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the fluorescence iron k@xmath2 line revealed by a 4.2-day asca observation in the seyfert 1 galaxy mcg 6 - 30 - 15 has distinct features which are unique characteristics of the emission line from a relativistic disk ( tanaka et al . the line is peaked at around 6.4 kev , with a broad red wing extending down to 4 kev . the skewness of the profile towards red and the sharp blue edge near the rest frame energy are a consequence of the combination of relativistic doppler effect and strong gravitational redshift , and are unique to a low - inclination relativistic accretion disk ; other mechanisms of the line formation are less successful to produce these features ( fabian et al . however , the alternative models such as thermal comptonization ( misra and kembhavi 1998 ) can not be ruled out completely . both the disk line fit ( tanaka et al . 1995 ) and the frequency extrema analysis ( bromley , miller & pariev 1998 ) suggested that the inner edge of line formation region is within a few schwarzschild radii , providing ever strongest evidence for a supper - massive black hole in this object . similar broad line profiles with lower statistics have also been seen in other agns ( mushotzky et al . 1995 , tanaka et al . 1995 , yaqoob et al . 1995 , nandra et al . 1997 , turner et al . 1998 and references therein ) . the line profile is strongly dependent on the inclination of the relativistic accretion disk ( fabian et al . 1989 ) , thus providing a method to estimate the inclination of inner accretion disk from line profile fitting . the inclinations from this method for a sample of seyfert 1 galaxies are strongly constrained to be nearly face - on ( nandra et al . 1997 ) , consistent with the expectation of the unification model of the two types of seyfert galaxies ( see antonucci 1993 for a review ) . however , there are complications to this simple picture . turner et al . ( 1998 ) found that type 2 seyfert galaxies also possess similar iron k@xmath2 line profiles , indicative of face - on accretion disk as well . this might be due to the strong contribution of a narrow component in these objects ( weaver & reynolds 1998 ) or due to complex absorption ( wang et al . moreover , the rapid variability of the line equivalent width ( ew ) and profile observed in mcg 6 - 30 - 15 ( iwasawa et al . 1996 ) and ngc 4051 ( wang et al . 1999b ) can not be readily explained by any current simple accretion disk line model . ngc 4151 is a bright nearby ( z = 0.0033 ) seyfert 1.5 galaxy . the edge - on orientation of its nucleus is strongly supported by the biconical geometry of the [ o iii ] @xmath55007 region ( evans et al . 1993 , pedlar et al . 1993 ) and the best estimated inclination is 65@xmath3 . yaqoob et al . ( 1995 ) presented the first measurement of its broad fe k line profile from the asca observations performed in 1993 may , november and december . the apparent line profile is asymmetric , consisting of a peak at @xmath06.3  6.4 kev and a broad red wing extending to @xmath04  5 kev . when a disk line model is fitted , the inclination angle of the disk ( @xmath6 = 0@xmath7 degrees ) is strongly constrained to be face - on , in contrary to the edge - on geometry of this source . this problem can be eased , but not solved , by assuming an additional narrow - line contribution , presumably from the torus , to the fe k line ( @xmath6 = 25@xmath8 degrees , see yaqoob & weaver 1997 ) . a much longer asca observation ( 3 days ) of ngc 4151 was carried out in 1995 , which provides data with better statistics ( leightly et al . 1997 ) . in this letter , we report measurement of the complex broad iron k line profile and its implication . ngc4151 was observed by asca from 1995 may 10 to 12 with the solid - state imaging spectrometer ( sis ) in 1ccd mode and the gas imaging spectrometer ( gis ) in ph mode . the gis data are contaminated by a nearby bl lac object due to their worse spatial resolution . in this letter , we will concentrate on the sis data , also because they offer much better energy resolution ( inoue 1993 ) , which is crucial to the fe k line profile analysis . the data were reduced with the asca standard software xselect and cleaned using the following screening criteria : satellite not passing through the south atlantic anomaly ( saa ) , geomagnetic cutoff rigidity greater than 6 gevc@xmath9 , and minimum elevation angle above earth s limb of 10@xmath3 and 20@xmath3 for nighttime and daytime observations , respectively . source counts were extracted from a circular area of radius 3.4 for the sis0 and sis1 , and the background counts were estimated from the blank sky