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the statistics of radio pulsars and supernova explosions as well as the abundance of heavy elements suggests that about @xmath3 neutron stars must have been born in our galaxy throughout its life ( narayan & ostriker 1990 , van den berg & tamman 1991 ) . as young objects , @xmath4yrs , neutron stars appear as rotation - powered bright x - ray and gamma - ray sources or radio pulsars . at intermediate ages , @xmath5yrs ,
nearby neutron stars can be detected by radiation from their cooling surface . at even later times ,
@xmath6yrs , isolated neutron stars ( ins ) become too cold to be detected as a result of internal processes and could eventually only be seen through their photospheric emission in the x - ray band , if they accrete enough interstellar matter to heat their surface ( ostriker , rees & silk 1970 ) . depending on the assumption about their space velocity distribution , several thousand old ins could be detectable e.g.  in the rosat all - sky - survey ( rass ) ( treves & colpi 1991 ; blaes & madau 1993 ; madau & blaes 1994 ) .
[ cols="<,^,^,^,^,^,^,^ , < " , ]     @xmath7 standard rosat hardness ratio hr1 : ( @xmath8 ) / ( @xmath9 ) + @xmath10 standard rosat hardness ratio hr2 : ( @xmath11 ) / ( @xmath12 ) + @xmath13 x - ray to optical flux ratio @xmath14 + @xmath15 @xmath16-magnitude -0.5truecm    recently , two very good ins candidates have been reported : rxj1856.53754 ( walter et al .  1996 ) , rxj0720.43125 ( haberl et al .  1997 ) .
both of them were detected in the rass as relatively bright sources ( see tab .
1 ) with soft , blackbody - like spectra and little interstellar absorption . pointed follow - up observations with the rosat pspc and/or hri yielded very small x - ray error circles which put stringent constraints on possible optical counterparts .
walter & matthews ( 1997 ) could identify rxj1856.53754 with a faint blue object in an hst observation ( @xmath17 ) .
the extremely large x - ray to optical flux ratio @xmath18 excludes anything other than an isolated neutron star for this object .
recent observations of the likely counterpart of rxj0720.43125 ( star x1 ) reveals @xmath19 ( motch & haberl 1998 ) and @xmath20 ( kulkarni & van kerkwijk 1998 , kvk98 ) . with @xmath21
the inferred ratio @xmath22 again indicates an ins origin for the x - ray emission . on long time
scales the x - ray emission of the two ins candidates is almost constant .
however , most remarkably , pulsations with a period of 8.4 seconds were detected for rxj0720.43125 , which , under the assumption of accretion , indicate a low magnetic field ( @xmath010@xmath23 g ) and a small space velocity ( @xmath010 km s@xmath24 ) for the neutron star ( haberl et al .
a third , fainter candidate ins , rxj0806.44123 , also detected in the rass has been reported by haberl et al .
its optical counterpart has @xmath25 such that @xmath26 .    assuming that only two ins with count rates @xmath02 pspc cts s@xmath24 exist in the rass
, we expect a number of order 20 over the whole sky with count rates @xmath00.2cts  s@xmath24 ( c.f .
neuhuser & trmper 1998 ) .
in order to improve the determination of the x - ray luminosity function of x - ray emitting source classes , mainly agns , galaxies and clusters of galaxies , we have almost completed a program of optical identifications of bright , high - galactic - latitude x - ray sources , termed the rosat bright survey rbs . the goal is to completely identify all bright x - ray sources , @xmath27 pspc cts s@xmath24 , detected in the rosat all - sky survey ( 1rxs catalogue , voges et al .
1996 ) at galactic latitudes @xmath28 , excluding lmc , smc and the virgo cluster ( hasinger et al .  1997
; fischer et al .  1998 ;
schwope et al .
the rbs is already to 98% spectroscopically complete .
we find a total of 757 stars , 659 agn , 297 clusters of galaxies , 171 bl lac candidates , 43 galaxies , 49 cataclysmic variables or x - ray binaries .
only 36 of the more than 2000 sources in the sample are not yet identified . among the known types of x - ray emitters , the bl lac objects are those with the most extreme x - ray to optical flux ratio .
yet the optically faintest bl lac objects at @xmath29 have @xmath30 ( fig .
