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agn have first been discovered in the radio and soon after searched in the optical band . consequently , they have been classified using their optical characteristics and mainly divided into two categories : type 1 ( agn1 ) and 2 ( agn2 ) according to the presence or not of broad emission lines in their optical spectra ( we will keep this definition of agn1 throughout this paper ) .    before the advent of the last generation of hard x - ray telescopes , agn samples where predominantly based on agn1 selected either in the optical or , later on , in the soft x - rays by _ einstein _ and _ rosat_. in these bands the evolution of agn1 has been well measured ( see e.g. della ceca et al . 1992 ; boyle et al . 2000 ; miyaji , hasinger , & schmidt 2000 ) . on the contrary the production of samples of agn2 has been difficult at any wavelength and limited to few local surveys . the general picture was in favor of a model in which agn1 objects were associated to agn with low absorption in the hard x - rays while agn2 to obscured sources with large column densities and spectra strongly depressed in the soft x - rays , as expected in the unification models ( e.g. antonucci 1993 ) .    in the last decade the advent of the _ asca _ and _ bepposax _ satellites has allowed for the first time the detection and identification of agn as the main counterparts of hard ( 2 - 10 kev ) x - ray sources down to fluxes @xmath14 erg @xmath3 s@xmath4 , more than 2 orders of magnitude fainter than _ heao1 _ ( wood et al . 1984 ) . these identifications accounted for about 30% of the 2 - 10 kev hard x - ray background ( ueda et al . 1998 ; fiore et al . recently the new generation of x - ray satellites such as _ chandra _ and _ xmm - newton _ , have reached fluxes 100 times fainter , identifying hundreds of sources and almost resolving the hard ( 2 - 10 kev ) x - ray background ( e.g. mushotzky et al . 2000 ; fiore et al . 2000 ; giacconi et al . 2001 ; hornschemeier et al . 2001 ; hasinger et al . 2001 ; tozzi et al . 2001 ; baldi et al . 2001 ) . thanks to their excellent angular resolution ( @xmath11 - 5@xmath15 ) , the first spectroscopic identifications projects have been able to observe faint ( i@xmath123 ) optical counterparts . at variance with the classical type-1/type-2 model in the optical , a significant number of the counterparts ( @xmath130% ) resulted to be apparently optical normal galaxies , with x - ray luminosities @xmath16@xmath10@xmath17 erg s@xmath4 typical of agn activity , and moreover part of the optical type 1 agns resulted to be absorbed in the hard x - rays ( see e.g. fiore et al . 2000 ; barger et al . 2001 ; tozzi et al . 2001 ; hornschemeier et al . 2001 ; comastri et al . 2002 ) . these observations have complicated the picture of the agn model . in this framework the computation of the density of agn has become an even more difficult task . in fact , it is not clear how to classify the sources and to take into account the selection biases introduced by the observation in the 2 - 10 kev range , where the absorption still play a relevant role .    these recent deep surveys with _ chandra _ and _ xmm - newton _ have reached fluxes @xmath1 @xmath18 erg @xmath3 s@xmath4(2 - 10 kev ) in quite small areas ( less than 1 deg@xmath19 ) . as a consequence these surveys are not able to provide statistical significant samples at brighter fluxes ( @xmath20 erg @xmath3 s@xmath4 ; 5 - 10 kev ) where the density of sources is about 5/deg@xmath19 ( fiore et al . 2001 ) and tens of square degrees are to be covered . such data are necessary to provide large numbers of spectroscopic identified sources in a wide range of x - ray fluxes in order to cover as much as possible the @xmath21 plane and hence to derive their x - ray luminosity function ( lf ) . in this paper we report the results of the spectroscopic identifications of one of such brighter samples . the x - ray sources have been detected by the _ bepposax_-mecs instruments in the 5 - 10 kev band in the framework of the high energy llarge area survey ( hellas ) . preliminary results have been presented in fiore et al . ( 1999 ) and la franca et al . the whole survey and the catalogue is described by fiore et al . the data have been analyzed in the framework of the synthesis models for the x - ray background by comastri et al . ( 2001 ) , and the correlation with the soft x - rays has been investigated by vignali et al . ( 2001 ) .    in section 2 we describe our x - ray and optical observations . in section 3 we present an analysis of the evolution of agn in the 2 - 10 kev band . because of the reasons previously described , the selection and definition of type 2/absorbed sources is still not clear , and thus we restricted our evolutionary studies to type 1 agn only . the results are discussed in section 4 . the spectroscopic follow up of the hellas sources has been carried out in a subsample enclosed in a region with @xmath22 , and outside @xmath23 and @xmath24 . in this region the number of sources is 118 out of a total of 147 . their flux distribution is shown in figure 1 and the sky coverage is shown in figure 2 and listed in table 1 . the _ bepposax _ x - ray positions have an uncertainty of about 1 - 1.5 arcmin , the larger at larger off - axis distances . we have thus searched for optical counterparts having r magnitude brighter than 21.0 in a circular region of 1 - 1.5 arcmin of radius around the hellas positions ( see below and section 3.1.1 for a discussion on the choice of this optical limit ) . in the case of large off - axis distances , the larger error - boxes ( 1.5@xmath25 ) have been used . 