data . spectra extracted from sis0 and sis1 were combined together and grouped to have @xmath1020 counts in each bin to allow the use of @xmath11 statistics . spectral analysis was carried out using xspec . the asca observation lasted for about three days , and the sis0 detector received a net exposure time of 93,000s and an average count rate of 1.49 cts / s in the 0.4  10.0 kev band , while these two parameters for the well - known 4.2-day asca observation on mcg 6 - 30 - 15 in 1994 july are 150,000s and 1.82 cts / s , respectively . although shorter exposure time and lower count rate may probably lead to worse statistics , we noticed that , because of its much harder spectrum ( due to strong absorption ) , the actual total counts ( 32,000 ) for ngc 4151 in the 5.0  7.0 kev band are almost three times of those ( 11,000 ) for mcg 6 - 30 - 15 in the same energy band . thus , the ngc 4151 fe k spectrum used in this paper has better statistics than the average mcg 6 - 30 - 15 spectrum and a best ever quality fe k line profile . following weaver et al . ( 1994 ) , we fit the underlying continuum in the 1.0  4.0 and 8.0  10.0 kev bands ( to exclude the possible broad iron line region ) with a model which consists of a dual absorbed power law with some fraction ( a best - fit value for this letter is @xmath05% ) of the direct continuum scattered into our line of sight and absorbed only by the galactic column of 2 @xmath12 10@xmath13 @xmath14 . we do not include a compton reflection component in the spectral fits because a clear reflection component has never been detected ( maisack & yaqoob 1991 ; yaqoob et al . 1993 ) . zdziarski et al . ( 1996 ) claimed that they detected a reflection component . however , considering the poor statistical significance of their detection ( they used only a delta - chi square of 2.7 for the their errors yet the model was complicated ) , the complex intrinsic continuum which was unknown , and the cross - calibration of two different satellites ( ginga and gro / osse ) which is not perfect , we think that their result is questionable . our model can describe the data reasonably well ( @xmath11 = 364 for 325 degrees of freedom ) , and the best - fit model parameters converge to @xmath15 = 1.30@xmath16 , n@xmath17(low ) @xmath18 3.2 @xmath12 10@xmath19 @xmath14 ( covering @xmath0 40% of the source ) , and n@xmath17(high ) = @xmath0 10.9 @xmath12 10@xmath19 @xmath14 ( covering @xmath0 60% of the source ) . the index is slightly flatter than that seen previously for ngc 4151 as derived from data weighted toward higher energies ( yaqoob et al . 1993 ) . we have also tried a model consisting of a power - law absorbed by ionized material ( zdziarski et al . 1995 ) plus a fraction of scattered underlying continuum , but failed to get a satisfactory fit ( @xmath15 = 1.00 , @xmath11 = 393 for 326 degrees of freedom ) . the profile of the iron k line is shown in figure 1a , which is similar to , and much better defined than , the one presented by yaqoob et al . the line shows a strong narrow peak around 6.4 kev and a huge red wing , containing a second slightly weak peak at around 5.2 kev , extending to @xmath0 4.5 kev . in this respect , the line profile of ngc 4151 is remarkably similar to that seen in mcg 6 - 30 - 15 ( see figure 1b for comparison ) . we also find a weak blue wing extending to about 8 kev , which is just visible in figure 2c of yaqoob et al . ( 1995 ) , is now clearly seen , thanks to the greatly improved statistics . the brightness of this source rules out the possibility that the blue wing is caused by improper background subtraction ( only 1@xmath20 of the count rate at 7 . kev is due to the background ) . first , we fit the line with a disk - line model ( fabian et al . 1989 ) plus a narrow core at 6.4 kev which is presumably from the torus and also statistically required ( @xmath21 = -46 ) . though expected in theory , the fluorescence fe k@xmath1 line is always ignored when fitted to data with much lower quality . considering the high quality of the data in this letter , we also include the corresponding fe k@xmath1 components ( disk - line plus narrow core ) in all of our models by amounts of 11.3% of fe k@xmath2 ( george & fabian 1991 ) . in fact , fe k@xmath1 components are also statistically required here ( @xmath21 = - 36 ) . the outer radius ( @xmath22 ) of the disk is fixed at 1000 @xmath23 ( @xmath23 = gm / c@xmath24 ) to minimize the numbers of free parameters because the fits are not sensitive to the value for the outer radius . the best fit value is also consistent with 1000 @xmath23 when it is allowed to vary . considering the strong peak at 6.4 kev , we fixed the disk - line energy at 6.4 kev in the source rest frame . results of this fit are given in table 1 ( model a ) . the inclination of the disk is 27@xmath3 , similar to the value obtained by yaqoob & weaver ( 1997 ) . however , the blue - wing of the line beyond the 6.8 kev has not been well fitted ( see figure 2a ) .    