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nasa s _ chandra _ x - ray observatory was launched on july 23 , 1999 .
the _ chandra _ multiwavelength project ( champ ) will combine radio to x - ray observations of serendipitous _
chandra _ sources , with emphasis on optical identification .
the champ is superior to previous x - ray surveys because of ( 1 ) unprecedented x - ray positional accuracy ( @xmath6 ) , ( 2 ) x - ray flux limits 20 times deeper than current wide area surveys ( down to @xmath7 erg  s@xmath1  @xmath2 ) , ( 3 ) larger sky coverage ( @xmath8 8 deg@xmath9 ) per year than current deep surveys .
* the x - ray luminosity function * at @xmath10 is described as    @xmath11    @xmath12 + where l@xmath13 is the x - ray luminosity in @xmath14 erg  s@xmath1 .
the redshift evolution of the luminosity function is characterized by    @xmath15    continuity of the luminosity function at the break luminosity requires that    @xmath16    the total number @xmath17 of quasars in the sample is obtained by integrating the luminosity function over luminosity and volume , i.e. ,    @xmath18    here @xmath19 is the solid angle covered by the survey as a function of redshift and luminosity .
the parameters of the x - ray luminosity function determined by boyle et al .
( 1993 ) are as follows : @xmath20 , @xmath21 , @xmath22 , @xmath23 . following comastri et al .
( 1995 ) , we have used k=2.6 and increased the the normalization @xmath24 by 20% .    * the x - ray logn - logs curve * : using the above luminosity function we derived the number density of quasars as a function of observed flux .
the luminosity function was integrated over the luminosity range @xmath25 erg s@xmath1 and the redshift range @xmath26 .
h@xmath27 and q@xmath28 were assumed throughout .
the predicted logn - logs curve is shown in figure 1 .    since the unabsorbed sources dominate at the faint end in the soft x - ray range , and since they are likely to be observed at high redshift , in the present analysis we will concentrate on unabsorbed sources only .
the absorbed sources would contribute an additional @xmath8 60% ( comastri et al .
1995 ) , making the total number consistent with the extrapolation of the empirical determination of logn - logs ( hasinger et al .
the flux of unabsorbed quasars is given by @xmath29 and in the soft x - ray band , @xmath30 is typically 1.3 .
* the champ sky coverage * : the champ cycle 1 consists of 85 extragalactic fields , |b|@xmath31 . from all the chandra cycle 1 fields we have excluded ( 1 ) deep fields of pi survey observations , ( 2 ) fields with extended sources & planetary targets , ( 3 ) acis sub - arrays and continuous clocking modes .
see figure 2 for champ sky coverage as a function of flux limit .
* cumulative number distribution in champ : * integrating the predicted logn - logs over the champ sky coverage , we obtained the cumulative number distribution of quasars in the champ fields ( figure 3 ) .
the total number in soft band is expected to be over 1500 for unabsorbed sources and over 2500 total .
* predicted redshift distribution : * the histogram ( figure 4 ) shows the predicted number distribution of quasars in champ fields . over 200 quasars will be detected in the redshift range @xmath3 and over 400 quasars in @xmath4 .
[ cols="<,>,>",options="header " , ]      boyle , b. , griffiths , r. , shanks , t. , stewart , g. , & georgantopoulus , i. 1993 , mnras , 260 , 49 comstri , a. , setti , g. , zamorani , g. , & hasinger , g. 1995 , a&a , 296 , 1 gioia et al .
1990 , apjs , 72 , 567 hasinger , g. et al .
1993 , a&a , 275 , 1
_ chandra _ x - ray observatory , ( formerly known as axaf ) , will observe down to the flux limit of 2@xmath0 erg  s@xmath1  @xmath2 . in its first year of operation _ 
 chandra _ 
 s ccd detectors will observe over 1500 quasars serendipitously in the soft ( 0.53.5 kev ) band . over 200 quasars will be detected in x - rays in the redshift range @xmath3 and over 400 quasars in @xmath4 . 
 this will enable us to determine the high redshift x - ray luminosity function . 
 this is the contribution by unabsorbed sources only . 
 the total numbers would be larger by @xmath5% .