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ngc  4261 is an early - type galaxy in the virgo west cloud , at the distance of 32 mpc ( @xcite ) . it belongs to a group of galaxies ( @xcite ) that does not show prominent sign of gravitational interactions . the h - band luminosity of ngc  4261 is @xmath0 corresponding to a dynamical mass of @xmath1 ( using a mass to light ratio of about 1.2 , from @xcite ) , ranking this galaxy among the most massive galaxies in virgo . the optical spectrum is consistent with a dominant population of old stars ( @xmath2 gyr , @xcite ) and the light distribution is smooth with no evidence of sub - structure ( @xcite ; @xcite ) . ngc  4261 is a fri radio galaxy ( 3c 270 ) showing two radio jets emanating from a nucleus hosting a supermassive black hole with a mass of about 5 @xmath3 10@xmath4 m@xmath5 ( @xcite ) surrounded by a dusty torus and a nuclear disk ( @xcite ; @xcite ) . the x - ray nuclear power from the underlying agn has been well studied by _ xmm _ and _ chandra _ : the x - ray luminosity is l@xmath6 erg s@xmath7 , and the x - ray spectrum is a power law absorbed by a column density n@xmath8 @xmath9 . the source appears to be embedded in diffuse hot gas ( @xmath10 kev ) and shows low amplitude , rapid variability ( @xcite ; @xcite ; @xcite ) . thanks to the high spatial resolution of _ chandra _ , @xcite ( zhfm03 hereafter ) have discovered the presence of about sixty bright off - nuclear x - ray sources in ngc  4261 , which stand out for their large - scale anisotropic pattern . the anisotropy extends over an angle of @xmath11 , corresponding to a linear dimension of @xmath12 kpc ( @xmath13 effective core radii large , since @xmath14 kpc for ngc  4261 ; @xcite ) . zhfm03 interpret this anisotropy as evidence that the x - ray sources are associated with young stellar population , possibly formed in a recent episode of star formation triggered by some dynamical agent along tidal tails . in spite of any optical evidence of either a young population ( the population in the nuclear regions is as old as 15 gyrs ; see @xcite and @xcite ) or of recent merging events , zhfm03 suggest a high mass binary origin for the majority of the sources , based on the significantly higher efficiency of the formation of high mass x - ray binaries relative to the low mass ones . this interpretation makes ngc  4261 a rather unique example , in clear contrast with _ observations of other early - type galaxies , where the x - ray sources are generally distributed nearly isotropically , trace the dominant ( old ) stellar population , and are believed to belong to the low mass x - ray binary population ( lmxb , e.g. @xcite ; @xcite ; @xcite ) . in addition , a significant fraction ( @xmath15 20% -70% ) of these lmxbs is associated to globular clusters ( gcs ; @xcite ; @xcite ; @xcite ; @xcite ; @xcite ) , with a preference to gcs with redder color index , i.e. , clusters that are more metal rich ( @xcite ; @xcite ; @xcite ) . in this perspective , the case of ngc  4261 stands out as a remarkable exception worth of further study . we have therefore chosen to study its case again , in order to deepen our understanding of the the nature of the x - ray sources in external galaxies . for this reason we have used archive observations of _ chandra _ , of the isaac newton telescope ( int ) , and of the hubble space telescope ( _ hst _ ) ( see table 1 for a summary of the dataset used ) to search for optical counterparts of the x - ray sources , and to study their relation with the gc population of ngc  4261 . the outline of the paper is as follows . in @xmath16 we describe the data reduction , in the x - ray and optical bands . in @xmath17 we proceed on studying the properties of the optical point - like sources , and we cross - correlate the optical data with the x - ray data . we then derive the luminosity function of the x - ray source population . in @xmath18 we investigate on the azimuthal anisotropy of the x - ray sources , and on that of the underlying gc candidate population . in the context of our new findings we discuss in @xmath19 the nature of the sources and their potential in tracing the history of assembly of ngc  4261 in the virgo cluster . ngc  4261 was observed by _ chandra _ @xcite with the acis - s3 ccd on 2000 may 6 for a total exposure of 37,210 s ( see table [ tab : obs ] for details ) . the data were collected in 1/2 sub - array mode to avoid the pile - up of the central agn source . the active field of view is shown in figure [ fig1 ] over - plotted on an optical image of the galaxy . the data reduction was performed with the package ` ciao ` v. 3.0.2 and ` caldb ` v. 2.2 . data were cleaned for high background periods due to particle induced flares and the resulting exposure time is of @xmath20 ks . individual sources were detected using a wavelet - detection algorithm ( _ wavdetect _ in ` ciao ` ) , with the same parameters used by zhfm03 , in three energy bands : full band ( @xmath21 kev ) , soft band ( @xmath22 kev ) and hard band ( @xmath23 kev ) . the