You will be presented with the full text of a science research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
the luminous ( l@xmath72.5@xmath8 l@xmath9 ) helium - rich o - type star  is one of the few hot subdwarfs that have been detected in the x - ray band .
x - rays from hot subdwarfs can have two different origins : they can be produced by accretion onto a neutron star or white dwarf companion or they can be emitted by shock - heated plasma in their stellar wind , as it occurs in early type stars of higher mass and luminosity . in both cases , x - ray observations of hot subdwarfs are useful because they provide a way to study the relatively weak winds of these low - mass stars ( see * ? ? ?
* for a review ) .
x - rays from  were discovered with an  observation carried out in 2011 @xcite .
the spectrum was fit by the sum of a blackbody with temperature kt=45@xmath1010 ev plus a faint power - law with photon index @xmath112 , giving a luminosity between @xmath12 and @xmath13 erg s@xmath1 ( for d=2 kpc , @xcite ) . the large uncertainty on the luminosity , due to the poorly constrained spectral parameters , left open both of the above possibilities for the origin of the x - ray emission in .
the  data also showed a periodicity at 19.2 s ( with a statistical significance of 3.2@xmath14 ) pointing to the presence of a compact companion .
however , no evidence for a binary nature had been reported in the literature @xcite and a recent campaign of high - resolution optical spectroscopy did not reveal radial velocity variations @xcite .
this means that either the orbital plane has a very small inclination and/or the orbital period is of the order of months , or that  is indeed a single star and the reported signal was spurious .
here we report the results of a longer follow - up  observation of  that we obtained in order to clarify the origin of its x - ray emission .
statistics vs. trial period for the 2016 observation of .
the dashed line indicates the period detected in the 2011 data .
, width=264 ]
was observed with the  satellite for 72 ks on 2016 february 1st .
the three ccd cameras of the epic instrument were operated in imaging mode with a time resolution of 73 ms for the pn camera @xcite and of 0.9 s for the two mos cameras @xcite .
the medium thickness filter was used for all cameras .
the data were processed with sas v15 .
we used only single- and double - pixel events for the pn ( pattern @xmath15 4 ) and single- and multiple - pixel events for the mos ( pattern @xmath15 12 ) .    for the timing analysis
, we extracted the source counts from a circle of radius 15@xmath16 and converted their arrival times to the solar system barycenter .
we used only the events in the energy range from 0.15 to 2 kev , as it was done for the 2011 data by @xcite .
this resulted in 1519 pn counts and 485 mos counts .
we estimate that the background contributes about 22% and 17% of these counts , respectively .    in order to take into account a possible spin - up or spin - down of the source during the @xmath74.5 years between the two  observations with plausible values of @xmath17 s s@xmath1 , we considered possible periods in the range from 19.1 to 19.2 s ( the value measured in 2011 was p@xmath18=19.156@xmath100.001 s ) . using the sum of the pn and mos counts
, we found a maximum value of the z@xmath19 statistics @xcite of 8.74 , for a period p=19.174 s ( see fig .
[ fig - z2 ] ) .
however , the corresponding probability of chance occurrence , taking into account the number of sampled periods , is too large to claim a significant detection .    through monte carlo simulations we found that a sinusoidal modulation with pulsed fraction of 33% , as found in the 2011 data , has a probability of 99.9% of being detected at a significance above 5@xmath14 in an observation with the same duration and counting statistics as the 2016 one .
the corresponding probability for a pulsed fraction of 25% ( the lower bound of the 2@xmath14 uncertainty of the 2011 value ) is 84% .
therefore , the lack of a detection in the new data strongly suggest that either the pulsations in  disappeared ( i.e. the pulsed fraction decreased making them undetectable ) or the peak at 19.2 s appearing in the 2011 periodogram was caused by a statistical fluctuation .    for the spectral analysis we used circular extraction regions with radii of 20@xmath16 for the source and 50@xmath16 for the background .
time intervals of high background were excluded .
we merged the spectra from the three cameras into a single epic spectrum and produced the appropriate response matrix using the task multixmmselect .
following exactly the same procedures , we extracted also the epic spectrum of  from the 2011 data , which we reprocessed using sas v15 .
