document: the broad emission iron lines are well - known features found in about two dozens of spectra of active galactic nuclei and black hole binaries . 
   they are supposed to originate close to the black hole by the reflection of the primary radiation on the accretion disc . 
   the spin of the black hole plays an important role in the forming of the line shape . 
   especially , it determines the position of the marginally stable orbit which is supposed to confine the inner edge of the accretion disc ( see figure  [ intro ] ) . 
   the innermost stable orbit occurs closer to a black hole with a higher spin value . 
   however , the spin affects also the overall shape of the line .    over almost two decades 
   the most widely used model of the relativistic disc spectral line has been the one by , which includes the effects of a maximally rotating kerr black hole . in other words 
   , the laor model sets the dimensionless angular momentum @xmath1 to the canonical value of @xmath0  so that it can not be subject of the data fitting procedure . 
   have relaxed this limitation and allowed @xmath1 to be fitted in the suite of ky models . 
   other numerical codes have been developed independently by several groups ( , , ) and equipped with similar functionality . 
   however , the laor model can still be used for evaluation of the spin if one identifies the inner edge of the disc with the marginally stable orbit . in this case 
   the spin is actually estimated from the lower boundary of the broad line . 
   the comparison of the laor and model is shown in the right panel of figure  [ intro ] . 
   the other parameters of the relativistic line models are inclination angle @xmath2 , rest energy of the line @xmath3 , inner radius of the disc @xmath4 , outer radius of the disc @xmath5 , emissivity parameters @xmath6 , @xmath7 with the break radius @xmath8 . 
   the emissivity of the line is given by @xmath9 for @xmath10 and @xmath11 for @xmath12 . 
   the angular dependence of the emissivity is characterized by limb darkening profile @xmath13 in the laor model . 
   the model enables to switch between different emission laws . 
   we used further two extreme cases , the with the same limb - darkening law as in the laor model and * with the limb - brightening law @xmath14 . 
   the aim of this paper is to compare the two models applied to the current data provided by the xmm - newton satellite , and to the artificial data generated for the on - coming x - ray mission . for this purpose 
   we have chosen two sources , mcg-6 - 30 - 15 and gx 339 - 4 , which exhibit an extremely skewed iron line according to recently published papers ( ) .     and marginally stable orbit @xmath15 . 
   right : comparison of the laor ( black , solid ) and ( red , dashed ) model for two values of the spin @xmath16 ( top ) and @xmath17 ( bottom ) . 
   the other parameters of the line are @xmath18kev , @xmath19 , @xmath20.,scaledwidth=98.0% ]     and marginally stable orbit @xmath15 . 
   right : comparison of the laor ( black , solid ) and ( red , dashed ) model for two values of the spin @xmath16 ( top ) and @xmath17 ( bottom ) . 
   the other parameters of the line are @xmath18kev , @xmath19 , @xmath20.,title="fig:",scaledwidth=98.0% ] +    and marginally stable orbit @xmath15 . 
   right : comparison of the laor ( black , solid ) and ( red , dashed ) model for two values of the spin @xmath16 ( top ) and @xmath17 ( bottom ) . 
   the other parameters of the line are @xmath18kev , @xmath19 , @xmath20.,title="fig:",scaledwidth=98.0% ] 
   we used the sas software version 7.1.2 ( http://xmm.esac.esa.int/sas ) to reduce the xmm - newton data of the sources . 
   further , we used standard tools for preparing and fitting the data available at http://heasarc.gsfc.nasa.gov ( ftools , xspec )    the galaxy mcg-6 - 30 - 15 is a nearby seyfert 1 galaxy ( @xmath21 ) . 
   the skewed iron line has been revealed in the x - ray spectra by all recent satellites . 
   the xmm - newton observed mcg-6 - 30 - 15 for a long 350ks exposure time during summer 2001 ( revolutions 301 , 302 , 303 ) . 
   the spectral results are described in . 
   we joined the three spectra into one using the ftool mathpha . 
   the black hole binary gx 339 - 4 exhibited a strong broadened line in the 76ks observation in 2002 ( ) when the source was in the very high state ( for a description of the different states see ) . 
   the observation was made in the _ burst mode _ due to a very high source flux . 
   the 97@xmath22 of photons are lost during the reading cycle in this mode , which results into 2.25ks total exposure time . 
   we rebinned all the data channels in order to oversample the instrumental energy resolution maximally by a factor of 3 and to have at least 20 counts per bin . 
   the first condition is much stronger with respect to the total flux of the sources 
   @xmath23erg@xmath24s@xmath25 in 210kev ( @xmath26cts ) for mcg-6 - 30 - 15 and @xmath27erg@xmath24s@xmath25 in 210kev ( @xmath28cts ) for gx 339 - 4 . 
   [ cols="^,^,^ " , ] 
   we investigated the iron line band for two representative sources 
   mcg-6 - 30 - 15 ( active galaxy ) and gx 339 - 4 ( x - ray binary ) . 
   the iron line is statistically better constrained for the active galaxy mcg-6 - 30 - 15 due to a significantly longer exposure time of the available observations  for comparison of count rates of the sources see table  3 . 
   the spectra of both sources are well described by a continuum model plus a broad iron line model . 
   we compared modeling of the broad iron line by the two relativistic models , laor and . 
   the model leads to a better defined minimum of @xmath29 for the best fit value . 
   the confidence contour plots for @xmath30 versus other model parameters are more regularly shaped . 
   this indicates that the model has a smoother adjustment between the different points in the parameter space allowing for more reliable constraints on @xmath30 . 
   the laor model has a less accurate grid and is strictly limited to the extreme kerr metric . 
   the discrepancies between the and laor results are within the general uncertainties of the spin determination using the skewed line profile when applied to the current data . 
   however , the results are apparently distinguishable for higher quality data , as those simulated for the xeus mission . 
   we find that the laor model tends to overestimate the spin value and furthermore , it has insufficient energy resolution which affects the correct determination of the high - energy edge of the broad line . 
   the discrepancies in the overall shape of the line are more visible especially for lower values of the spin @xmath30 . as a side - product 
   , we have found that the correct re - binning of the data with respect to the instrumental energy resolution is crucial to obtain statistically the most relevant results . 
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summary: the analysis of the broad iron line profile in the x - ray spectra of active galactic nuclei and black hole x - ray binaries allows us to constrain the spin parameter of the black hole . 
   we compare the constraints on the spin value for two x - ray sources , mcg-6 - 30 - 15 and gx 339 - 4 , with a broad iron line using present relativistic line models in xspec  laor and . 
   the laor model has the spin value set to the extremal value @xmath0 , while the model enables direct fitting of the spin parameter . 
   the spin value is constrained mainly by the lower boundary of the broad line , which depends on the inner boundary of the disc emission where the gravitational redshift is maximal . 
   the position of the inner disc boundary is usually identified with the marginally stable orbit which is related to the spin value . in this way 
   the laor model can be used to estimate the spin value . 
   we investigate the consistency of the laor and models . 
   we find that the spin values evaluated by both models agree within the general uncertainties when applied on the current data . 
   however , the results are apparently distinguishable for higher quality data , such as those simulated for the international x - ray observatory ( ixo ) mission . 
   we find that the laor model tends to overestimate the spin value and furthermore , it has insufficient resolution which affects the correct determination of the high - energy edge of the broad line . +