catalog that we obtained includes 54 sources detected above the probability threshold of 10@xmath24 that implies @xmath25 1 false detection due to statistical fluctuations . the positions of the x - ray sources are shown in figure [ fig2 ] . in table [ tab : sources ] we give the source number ( column 1 ) , the right ascension and declination j2000 ( column 2 and 3 ) . count rates ( column 4 ) are converted into unabsorbed luminosities ( @xmath26 kev ) assuming a power - law model with photon index 1.7 and a galactic line of sight column density of n@xmath27= 5.8 @xmath3 10@xmath28 @xmath9 ( @xcite ) . zhfm03 do not publish a catalog of their x - ray sources , so a comparison of the two source lists is not possible . however , a visual inspection of their figure 1 indicate consistency . data relative to the central region of this galaxy has also been analyzed by @xcite and @xcite , who have mostly focused on the agn emission in the x - ray , radio and optical bands . in particular @xcite detect 5 off - nuclear sources : a , b , c , e coincide with our sources 38 , 33 , 31 , and 14 respectively , and d , which we do not detect ( our source 19 is the closest , at @xmath29 ) . although these authors comment of the fact that they do not find optical counterparts for any of their sources , they do not comment on the possible connection of these sources with the jet , in spite of the evidence that two of them , ( a and e ) are aligned and at opposite ends of it . we have used archive observations of int and _ hst _ wfpc2 for a systematic search of the optical counterparts of the x - ray sources . we extracted a deep @xmath30 band image from the int - wide field camera ( wfc ) archive , consisting of three exposures of 500 seconds each , for a total integration time of 1500 seconds . the image covers the whole galaxy ( the field of view of the int wfc is of about @xmath31 deg@xmath32 ) and has a mean seeing of @xmath33 ( @xcite ) . the reduction of the images was carried out using standard tasks in the ` iraf ` package . bias subtraction and flat - field normalization were applied to each exposure using the median of several bias frames and flat - field exposures obtained during the observations . the three exposures were combined using a median filter , that automatically removes cosmic rays . using reference catalogues of astrometric stars , we obtained a wcs solution for the whole field having a mean rms of @xmath151 arcsec . wfpc2 observations cover the central portion of the galaxy , as shown by the white boxes in figure [ fig1 ] . the south - east ( se ) field was observed using three different filters : f675w , f547 m , f791w while only the f702w filter is available for the north - west ( nw ) field . the exposure times of the four images are given in table [ tab : obs ] . for our analysis we started from pipelined processed data . hst _ wfpc2 and int , we first fitted ellipses to the isophote of the galaxy using the task ` ellipse ` from the iraf package , and subtracted the model from the data . the resulting image was then used to detect sources with sextractor ( @xcite ) . the last isophote has a major and minor axis of @xmath34 and @xmath35 arcmin respectively . we improve the relative hst astrometry from @xmath36 to @xmath37 arcsec comparing the overlap region between the nw and se field . on the contrary we did not try to make any improvement in the relative astrometry between chandra and int / hst images . we note that , for both hst and int images , the model subtraction leaves a quadrupole structure in the central region . although this region is smaller in the hst data , we exclude a wider area of @xmath150.5 square arcmin , due to the presence of residuals in the int data that could induce spurious results in our comparisons . this structure was already noticed by @xcite for this object , and was attributed to the boxyness of the galaxy . using sextractor we extracted point - like objects from the model subtracted images of both int and _ hst _ datasets . we set a sextractor detection threshold of 2.5 @xmath38 over the local background . we select as point - like sources objects with the following properties : ( i ) a magnitude error smaller than 0.1 , ( ii ) ellipticity smaller than 0.5 , ( iii ) fwhm between 1 and 4 pixels ( 0.1 - 0.4 arcsec ) for _ hst _ , as suggested by @xcite , and fwhm between 3 and 10 pixels ( 1 - 3 arcsec ) for int . note that for the _ hst_-se field we based our selection upon the f675w filter owing to its longer exposure . as a final step , we have checked the sextractor results by visual inspection , and we have removed objects on or close to the ccds edges for the hst images . for the se field of the _ hst _ observation , we have been able to estimate the color of the sources presented in @xmath39 . the instrumental magnitudes , provided by sextractor , were converted in the johnson - cousin v , r and i systems using the synthetic transformations given in @xcite . in order to estimate the photometric error due to model subtraction we measured the photometry on the unsubtracted image using local background and apertures from the subtracted image . the difference in the v - i color of each globular cluster obtained from the two different estimates is shown in figure [ fig3 ] . the distribution appears fairly gaussian with a mean difference of @xmath40 mag ( see figure [ fig3 ] ) . the total photometric uncertainty is thus given by the combination of the error on the measure of gcs magnitudes ( given by sextractor ) , the error on the zero - point ( taken from @xcite ) and the uncertainty due to the model subtraction ( 0.07 mag , fig.