the effective exposure times of the 2016 and 2011 spectra are 48 ks and 28 ks , respectively . by comparing the two spectra we found no evidence that the flux or spectral shape changed between the two observations .
therefore , we performed the spectral analysis by jointly fitting the 2011 and 2016 data in the 0.2 - 2 kev energy range using the xspec software .
simple models ( power law , blackbody , thermal bremsstrahlung ) modified by interstellar absorption could not fit the data ( values of reduced @xmath202.5 ) .
thermal plasma emission models , with abundances fixed at solar values @xcite were also rejected , but they could give satisfactory fits if some of the abundances were let free to vary .
for simplicity , and considering that the quality of the data does not allow us to constrain a large number of spectral parameters , we considered only a single - temperature plasma described by the apec model .
we found strong evidence for an overabundance of c and ne , the latter being required to fit the significant excess of emission visible in the spectra at @xmath70.9 kev .
the derived c and ne abundances were found to depend on that of he , which was however poorly constrained by the x - ray data .
therefore , considering that  is an extreme he - rich star , we fixed the abundance of this element to a mass fraction of x@xmath21=0.99 . in this case
we obtained a good fit with temperature kt=0.22 kev , absorption n@xmath22 @xmath23 , and solar abundances relative to hydrogen for all the other elements ( see fig .
[ fig - sp ] and model a in table  [ tab1 ] ) . from optical studies we know , however , that also other elements are overabundant in this star .
indeed , an equally good fit was obtained with the abundances of he , c , n ,
si and fe fixed at the values of @xcite , but also in this case an overabundance of ne was required ( model b in table  [ tab1 ] ) .
the two models result in slightly different values of the unabsorbed flux , both of a few @xmath24 erg @xmath23 s@xmath1 ( 0.2 - 2 kev ) , which , for a distance of 2 kpc @xcite , correspond to x - ray luminosites in the range @xmath25 erg s@xmath1 .
we performed also spectral fits fixing the interstellar absorption at n@xmath26=6.2@xmath2710@xmath28 @xmath23 , based on the  reddening of e(b 
v)=0.09 @xcite and the relation derived by @xcite . in the case of model
a , we obtained a good fit ( @xmath29=1.03 ) with similar abundances and a slightly lower temperature kt=0.20@xmath100.01 kev , while model b resulted in a worse fit to the data ( @xmath29=1.7 ) .
.results of the joint fits of the 2011 and 2014 epic spectra of  with two different assumptions for the element abundances . [ cols="<,^,^",options="header " , ]     notes : abundances are given in mass fraction ; the flux refers to the 0.2 - 2 kev range and is corrected for the absorption ; errors are at 1@xmath14 .
the non - detection of a significant periodicity at 19.2 s in the new data ( which have a higher counting statistics than the previous ones ) casts doubts on the presence of a neutron star companion in .
this result , coupled to the lack of radial velocity variations in the optical spectra @xcite , leads us to conclude that  is most likely a single star . indeed ,
one of the scenarios proposed to explain the origin of extreme helium - rich stars is that they result from the merger of a binary composed of a he white dwarf and a more massive co white dwarf .
the lighter white dwarf is disrupted and a single he - enriched star is formed @xcite .
if  is a single star , the observed x - ray emission can not be powered by mass accretion onto a neutron star companion , with the interesting consequence that it has to originate in  itself .
the x - ray emission from massive o - type stars is related to the presence of powerful radiation - driven winds .
x - rays are produced in the winds of single stars by shock - heated plasma resulting from various instabilities ( see , e.g. , * ? ? ? * and references therein ) .
an empirical correlation has been found between the x - ray and bolometric luminosities of early type stars : @xmath30 @xcite .
the x - ray luminosity we derived for   corresponds to a value of @xmath31 , fully consistent with the observed dispersion around the above average relation .
it is thus natural to ascribe the x - rays observed in  to the same , or similar , processes that are at work in the stellar winds of massive early type stars .
there is in fact evidence from uv and optical spectroscopy that , despite its low luminosity compared to normal o - type stars , has a stellar wind .
the inferred mass - loss rate is @xmath32=3@xmath33 @xmath5 yr@xmath1 @xcite , in good agreement with the predictions of the most recent theoretical models of radiatively - driven winds in low luminosity hot stars @xcite .    in fig .
