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more than 54% of all wolf - rayet ( wr ) stars are in binary systems that consist of a wr and an ob - type star @xcite . the collision between two stellar winds in these systems produces characteristic signatures in different wavelength bands that can include non - thermal radio emission @xcite , copious and variable x - rays @xcite , and dust emission in the infrared ( ir ) @xcite . subject of this _ letter _ is the wc9d star wr65 . @xcite reported the presence of warm dust in this object . they discussed that in early - type wc stars an increase in the wind density provided by e.g.  shock compression in colliding winds in binary systems may be sufficient to result in grain formation . @xcite found no need to invoke binarity to explain the presence of dust in late wc stars . he fitted the observed ir spectrum of  assuming a single star with stellar mass - loss in the form of dust with @xmath0 and surrounded by a dust shell with an inner radius of 520@xmath1 . @xcite detected radio emission from , but could not constrain its spectral index .    in a previous paper @xcite we demonstrated that single wc type stars are x - ray quiet and proposed that all x - ray active wc stars must be in binary systems . x - ray emission from  was detected and tentatively explained as a result of wind - wind collision . in this paper we report on the x - ray light curve of   and changes in its x - ray spectra that unambiguously confirm its status as a colliding - wind binary ( cwb ) . .coordinates of  from different sources [ cols="^,^,^,^,^",options="header " , ] the coordinates of  from different sources are compiled in table[tab : coor ] . the coordinates from simbad and the usno - b1.0 catalog agree well with the source location in the  hri - i image . except for there are no other known objects at this position . wr65 is a probable member of the cluster pismis20 , which contains one more wr star , wr67 , and has a distance of @xmath2pc @xcite . m ) .  is the bright object at the bottom , close to the detector edge . the coordinates are galactic . ] we fit the optical spectrum of  and its available photometry using the potsdam wolf - rayet ( powr ) stellar atmosphere models @xcite . the deduced stellar parameters are listed in table[tab : par ] . a wind - blown bubble with a radius of @xmath3 is known around   @xcite . @xcite discovered a hi shell with a radius of 22pc surrounding  and wr67 . the infrared _ spitzer _ irac image of  at @xmath4 m is shown in fig.[fig : irac ] . despite its unfavorable location close to the edge of the detector , a rather compact nebula around is visible . the radius of the nebula is @xmath5 or 1.4pc . given the strong mass - loss rate of  and its strong uv field , it is plausible that the nebula is physically associated with this star . cccc @xmath6 & @xmath7 & @xmath8 [ kms@xmath9 & @xmath10^{\rm b}$ ] + 2.4 & 5.8 ... 6.4 & @xmath11 & @xmath12 +   +   +    in the x - ray sky ,  is located in a very interesting neighbourhood ( see fig[fig : acis ] ) . the star is @xmath13 away from the supernova remnant ( snr ) g320.4 - 1.2 and @xmath14 away from the x - ray bright `` cir pulsar '' and its associated pulsar wind nebula ( pwn ) . the snr and the cir pulsar are at the distance @xmath15kpc @xcite . this region of the sky is often observed , and serendipitous x - ray observations of  are in archives .        the most homogeneous data set found in archives is from _ chandra _ acis - i observations conducted from year 2000 to 2005 . in 2005 was also observed on one occasion by _ hrc - i and twice by _ xmm - newton _ , but was not detected during _ xmm - newton _ observations affected by high radiative background . the local x - ray background is high because  is projected on the peripheral region of the pwn mentioned above . therefore we paid special attention to the background subtraction.conveniently , the x - ray emission from the pwn is well studied @xcite . @xcite find that it is constant over at least twelve years . we defined background regions as annuli around a stellar point source . figure[fig : lc ] shows the background - subtracted x - ray light - curve of  that displays a surprisingly high level of variability . such strong x - ray variability in massive stars is observed only in binary systems @xcite . the  x - ray light curve gives strong evidence that the star is a colliding wind binary ( cwb ) . unfortunately , the light - curve is too sparse to search for the period . given the time interval between the two recorded maxima , one may conclude that the period is not longer than about five years . five of the  observations of  yielded enough counts for a crude x - ray spectral analysis . the background - subtracted spectra obtained at different epochs are displayed in fig.[fig : time ] . the spectral energy distribution is harder than for single wr stars @xcite . the unresolved emission features coincide with the location of lines of fe , ca , ar , s , si , and mg ( cf . figs.