25 sources have been identified with cross - correlation with existing catalogues ( ned ) , and 49 have been investigated at the telescope . the total resulting sample of 74 sources has been built up in such a way that to a ) randomly sample the flux distribution of the parent catalogue of 118 sources ( see figure 1 ) , and b ) to avoid possible biases introduced by the cross - correlation with existing catalogues ( see appendix a for a detailed discussion ) . cl    = 0.2 cm 1998 dec 28 : 3 nights & eso/3.6 m efosc2 1999 jun 8 : 3 nights & eso/3.6 m efosc2 2000 jan 3 : 3 nights & eso/3.6 m efosc2 2000 jul29 : 3 nights & eso/3.6 m efosc2 [ tab2 ]    the follow - up observations have been carried out using the efosc2 at the 3.6 m eso telescope . previously published identifications ( fiore et al . 1999 ) have been carried out with the rc spectrograph ( rcsp ) at the kitt peak 4 m telescope , the fast spectrograph at the whipple 60@xmath15 telescope , and the hawaii 88@xmath15 telescope . long slit and multi object spectroscopy have been carried with resolution between 7 and 16 . the reduction process used standard midas facilities ( banse et al . the raw data were sky - subtracted and corrected for pixel - to - pixel variations by division with a suitably normalized exposure of the spectrum of an incandescent source ( flat - field ) . wavelength calibrations were carried out by comparison with exposures of he - ar , he , ar and ne lamps . relative flux calibration was carried out by observations of spectrophotometric standard stars ( oke 1990 ) . the campaign dates of the observations carried out at eso are listed in table 2 . we have divided the identified sources in agn1 , agn2 , galaxies with narrow emission lines showing moderate - to - high degree of excitation ( elg ) , bl lac objects , clusters of galaxies and stars . agn2 includes agn1.5 , agn1.8 and agn1.9 . no distinction has been made between radio - loud and radio quite objects . the radio properties of the hellas sources will be discussed in a forthcoming paper ( p. ciliegi , in preparation ) . the r - band magnitudes of these identifications have been pushed down to the limit of obtaining about less than 10@xmath26 probability of having a spurious identification , due to chance coincidences , for each of the hellas sources ( i.e. 90@xmath26 reliability for the whole sample ) . for agn1 we have chosen a limit of r=21 where the surface density is @xmath160 - 70/deg@xmath19 ( zitelli et al . tresse et al . ( 1996 ) found that 17@xmath26 of galaxies with @xmath27 host an emission line spectrum not typical of excitation from starburst ( but typical of agn2 activity ) , this percentage reduces to at minimum 8% if the maximum likely effect of absorption under the balmer lines is taken into account . they also found that the galaxies having forbidden emission lines without being agn2 are at least 36@xmath26 . these objects are mainly starburst galaxies and low ionization narrow emission line galaxies ( liner ) . we will call here all these sources simply emission line galaxies ( elg ) . we have assumed roughly constant these percentages and thus chosen a limit of r=19 for agn2 where the surface density of all galaxies is about 500/deg@xmath19 and consequently the expected density of agn2 is @xmath170 - 80/deg@xmath19 . for elg we have chosen a limit of r=17.5 where the surface density of all galaxies is about 160/deg@xmath19 , and therefore the expected density of elg is @xmath160/deg@xmath19 . the classification of the narrow emission line galaxies has been carried out using standard line ratio diagnostics ( tresse et al . some of the spectra showed h@xmath28 in emission and h@xmath29 having a ) very small emission , or b ) absent , or c ) small absorption . such spectra show however spectral features typical of strong - emission - line spectra such as [ oii]@xmath30 , [ oiii]@xmath31 , and [ sii]@xmath32 . in such cases the [ oiii]/h@xmath29 push the diagnostic ratio in the locus of agn2 , but stellar absorption at h@xmath29 could be significant affecting the classification . following tresse et al . ( 1996 ) , we have tested the classification under the assumption of a possible absorption of ew=2  over h@xmath29 . we have spectroscopically identified 36 out of the 49 hellas sources whose field has been investigated at the telescopes . for 13 sources we have found no optical counterparts brighter than the chosen magnitude limits ( see discussion above ) . the astrometric accuracy of the new generation x - ray telescopes ( _ chandra _ and _ xmm - newton _ ) would probably allow an unambiguous identification of the counterparts of these hellas sources , but in the framework of this work these fields have been declared _ empty_. these sources have harder x - ray spectra than the total sample . they have an average softness ratio of @xmath33 in comparison with the average value @xmath34 of the total sample of 118 sources , where s and h are the 1.3 - 4.5 kev the 4.5 - 10 kev counts , respectively ( see fiore et al . the absence of bright counterparts and the average harder x - ray spectrum favor the hypothesis that most of these sources preferably harbor absorbed agn . if these sources correspond to optical normal  galaxies , such as those observed by _ chandra _ and _ xmm - newton _ ( see e.g. fiore et al . 2000 ; barger et al . 2001 ; tozzi et al . 2001 ; hornschemeier et al . 2001 ; comastri et al . 2002 ) , we would have not been able to properly identify them inside our error - boxes . however , our sample is statistically well defined , and from the number of empty fields we can directly derive an upper limit of @xmath118% ( 13/74 ) for the presence of these optical normal galaxies in our survey .    