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none of the previous analysis of the x - ray data of rx j1856.5 - 3754 ( henceforward rxj1856 ) revealed any significant periodicity @xcite . on the contrary a recent xmm - newton observation of rxj1856 has discovered that this isolated neutron star pulsates with a period of @xmath3 s @xcite .
this object has a completely featureless x - ray and optical spectra .
the lack of any significant spectral features in the x - ray spectrum argues against a heavy element atmosphere @xcite , whereas single temperature hydrogen atmosphere fits over - predict the optical flux by a large factor @xcite . as the soft x - ray spectrum
is well fitted by the planckian spectrum with a temperature @xmath4ev @xcite .
it has been proposed that the star has no atmosphere , but a condensed matter surface @xcite .
such a surface might emit a featureless , likely a blackbody spectrum , at a temperature close to that of the surface , as suggested by @xcite .
the overall spectra of this source has often been described by a two - temperature blackbody models @xcite , because the parameters derived from x - rays do not fit the optical data , which shows the rayleigh - jeans slope with an intensity a factor of 6 larger than that of a x - ray emission .
alternatively a recent paper by @xcite explains the observed featureless spectra of this object assuming that the star has a thin magnetic , partially ionized hydrogen atmosphere on top of a condensed surface .
though , one of important uncertainties for this model appears to be a creation of thin hydrogen atmospheres .
however , to make these models work , the ns has to have a condensed matter surface , which requires the specific conditions @xcite .
also , it remains to be seen weather a detailed analysis of magnetically condensed matter confirms the required non - uniform distribution of the surface temperature ( two - component blackbody model ) .
so we must conclude that existing models , based on an assumption that the emission of rxj1856 has a thermal nature face numerous problems .    in the present paper
we propose our explanation of the detected pulsations in the x - ray emission of rxj1856 , which is naturally possible based on emission model developed by @xcite . in this paper
it is assumed that the emission of this object is generated by the synchrotron radiation , which is created as the result of appearance of pitch angles during the quasi - linear stage of the cyclotron instability .
we suppose that the model proposed by @xcite works , which assumes the case of a nearly aligned rotator , whereas the periodic variations of the observed emission may be caused by the presence of a very low frequency , nearly transverse drift waves in the pulsar magnetosphere .
these waves propagate across the magnetic field and encircle the open field line region of the pulsar magnetosphere @xcite .
they are not directly observable but only cause the periodic change of the direction of the pulsar emission @xcite .    in this paper , we give a description of the emission mechanism in section 2 .
the mechanism of change of the pulsar radiation direction is described in section 3 , our model is presented in section 4 and conclusions are done in section 5 .
as it is known the pulsar magnetosphere is filled by a dense relativistic electron - positron plasma .
the ( e@xmath5e@xmath6 ) pairs are generated as a consequence of the avalanche process ( first described by @xcite ) and flow along the open magnetic field lines .
the plasma is multi - component , with a one - dimensional distribution function ( see fig.1 from @xcite ) and consists of the following components : the bulk of plasma with an average lorentz - factor @xmath7 ; a tail on the distribution function with @xmath8 and the primary beam with @xmath9 .
the main mechanism of wave generation in plasmas of the pulsar magnetosphere is the cyclotron instability .
generation of waves is possible if the condition of the cyclotron resonance if fulfilled @xcite : @xmath10 where @xmath11 is the particle velocity along the magnetic field , @xmath12 is the lorentz - factor for the resonant particles and @xmath13 is the drift velocity of the particles due to curvature of the field lines ( @xmath14 is the radius of curvature of the field lines and @xmath15 is the cyclotron frequency ) . here
cylindrical coordinate system is chosen , with the @xmath16-axis directed transversely to the plane of field line , when @xmath17 and @xmath18 are the radial and azimuthal coordinates . during the quasi - linear stage of the instability
a diffusion of particles arises as along , also across the magnetic field lines .
therefore , plasma particles acquire transverse momenta and begin to rotate along the larmor orbits .
the synchrotron emission is generated as a result of the appearance of pitch angles .
in @xcite it has been assumed that the emission of rxj1856 is generated by the synchrotron mechanism , which is created as the result of the cyclotron instability .
though , the original waves excited during the cyclotron resonance come in the radio domain , the radio emission is not observed from rxj1856 .
this waves as well as the x - ray and optical emission propagate along the local magnetic field lines .
one of the possible explanations why the radio emission is not detected from this object is that it is generated at lower altitudes in contrast to the x - ray and optical emission and might miss our line of sight .
another explanation is that the radio emission covers a large distance in the pulsar magnetosphere ( since the model of the aligned rotator is used ) .
so there is a high probability for it , to come in the cyclotron damping range @xmath19 @xcite . in this case
the radio emission will not reach an observer .