[fig : time ] , [ fig:2sp ] ) in collisionally ionized x - ray spectra . interestingly , the iron lines at @xmath16kev ( 1.8 ) are weaker at some epochs compared to others ( e.g.  they seem to disappear completely on 2005 february 07 ) . if real , this may reflect the change of the temperature in the hot plasma .    the x - ray spectrum of  in high states displays strong emission features in the 2 - 4  range ( see top panel in fig.[fig : time ] ) . weak lines of caxix(@xmath17 ) , caxx(@xmath18 ) , and arxviii(@xmath19 ) are sometimes observed in the x - ray spectra of cwbs , e.g.  the wc8 binary @xmath20vel @xcite , the wn6 binary wr25 @xcite , and the lbv binary @xmath21car @xcite . it seems plausible to assume that the unresolved ca and ar lines contribute to the emission `` bump '' at 2 - 4  in the  spectrum . however , the strength of this emission complex in is much higher than observed in any other cwb . we rule out its origin from the background pwn emission , because there are no emission features at 2 - 4  in the spectra extracted from an annulus region around . in colliding wind binaries consisting of an ob star and a wc star , the latter has a significantly denser , c , o - enriched wind that is basically opaque for x - rays @xcite . when in the course of the orbital motion the wind collision region ( wcr ) is at superior conjunction relative to the wc star ( with respect to the observer ) , the x - rays generated in the wcr suffer strong absorption . despite many still unclear details , this general picture is observationally confirmed by the x - ray spectral analysis of the wc+o type binaries wr140 and @xmath20vel . in these systems the strongest photo - attenuation is seen in low x - ray states , while the lowest photo - attenuation is found when the x - ray flux is high @xcite .    to investigate how x - ray spectral variations are coupled with the variation of the absorption column , we fit a model to the observed spectra and check how the model parameters vary between the different epochs .    following a method used in @xcite to successfully analize x - ray spectra of wr140 , we tried to fit the observed  spectra alternatively with non - equilibrium ionization , pshock , and power - law models using the xspec software @xcite . however , no meaningful constraints on model parameters could be obtained because of the poor quality of the  spectra . @xcite found that only models with non - equilibrium ionization can reproduce the _ detailed _ x - ray spectra of cwbs . however , he noticed that these spectra can be crudely described by thermal plasma emission with some `` average '' temperature . finally , we select a two - temperature thermal plasma model ( * ? ? ? * apec ) where each component is allowed to suffer different degrees of attenuation  the `` 2t2@xmath22 '' model . this model provides a first insight into the properties of the system by constraining the model parameters . we refer to the plasma with lower temperature @xmath23 as the `` soft '' model component , and to the plasma with higher temperature @xmath24 as the `` hard '' model component . as demonstrated in figs.[fig : fit],[fig : fit1 ] the 2t2@xmath22-model sufficiently well reproduces the shape of the  spectra observed at different epochs . we assume solar composition . selecting wc - wind abundances results in scaling down of the parameter that describes normalization of the emission measure ( em ) , but the effect of abundances on _ spectral shape _ can not be discriminated given the poor quality of available data . the spectrum of  with highest signal - to - noise and its best - fit model are shown in fig.[fig : fit ] . the temperatures inferred from spectral fitting are @xmath25kev and @xmath26kev . such temperatures can be expected from the collision of metal enriched winds in wide binaries . the temperature is highest at the point of head - on collision , and is decreasing along the shock cone @xcite .    the interstellar absorption column density towards the cir pulsar , @xmath27@xmath28 , is well constrained and is consistent with the color excess inferred from our fitting of the  spectral energy distribution . the neutral hydrogen column densities are @xmath29@xmath28 for the soft component and @xmath30@xmath28 for the hard component . the column density to the soft component , @xmath31 , exceeds the interstellar column by a factor of more than two , but it is lower than the column density to the hard component , @xmath32 . this indicates that the hotter plasma is more deeply embedded .    from the spectrum obtained at 2005/10/18 the model flux is @xmath33ergs@xmath34@xmath28 , which corresponds to @xmath35ergs@xmath34 , or in terms of stellar bolometric luminosity to @xmath36 . the flux of the soft spectral component , @xmath37ergs@xmath34@xmath28 , corresponds to @xmath38ergs@xmath34 which is similar to the x - ray luminosity of a single o dwarf @xcite . spectra observed at other epochs were also fitted with the 2t2@xmath39-model . for illustration , the spectrum obtained in the low state and its corresponding best - fit model are shown in fig.