[ f : colcol ] shows the ratio @xmath31 for bl lac objects , ins candidates and unidentified rbs sources ) .    for all unidentified x - ray sources we have produced finding charts from the roe / nrl and apm digitised sky survey catalogues .
the position errors of bright rass - sources are of sufficient quality that we can immediately localize x - ray sources with empty rass error circles on the sky survey plates .
for quite a number of such objects we have been awarded short follow - up observations with the rosat hri in the last years or looked in the archive , in order to investigate whether the source is a fluke or has an exceptionally large x - ray position error ( a number of such objects existed in the first processing of the all - sky - survey , rass - i ) .
another possibility is that the x - ray source is extended ( cluster or group ) and therefore has a formally empty error circle . finally , in one case so far the hri confirmed the position and flux of the x - ray source and has put very strong constraints on the magnitude of an optical counterpart .
the field of rbs1223 has been observed with the rosat hri on june 13 , 1997 at 19:58 ut for a net exposure time of 2218s .
we have corrected the individual x - ray photons for the hri plate - scale factor of 0.9972 ( hasinger et al .
1998 ) and then analysed the data using the standard interactive analysis system exsas ( zimmermann et al .
the hri observation showed a point - like x - ray source consistent with the position of 1rxsj130848.6 + 212708 .
positions of rosat pointed sources have a routine accuracy of @xmath010  due to inherent attitude errors .
the hri position of rbs1223 however , could be related to an absolute celestial reference system through the fortuitous detection of a second , fainter hri source about 4.5 arcmin west of rbs1223 , which could be identified with the bright ( @xmath32 ) star ag+21 1341 ( using simbad ) .
the x - ray position of the second hri source is within @xmath33 of the optical position of ag+21 1341 , as determined from the apm catalogue .
we did not consider it necessary to correct for this shift .
the final position of rbs1223 is therefore ra(2000)@xmath34 , dec(2000)@xmath35 .
taking into account the statistical accuracy of the centroiding for both sources , the 90% position error for rbs1223 is 1.6 arcsec . fig .
[ f : xcont_over ] shows a superposition of the x - ray contours of the rbs1223 field on a digitised copy of the poss - ii plate , clearly indicating an empty error box down to the plate limit .
the hri count rate of rbs1223 , @xmath36  cts  s@xmath24 , is comparable to the pspc count rate of @xmath37  cts  s@xmath24 .
rbs1223 has pspc hardness ratios which can be fit with a blackbody temperature of @xmath38ev , and an interstellar absorption column density of @xmath39  @xmath40 , consistent with the total galactic value in this direction , @xmath41  @xmath40 .
the unabsorbed blackbody flux at earth is about @xmath42  erg  @xmath40  s@xmath24 .
we have analysed the @xmath43 hri photons from rbs1223 for periodicities in the x - ray flux , but did not detect any significant signal in the fourier transform of the data .
-0.5truecm    ( 8,8 )    -0.5truecm    -0.5truecm
images of the rbs1223 field have been obtained with the low resolution imaging spectrometer ( oke et al .
1995 ) at the cassegrain focus of the keck ii telescope on the photometric night of march 19th , 1998 .
the @xmath44 pixel loral ccd was read out in single amplifier mode . a 15 min exposure through the r filter
was taken at 13:33 ut and a 15 min exposure through the b filter at 13:52 ut .
the reduction was done using midas .
the two frames were bias subtracted , cleaned of cosmic rays and flat - fielded using twilight flats .
no photometric standards were taken . using the brightness limits derived by kvk98 for the same instrumental setup and same observing conditions we estimate the limiting magnitude of our images at about @xmath45 by scaling with the sqaureroot of the integration time . for our estimate of the x - ray to optical flux ratio
we used the conservative limit of @xmath46 .
[ f : xcont_detail ] shows the hri x - ray contours of rbs1223 superposed on the @xmath47 keck image .
no optical counterpart is detected in the x - ray error circle in either the @xmath47 or the @xmath48 image . using the observed x - ray flux from above we derive a lower limit of @xmath49 , using the relation @xmath50 ( maccacaro et al .
1988 ) under the assumption of @xmath51 .