in the accretion disk model context , the high energy blue wing can be produced by an accretion disk with high inclination angle . the blue wing , as well as the edge - on geometry of ngc 4151 inferred from optical and uv observations ( evans et al . 1993 ) , motivated us to add an extra component of a disk line with large inclination to model a ( model b , fe k@xmath1 components also included ) . we assume the inner and outer radii of the disk for these two disk - line components are the same , while the emissivity index q ( f@xmath25 ) is allowed to vary independently considering the fact the line photons from different parts of the disk may have different scattering fraction . the results are also given in table 1 . the model can fit the observed line profile fairly well ( see figure 2b ) . the improvement of model b to model a is significant ( @xmath21 = -19 ) . as expected , the second disk component requires a high inclination of @xmath26 degrees , while the first component requires a face - on disk ( @xmath27 degrees ) . alternatively , the high energy blue wing between 6.8 and 8.0 kev may also be produced by the ni k@xmath2 line emission at 7.48 kev . in order to reduce the excesses in the residuals between 6.8 and 8.0 kev in figure 2a , corresponding ni k@xmath2 components by amounts of 12@xmath28% of fe k@xmath2 have to be added to model a ( see figure 2c , @xmath21 = -13 ) . zdziarski et al . ( 1996 ) argued that the asca data of ngc 4151 obtained in may 1993 can be modeled by complex absorption and a narrow fe k line only , with no broad line required , contrary to the results of yaqoob et al . ( 1995 ) and this letter . however , the asca data used by zdziarski et al have much lower signal - to - noise . through fitting the unprecedented high quality spectra of ngc 4151 obtained by asca in 1995 , we find a prominent broad fe k line profile in ngc 4151 ( see figure 1 ) , which is remarkably similar to that of mcg 6 - 30 - 15 . a model consisting a dual absorbed power law plus a scattered component and a narrow fe k line at 6.4 kev ( @xmath29 = 0.11@xmath30 ) fits the data ( 1.0  10.0 kev ) poorly ( @xmath11 = 706 for 598 degrees of freedom ) and results broad systematic positive residuals around 6.0 kev ( see figure 2d ) . an extra broad line is statistically required ( @xmath21 = -78 ) .    the high signal - to - noise profile of ngc 4151 obtained in this letter shows a strong narrow peak around 6.4 kev and a huge red wing , containing a second slightly weak peak at around 5.2 kev , extending to @xmath0 4.5 kev . in this respect , the line profile of ngc 4151 is remarkably similar to that seen in mcg 6 - 30 - 15 . we also find a weak blue wing extending to about 8.0 kev , which is just visible in figure 2c of yaqoob et al . ( 1995 ) , is now clearly seen ( figure 1a ) , thanks to the greatly improved statistics . there may also be such weak blue wing in the fe k profile of mcg 6 - 30 - 15 ( see figure 1b ) . when fitted by the disk - line model plus a narrow core , a face - on disk is also required ( @xmath6 = 27@xmath3 ) , contrary to the edge - on geometry inferred from optical and uv observations ( evans et al . 1993 , pedlar et al . we consider that the compton mirror of poutanen et al . ( 1996 ) , who proposed that the central source of ngc 4151 is completely hidden from our line of sight and observed x - ray radiation is produced by scattering in the higher , cooler parts of the accretion disk corona , or in a wind , could solve the problem well . we also want to point out that scattering by an `` ionizing cone '' would also explain the similarity of fe k line profiles seen in seyfert 2 galaxies and in seyfert 1 galaxies ( turner et al . 