[fig : fit1 ] . figure[fig : nh ] shows the model parameters depending on the time of observation . within the error bars , the temperatures @xmath23 and @xmath24 , and the absorption column to the source for the soft component , @xmath31 are similar at all observed epochs , and therefore are not plotted in fig.[fig : nh ] . in contrast , the column density to the hard component , @xmath32 , changes at different epochs ( panel a of fig.[fig : nh ] ) . variations in the em of the hard and soft components are shown in panels b and c of fig.[fig : nh ] . the em of the soft component changes strongly , reflecting the x - ray light - curve variations , while the changes in the em of the hard component are less pronounced .    . _ panel a : _ variations of absorption column density of the hard component obtained from spectral fits . _ panel b : _ normalization factor that characterizes the emission measure of the hard component . _ panel c : _ normalization factor of the soft component . ] the main findings from the analysis of  x - ray spectra obtained at different epochs are : ( i ) the spectra display emission features at the wavelengths of the lines of strongly ionized fe , ca , ar , s , si , and mg ; ( ii ) the star shows strong spectral x - ray variability ; ( iii ) the spectra can be fitted with a two - temperature model , consisting of a soft component with @xmath40kev and a hard component with @xmath41kev ; ( iv ) the absorption for the hard component is strong and variable , while the absorption for the soft component is less severe and nearly constant ; ( v ) the changes in the em are stronger for the soft component .    can these observations be explained in the framework of a colliding wind model ? @xcite compare the 2.0 - 10.0kev light curves of wr140 and @xmath21 car ( both are systems with high eccentricity and high inclination ) and note that they are very similar . @xcite explain the light - curve ( 2.010.0kev ) of @xmath21car using a hydrodynamic simulation of colliding winds . they show that the data are broadly consistent with the x - ray emission being moderately low throughout most of the orbit . however , a sharp peak occurs near periastron , when a greater fraction of the stellar winds collides , and the higher pre - shock densities and the lower pre - shock velocities result in stronger radiative cooling . the eclipse occurs immediately after periastron passage when the companion of @xmath21car swings behind the lbv star with its dense wind . it is hard to see how a similar model can be used to explain the x - ray variability of wr65 . strong variations in the absorbing column indicate that the orbit of  has high inclination too . considering the light - curve ( fig.[fig : lc ] ) we can rule out that  has a period of a few days : the two data points obtained in april 2003 indicate constant count rates over at least a week . moreover , two data points in the light - curve separated by two months ( 12/2004 and 02/2005 ) are similar , while there is an increase of emission by nearly 70% in the next two months . therefore ,  has most likely an orbital period of a few years . in this case the long duration of the minimum is inconsistent with its interpretation as attenuation by the wind of the wc star immediately after periastron passage . quite different from the short - duration rise of column density observed in wr140 and @xmath21 car , the column density in  is persistently very high , @xmath42 , sharply decreasing only at the maximum x - ray light . the temporary variations of column density in  are similar to those measured in the wc8+o binary @xmath20vel @xcite . its hard x - ray emission increases by a factor of @xmath43 when column density decreases by factor of @xmath44few during the passage of weak - wind inner side of the shock cone across the line of sight @xcite .    in  the soft x - ray emission is present at all observed epochs . its variation can be explained by changing emission measure , while the absorption is nearly constant and comparably low . a soft spectral component in the low state is also observed in @xmath20vel , v444cyg and @xmath21car . the origin of soft x - rays in these stars is attributed either to individual stellar winds or to a circumstellar nebula . accordingly , in these objects the absorbing column to the soft component is similar to the interstellar absorbing column . in contrast , we find for  that @xmath45 , and the em of the soft component changes with the x - ray light - curve . thus it is unlikely that the intrinsic wind emission of a companion is solely responsible for the soft x - rays from wr65 . wr65 is a persistent dust maker @xcite . @xcite notice that binary stars that are persistent dust makers have nearly circular orbits . we can assume that the same holds for . there is a