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massive ( @xmath4  10  m@xmath5 ) , runaway ( @xmath6  30  km  s@xmath7 ) stars are able to produce large - scale bow shocks in the interstellar medium ( ism ) .
these shocks are driven by the interaction of the fast stellar wind ( @xmath8  1000  km  s@xmath7 ) , large proper motion , and the ism .
the gas and dust in the pile - up material are heated and ionized by the strong uv radiation from the star which makes the bow shock observable at optical and infrared ( ir ) wavelengths ( e.g. , * ? ? ?
* ; * ? ? ?
* and references therein ) .
@xcite analyzed very large array ( vla ) observations of the runaway o star bd@xmath243@xmath33654 and concluded that the radio emission is spatially coincident with the bow shock detected in ir images .
more importantly , this extended radio emission was found to have a non - thermal origin .
benaglia et al .
argued that the non - thermal origin of the radio emission is produced by syncrotron emission .
the electrons that generate this emission could upscatter photons from stellar and dust radiation fields through the inverse compton process , producing high - energy emission .
this interesting detection opened a new window for exploring the production of non - thermal emission around massive stars , and a number of theoretical works addressing this phenomenon have been published ( e.g. , * ? ? ? * and references therein ) .
@xcite presented detailed analytical predictions for the non - thermal emission from bow shocks around o - type stars .
these authors applied their model to the well - know and closest runaway star @xmath1oph , concluding that high - energy emission should be detected toward its bow shock .
however , x - ray and @xmath0-ray emission has been eluding detection towards known runaway stars .
@xcite presented fermi @xmath0-ray space telescope observations of a sample of 27 bow shocks ( including @xmath1oph ) accumulated over 57 months with no positive detections .
@xcite extended this study up to 73 bow shocks using the h.e.s.s .
telescopes in the tev regime with the same conclusions . in x - rays
no detections were obtaned either , despite the dedicated observational campains using _ chandra _ , _ xmm - newton _ , and _ suzaku _ x - ray telescopes @xcite .
[ fig : wise_rgb ]    there has been only one claim of detection of non - thermal x - ray emission toward a runaway star .
@xcite presented _ xmm - newton _ observations of ae  aur and reported the discovery of a `` blob '' of x - ray emission at @xmath930@xmath10 northeast from the star .
these authors also presented a model to explain their results , but we notice that their spectral analysis can not be used to discriminate between a thermal and a non - thermal origin .
furthermore , these authors compare their _ xmm - newton _ observations with low - resolution mid - ir _ wise _ observations . under the assumption that this detection is related to ae  aur
, @xcite presented further analytical modelling for this x - ray emission .
they concluded that non - thermal processes in bow shocks around runaway stars are responsible for a significant fraction of the high - energy photons produced in our galaxy .    in this paper
we present a search for non - thermal diffuse x - ray emission in bow shocks around o - type stars .
we use archived _ xmm - newton _ observations of a sample of well - determined galactic runaway stars .
section  2 presents our sample and describe the _ xmm - newton _ observations . in section  3 and 4
we presents and discuss our results , respectively .
finally , we present our conclusions in section  5 .
to obtain a clear sample of runaway o stars we searched the list presented by @xcite .
these authors identified runaway stars using the proper motions reported by the first gaia data release ( dr1 ; * ? ? ? * ) .
their table  1 presents a list of confirmed candidates as well as a list of new discoveries .
we cross - corretated that table with archived _ xmm - newton _ epic observations and clear detections of bow shocks in the _ wise _ w4 22  @xmath11 m or _ spitzer _ mips 24  @xmath11 m band . as a result ,
our sample consists of 6 objects : bd@xmath1214@xmath35040 , hd24760 ( @xmath13per ) , hd57682 , hd153919 , hd188001 ( 9sge ) , and hd210839 ( @xmath14cep ) .
figure  1 presents mid - ir colour - composite _ wise _ images of the six objects studied in this paper .
details of the x - ray observations used in this paper are given in table  1 .
columns 5 , 6 , and 7 of table  1 present the total exposure time for the pn , mos1 , and mos2 epic cameras , respectively .
it is worth mentioning that almost all observations were performed with deep exposures ( @xmath15  ks ) , except for the cases of hd24760 and hd57682 ( @xmath16  ks ) .
observations of bd@xmath1214@xmath35040 were only performed using the mos cameras but with deep exposure times of @xmath17  ks .
finally , we remark that the epic cameras have a fwhm@xmath186 .    [ cols="^,^,>,^,^,^,^,^,^,^ " , ]
the detection of non - thermal radio emission associated with the bow shock around bd@xmath243@xmath33654 @xcite opened a door for studies of particle acceleration by massive stars .
those vla observations showed that the non - thermal emission has an extended distribution , spatially coincident with the bow shock observed by the midcourse space experiment ( msx ) in the d ( 14.65  @xmath11 m ) bandm image presented by @xcite . ] .