[fig3 ] ) . this results in a mean error on the v - i color of @xmath15 0.11 mag . we used f547 m and f791w filters to determine the color index ( v - i ) of the extracted objects . we select 325 objects detected in each of the three filters with v - i color between 0 and 2 ( @xcite ) , that we consider gc . their color index and magnitude distributions shown in figure [ fig4 ] are consistent with those of the globular cluster populations observed in other massive early - type galaxies ( @xcite ; @xcite ; @xcite ; @xcite ; @xcite ) , reinforcing our definition . the color index distribution spreads from v - i @xmath41 to @xmath42 , indicating that both populations of gc ( red and blue ) are present . thus the unimodal distribution observed in the v - i color histogram is probably due to the blending of different gcs populations . since the color information is restricted to the _ hst_-se field , we can not classify as gcs the sources extracted from the other optical images at the same confidence . however , we have first considered sources in the region common to all three datasets ( @xmath43 arcmin@xmath32 ) . the cross - correlation between the three lists indicates that @xmath44% ( 37/38 ) and @xmath45% ( 49/59 ) of the sources in the _ hst_-nw and int field respectively coincide with a gc in the _ hst_-se field . given this high fraction , we are confident that even the point - like sources from the int dataset are good `` gc candidates '' , in spite of the lower resolution of the instrument . in what follows , we will use the `` gc candidate '' list ( from the int observation ) , that provides an homogeneous set of optical sources over the whole galaxy .    as a final check , we have also compared the sky projected distribution of the int and _ hst _ gc candidates . since _ hst_-se and _ hst_-nw fields have different exposure times , we restrict the _ hst _ sample to a magnitude @xmath46 = @xmath47 ( i.e. the magnitude limit of the shallowest field ) . the comparison is done through an adaptive kernel density analysis . this technique is a two - step procedure which first applies a pilot smoothing to estimate the local density of gcs , starting from the list of their positions only , and then uses a smoothing window variable in size that decreases with increasing local density . in this way the statistical noise in low - density regions can be reduced without smearing out the high density peaks @xcite . as shown in figure [ fig5 ] , the iso - density contours from the adaptive kernel analysis are in very good agreement , confirming the reliability of the int gcs dataset . what is apparent in the figure is the peculiar distribution of the gc population , that shows two main concentrations ne and sw of the nuclear region . the presence of the same structures in both datasets ensures that it is not a feature derived from the poor subtraction in the central region ( the black area in figure [ fig5 ] ) since the equivalent area in the hst data is negligible .      in order to investigate the nature of the x - ray sources we have looked for optical counterparts . the central source ( n. 29 ) is identified with the agn ( @xcite ; @xcite ; @xcite ) , and sources no . 27 , 43 and 53 with background / foreground galaxies . the positions of the remaining x - ray sources have been cross - correlated with the gc sample , from the hst fields , with a matching radius of @xmath31 arcsec , and from the int dataset , with a matching radius of @xmath48 arcsec . these choices take into account only the intrinsic astrometric uncertainties of the three datasets ( see sections 2.1 and 2.2 ) and not the relative uncertainties eventually present between the optical and x - ray observations . source 54 falls outside the model subtracted area , while sources no . 