the list of the observed hellas sources with their most probable spectroscopic identification is shown in table 3 . 80% of our identifications are inside an error - box of 60 arcsecs . 2 bright galaxies ( r@xmath115 - 16 ) are at a distance of about 110@xmath15 . the 13 empty fields are listed in table 4 . the optical spectra are shown in figure 3 ; as a reference , a list of the most typical emission lines for agn are over - plotted with the corresponding redshift . in table 5 the ew , fwhm , fwzi in the observed frame of the most relevant emission lines are listed .    in total , 63@xmath26 ( 74/118 ) of our hellas subsample has been searched for spectroscopic identification . 61 have been identified : 37 agn1 , 9 agn2 , 5 elg , 6 clusters , 2 bl lac , 1 radio galaxy and 1 star . ccrrrllll    = 0.2 cm 00 26 36.5 @xmath3519 44 14 & 00 26 36.1 @xmath3519 44 16 & 7 & 13 & 3.4 & 18.1 & agn2 & 0.238 + 00 27 09.9 @xmath3519 26 31 & 00 27 09.8 @xmath3519 26 14 & 18 & 6 & 1.8 & 17.7 & agn1 & 0.227@xmath36 + 00 45 49.6 @xmath3525 15 14 & 00 45 46.3 @xmath3525 15 50 & 59 & 24 & 3.3 & 17.5 & agn2 & 0.111 + 01 21 56.8 @xmath3558 44 05 & 01 21 56.9 @xmath3558 44 42 & 37 & 16 & 2.6 & 16.8 & agn2 & 0.118 + 01 34 33.3 @xmath3529 58 38 & 01 34 33.8 @xmath3529 58 16 & 24 & 6 & 0.9 & 18.0 & agn1 & 2.217 + 01 35 30.2 @xmath3529 51 22 & 01 35 32.7 @xmath3529 52 02 & 53 & 8 & 0.9 & 17.7 & agn1 & 1.344 + 01 40 08.9 @xmath3567 48 13 & 01 40 14.7 @xmath3567 48 54 & 53 & 8 & 2.8 & 12.4 & star & 0.000 + 02 42 01.8 @xmath3700 00 46 & 02 42 00.9 @xmath3700 00 22 & 29 & 10 & 1.5 & 18.6 & agn1 & 1.112@xmath38 + 03 15 45.0 @xmath3555 29 26 & 03 15 47.5 @xmath3555 29 04 & 31 & 15 & 2.7 & 17.9 & agn1 & 0.464@xmath38 + 03 17 32.4 @xmath3555 20 12 & 03 17 32.7 @xmath3555 20 26 & 14 & 21 & 4.1 & 17.5 & agn1 & 0.406@xmath38 + 03 33 09.6 @xmath3536 19 40 & 03 33 12.2 @xmath3536 19 48 & 33 & 15 & 4.0 & 17.5 & blac & 0.308@xmath38 + 03 34 07.4 @xmath3536 04 22 & 03 34 07.5 @xmath3536 04 04 & 19 & 5 & 1.9 & 20.1 & agn1 & 0.904 + 03 36 51.3 @xmath3536 15 57 & 03 36 54.0 @xmath3536 16 06 & 34 & 15 & 3.7 & 17.7 & agn1 & 1.537@xmath38 + 04 37 14.5 @xmath3547 30 58 & 04 37 11.8 @xmath3547 31 48 & 57 & 16 & 2.7 & 17.3 & agn1 & 0.142 + 04 38 47.9 @xmath3547 29 07 & 04 38 47.0 @xmath3547 28 02 & 67 & 20 & 4.7 & 20.5 & agn1 & 1.453 + 06 46 39.3 @xmath3544 15 35 & 06 46 37.6 @xmath3544 15 34 & 19 & 17 & 4.3 & 16.6 & agn1 & 0.153 + 07 21 29.6 @xmath3771 14 04 & 07 21 36.2 @xmath3771 13 24 & 52 & 8 & 0.8 & 17.7 & agn1 & 0.232@xmath38 + 07 41 40.3 @xmath3774 14 58 & 07 41 50.0 @xmath3774 14 48 & 41 & 23 & 30.7 & .... & clust . & 0.216@xmath38 + 07 43 09.1 @xmath3774 29 20 & 07 43 12.6 @xmath3774 29 36 & 22 & 7 & 6.0 & 16.4 & agn1 & 0.312@xmath38 + 08 37 37.2 @xmath3725 47 49 & 08 37 37.1 @xmath3725 47 52 & 4 & 12 & 2.6 & 16.9 & agn1 & 0.077 + 08 38 59.9 @xmath3726 08 14 & 08 38 59.2 @xmath3726 08 14 & 10 & 23 & 16.4 & 15.3 & elg & 0.048 + 10 32 15.8 @xmath3750 51 04 & 10 32 16.2 @xmath3750 51 20 & 18 & 10 & 3.1 & 15.9 & agn1 & 0.174 + 10 54 19.8 @xmath3757 25 09 & 10 54 21.2 @xmath3757 25 44 & 38 & 13 & 2.6 & 18.5 & agn2 & 0.205@xmath38 + 11 01 46.4 @xmath3772 26 11 & 11 01 48.8 @xmath3772 25 38 & 36 & 22 & 7.3 & 16.7 & agn1 & 1.460@xmath38 + 11 02 37.2 @xmath3772 46 38 & 11 02 36.8 @xmath3772 46 40 & 3 & 21 & 7.9 & 15.1 & agn1 & 0.089@xmath38 + 11 06 14.0 @xmath3772 43 16 & 11 06 16.1 @xmath3772 44 14 & 59 & 9 & 1.7 & 18.5 & agn1 & 0.680@xmath38 + 11 18 11.9 @xmath3740 28 33 & 11 18 13.8 @xmath3740 28 38 & 23 & 4 & 0.8 & 18.7 & agn1 & 0.387 + 11 18 46.2 @xmath3740 27 39 & 11 18 48.7 @xmath3740 26 48 & 59 & 5 & 1.4 & 18.5 & agn1 & 1.129 + 12 04 07.6 @xmath3728 08 31 & 12 04 04.0 @xmath3728 07 24 & 82 & 16 & 5.1 & .... & clust . & 0.167@xmath38 + 12 17 50.3 @xmath3730 07 08 & 12 17 52.1 @xmath3730 07 02 & 25 & 20 & 3.5 & 14.0 & bllac & 0.237@xmath38 + 12 18 55.0 @xmath3729 58 12 & 12 18 52.5 @xmath3729 59 02 & 59 & 13 & 2.0 & 18.6 & agn2 & 0.176 + 12 19 45.7 @xmath3747 20 43 & 12 19 52.3 @xmath3747 20 58 & 69 & 8 & 1.2 & 19.3 & agn1 & 0.654@xmath38 + 12 22 06.8 @xmath3775 26 17 & 12 22 07.0 @xmath3775 26 18 & 2 & 7 & 2.5 & .... & clust . & 0.240@xmath38 + 12 40 26.0 @xmath3505 13 20 & 12 40 27.8 @xmath3505 14 02 & 50 & 12 & 3.1 & 18.8 & agn1 & 0.300 + 12 40 29.6 @xmath3505 07 46 & 12 40 36.4 @xmath3505 07 52 & 102 & 17 & 1.9 & 15.2 & elg & 0.008 + 13 04 38.2 @xmath3510 15 47 & 13 04 35.6 @xmath3510 15 48 & 39 & 6 & 1.4 & 20.1 & agn1 & 2.386 + 13 05 32.3 @xmath3510 32 36 & 13 05 33.0 @xmath3510 33 20 & 45 & 22 & 19.3 & 14.9 & agn1 & 0.278@xmath38 + 13 36 34.3 @xmath3533 57 48 & 13 36 39.0 @xmath3533 57 58 & 60 & 22 & 3.2 & 10.5 & radiog . & 0.013@xmath38 + 13 42 59.3 @xmath3700 01 38 & 13 42 56.5 @xmath3700 00 58 & 59 & 21 & 3.2 & 18.7 & agn1 & 0.804@xmath38 + 13 48 20.8 @xmath3530 11 06 & 13 48 19.5 @xmath3530 11 56 & 52 & 14 & 2.2 & 15.3 & agn2 & 0.128 + 13 48 45.4 @xmath3530 29 37 & 13 48 44.7 @xmath3530 29 46 & 13 & 14 & 5.1 & 17.1 & agn1 & 0.330 + 13 50 09.4 @xmath3530 19 55 & 13 50 15.4 @xmath3530 20 10 & 80 & 11 & 5.1 & 16.5 & elg & 0.074 + 13 53 54.6 @xmath3718 20 33 & 13 53 54.4 @xmath3718 20 16 & 18 & 18 & 6.8 & 17.3 & agn1 & 0.217 + 14 17 12.5 @xmath3724 59 29 & 14 17 18.8 @xmath3724 59 30 & 86 & 13 & 0.7 & 19.5 & agn1 & 1.057@xmath38 + 14 18 31.1 @xmath3725 11 07 & 14 18 31.2 @xmath3725 10 50 & 17 & 9 & 6.1 & .... & clust . & 0.240@xmath38 + 15 19 39.9 @xmath3765 35 46 & 15 19 33.7 @xmath3765 35 58 & 41 & 14 & 9.4 & 14.4 & agn2 & 0.044 + 15 28 47.3 @xmath3719 39 10 & 15 28 47.7 @xmath3719 38 54 & 18 & 5 & 1.6 & 20.3 & agn1 & 0.657@xmath39 + 16 26 59.9 @xmath3755 28 21 & 16 26 59.0 @xmath3755 27 24 & 57 & 11 & 12.1 & .... & clust . & 0.130@xmath38 + 16 34 11.8 @xmath3759 45 29 & 16 34 18.5 @xmath3759 45 44 & 53 & 3 & 0.8 & 19.0 & agn2 & 0.341@xmath40 + 16 49 57.9 @xmath3704 53 33 & 16 50 00.0 @xmath3704 54 00 & 42 & 20 & 9.5 & .... & clust . & 0.154@xmath38 + 16 50 40.1 @xmath3704 37 17 & 16 50 42.7 @xmath3704 36 18 & 71 & 25 & 12.2 & 14.6 & agn2 & 0.031 + 16 52 38.0 @xmath3702 22 18 & 16 52 37.5 @xmath3702 22 06 & 14 & 5 & 0.7 & 20.7 & agn1 & 0.395 + 20 42 47.6 @xmath3510 38 31 & 20 42 53.0 @xmath3510 38 26 & 80 & 21 & 5.3 & 17.9 & agn1 & 0.363 + 20 44 34.8 @xmath3510 27 34 & 20 44 34.8 @xmath3510 28 08 & 35 & 15 & 2.0 & 17.7 & agn1 & 2.755 + 22 26 30.3 @xmath3721 11 57 & 22 26 31.5 @xmath3721 11 38 & 26 & 14 & 3.9 & 17.6 & agn1 & 0.260 + 23 15 36.4 @xmath3559 03 40 & 23 15 46.8 @xmath3559 03 14 & 85 & 8 & 2.0 & 11.2 & elg & 0.044@xmath38 + 23 19 22.1 @xmath3542 41 50 & 23 19 32.0 @xmath3542 42 28 & 116 & 21 & 5.7 & 16.5 & elg@xmath41 & 0.101 + 23 27 28.7 @xmath3708 49 30 & 23 27 28.7 @xmath3708 49 26 & 4 & 7 & 0.5 & 18.5 & agn1@xmath42 & 0.154 + 23 29 02.4 @xmath3708 34 39 & 23 29 05.8 @xmath3708 34 16 & 56 & 20 & 2.9 & 20.3 & agn1 & 0.953 + 23 31 55.6 @xmath3719 38 34 & 23 31 54.3 @xmath3719 38 36 & 19 & 17 & 3.7 & 18.8 & agn1 & 0.475 + 23 55 53.3 @xmath3728 36 06 & 23 55 54.3 @xmath3728 35 58 & 16 & 12 & 4.2 & 17.9 & agn1 & 0.731@xmath38 + [ tab3 ]    cc    = 0.2 cm 01 34 28.6 @xmath3530 06 04 & 1.33 + 01 34 49.6 @xmath3530 02 34 & 0.71 + 09 46 05.3 @xmath3514 02 59 & 2.82 + 09 46 32.8 @xmath3514 06 16 & 3.22 + 13 04 24.3 @xmath3510 23 53 & 1.28 + 13 04 45.1 @xmath3505 33 37 & 1.26 + 13 48 24.3 @xmath3530 25 47 & 3.15 + 14 11 58.7 @xmath3503 07 02 & 3.93 + 22 03 00.5 @xmath3532 04 18 & 2.83 + 22 31 49.6 @xmath3711 32 08 & 1.94 + 23 02 30.1 @xmath3708 37 06 & 2.67 + 23 02 36.2 @xmath3708 56 42 & 3.17 + 23 16 09.8 @xmath3559 11 24 & 1.32 + [ tab4 ] because of the difficulties previously described in quantifying all the selection biases in building up samples of type 2 ( and/or absorbed ) agn , we have decided to limit our evolutionary analysis to agn1 only . see appendix a for a discussion on the absence of substantial biases on our sample of agn1 . we have assumed h@xmath43=50 km / s / mpc and either the ( @xmath12,@xmath13)=(1.0,0.0 ) or the ( @xmath12,@xmath13)=(0.3,0.7 ) cosmologies . we have computed the 2 - 10 kev luminosity for agn1 assuming a typical spectral energy distribution as computed by pompilio , la franca , & matt ( 2000 ) , which assumes a power slope in energy with index @xmath44=0.9 ( @xmath45 ) and takes into account the reflection . this spectrum is roughly approximated by a single power @xmath44=0.6 in the range 2 - 50 kev ( i.e. up to @xmath46=4 ) . this is in agreement with the average slope of the spectra of agn1 from the hellas sample ( vignali et al . 2001 ) .        as our spectroscopic identifications are limited to magnitudes brighter than @xmath47 , in order to estimate the density of agn1 , we have evaluated which is their completeness in our sample . we have assumed that all agn1 follow the same distribution of the ratio of the 5 kev to r - band optical luminosity log(l@xmath48/l@xmath49 ) without dependencies on luminosity and/or redshift . we have estimated the relationship between optical and hard x - ray luminosity for agn1 as follow . we selected a mixed sample of both 25 optical and 25 hard x - ray selected type 1 agn , mostly in the redshift range 0@xmath50@xmath46@xmath502 , from mineo et al . ( 2000 ) , george et al . ( 2000 ) and akiyama et al . the 25 optically selected agn1 have an average log(l@xmath48/l@xmath49 ) ratio of -4.78 with a standard deviation of 0.59 , while the 25 x - ray selected agn1 have an average log(l@xmath48/l@xmath49 ) ratio of -4.38 with a standard deviation of 0.58 . in figure 4 the histogram of the ratio of 5 kev and r - band optical luminosity of the total sample of 50 agn1 at @xmath46=0 is shown . we fitted the distribution with a gaussian having mean -4.58 and standard deviation 0.61 . we have estimated that with this distribution , a limit of @xmath47 corresponds to only 1 agn1 lost from our hellas survey , and consequently we can say that a limit of @xmath47 assure a high level of completeness for the identification of agn1 at the fluxes of our hellas survey . at variance , as already discussed , our hellas identification programme is biased against absorbed type 2 sources which probably harbor in the 13 empty fields found . although the incompleteness for agn1 in the hellas sample is quite low , during the computation of the lf of agn1 , we have estimated the correct area coverage for our survey by multiplying the area coverage corresponding to each point of the @xmath21 plane for : a ) the spectroscopic completeness of the hellas sample ( 74/118 ) , and b ) the fraction of found agn1 as a function of @xmath46 , as described before .      in order to increase the statistical significance of our analysis , the hellas data have been combined to other hard x - ray samples of agn1 identified by grossan ( 1992 ) , boyle et al . ( 1998 ) , and akiyama et al . ( 2000 ) . the sample of grossan ( 1992 ) consists of 84 agn1 and 12 agn2 detected by _ predominantly at low redshift @xmath27 . the sample covers an area of 26919 deg@xmath19 down to a flux limit of @xmath51 erg @xmath3 s@xmath4 . the quoted fluxes at 5 kev have been converted to fluxes in the 2 - 10 kev assuming @xmath52 .    the sample of boyle et al . ( 1998 ) consists of 12 agn1 and 6 agn2 detected by _ asca_. 