since then , the idea that charged particles in the compress ism ( the bow shock ) can cool down by non - thermal processes ( such as syncrotron radiation ) , has been studied extensively in theoretical studies ( see * ? ? ? * and references therein ) . but the absence of firmly confirmed detections of high - energy non - thermal emission is pushing the limits of theory , even for the case of bd@xmath243@xmath193654 .    in order to estimate upper limits to the non - thermal diffuse x - ray emission , we extracted background - subtracted spectra from regions spatially coincident with the bow shocks in our sample .
the obtained background count rates in the 0.35.0  kev energy range along with the estimated hydrogen column densities ( @xmath20 ) were used to obtain absorbed ( @xmath21 ) and unabsorbed ( @xmath22 ) x - ray fluxes . using the _ chandra _ pimms tool we estimated the fluxes assuming that the emission can be modeled by a power - law spectrum with @xmath23 .
table  2 shows that our estimated observed fluxes are comparable to those reported by @xcite for @xmath1oph and bd@xmath243@xmath33654 .
our systematic search using a clean sample of runaway massive stars add to the list of bow shocks around runaway stars without non - thermal diffuse x - ray emission @xcite .
our present results encouraged us to question the previously claimed detection of non - thermal x - ray emission in the bow shock around ae  aur @xcite . in appendix
a , we show that the detected emission is a point - like source unrelated to the bow shock around ae  aur .
it has become evident that current theoretical models overpredict the flux of the non - thermal diffuse x - ray emission in bow shocks around runaray . to start with
, @xcite adopted a mass - loss rate a factor of @xmath245 greater than that reported by @xcite , overestimating the density of high - energy particles . on the other hand , unlike the cases of supernova remnants which are known to emit considerably non - thermal x - ray emission ( e.g. , * ? ? ? * ) , the open morphologies of bow shocks around runaway stars might reduce the injection efficiency of energy from thermal plasma to accelerate particles and produce non - thermal emission .
we have searched for non - thermal diffuse x - ray emission associated with bow shocks around runaway o - type stars .
xmm - newton _ observations of a sample of 6 well determined runaway stars and found no evidence of such emission .
we also revised the only claimed case of non - thermal diffuse x - ray emission detected from a bow shock , ae  aur .
there is emission ; however , its spatial distribution is consistent with being that of a point source .
moreover , this x - ray source is not spatially coincident with the bow shock .
thus , we conclude that this x - ray emission is not associated with the bow shock .
thus far , there are 9 bow shocks around o  stars that stand in defiance of the recent and growing body of theoretical predictions for non - thermal diffuse x - ray emissions from such structures .
we conclude that , if this predicted non - thermal diffuse x - ray emission is present in bow shocks around runaway o stars , it is below the detection limits of the current x - ray satellites .
+      [ fig : wise_rgb ]    the authors would like to thank the anonymous referee for valuable comments that improved our manuscript .
this work was based on observations obtained with
_ xmm  newton _ , an esa science mission with instruments and contributions directly funded by esa member states and nasa .
this publication also makes use of data obtained with _
wise _ and _ spizer_. ( _ wise _ )
the absence of non - thermal diffuse x - ray emission towards the sample of six bow shocks presented in this paper , along with previously reported undetections ( see section  1 ) , questions the nature and presence of the x - ray emission towards ae  aur reported by @xcite . to confirm previous results ,
we have analyzed the _ xmm - newton _ observations of ae  aur in a similar way as described for other sources studied here .
we compare our x - ray images to _ spizer _ mips and irac images .
the left panel of figure  3 shows the higher - resolution image of the _ spitzer _ data as compared to the _ wise _ w3 image ( see figure 1 in * ? ? ?
on the other hand , the right panel of figure  3 presents a comparison of the _ spizer _ irac 8  @xmath11 m and the medium and hard x - ray bands .
this panel confirms that the blob of x - ray emission has a point - like shape with an angular separation of 35 from ae  aur , but also that this emission is not spatially coincident with the bow shock around ae  aur .
the position of the blob of x - ray emission is shown in fig .
3-right panel with a ( white ) solid line circular aperture .
this x - ray blob is not one of the dense molecular globules detected in co @xmath2425 from ae  aur ( see globule # 5 in figure  2 of * ? ? ?
thus , we can not confirm the previous claims on non - thermal emission associated with the bow shock around ae  aur .
theoretical models predict that the compressed interstellar medium around runaway o stars can produce high - energy non - thermal diffuse emission , in particular , non - thermal x - ray and @xmath0-ray emission . 
 so far , detection of non - thermal x - ray emission was claimed for only one runaway star ae  aur . 
 we present a search for non - thermal diffuse x - ray emission from bow shocks using archived _ xmm - newton _ 
 observations for a clean sample of 6 well - determined runaway o stars . 
 we find that none of these objects present diffuse x - ray emission associated to their bow shocks , similarly to previous x - ray studies toward @xmath1oph and bd@xmath243@xmath33654 . 
 we carefully investigated multi - wavelength observations of ae  aur and could not confirm previous findings of non - thermal x - rays . 
 we conclude that so far there is no clear evidence of non - thermal extended emission in bow shocks around runaway o stars .