19 , 31 and 33 fall in the central region excluded from our previous analysis . we found an optical counterpart for 23 of the 46 discrete x - ray sources that we assume all belonging to ngc  4261 . the identification is indicated in table [ tab : sources ] . to calculate the chance coincidence probability we performed 1000 monte - carlo simulations by shifting the positions of the int and hst optical samples by a random offset and performing again the cross - correlation . we adopted a shift larger than the search radius but small enough so the sources do not fall outside the distribution of the x - ray sources ( @xmath151.5 arcmin ) . the probability of finding more than one spurious association is @xmath152.5% and @xmath150.1% in the int and hst sample , respectively . in figure [ fig6 ] we give a few examples that show the positions of the x - ray sources over - plotted onto the _ hst _ se , _ hst _ nw and int fields . the figure also demonstrates the good correspondence between the _ hst _ and int counterparts , indicating again that the identifications obtained in the int field not covered by _ hst _ are not severely hampered by the lower resolution of the int image . note that , where the color information is available , we found that all the x - ray sources inhabit a red ( color index v - i @xmath49 ) gc , see figure [ fig4 ] . the optical identification with gc candidates for @xmath50% of the x - ray sources suggests a lmxb origin for them ( @xcite ) . we verified the above statement by comparing the cumulative x - ray luminosity function ( xlf ) of our sources with those derived from observations of other galaxies ( figure [ fig7 ] ) . in particular , we use the functional forms derived by @xcite and @xcite considered by these authors as a signature of a universal behavior for the hmxb and the lmxb populations , respectively ; namely a single power - law for hmxb , with slope of @xmath51 , and a broken power - law for lmxb , with slopes of @xmath52 and @xmath53 for the high luminosity end . in testing the consistency with these universal functions , we have derived the xlf using all sources in table [ tab : sources ] , except for the agn ( source no . 29 ) and the 3 interlopers ( sources no . 27 , 43 and 53 ) . we performed a log - log maximum likelihood fitting procedure for both hmxb and lmxb functional forms assuming fixed slopes . we considered as free parameter the normalization factors and , in the case of a broken power law , the luminosity of the break . the resulting xlfs are shown in figure [ fig7 ] . it is unquestionable that a single power - law can not reproduce the lf of the sources in ngc  4261 . it is also apparent that the lmxb broken - power law is a good representation of the data given also the value of the break luminosity , that we have left as a free parameter in the fitting procedure , and that is consistent with the gilfanov value of @xmath54 . this result is consistent with the hypothesis that the x - ray sources in this galaxy are associated to a population of lmxbs . note that we do not take into account incompleteness at low luminosities @xcite ; however , we point out that at least at the high luminosity end ( where incompleteness should not significantly affect the xlf slope ) the luminosity function is much steeper than the xlf of the hmxbs . zhfm03 have shown with a kolmogorov - smirnov test that the spatial distribution of the x - ray sources in ngc  4261 is anisotropic . using our list of 50 sources ( sources n. 29 , 27 , 43 and 53 excluded ) we confirm the spatial anisotropy over a scale of about @xmath55 with very high confidence ( p@xmath56% from the kolmogorov - smirnov test ) . in the preceding sections we have shown that the properties of the x - ray sources in ngc  4261 are consistent with a population of lmxbs . this however does not explain yet their spatial anisotropy , which is in stark contrast with the smooth isophotal distribution of the optical light . given the close association with the gc population , which we find also has a peculiar distribution , we have considered the possibility that the anisotropy is nothing but a manifestation of an anisotropy seeded in the gc population . in order to test this hypothesis we have first done a statistical analysis on the spatial distribution of the gc candidates .    in order to investigate both the details of the spatial distributions of the x - ray and optical sources separately and their relation we have compared the normalized surface density of sources again with an adaptive kernel density analysis on the two sets of sources . in figure [ fig8 ] we show the iso - intensity contours extracted from the kernel analysis on the gc candidate sample ( thin line ) and on the x - ray sources ( thick line ) . with this figure as a guide , we have defined 6 regions ( see figure [ fig9 ] , right panel ) in which we have simply computed the surface number density of sources for both gcs and x - ray sources , which plot in figure [ fig9 ] ( left panel ) as a function of the region number . the errors associated with each entry reflects the total number of sources in the region , and gives a direct measure of the significance of the observed region - to - region variations , we also test the correlation between the two datasets using the iraf task ` crosscor ` and we find that the highest correlation between the two datasets is consistent with a null shift . our combined analysis of the x - ray and optical material available for ngc  4261 shows that the large majority of the x - ray sources in ngc  4261 are accreting lmxbs . about 50% of these sources have an optical counterpart , that we identify with a gc . where color information is available , the sources are associated with