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ce gru ( also known as grus v-1 and hawkins v-1 ) was discovered by hawkins ( 1981 , 1983 ) during a search for variable objects using uk schmidt plates .
further observations by tuohy et al ( 1988 ) confirmed its binary nature ( an orbital period of 108.5 mins ) while cropper et al ( 1990 ) found strong circular polarisation 
a characteristic of the polar ( or am her ) class of cataclysmic variable .
these objects are interacting binaries in which material flows from a dwarf main sequence star onto a magnetic ( @xmath210200 mg ) white dwarf .
this strong magnetic field is high enough to force the spin of the white dwarf to be synchronised with the binary orbital period .
tuohy et al ( 1988 ) demonstrated that there are two accretion poles which are characterised by different emission properties : one pole which is always in view and is stronger in blue light , while the other pole which is visible for only @xmath00.35 of the white dwarf spin period is stronger in red light .
cropper et al ( 1990 ) showed that the pole always in view was positively circularly polarised , with the other being negatively polarised .
the uk schmidt plates show that ce gru exhibits two distinct levels of brightness , ( @xmath218 and @xmath221 ) . in the fainter state
the accretion flow is much reduced , or stopped altogether .
this may be the reason why ce gru was not detected during the _ rosat _ all - sky survey ( verbunt et al 1997 ) .
indeed , ce gru has never before been detected in x - rays . in this paper
, we report the first x - ray detection of ce gru which were made as part of a survey of polars using _ xmm - newton_.
the satellite _ xmm - newton _ was launched in dec 1999 by the european space agency .
it has the largest effective area of any x - ray satellite and also has a 30 cm optical / uv telescope ( the optical monitor , om : mason et al 2001 ) allowing simultaneous x - ray and optical / uv coverage .
ce gru was observed using _ xmm - newton _ on 2001 oct 31 .
the epic instruments ( imaging detectors covering the energy range 0.110kev with moderate spectra resolution ) were operated in full frame mode ( the count rate was not high enough to cause pile - up problems ) .
the rgs detectors ( high resolution spectrographs operating in the 0.32.0kev range : den herder et al 2001 ) were configured in the standard spectroscopy mode .
we clearly detect ce gru in the x - ray band .
om data were taken in two uv filters ( uvw1 : 24003400  , uvw2 : 18002400  ) and one optical band ( @xmath3 band ) . the observation log is shown in table [ log ] .
the data were processed using the _ xmm - newton _ _ science analysis software _ v5.2 .
the rgs spectra were of low signal to noise and showed no evidence for significant line emission : we do not consider them further . for the epic pn detector ( strder et al 2001 ) , data were extracted using an aperture of 40@xmath4 arc sec centered on the source .
background data were extracted from a source free region . for the epic mos detectors ( turner et al 2001 ) we extracted data in a similar way , but extracted the background from an annulus around the source .
the background data were scaled and subtracted from the source data . in extracting the epic pn spectrum
, we used only single pixel events and used the response file epn_ff20_sy9_thin.rmf . in the case of the mos data we used the response files m[1 - 2]_thin1v9q19t5r5_all_15.rsp
the om data were analysed in a similar way using omichain and omfchain ( this latter task was not incorporated in sas v5.2 but will be in a later version ) .
data were background subtracted and corrected for coincidence losses ( mason et al 2001 ) .
the optical monitor data shows that ce gru had a mean brightness of @xmath3=17.9 and a maximum @xmath3=17.5 . since the @xmath3 band observations started at the descent from maximum its likely that its true maximum was brighter than this . assuming a similar colour to that found by tuohy et al ( 1988 ) ( @xmath5=0.53 ) this places ce gru in a high accretion state at the time of the _ xmm - newton _ observations and a similar brightness to observed by tuohy et al ( @xmath618 ) .
the length of the observation in the epic mos detectors covered just over 1 orbital cycle .
the flux in the uv filters corresponds to : uvw1 @xmath7 ergs s@xmath8 @xmath9 , uvw2 @xmath10 ergs s@xmath8 @xmath9 , ( based on om observations of isolated white dwarfs ) .
the mean flux in the @xmath3 filter corresponds to @xmath11 ergs s@xmath8 @xmath9  at 5000  .
