You will be presented with the full text of a science research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
in an earlier paper ( romer et al . 2001 , r01 hereafter ) we described the expected catalog properties and scientific applications of an _ xmm - newton _ cluster survey ( xcs ) based on serendipitous detections in pointed observations .
we revisit these predictions below , in light of improved knowledge of the in flight performance of _ xmm_. we also compare the xcs to other on - going x - ray surveys .
[ cluster - numbers ]    .the expected number of @xmath2 kev clusters detected by the xcs as a function of cosmology .
column @xmath3 , the original xcs predictions for @xmath4 detections from r01
. columns @xmath5 and @xmath6 , updated xcs predictions based on the measured in flight internal background count rate .
all the results assume a final survey area of 800 sq .
@xmath7there was a typo in the the printed version of r01 , this is the correct value . [ cols="^,^,^,^,^,^,^,^,^,^ " , ]
recently it has come to light that the quiescent internal background count rates in the epic detectors are roughly ten times higher than was anticipated prior to the _ xmm - newton _ launch ( see lumb 2001 ) .
we have , therefore , recomputed the expected properties of the xcs cluster catalog using a total ( external plus internal ) background level that is roughly double that used in r01 . in table  1
, we compare the new predictions ( column _ ii _ ) with those published in table  4 of r01 ( column _ i _ ) .
for example , for an open model ( @xmath8 , @xmath9 ) , we now predict that the xcs will detect 390 @xmath2 kev , @xmath1 clusters at @xmath10 , compared to 480 in r01 .
changes at that level should not severely limit the ability of xcs to differentiate between open and flat cosmological models .
we have also included in table  1 predictions for the number of xcs clusters that would be detected at @xmath0 .
we did not include low signal to noise detections in r01 because they will be hard to identify using an extent criterion alone .
however , when combined with planck and/or sdss data , it may be possible to identify these objects in a timely and quantifiable manner , so we include them here for completeness .
the redshift distribution of the @xmath11 and @xmath10 xcs detections can be seen in figure  1 .
the massive cluster survey ( macs , ebeling , edge & henry 2001 ) has been very successful at identifying high redshift , high luminosity , clusters detected in 22,735 square degrees of the rosat all sky survey . the wide areal coverage of the macs ( nearly 30 times that of the xcs ! ) results in sensitivity to a very interesting class of clusters , those at medium redshift and high luminosity ( @xmath12 erg  @xmath13  s@xmath14 ; 0.1 - 2.0 kev ) .
macs clusters are already being used for a variety science programs including studies of cluster evolution and the sunyaev - zeldovich effect .    using the r01 methodology and based on the macs selection function presented in figure  5 of ebeling et al .
( 2001 ) we have been able to predict the number of clusters that would be detected by macs as a function of cosmology .
we present those results in figure  1 and table  1 .
overall , we can expect macs to detect between 100 and 500 clusters at @xmath15 , depending on cosmology .
an _ xmm _ slew survey has been proposed ( _ e.g. _ lumb & jones 2000 ) . an _
xmm _ slew survey would have two important advantages over the xcs ; it would be drawn from random parts of the sky ( rather than from regions that surround known x - ray targets ) and it would cover significantly more area .
lumb & jones ( 2000 ) estimated an annual survey rate of 4000 square degrees to a flux limit of @xmath16 erg  @xmath13  s@xmath14 ( 0.5 - 2.0 kev ) .
this compares to an estimated annual survey rate for the xcs of @xmath17 square degrees ( the xcs catalog will not have a single flux limit , but a typical value will be @xmath18 erg  @xmath13  s@xmath14 in the 0.5 - 2.0 kev band , see r01 ) .    we have attempted to predict the properties of a slew survey cluster catalog using the r01 methodology . at the time of writing , the slew survey had only just begun and the actual sensitivity level was unknown ( lumb , private communication ) .
we , therefore , present results for two different ( 0.5 - 2.0 kev ) flux limits ; @xmath16 erg  @xmath13  s@xmath14 and @xmath19 erg  @xmath13  s@xmath14 , see figure  2 . we have assumed that 4000 square degrees would be covered in one year by the slew survey . for comparison
we have shown the expected numbers of clusters that would be detected at @xmath11 in one year of the xcs ( or 80 square degrees ) .
a slew survey with @xmath16 erg  @xmath13  s@xmath14 sensitivity would produce a very impressive cluster catalog at low and medium redshifts .
such a catalog would have a variety of applications including constraints on @xmath20 and @xmath21 and studies of cluster evolution and planck foregrounds .    in summary , the macs and the _ xmm _ slew surveys have much better low redshift sensitivity than xcs by virtue of their large areal coverage .
however , neither will be able to differentiate between open and flat low @xmath22 models , because they lack sensitivity to high redshift clusters .
these surveys should be seen as complementary too , not in competition with , the xcs .
the xcs faces many challenges , including the complexity of its selection function and our lack of understanding of cluster evolution .
the future existence of the xcs should not deter the launch of a dedicated x - ray survey satellite .
this paper describes updated predictions , as a function of the underlying cosmological model , for a serendipitous galaxy cluster survey that we plan to conduct with the _ xmm - newton _ x - ray satellite . 
 we have included the effects of the higher than anticipated internal background count rates and have expanded our predictions to include clusters detected at @xmath0 . even with the enhanced background levels , we expect the xcs to detect sufficient clusters at @xmath1 to differentiate between open and flat cosmological models . we have compared the xcs cluster redshift distribution to those expected from the _ xmm _ slew survey and the _ rosat _ massive cluster survey ( macs ) and find them to be complementary . 
 we conclude that the future existence of the xcs should not deter the launch of a dedicated x - ray survey satellite .    
 # 1_#1 _ # 1_#1 _ =    # 1 1.25 in .125 in .25 in