2 - 10 kev counts have been converted to fluxes with a count - rate - to - flux conversion factor of @xmath53 erg @xmath3 s@xmath4/count , assuming @xmath52 ( georgantopoulos et al . , 1997 ) . the sky coverage has been taken from georgantopoulos et al . ( 1997 , see their figure 1 ) . the sample of akiyama et al . ( 2000 ) consists of 25 agn1 , 5 agn2 , 2 clusters , 1 star and 1 unidentified source . the sources have been detected by _ asca_. in order to convert the counts of the sky coverage presented in their table 1 , a conversion has been applied assuming that 1 count corresponds to @xmath54 erg @xmath3 s@xmath4 in the 2 - 10 kev band for an x - ray source with a slope @xmath44=0.6 . a total of 158 agn1 have been used in the redshift range 0@xmath50@xmath46@xmath503 . their luminosity - redshift distribution is shown in figure 5 . in this distribution a gap between the sample of grossan from _ hea01 _ and the other fainter samples from _ asca _ and _ bepposax _ is evident . this is because surveys at fluxes in the range @xmath55@xmath56 erg @xmath3 s@xmath4 in the 2 - 10 kev energy band where some hundreds of deg@xmath19 need to be covered are still missing . these surveys are in program with _ xmm - newton _ ( see e.g. barcons et al . 2001 ) . the best - fit parameters for the 2 - 10 kev lf and its cosmological evolution have been derived by minimization of the @xmath57 computed by comparison of the observed and expected number of agn1 in 6@xmath583 bins in the @xmath21 plane . the adopted luminosity / redshift grid was : 6 log@xmath16 bins , with @xmath59log@xmath16=1 in the range 42.2 - 48.2 , and 3 redshift bins ( 0.0 - 0.2 , 0.2 - 1.0 , 1.0 - 3.0 ) . table 6 summarizes the results for a number of different models . the errors quoted for the parameters are 68@xmath26 ( 1@xmath60 ) confidence intervals . they correspond to variations of @xmath61=1.0 , obtained perturbing each parameter in turn with respect to its best - fit value , and looking for a minimization with the remaining parameters free to float . the fitted lf have been tested with the 2d kolmogorov - smirnov ( 2dks ) test . two different functional forms have been used . following ceballos and barcons ( 1996 ) and boyle et al . ( 1998 ) , we used the pure luminosity evolution ( ple ) for the agn1 lf . we also used the luminosity dependent density evolution ( ldde ) model similar to the one fitted in the soft x - rays by miyaji et al . ( 2000 ) . the local ( @xmath46=0 ) agn1 lf used for the ple model has been represented by a two - power - law : @xmath62    where @xmath63 is expressed in units of @xmath64 . a standard power - law luminosity evolution model was used to parameterize the cosmological evolution of this lf : @xmath65 .    in the case of the ldde model , as an analytical expression of the present day ( @xmath46=0 ) luminosity function , we used the smoothly connected two power - law form :    @xmath66^{-1}.\ ] ]    the description of the evolution is given by a factor @xmath67 such that :    @xmath68    where @xmath69 } ) }            & ( z \leq z_{\rm c } ; l_{\rm x}<l_{\rm a } ) \\         ( 1+z)^{p1 }           & ( z \leq { z_{\rm c } } ; l_{\rm x}\ge l_{\rm a})\nonumber \\           e(l_{\rm x},{z_{\rm c } } )           \left[(1+z)/(1+{z_{\rm c } } ) \right]^{p2 }            & ( z>{z_{\rm c } } ) . \\      \end{array }         \right . \ ] ]    lclllllllr    = 0.2 cm 0 ) ple from boyle & 1.0,0.0 & 1.73 & 2.96 & 44.16 & 2.00 & ... & @xmath70 & @xmath71 & 24 + 1 ) ple & 1.0,0.0 & 1.83 & 3.00 & 44.17 & 2.12 & ... & @xmath72 & @xmath73 & 35 + 2 ) ple with @xmath74 & 1.0,0.0 & 1.87 & 3.03 & 44.17 & 2.52 & 1.39 & @xmath75 & @xmath76 & 34 + 3 ) ldde & 1.0,0.0 & 0.68 & 2.02 & 44.03 & 4.66 & 1.55@xmath40 & @xmath77 @xmath78 & @xmath79 & 40 + 4 ) ple & 0.3,0.7 & 1.93 & 2.97 & 44.24 & 2.22 & ... & @xmath80 & @xmath79 & 56 + 5 ) ple with @xmath74 & 0.3,0.7 & 1.94 & 2.97 & 44.24 & 2.26 & 2.38 & @xmath80 & @xmath81 & 54 + 6 ) ldde & 0.3,0.7 & 0.69 & 1.95 & 43.96 & 4.56 & 1.58@xmath40 & @xmath82 @xmath41 & @xmath83 & 60 +   + 7 ) ldde softx & 1.0,0.0 & 0.62@xmath40 & 2.25@xmath40 & 44.13 & 5.40@xmath40 & 1.55@xmath40 & @xmath84 @xmath85 & @xmath86 & 48 +   + 68@xmath26 confidence errors & & @xmath87 & @xmath88 & @xmath89 & @xmath90 & @xmath91 & 8@xmath26   + [ tab6 ]    the parameters are defined as in miyaji et al . in particular , @xmath44 is proportional to the decrease of the density evolution of the faint objects . in this parameterization of the lf @xmath16 is expressed in units of @xmath92 . in both models the lf has been computed for luminosities brighter than logl@xmath93=42.2 . using this parameterization , for ( @xmath12,@xmath13)=(1.0,0.0 ) , a fit was done by keeping fixed the parameters @xmath94=@xmath95 ( the redshift at which the evolution stops ) , @xmath44=@xmath96 , and the ratio @xmath97 to the value found by miyaji et al . ( 2000 ) for agn1 only ( their ldde1 model ) in the soft x - rays , and leaving all the remaining parameters ( the two slopes @xmath98 and @xmath99 , the break luminosity @xmath100 , and the speed of the density evolution @xmath101 ) free to vary . in the case of an ( @xmath12,@xmath13)=(0.3,0.7 ) universe , as the fit for agn1 only from miyaji et al . ( 2000 ) was not available , their parameters from the fit for all agns ( agn1+agn2 ) have been used : @xmath94=@xmath102 , @xmath44=@xmath103 , @xmath104 . we started our computation of the lf in an ( @xmath12,@xmath105)=(1.0,0.0 ) cosmology . previous estimates of the shape and evolution of the lf of agn1 in the 2 - 10 kev range are from ceballos and barcons ( 1996 ) , and boyle et al . boyle et al . ( 1998 ) combined the local sample of 84 agn1 observed by _ heao1 _ from grossan ( 1992 ) with a fainter sample of 12 agn1 observed by _ asca _ ( see their distribution in the l@xmath106-@xmath46 plane in figure 4 ) . they found that the 2 - 10 kev agn x - ray lf is best represented by a two - power - law function evolving according to a pure luminosity evolution ( ple ) model : @xmath5@xmath6@xmath107 ( see model 0 in table 6 ) .    