.the log of _ xmm - newton _ observations of ce gru . [ cols="<,>,>,>",options="header " , ]     ( 16,12 ) ( 0,12.8 )    ( 8.,13.2 )    ( -0.9,6.5 )    ( 7.7,6.6 )      in the bright phase the hard x - ray bolometric luminosity is @xmath12 ergs s@xmath8 @xmath13 . to place a very crude estimate on the distance to ce gru we compare this luminosity to the hard x - ray luminosity of am her ( whose distance is reasonably well determined ) .
ishida et al ( 1997 ) observed am her using _ asca _ when it was in a high accretion state and find @xmath14 ergs s@xmath8using a distance of 75 pc .
we find that ce gru would have to lie @xmath0400 pc distance to equal the @xmath15 found for am her .
although ce gru was detected in x - rays for the first time and was clearly in a high accretion state , no distinct soft x - ray component was observed . on the face of it , this is surprising since a strong soft x - ray flux has long been considered one of the defining properties of polars .
observations of polars using _ exosat _ ( eg osborne 1988 ) and _ rosat _ ( eg ramsay et al 1994 ) found a strong distinct soft x - ray component , and in general , the ratio @xmath16 .
further , the ratio was correlated with the magnetic field strength of the white dwarf : a high field gave a high ratio . to account for the large ` soft x - ray excess ' seen in many polars , dense ` blobs ' of material which do not form a shock and radiate in soft x - rays were proposed ( kuijpers & pringle 1982 , frank , king & lasota 1988 ) .
currently , four polars have been observed using _ xmm - newton_. of those systems observed in an intermediate or high accretion state , ww hor showed no distinct soft x - ray component ( ramsay et al 2001 ) while by cam has one pole which did show such a component and one which did not ( ramsay & cropper 2002 ) .
indeed only one other system , dp leo , has shown a distinct soft x - ray component ( ramsay et al 2001 ) .
even for those polars which showed the lowest ratios using _ rosat _ ( ef eri and am her ) a distinct soft x - ray component was still observed .
a variation of the standard shock model has been proposed by heise & verbunt ( 1988 ) and gnsicke , beuermann & de martino ( 1995 ) who suggest that the reprocessed x - ray component lies in the euv band while the strong soft x - ray component seen in many polars originates from dense ` blobs ' of material which do not form an exposed shock .
our data are consistent with this view if the temperature of the reprocessed component is @xmath17ev : below 2ev the flux of the reprocessed component is too small , while above 5ev it would be too high .
a component with a temperature above 10ev would be evident in the spectrum ( figure  [ bb ] ) .
the implication in this case is that the fraction of blobby accretion is small in ce gru .
again this is consistent with the unfolded light curves ( not shown ) which do not show strong flaring seen in many systems ( such as by cam 
ramsay & cropper 2002 ) .
the amount of irradiation that the white dwarf receives is a function of height in the post - shock region , with greater temperatures from higher up , but more overall flux ( even at higher energies ) from near the base ( cropper , wu & ramsay 2000 ) . on the other hand , because of the curved surface of the white dwarf , the illumination is decreasing , with slightly more than the square of the distance from the axis of the post - shock region .
further , albedo varies as a function of temperature . although the effects of irradiating a white dwarf atmosphere have been explored to some extent ( williams
et al 1987 , heise 1995 ) , this is an area which needs further work to determine how the temperature of the reprocessed spectrum is effected by parameters such as the accretion rate and magnetic field .
we note , however , that higher specific accretion rates result in lower shock heights , and higher magnetic fields also reduce the height of the post - shock region ( see for example cropper et al 1999 ) , so the solid angle of emission from the post - shock flow intercepted by the white dwarf photosphere is increased . in these high - state data of ce gru , we have no reason to expect anomalously low specific accretion rates , and ce gru has a magnetic field strength typical of polars .
therefore there is no obvious reason for the reprocessed component to have moved into the euv if it is normally in the soft x - ray band in polars .
this gives support to the heise & verbunt ( 1988 ) suggestion that the soft x - ray component is indeed caused by blobby accretion .
some factor(s ) must determine the number , length and density of blobs .
obvious parameters include the magnetic field strength and orientation of the white dwarf , the orbital period and the mass transfer rate .
ramsay et al ( 1994 ) identified the magnetic field strength as one important parameter .
further progress in this regard awaits a systematic analysis of the strength of the soft x - ray component as a function of these parameters in a sufficiently large sample of polars .
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we report the detection of the polar ce gru in x - rays for the first time . 
 we find evidence for a dip seen in the hard x - ray light curve which we attribute to the accretion stream obscuring the accretion region in the lower hemisphere of the white dwarf . 
 the x - ray spectrum can be fitted using only a shock model : there is no distinct soft x - ray component . 
 we suggest that this is because the reprocessed component is cool enough so that it is shifted into the uv . 
 we determine a mass for the white dwarf of @xmath01.0@xmath1 .    
 stars : individual : ce gru  stars : binaries  stars : cataclysmic variables  x - rays : stars