-0.5truecm    ( 8,8 )    -0.5truecm    -0.5truecm
rbs1223 has all properties that single out the new class of isolated neutron stars identified by rosat : a very high x - ray to optical flux ratio , a rather soft x - ray spectrum and a constant x - ray flux over long time scales . with its very small x - ray error box
it will provide a further challenge for optical identification . scaling from the counterpart of rxj1856.53754 ( walter & matthews 1998 ) we expect its optical magnitude in the range @xmath52 , within reach of the hst and ground - based 810 m telescopes .
the direct prediction of the optical brightness assuming pure blackbody radiation with the spectral parameters of tab .
1 gives only @xmath53 , beyond the limit of current telescopes .
[ f : colcol ] shows the extreme position of rbs1223 and the other ins candidates of tab
.  1 plotted in a kind of x - ray / optical colour - colour diagram together with the otherwise most extreme point - like rbs sources , the bl lac objects . only sources with a very peaked x - ray - optical spectrum can be in the upper left corner of this diagram .
rbs1223 is the faintest of all known ins candidates and has also the highest x - ray temperature . among the 36 unidentified rbs sources ,
there is only a handful of objects with x - ray hardness ratios consistent with ins .
( we have , however , to be careful about possible misidentified objects in x - ray surveys ) . the next best ins candidate from the rosat bright survey is rbs1556 , which lies in the same part of the diagram . at somewhat lower values of @xmath54
there are a number of unidentified objects , which could be either bl lac candidates or ins . assuming between 2 and 10 candidate objects in the rbs , a rough estimate of the surface density of ins brighter than 0.2 pspc cts / s is @xmath55  sources  sr@xmath24 .
the lower and upper limit can provide strong constraints on the ins @xmath56 function and therefore on the nature of the population as a whole .
the accreting old neutron star scenario was recently criticized because of a number of highly uncertain assumptions or unlikely requirements ( see the discussion in kvk98 ; wang et al .
a very low magnetic field is required to overcome the propeller effect and allow accretion .
ism accretion would most likely be highly variable on time - scales of months to years .
finally , accreting neutron stars would quickly build up a hydrogen - rich envelope , which should appear much brighter in the optical band than is detected for ins ( wang et al .
1998 ) .
several authors have therefore proposed alternatives or variants for the nature of isolated neutron star candidates .
haberl et al .
( 1997 ) suggest a connection of rxj0720.43125 to the class of anomalous pulsars ( mereghetti & stella 1995 ) which might be born with low space velocities and magnetic fields .
kvk98 discuss the interesting possibility that the ins candidates are cooling magnetars , very high magnetic field isolated neutron stars with long spin period , which are associated with soft gamma ray repeaters . at any rate ,
the detection of more objects of this class and their detailed study with existing and upcoming x - ray and optical instruments is of vital importance for the understanding of their nature .
we thank the referee frederick m.  walter for helpful comments .
the rosat project is supported by the bundesministerium fr bildung , forschung und wissenschaft ( bmbf ) , by the national aeronautics and space administration ( nasa ) , and the science and engineering research council ( serc ) .
the w.m .
keck observatory is operated as a scientific partnership between the california institute of technology , the university of california , and the national aeronautics and space administration .
it was made possible by the generous financial support of the w.m .
keck foundation .
this research has made use of the simbad database operated at cds , strasbourg , france and was greatly facilitated by use of the apm catalogue based on scans of the poss plates performed at the institute of astronomy , cambridge , uk and the cosmos scans of the eso / serc j plates performed at the royal observatory edinburgh .
this work has been supported in part by dlr grant 50 or 9403 5 ( ads , gh ) and by nasa grant nag5 - 1531 ( ms ) .
in the rosat bright survey ( rbs ) we have almost completely optically identified the brightest @xmath02000 high - galactic latitude sources from the rosat all - sky survey bright source catalogue ( 1rxs ) . 
 a small number of sources has empty x - ray error circles on optical images . 
 rosat hri follow - up observations of rbs1223 ( = 1rxsj130848.6 + 212708 ) , a soft object with extreme x - ray to optical flux ratio , have confirmed a relatively bright x - ray source , whose position could be determined to an accuracy of 1.6 arcsec ( 90% ) due to the presence of a nearby , x - ray detected bright star . 
 deep keck r- and b - band images of the field were taken , but the refined x - ray error circle remains empty to a limiting magnitude @xmath1 . with an x - ray to optical flux ratio of @xmath2 
 this object is almost certainly an isolated neutron star , similar to the two so far best - known examples rxj1856.43754 and rxj0720.43125 . 
 we discuss limits on the number of similar objects in the rbs catalogue .