in figure 6 the lf from only the 37 agn1 from hellas in three redshift intervals ( 0.0@xmath50@xmath46@xmath500.2 , 0.2@xmath50@xmath46@xmath501.0 , and 1.0@xmath50@xmath46@xmath503.0 ) is shown . for the sake of comparison , the best - fit lf from boyle et al . ( 1998 ) is also shown . the data have been represented by correcting for evolution within the redshift bins as explained in la franca & cristiani ( 1997 ) . the error bars in the figures are based on poisson statistics at the 68@xmath26 confidence level . the data are in rough agreement with the previous estimate of boyle et al . ( 1998 ) , but show an excess of faint agn1 at redshift larger than @xmath46@xmath11.0 . this feature is still present after combining our data with the other agn1 samples from grossan ( 1992 ) , boyle et al . ( 1998 ) , and akyiama et al . ( 2000 ) ( figure 7 ) . as already discussed , these samples all together collect 158 agn1 . these data have a @xmath57 probability of 0.04 to be drawn from a ple model such as that computed by boyle et al . ( 1998 ) ( model 0 in table 6 ) . we did not use the 2dks as this test is not appropriate in this case : it uses the cumulative distributions in the @xmath21 plane regardless of the normalization of the lf . our best - fit to the data with a ple model found a lf with similar slopes and break luminosity as that one of boyle et al . ( 1998 ) but with a slightly larger evolution ( @xmath108 instead of @xmath8=@xmath109 ) , and a significantly 20(@xmath110)@xmath26 larger normalization ( the normalization has a poisson uncertainty of 8@xmath26 . see model 1 in table 6 ) . our larger values of the evolution parameter @xmath8 and normalization @xmath111 are originated by the necessity of better fitting the observed higher density of faint agn1 at high redshift . the 2dks test gives a probability of 0.22 for this fit ( see table 6 ) . a even better probability of 0.31 is obtained if a stop in the evolution is applied at redshift @xmath74=@xmath112 ( model 2 in table 6 ) and a larger evolution ( @xmath8=@xmath113 ) is used . although our fits are already statistically adequate , our ple models do not fully describe an over - density of faint agn1 which is observed at high redshift . the data are not sufficiently faint to properly probe this part of the lf , but as this feature is similar to what observed in the soft x - rays ( miyaji et al . 2000 ) , we tried to obtain a even better fit of the data , by using the luminosity dependent density evolution ( ldde ) model similar to the one fitted in the soft x - rays , as described in the previous section . this model differentiates from the ple especially in the faint part of the lf , at luminosities lower than @xmath100 . in this part of the lf the density evolution decreases in proportion to the faintness of the objects . as our data just start to probe the part of the lf which is fainter than @xmath100 , we can not expect to find directly a fit to all the parameters of the ldde model to our data , and we limited our analysis to a check of the compatibility ( with limited changes ) of the ldde parameters found by miyaji et al . ( 2000 ) to our data . with this model we found the best - fit to our data ( model 3 : ldde , figure 8) by keeping fixed the parameters describing the stop in the evolution ( @xmath94=@xmath95 ) , and the dependence on luminosity of the density evolution ( @xmath44=@xmath96 ) to the value found by miyaji et al . ( 2000 ) for agn1 only , and leaving all the remaining parameters free to vary ( see section 3.2 ) . the 2dks test gives , for this model , a probability of 0.47 .      if an ( @xmath12,@xmath13)=(0.3,0.7 ) cosmology is assumed , the ple models provide an ever better representation of the data in comparison with what found in the ( @xmath12,@xmath13)=(1.0,0.0 ) universe ( see figure 9 ) . the 2dks probability is 0.70 and 0.47 , with and without the introduction of the @xmath74 parameter , respectively . in this case even the simple ple model obtain a quite good fit of the data , and the introduction of the @xmath74 parameter is necessary to stop the evolution only at redshifts larger than 2.4 . however our data contain not enough agn1 at redshift larger than 2 in order to obtain an accurate measure of the @xmath74 parameter ( see figure 4 ) , therefore the errors in this parameter are quite large . also the ldde model obtain a satisfactory fit of the data ( see figure 10 ) . thanks to the identification of 61 sources of the hellas sample we have been able to double the number of hard x - ray agn1 available for statistical analysis at fluxes in the range @xmath114@xmath55 erg @xmath3 s@xmath4 . in total we can use 74 agn1 at these fluxes ( 37 from hellas ) , which combined with the local sample of grossan have allowed to show directly the shape of the lf of agn1 as function of redshift and measure its evolution . the ple models provide satisfactory fits of the data both in the ( @xmath12,@xmath13)=(1.0,0.0 ) and in the ( @xmath12,@xmath13)=(0.3,0.7 ) cosmologies . our estimate of the lf in the ( @xmath12,@xmath13)=(1.0,0.0 ) has a significantly larger normalization in comparison to the previous measure from boyle et al . ( 1998 ) . the data start to probe in the hard x - rays the faint part of the lf where the excess of density of agn1 has been observed in the soft x - rays , justifying the implementation of the ldde models . however , in both cosmologies , the statistic is not significant enough to distinguish between the ple and ldde models ( see table 6 ) . in fact , in the prediction of the differential counts of agn1 shown in figure 11 , the models differentiates at fluxes fainter than @xmath115 erg @xmath3 s@xmath4 , where the statistic is still poor . the new upcoming fainter surveys from _ chandra _ and _ xmm - newton _ will easily test which model is correct .    