* - rotation , * k * - emission and * a * - observers axes . angles @xmath20 and @xmath21 are constants , while @xmath22 and @xmath23 are oscillating with time.,width=5 ]
considered distribution function should generate various wave - modes in certain conditions .
particularly it has been shown @xcite that a very low frequency , nearly transverse drift waves can be excited .
they propagate across the magnetic field , so that the angle between @xmath24 and @xmath25 is close to @xmath26 . in other words , @xmath27 , where @xmath28 .
the period of the drift waves @xmath29 can be written as @xcite : @xmath30 where @xmath31 is the pulsar spin period , @xmath32 is the lorentz - factor of the relativistic particles and @xmath33 is the magnetic field in the wave excitement region ( @xmath34 is the magnetic field at the pulsar surface and @xmath35 is the radius of the neutron star ) .
it appears that the period of the drift waves can vary in a broad range .
the magnetic field of drift wave adds with the pulsar magnetic field as @xmath17 component and causes changing of the curvature of field line .
even a small change of @xmath36 causes significant change of @xmath37 .
variation of the field line curvature can be estimated as : @xmath38 here @xmath39 is a longitudinal component of wave vector and @xmath17 is distance to the center of the pulsar .
it follows that even the drift wave with a modest amplitude @xmath40 alters the field line curvature substantially @xmath41 .    since the pulsar emission propagates along the local magnetic field lines
, curvature variation causes change of the emission direction , with the period of the drift waves .
there is unequivocal correspondence between the observable intensity and @xmath42 ( the angle between the line of sight of an observer and the emission direction , see fig .
[ fig1 ] ) .
maximum of intensity corresponds to the minimum of @xmath43 .
the period of pulsar is the time interval between neighboring maxima of observable intensity i.e. minima of @xmath43 . according to this fact
, we can say that the observable period depends on time behavior of @xmath43 and as it appears below it might differ from the real spin period of the pulsar .    from pulsar geometry
it follows that @xmath23 can be expressed as @xcite : @xmath44,\end{aligned}\ ] ] where @xmath45 is the angular velocity of the pulsar , @xmath46 is the angle between the rotation and the observer s axes , @xmath47 is the angle between the rotation and emission axes ( see fig .  [ fig1 ] ) and @xmath48 is the amplitude of changing of @xmath22 .    in the absence of the drift wave @xmath49 and consequently the period of @xmath43
equals to @xmath50 . on the other hand , if the angle between the rotation and emission axes is too small i.e. @xmath51 , then the period of @xmath23 equals to @xmath52 . in this case
the observable period @xmath53 does not represent the real spin period of the pulsar , but equals to the period of the drift wave , which we suppose to be 7.055 s. when the real spin period of this object has been estimated by @xcite to be @xmath54 .
hence , for some values of parameters @xmath47 , @xmath55 , @xmath56 , @xmath57 and @xmath21 it is possible to explain the 7 s pulsations of rxj1856 .
if we consider this object in the framework of our model its angular parameters will get the values shown in table 1 .
simulated lightcurve for rxj1856 is presented on fig .
[ fig2 ] , which well expresses the value of the pulsed fraction of only @xmath2 ( the smallest ever seen in the isolated x - ray pulsars ) , obtained from observations @xcite .
in the present paper we propose our explanation of recently discovered 7 s pulsations in the isolated neutron star rxj1856 , based on model drawn by @xcite .
the main feature of this work is that the spin period of pulsar might differ from the observable one , which is the consequence of existence of very low frequency drift waves in the region of excitement of the pulsar emission .
these particular waves are not detected but only result in a periodical change of curvature of the magnetic field lines , which in turn cause the change of observed radiation with a period of the drift wave .
drift wave driven model is very convenient , since it makes able to explain almost every feature of known extraordinary pulsars such as : extremely long period radio pulsars @xcite , rotation radio transients @xcite , anomalous x - ray pulsars and soft gamma - ray repeaters @xcite .
we suppose that treatment of the real period of rxj1856 can be achieved by more detailed observations . in the case of nonzero @xmath20 , which is most likely ,
variations with different time - scale should be appeared with value of the pulsar spin period , see fig .
if this is confirmed , it will benefit to our model .
we are grateful to george machabeli for valuable discussions .
this work was partially supported by georgian nsf grant st06/4 - 096 .
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an explanation of the recently discovered @xmath0 s pulsations from the isolated neutron star rx j1856.5 - 3754 is presented . 
 it is assumed that the real spin period of this source is @xmath1 s , whereas the observed spin - modulation is caused by the presence of a nearly transverse , very low frequency drift waves in the pulsar magnetosphere . 
 it is supposed that the period of the drift wave is equal to a recently observed one . 
 the simulated lightcurve is plotted , the angular parameters are defined and the value of the pulsed fraction of only @xmath2 is explained .    
 ( stars : ) pulsars : individual rx j1856.5 - 3754 , stars : magnetic fields , radiation mechanisms : non - thermal    97.60.gb , 94.20.bb