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in recent years , optical surveys ( e.g , the sloan digital sky survey and the digital palomar sky survey ) have discovered a large number ( @xmath2  1000 ) of quasars at @xmath0 . from the pioneering study of kaspi et al .
( 2000 ; see fig .
1a ) , the number of x - ray detected agn at @xmath0 has increased to more than 110 ( fig .
1b ) , mostly thanks to exploratory observations with _
( e.g. , vignali et al .
2001 , 2005 ; brandt et al . 2002 ; bassett et al . 2004 ; lopez et al . 2006 ; shemmer et al .
2006a ) and longer exposures with xmm-_newton _ ( e.g. , shemmer et al .
2005 ) . at the very faint x - ray fluxes ,
x - ray surveys have provided detection of several @xmath0 agn and quasars ( e.g. , schneider et al .
1998 ; silverman et al . 2002 ; vignali et al .
2002 ) .
magnitude for @xmath0 agn and quasars . *
( a ) * the situation after the kaspi et al .
( 2000 ) work using _ rosat _ data ; * ( b ) * the updated census of x - ray observations of @xmath0 agn , including the results from moderate - depth and ultra - deep x - ray surveys .
, title="fig:",scaledwidth=45.0% ]   magnitude for @xmath0 agn and quasars . *
( a ) * the situation after the kaspi et al .
( 2000 ) work using _ rosat _ data ; * ( b ) * the updated census of x - ray observations of @xmath0 agn , including the results from moderate - depth and ultra - deep x - ray surveys . , title="fig:",scaledwidth=45.0% ]      @xmath3 x - ray emission is a universal property of agn .
the x - ray properties of high - redshift agn and quasars ( derived from either stacked or individual x - ray spectra ) are similar to those of local quasars , with no evidence for widespread absorption . for radio - quiet quasars ( rqqs ) , a photon index of @xmath41.92.0 is obtained ( e.g. , vignali et al . 2005 ; shemmer et al .
2005 ) , also at @xmath5 ( shemmer et al .
2006a ) , while for `` moderate '' radio - loud quasars ( rlqs ) and blazars , @xmath41.7 and @xmath41.5 are obtained ( lopez et al .
2006 ) , respectively .
@xmath3 the comparison with the lower redshift ( luminosity ) palomar - green quasars observed by xmm-_newton _
( piconcelli et al . 2005 ) indicates that the photon index does not vary significantly with redshift and luminosity , but seems to depend primarily on the accretion rate ( i.e. , steeper x - ray slopes are associated with higher eddington ratio sources ; shemmer et al .
2006b ) .
@xmath3 following x - ray studies of early 80 and 90 , the relation between x - ray and longer wavelength emission has been investigated by means of the point - to - point spectral slope between 2500    and 2  kev in the source rest frame ( @xmath6 ) .
any changes in the accretion rate over cosmic time might lead to changes in the fraction of total power emitted as x - rays .
using 333 agn at @xmath1@xmath206.3 ( 88% x - ray detections ) , steffen et al .
( 2006 ) confirmed that @xmath7 correlates with @xmath8 with an index @xmath9 , and @xmath6 depends upon @xmath7 ( with the slope perhaps depending on @xmath10 ) .
the research field related to @xmath0 agn still offers plenty of opportunities .
in particular , the detection of x - ray variability in some @xmath0 quasars over time scales of month - year ( shemmer et al . 2005 ) needs further investigations to check the possibility that quasars are more variable in the early universe .
furthermore , detailed x - ray spectra of @xmath0 rlqs filling the observational gap between `` moderate '' rlqs and blazars are still needed , as well as studies of `` peculiar '' quasars and faint agn population at the highest redshifts .
bassett , l.c . , et al .
2004 , , 128 , 523 brandt , w.n . , et al .  2002 , , 569 , l5 kaspi , s. , brandt , w.n . , &
schneider , d.p .
2000 , , 119 , 2031 lopez , l.a . , et al .
2006 , , 131 , 1914 piconcelli , e. , et al .
2005 , , 432 , 15 schneider , d.p . , et al .   1998 , , 115 , 1230 shemmer , o. , et al .   2005 , , 630 , 729 shemmer , o. , et al .  2006a , , 644 , 86 shemmer , o. , brandt , w. , netzer , h. , maiolino , r. , & kaspi , s.  2006b , , 646 , l29 silverman , j.d .
, et al .
2002 , , 569 , l1 steffen , a.t . , et al .
2006 , , 131 , 2826 vignali , c. , et al .
2001 , , 122 , 2143 vignali , c. , et al .
2002 , , 580 , l105 vignali , c. , brandt , w.n . ,
schneider , d.p . , & kaspi , s. 2005 , , 129 , 2519
we review the x - ray studies of the highest redshift quasars , focusing on the results obtained with _ chandra _ and xmm-_newton_. overall , the x - ray and broad - band properties of @xmath0 quasars and local quasars are similar , suggesting that the small - scale x - ray emission regions of agn are insensitive to the significant changes occurring at @xmath1@xmath206 .