in table 6 the percentages of the contribution to the 2 - 10 kev x - ray background are shown . the x - ray background has been computed integrating the lf up to @xmath116 for @xmath16@xmath11710@xmath118 @xmath119 . at variance with the results of 24% obtained from the evolution of the lf derived by boyle et al . ( 1998 ) , our models reproduces from @xmath135% up to @xmath160% of the xrb . we used @xmath120 erg @xmath3 s@xmath4 deg@xmath121 from chen , fabian and gendrau ( 1997 ) . the highest percentages would probably imply that part of the absorbed population necessary to reproduce the xrb is already included in the agn1 at high redshift . however , more detailed analysis of this issue are beyond the scope of this paper . the percentages would decreas if a value f @xmath122 erg @xmath3 s@xmath4 deg@xmath121 from vecchi et al . ( 1999 ) is assumed . it is interesting to notice that agn1 in the 2 - 10 kev range show an evolution up to @xmath123 which is fairly well compatible with what observed in the soft x - rays . in fact we found a good fit of the data in the ( @xmath12,@xmath13)=(1.0,0.0 ) universe if we assume exactly the same parameters found in the soft x - rays by miyaji et al . ( 2000 ) for agn1 , by only looking for a fit with the break luminosity @xmath100 ( model 7 in table 6 ) . miyaji et al . ( 2000 ) found log@xmath100=43.78 . the value @xmath100 found by our fit in the 2 - 10 kev band is log@xmath100=44.13 . this luminosity difference implies an x - ray spectrum for agn1 with slope @xmath44=0.6 , which is the same slope used in our computation of the lf . we note however that miyaji et al . ( 2000 ) used a slope @xmath44=1.0 in their computations of the soft x - ray lf . therefore , taken at a face value , the match between the soft and hard x - rays lfs implies that agn1 have a broad band concave spectrum getting steeper going toward lower energies . as already discussed in the previous section , the ( @xmath12,@xmath13)=(0.3,0.7 ) cosmology for agn1 alone has not been analyzed by miyaji et al . ( 2000 ) . the agreement of the evolution measured in the soft and hard x - rays is also shown in figure 12 , where the evolution of the density of agn1 brighter than logl@xmath93=44.8 is shown . the soft x - ray data from miyaji et al . ( 2000 ) have been over - plotted assuming a slope @xmath52 , which corresponds to a limit logl@xmath106(0.5 - 2 kev)@xmath11744.5 . the continuous line are the predictions of our ldde model for ( @xmath12,@xmath13)=(1.0,0.0 ) .    based on observations collected at the european southern observatory , chile , eso n@xmath124 : 62.p-0783 , 63.o-0117(a ) , 64.o-0595(a ) , 65.o-0541(a ) . this research has made use of the nasa / ipac extragalactic database ( ned ) which is operated by the jet propulsion laboratory , california institute of technology , under contract with the national aeronautics and space administration . this research has been partially supported by asi contract ars-99 - 75 , murst grants cofin-98 - 02 - 32 , cofin-99 - 034 , cofin-00 - 02 - 36 and a 1999 cnaa grant . as already described , 25 sources have been identified by cross - correlation with existing catalogues . however , as the sample is not fully spectroscopic identified , the cross - correlation with the existing catalogues could alter the average characteristics ( fluxes , percentages of classes of counterparts , etc . ) of the subsample of identified sources . namely , the subsample could not be representative of the whole sample . let s explain this with an example . in an ideal sample of 50 agn1 and 50 agn2 a random identification of a subsample of 40 sources will identify about 50@xmath26 agn1 and 50@xmath26 agn2 according to sampling errors . but if a cross - correlation is first made with a catalogue of only agn1 ( let say 30 ) and later only 10 sources are randomly identified , the fraction of agn1 in the total identified subsample of 40 sources will be artificially increased ( they will be at minimum 75@xmath26 ) . our subsample is risking the same sort of bias , and we wish to quantify it . a kolmogorov - smirnov ( ks ) two - sample test gives a 9@xmath26 probability that the x - ray flux distribution of the 25 sources identified through the cross - correlation with catalogues belong to the same parent population of the total sample of 118 sources . although not significant , this low probability is due to the average slightly brighter fluxes of the cross - correlated subsample ( @xmath125 ) in comparison to the total sample ( @xmath126 ) . we have thus tried to populate the faintest bins during the observing runs at the telescope , and , indeed , the ks test gives a 66@xmath26 probability that the whole subsample of the 74 identified sources ( included the empty fields ) belong to the same parent population of the total sample of 118 sources ( see figure 1 ) .    