1998 ) , indicating reprocessing by face - on disks and contradicting the expectation of the unification scheme . the variability properties of the fe k line are important to test such scattering hypothesis . besides the broad profile of the fe k line , the blue wing extending to about 8.0 kev ( see figure 1a ) , which could not be well fitted by a simple disk line plus a narrow core even after the corresponding fe k@xmath1 components included , should also be paid much attention to . one possible explanation is that we observe not only a broad fe k line in ngc 4151 , but also a corresponding ni k@xmath2 line by an amount of 12@xmath28% of fe k@xmath2 . this may be the first detection of ni k@xmath2 fluorescence line emission in the x - ray spectrum of agns . however , we notice that the energy of ni k@xmath2 line is just above that of the fe k absorption edge . the line flux , which is sensitive to the ni abundance and x - ray spectral slope , can be substantially reduced due to photo - electron absorption by fe atoms . so , the contribution of ni k@xmath2 emission line might be much weaker than expected . an alternative explanation to the weak blue wing is an extra fe k disk line with high inclination angle ( it is also possible that both the ni k@xmath2 components and the extra disk line are important in reality ) . poutanen et al . ( 1996 ) assumed that the central source is completely hidden from our line of sight by the thick part of the accretion disk wind or by the broad emission - line clouds . however , it is plausible to suppose that the thick material covers the x - ray only partially , and part of the central continuum is seen directly . the fast variability and the complex absorption of the x - ray continuum seem to support this idea . the results of two - disk component model fit ( model b ) is fully consistent with this picture . poutanen et al . ( 1996 ) assumed that the scattering region is situated along the axis of the accretion disk in the form of a cone . the half opening angle of the `` ionized cone '' determined from the optical and uv observation is about 35 degree ( see fig . 10 of evans et al . 1993 ) , the angle - averaged line profile in the scattered light should resemble the one from a face - on disk , which corresponds to the low inclination line component in our two disk - line fit . the direct component should show the same inclination as the system , which is 65@xmath3 inferred from optical and uv observations . we got an inclination of 58@xmath31 degrees for this line component , which is in good agreement with interpreting it as a direct component . comparable ew of the two line components suggests that the flux of the x - ray scattered should be commensurate roughly with the directly observed flux . the little lower parameter @xmath32 of the direct disk - line indicates that more direct fe k photons can be observed from large disk radii . this work is supported by chinese national natural science foundation , pandeng project and foundation of ministry of education . we thank the referee for many critical comments , especially on the possility of ni k@xmath2 contribution , which significantly improve the presentation of this paper . +   + @xmath33errors are given at 90% level . the energy of narrow core is fixed at 6.4 kev in the galaxy rest frame and the line width fixed at zero . model a : a single cold disk line ( 6.4 kev in the galaxy rest frame ) plus a narrow core ; model b : two cold disk line with different inclinations plus a narrow core . fe k@xmath1 components by amounts of 11.3% of fe k@xmath2 are included in both models .
we present an analysis of the fe k line profile of ngc 4151 by using long asca observation data obtained in may 1995 . 
 the unprecedented good data quality , which is much better in the energy band around 6.4 kev than that of the famous 4.2-day asca observation of mcg 6 - 30 - 15 in july 1994 , offers a unique opportunity to study the details of fe k line profile . 
 apart from those characteristics already noticed in earlier asca observations on this object ( yaqoob et al . 
 1995 ) : a broad and skewed profile , with a strong peak at about 6.4 kev and a large red wing extending to @xmath04  5 kev , which is remarkably similar to that of mcg 6 - 30 - 15 , we also find a weak blue wing extending to about 8 kev , thanks to the good quality of the data . 
 when fitted by a relativistic accretion disk line plus a narrow core at 6.4 kev , the data constrain the accretion disk to be nearly face - on , contrary to the edge - on geometry inferred from optical and uv observations . 
 however , the extended blue wing can not be well fitted even after we include corresponding fe k@xmath1 components . 
 ni k@xmath2 line emission by an amount of 12% of fe k@xmath2 is statistically required . 
 an alternative explanation is a model consisting of a narrow core and two disk lines with inclinations of 58@xmath3 and 0@xmath3 , respectively . 
 we suppose that the component with inclination of 58@xmath3 was observed directly , consistent with its edge - on geometry , and the component with inclination of 0@xmath4 was scattered into our line of sight by a compton mirror , which might be the cool accretion disk corona proposed by poutanen et al . 
 ( 1996 ) . 
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