[ fig_lxlb ] we plot the x - ray and bolometric luminosities of the five sdo stars that have been detected in the x - ray range . for comparison ,
the values for a sample of normal o stars seen in the rosat all sky survey @xcite are also shown in the figure .
@xcite is another single , he - rich star very similar to  for what concerns its optical spectrum , composition and mass - loss properties @xcite .
these two stars lie very close in the @xmath34-@xmath35 plane .
the same is true for the spectroscopic binary , which is in a 1.55 day orbit with a neutron star or white dwarf companion @xcite . for this star we plot in fig .
[ fig_lxlb ] the x - ray luminosity observed when the compact companion is eclipsed by the sdo star and likely due to wind emission ; out of the eclipse the luminosity is a factor ten higher and pulsations at 13.2 s are observed .
the x - ray spectra of these three stars are similar , except for the presence of a harder component in  which might be related to its binary nature .
the two other sdo stars plotted in fig .
[ fig_lxlb ] ( feige 34 and ; @xcite ) are less luminous and their faint x - ray emission has not been studied in detail .
it is however remarkable that they lie on the extrapolation of the average l@xmath2-l@xmath36 relation observed at higher luminosity .
the temperature found in the spectral fits of  is in the lower range of the values seen in the sample of o - type stars observed with the  epic instrument , when a single - temperature plasma is sufficient to describe their low resolution x - ray spectra @xcite .
this might be related to the weaker wind of  compared to that of normal o - type stars .
it is also interesting to note that the weak - wind o - type dwarf @xmath37 col ( hd 38666 ) , with a mass - loss rate @xmath32 = 10@xmath38 @xmath5 yr@xmath1 @xcite similar to that of , has an x - ray emitting wind with higher temperature ( kt@xmath70.4 kev , @xcite ) , despite its lower wind velocity ( v@xmath39=1,200 km s@xmath1 wrt v@xmath39=2,000 km s@xmath1 , @xcite ) .
however , we caution that the temperatures derived from low - resolution spectroscopy of faint x - ray sources , such as the hot subdwarfs discussed here , might be affected by the uncertainties in the element abundances .
instruments with large collecting area and high spectral resolution are needed to address these issues in more detail and fully exploit the study of subdwarfs in the context of a more general understanding of stellar wind in hot stars .
this work , supported through financial contributions from the agreement asi / inaf i/037/12/0 and from prin inaf 2014 , is based on data from observations with _ xmm - newton _ , an esa science mission with instruments and contributions directly funded by esa member states and the usa ( nasa ) .
pe acknowledges funding in the framework of the nwo vidi award a.2320.0076 .                    , u. , geier , s. , irrgang , a. , et  al .
2014 , in astronomical society of the pacific conference series , vol .
481 , 6th meeting on hot subdwarf stars and related objects , ed .
v.  van grootel , e.  green , g.  fontaine , & s.  charpinet , 307
we report the results of a new  observation of the helium - rich hot subdwarf  carried out in february 2016 . 
 the possible periodicity at 19 s seen in a 2011 shorter observation is not confirmed , thus dismissing the evidence for a binary nature . 
 this implies that the observed soft x - ray emission , with a luminosity of a few 10@xmath0 erg s@xmath1 , originates in  itself , rather than in an accreting neutron star companion . 
 the x - ray spectrum is well fit by thermal plasma emission with a temperature of 0.22 kev and non - solar element abundances . besides the overabundance of he , c and n already known from optical / uv studies , the x - ray spectra indicate also a significant excess of ne . 
 the soft x - ray spectrum and the ratio of x - ray to bolometric luminosity , l@xmath2/l@xmath3 , are similar to those observed in massive early type stars . 
 this indicates that the mechanisms responsible for plasma shock - heating can work also in the weak stellar winds ( mass loss rates @xmath4 @xmath5 yr@xmath1 ) of low - mass hot stars .    
 [ firstpage ]    stars : subdwarfs , individual : bd+37@xmath6442