the red ( metal - rich ) population , as already observed in several other galaxies . further evidence of a lmxb origin comes from the interpretation of the luminosity function , consistent with the `` universal '' distribution proposed by @xcite . the photometric analysis of the gc population of ngc  4261 carried on a portion of the galaxy ( _ hst_-se field ) shows that the properties of the gcs are similar to those observed in other early - type galaxies ( @xcite ; @xcite ; @xcite ) . furthermore this similarity extends to the connection between x - ray sources and gc population ( @xcite ) . in particular we note that the x - ray sources are preferentially associated with the brightest and reddest gcs . as in other ellipticals , we find that the gcs hosting a x - ray source are @xmath157% and that the x - ray sources associated with gcs are @xmath1550% , consistent with the result of @xcite . it is therefore not surprising that ngc  4261 ranks among those galaxies having a sizable fraction of x - ray sources in gcs . the peculiarity of ngc  4261 resides mainly in the large scale anisotropy of the x - ray sources . our adaptive kernel density analysis has highlighted the presence of a peculiar spatial distribution in the gc candidate population , and the existence of a correlation between the spatial anisotropy of the x - ray sources and the main over - densities seeded in the gc system . to our knowledge , this is the first time that x - ray sources indicate the presence of substructures in the distribution of gc . a non uniform spatial distribution of the gc system of ngc  4261 may reflect a peculiar history of formation of this galaxy . in the paradigm of hierarchical clustering cosmogony , elliptical galaxies may be the end result of binary or multiple major mergers among gas - rich progenitors . major mergers likely redistribute the original gc systems of the progenitor galaxies and trigger the formation of new gcs in the center as well as along the tidal tails of the new object arising from the merger . this would result in the coexistence of two populations , metal rich and metal poor clusters , that have different photometric properties , spatial distribution and dynamical characteristics , as shown in a recent work by @xcite . hierarchical build - up also implies a second way by which galaxy can grow , namely accretion of dwarf satellites that would lose their gcs to the primary system . gcs coming from shredded satellites could also contribute to spatial anisotropies and would be mostly metal - poor if such satellites are similar to nearby dwarfs . evidence that either of these two mechanisms can account for a large fraction of the gc systems in ellipticals is still lacking ( @xcite ) . a close inspection of the photometric and kinematical properties of the gcs can in principle help disentangle many aspects of such complex galaxy genesis . ngc  4261 could be an excellent target for further investigations : combined chandra and hst observations can thus potentially shed light on how early - type galaxies were assembled . in parallel , we are undertaking a theoretical study running high - resolution simulations of galaxy mergers similar to those carried on in @xcite , including a gc formation algorithm . we thank the anonymous referee for useful comments that helped us to improve the paper . we thank marco scodeggio for providing us the implementation of the algorithm used for the adaptive kernel density analysis , and michele bellazzini for sharp comments and suggestions . this work was supported by the italian ministry of university and research ( miur ) under the national program prin 2003 .
_ chandra _ images of the elliptical galaxy ngc  4261 have revealed an anisotropy in the spatial distribution of the off - nuclear x - ray sources , interpreted by zezas et al . 
 as evidence of an association with a young stellar population . 
 our independent analysis of archive x - ray ( _ chandra _ ) and optical ( int and _ hst _ ) observations confirms the anisotropy of the x - ray sources but conducts to a different interpretation for their origin . 
 we find that nearly half of the x - ray sources are associated to a globular cluster ( optical counterpart ) suggesting that they are accreting low - mass x - ray binaries . 
 where color index information is available , the x - ray sources are found to reside in red ( metal - rich ) systems . 
 the luminosity function of the x - ray sources is also consistent with the one drawn from a population of low mass x - ray binaries . 
 we further investigate the properties of the sample of point - like sources obtained from archival optical images that we suggest are good globular cluster candidates and for which we find that the projected spatial distribution is non - homogeneous . 
 in addition , we show that the distributions of the optical and x - ray populations are very similar , which leads us to conclude that the spatial anisotropy of the x - ray sources in ngc  4261 is mostly a reflection of the anisotropy of the globular cluster population .