpo to the spectral properties of the source , but it does not address the mechanism that determines the emission properties of the qpos . for example , the steep increase of the rms amplitude of the khz qpos with energy ( e.g. , * ? ? ? * ; * ? ? ? * ; * ? ? ? * ) implies a modulation of the emitted flux of up to @xmath68% at @xmath69 kev , where the contribution of the disc is negligible ( see figure [ fig : sa_ga_pha ] ) . our findings suggest that the mechanisms that lead to the emission of the lower and the upper khz qpo are not the same . this is similar to what was proposed by @xcite based on the energy and frequency dependence of the time lags of the khz qpos in this source . we detect the upper khz qpo over all source spectral states , except the extreme soft state , whereas we only detect the lower khz qpo in the transition from the hard to the soft state , at @xmath70 , where @xmath2 drops abruptly from @xmath37 to @xmath47 . the parameter @xmath2 , together with the electron temperature , @xmath55 , in the nthcomp spectral model component is related to @xmath53 , the optical depth of the corona responsible for the comptonised emission , through eq . ( [ eq_gamma ] ) . since during the observations in the transitional state , where the lower khz qpo is present , @xmath2 drops abruptly ( see , e.g. , the upper left panel of figure  [ fig : sa_ga_pha ] ) , whereas @xmath71 continues decreasing gradually ( see the upper right panel of figure  [ fig : sa_ga_pha ] ) , in the context of the nthcomp model the drastic changes of @xmath2 likely reflect substantial changes of the optical depth of the corona . the lower panel of figure  [ fig : sa_qpo_sigma ] shows that in the observations with the lower khz qpo @xmath53 is high and increases very sharply with @xmath7 , from @xmath62 to @xmath63 . notwithstanding that in several of these observations we also detected the upper khz qpo , this qpo is also present in observations in which the optical depth of the corona is low in comparison , @xmath72 , and does not change much with @xmath7 . notice , by the way , that the drop of @xmath2 in the transitional and soft states does not contradict the fact that at @xmath73 the spectrum of the source softens . indeed , although in the transitional and soft states @xmath2 decreases to values that are similar to those in the hard state , suggesting an overall hardening of the spectrum , @xmath55 is much higher in the hard than in the transitional and soft states , such that the @xmath0 kev spectrum of the source does in fact softens as @xmath7 increases ( cf . figure  [ fig : sa_ga_pha ] ) . @xcite ( see also * ? ? ? * ) proposed a model in which a feed - back loop between the corona and the source of soft photons ( the disc and/or the neutron - star surface ) excites a global mode in the system , such that the temperature and electron density of the corona and the temperature of the source of soft photons oscillate coherently at a given frequency , giving rise to a qpo . this model can explain the rms and time - lag energy spectra of the lower khz qpo @xcite in the ns lmxb 4u 160852 . this scenario also provides a framework to interpret our findings in terms of a coupling and an efficient energy exchange between the disc and the corona . since in the observations in which the lower khz qpo is present the changes of the temperature of the corona are relatively small ( at least compared to the relative changes of the optical depth ) , the frequency range spanned by this qpo likely reflects the range of optical depths ( electron density ) in the corona at which the resonance is excited . we define the optical depth of the corona as @xmath74 @xcite , where @xmath75 is the size of the corona , @xmath76 @xmath77 is the thomson cross section of the electron , and @xmath78 is the electron density . if we take 5 and 20 as extreme values of @xmath53 in our fits ( see figure [ fig : sa_qpo_sigma ] ) , then @xmath79 @xmath80 @xmath81 and @xmath82 @xmath81 for @xmath75 in the range of 4 and 15 km @xcite , respectively . these values are consistent with the typical electron density in the corona of neutron star or black hole systems ( e.g. , * ? ? ? it is interesting that in 4u 163653 the frequency at which ( i ) the lower khz qpo is most often detected @xcite , ( ii ) the quality factor ( @xmath83 , defined as the qpo frequency divided by the full - width at half - maximum ) of the lower khz qpo is the highest @xcite , and ( iii ) the coherence between the low- and high - energy signals is the highest @xcite , is always @xmath84 hz . furthermore , @xcite recently showed that the phase lag between the low- and high - energy photons in the lower khz qpo in 4u 163653 is largest at @xmath85 , which also corresponds to a qpo frequency of @xmath84 hz . all the above suggests that this frequency corresponds to a global oscillation mode of the accretion flow in this source ( see * ? ? ? the mode could possibly be excited over a fairly large range of values of the properties of the corona ( e.g. , @xmath53 , @xmath55 and @xmath75 ) , and hence over a range of frequencies of the qpo , as observed . on the other hand , the excitation and damping mechanisms of this global mode of the corona will play a role upon the range of frequencies and the coherence of the qpo . in a follow - up paper ( ribeiro et al . 2017 , in preparation ) we discuss correlations between the rms amplitude of the qpos and the spectral properties of the source , which provide further support to this scenario . this research has made use of data obtained from the high energy astrophysics science archive research center ( heasarc ) , provided by nasa s goddard space flight center and nasa s astrophysics data system bibliographic services . we thank tomaso belloni for useful comments and discussions and wenfei yu for carefully reading and commenting the manuscript . er acknowledges the support from conselho nacional de desenvolvimento cientfico e tecnolgico ( cnpq - brazil )                      m. , van der klis m. , van paradijs j. , lewin w.  h.  g. , lamb f. , vaughan b. , kuulkers e. , augusteijn t. , zhang w. , marshall f.  e. , swank j.  h. , lapidus i. , lochner j.  c. , strohmayer t.  e. , 1996 , apj , 469 , l13
we fitted the @xmath0-kev spectrum of all the observations of the neutron - star low - mass x - ray binary 4u 1636@xmath153 taken with the _ rossi x - ray timing explorer _ using a model that includes a thermal comptonisation component . 
 we found that in the low - hard state the power - law index of this component , @xmath2 , gradually increases as the source moves in the colour - colour diagram . when the source undergoes a transition from the hard to the soft state @xmath2 drops abruptly ; once the source is in the soft state @xmath2 increases again and then decreases gradually as the source spectrum softens further . 
 the changes in @xmath2 , together with changes of the electron temperature , reflect changes of the optical depth in the corona . 
 the lower kilohertz quasi - periodic oscillation ( khz qpo ) in this source appears only in observations during the transition from the hard to the soft state , when the optical depth of the corona is high and changes depends strongly upon the position of the source in the colour - colour diagram . 
 our results are consistent with a scenario in which the lower khz qpo reflects a global mode in the system that results from the resonance between , the disc and/or the neutron - star surface , and the comptonising corona . 
 [ firstpage ]    accretion , accretion discs  stars : neutron  x - rays : binaries  stars : individual : 4u 163653