growing number of wc9d binary stars that constitute the `` pinwheel '' class of objects @xcite . in these binary systems the dust is formed in the wind collision region and is distributed along a wide ( compared to the orbital separation ) archimedean spiral . the peak of dust production occurs at some distance downstream from the colliding wind stagnation point . so far there is no direct evidence that  is a pinwheel . however , the topology and structure of circumstellar matter in a pinwheel could help to explain x - ray spectral variability in . @xcite presented a model for the absorption of x - rays that includes a treatment of dust grains . their work confirmed that large - grain absorption for soft x - rays ( @xmath46kev ) is reduced due to the self - blanketing of grains . interestingly , from the point of view of radiative transfer this is identical to the reduction of opacity for the x - rays in clumped stellar winds @xcite . winds of wc stars are extremely rich in c and o and could have enhanced abundance of mg and ne . c and o are dominant absorbers for the softer x - rays ( @xmath47kev ) , while mg and ne strongly contribute to the absorption of harder x - rays ( @xmath48kev ) . in the dusty shell around a wc9d star , c would be depleted from the gaseous phase and form grains , reducing the opacity for the softer x - rays . this may help to explain the large differences between the column densities that we determine from soft and hard x - rays . recently , @xcite presented 3-d hydrodynamical simulations of colliding winds . scaling the results of their simulation to the parameters of , soft x - ray emission could originate from a large volume extending above the orbital plane . this emission would show little variation with orbital phase and suffer comparably mild absorption in the outer regions of the wc star wind , similarly to what is observed in wr65 . on the other hand , a high postshock temperature of @xmath49kev is predicted for a confined region near the point where the winds collide head - on . densities up to @xmath50g@xmath51 can be expected in the dusty spiral arms . assuming the wind opacity for hard x - rays as @xmath52@xmath53g@xmath34 @xcite , a spiral arm width of @xmath54 cm would explain the observed high absorption . this width is consistent with the observed width of the dusty spiral arms in wr104 @xcite . the lowest absorbing column can be expected at the phase when the line of sight crosses only the outer low density part of the spiral arms . it is possible that the  observations of at high state have caught this orbital phase .    to conclude , our first brief study of the variable x - ray emission from  indicates that it is a colliding wind system , where the dust and complex geometry of its colliding wind region are pivotal in explaining its x - ray properties . at present ,  is the only known wc9d star where x - ray emission is detected . future observations are needed to constrain the orbital parameters of and allow for detailed modeling . the authors are grateful to p. m. williams for initiating this study and to n. s. schulz for useful discussions on the acis - i spectra . the authors thank the referee ( a.f.j . moffat ) for detailed and helpful comments . based on  data : obsid 754 , 6117 , 6116 , 5535 , 5534 , 5515 , 4384 , 3834 , 3833 . this research has made use of software provided by the chandra x - ray center and data obtained through the nasa / ipac infrared science archive and high energy astrophysics science archive research center . the simbad database , operated at cds , strasbourg , france , and nasa s astrophysics data system were extensively used during this work . corcoran m. f. , pittard j. m. , stevens i. r. , henley d. b. , pollock a. m. t. , 2005 , in : x - ray and radio connections , eds . sjouwerman and k.k dyer . published electronically by nrao , http://www.aoc.nrao.edu/events/xraydio
we report the discovery of variability in the x - ray emission from the wolf - rayet type star wr65 . using archival data spanning over 5yr 
 we detect changes of the x - ray flux by a factor of 3 accompanied by changes in the x - ray spectra . 
 we believe that this x - ray emission originates from wind - wind collision in a massive binary system . 
 the observed changes can be explained by the variations in the emission measure of the hot plasma , and by the different absorption column along the binary orbit . 
 the x - ray spectra of  display prominent emission features at wavelengths corresponding to the lines of strongly ionized fe , ca , ar , s , si , and mg . 
 wr65 is a carbon rich wc9d star that is a persistent dust maker . 
 this is the first investigation of any x - ray spectrum for a star of this spectral type . 
 there are indications that the dust and the complex geometry of the colliding wind region are pivotal in explaining the x - ray properties of . 
 [ firstpage ]    stars : early - type  stars : wolf - rayet  stars : individual : wr65  x - ray : stars