in this way , our observing runs at the telescope have recovered the possible alteration on the fraction of classes of sources which are identified . as the catalogues used for the cross correlation are mainly populated by agn1 ( we used ned ) , their fraction in our sample , as explained before , could be artificially increased . agn1 are 14 out of the 25 cross - correlated sources ( 56@xmath26 ) . the fraction of agn1 is , as expected ( but not significantly ) , lower for the sources observed at the telescope : 23 out of the 49 ( 47@xmath26 ) . the two values are not statistically distinguishable from the observed fraction 50@xmath26 of agn1 in the whole sample , which we thus consider representative of the whole hellas sample . la franca f. , fiore f. , vignali c. , comastri a. , pompilio f. , 2001 , in asp conf . 232 , the new era of wide field astronomy , ed . clowes , a.j . adamson & g.e . bromage ( san francisco : asp ) , 96 , preprint(astro - ph/00111008 )                              = 0.2 cm -10.34 & 55.51 -11.85 & 55.51 -11.89 & 55.48 -11.93 & 55.39 -11.97 & 55.28 -12.00 & 55.05 -12.04 & 54.66 -12.08 & 54.04 -12.11 & 53.16 -12.15 & 51.97 -12.19 & 50.63 -12.22 & 49.04 -12.26 & 47.13 -12.30 & 44.88 -12.34 & 42.44 -12.37 & 39.93 -12.41 & 37.49 -12.45 & 34.88 -12.48 & 31.93 -12.52 & 28.83 -12.56 & 25.67 -12.59 & 22.75 -12.63 & 20.03 -12.67 & 17.53 -12.70 & 15.23 -12.74 & 13.16 -12.78 & 11.32 -12.82 & 9.73 -12.85 & 8.30 -12.89 & 6.96 -12.93 & 5.67 -12.96 & 4.56 -13.00 & 3.60 -13.04 & 2.88 -13.07 & 2.35 -13.11 & 1.96 -13.15 & 1.60 -13.19 & 1.27 -13.22 & 0.94 -13.26 & 0.68 -13.30 & 0.47 -13.33 & 0.33 -13.37 & 0.22 -13.41 & 0.14 -13.44 & 0.08 -13.48 & 0.05 -13.52 & 0.03 -13.56 & 0.01 -13.59 & 0.00 [ tab1 ]      = 0.2 cm h002636@xmath35194416 & 0.238 & fwzi & ... & 62.1 & 38.1 & 62.1 & ... & ... + & & fwhm & ... & 19.7 & 15.4 & 18.5 & ... & ... + & & ew & ... & 82.4 & 14.6 & 133.1 & ... & ... +   + h004546@xmath35251550 & 0.111 & fwzi & ... & ... & ... & 40.1 & 104.2 & 44.1 + & & fwhm & ... & ... & ... & 14.6 & 69.7 & 50.4 + & & ew & ... & -1.3 & -0.2 & 5.6 & 22.2 & 2.5 +   + h012157@xmath35584442 & 0.118 & fwzi & ... & 44.1 & 38.1 & 64.1 & 120.2 & 48.1 + & & fwhm & ... & 19.7 & 13.1 & 16.8 & 45.0 & 80.0 + & & ew & ... & 30.4 & 2.9 & 21.4 & 53.5 & 20.8 +   + h013434@xmath35295816 & 2.217 & fwzi & @xmath127452.7 & ... & ... & ... & ... & ... + & & fwhm & @xmath127123.6 & ... & ... & ... & ... & ... + & & ew & @xmath127156.5 & ... & ... & ... & ... & ... +   + h013533@xmath35295202 & 1.344 & fwzi & 282.5 & ... & ... & ... & ... & ... + & & fwhm & 67.6 & ... & ... & ... & ... & ... + & & ew & 61.2 & ... & ... & ... & ... & ... +   + h033408@xmath35360403 & 0.904 & fwzi & 412.6 & 40.1 & ... & ... & ... & ... + & & fwhm & 174.0 & 26.5 & ... & ... & ... & ... + & & ew & 120.5 & 6.1 & ... & ... & ... & ... +   + h043712@xmath35473148 & 0.142 & fwzi & ... & 36.0 & 122.2 & 64.1 & 162.3 & 70.1 + & & fwhm & ... & 16.4 & 36.4 & 20.9 & 39.7 & 73.0 + & & ew & ... & 5.2 & 65.4 & 23.6 & 241.2 & 17.0 +   + h043847@xmath35472802 & 1.453 & 372.6 & ... & ... & ... & ... & ... + & & 64.9 & ... & ... & ... & ... & ... + & & 61.4 & ... & ... & ... & ... & ... +   + h064638@xmath35441534 & 0.153 & fwzi & ... & 60.1 & 128.2 & 60.1 & 122.2 & 46.1 + & & fwhm & ... & 18.3 & 47.2 & 21.3 & 36.1 & 29.0 + & & ew & ... & 7.9 & 61.0 & 78.9 & 168.4 & 10.0 +   + h083737@xmath37254752 & 0.077 & fwzi & ... & 52.1 & 69.4 & 56.8 & 327.2 & 60.1 + & & fwhm & ... & 40.3 & 28.8 & 15.2 & 71.8 & 43.0 + & & ew & ... & 51.8 & 16.0 & 11.3 & 291.6 & 17.0 +   + h083859@xmath37260814 & 0.048 & fwzi & ... & ... & 36.7 & 40.1 & 110.2 & 70.1 + & & fwhm & ... & ... & 11.7 & 23.2 & 42.1 & 32.2 + & & ew & ... & -8.9 & 1.9 & 4.1 & 46.0 & 6.7 +   + h103216@xmath37505120 & 0.174 & fwzi & ... & 30.0 & 203.7 & 36.7 & 150.2 & 33.4 + & & fwhm & ... & 12.0 & 68.1 & 13.2 & 79.9 & 15.0 + & & ew & ... & 5.8 & 65.1 & 13.1 & 163.2 & 6.5 +   + h111814@xmath37402838 & 0.387 & fwzi & 340.5 & 50.1 & ... & 40.1 & ... & ... + & & fwhm & 184.0 & 46.7 & ... & 10.3 & ... & ... + & & ew & 393.8 & 6.7 & -0.7 & 6.7 & ... & ... +   + h111849@xmath37402648 & 1.129 & fwzi & @xmath127200.3 & ... & ... & ... & ... & ... + & & fwhm & @xmath12765.6 & ... & ... & ... & ... & ... + & & ew & @xmath12759.9 & ... & ... & ... & ... & ... +   + h121853@xmath37295902 & 0.176 & fwzi & ... & 43.4 & 36.1 & 56.8 & 153.6 & 50.1 + & & fwhm & ... & 12.4 & 12.1 & 12.5 & 26.8 & 20.9 + & & ew & ... & 25.4 & 17.8 & 199.6 & 186.1 & 22.6 +   + h124028@xmath35051402 & 0.300 & fwzi & ... & 80.1 & 166.9 & 63.4 & ... & ... + & & fwhm & ... & 25.6 & 61.3 & 23.9 & ... & ... + & & ew & ... & 36.1 & 90.8 & 67.1 & ... & ... + [ tab5 ]      = 0.18 cm h124036@xmath35050752 & 0.008 & fwzi & ... & ... & 20.0 & -10.0 & 90.1 & 76.8 + & & fwhm & ... & ... & 13.9 & -7.0 & 34.0 & 26.0 + & & ew & ... & ... & 0.8 & -0.4 & 26.3 & 6.8 +   + h130436@xmath35101549 & 2.386 & fwzi & @xmath36468.7 & ... & ... & ... & ... & ... + & & fwhm & @xmath127128.4 & ... & ... & ... & ... & ... + & & ew & @xmath127537.0 & ... & ... & ... & ... & ... +   + h134820@xmath35301156 & 0.128 & fwzi & ... & 52.1 & ... & 56.1 & 112.2 & ... + & & fwhm & ... & 21.5 & ... & 22.0 & 53.9 & ... + & & ew & ... & 19.0 & -0.4 & 12.7 & 63.0 & ... +   + h134845@xmath35302946 & 0.330 & fwzi & ... & 36.0 & 220.4 & 72.1 & ... & ... + & & fwhm & ... & 15.8 & 83.0 & 20.5 & ... & ... + & & ew & ... & 2.3 & 84.2 & 55.8 & ... & ... +   + h135015@xmath35302010 & 0.074 & fwzi & ... & 60.1 & ... & 52.1 & 168.3 & 56.1 + & & fwhm & ... & 23.1 & ... & 24.5 & 64.4 & 31.7 + & & ew & ... & 100.5 & -0.6 & 16.0 & 48.8 & 8.0 +   + h135354@xmath37182016 & 0.217 & fwzi & ... & 52.1 & 24.0 & 64.1 & 252.4 & 40.1 + & & fwhm & ... & 13.7 & 9.3 & 19.7 & 95.0 & 47.9 + & & ew & ... & 47.3 & 2.1 & 22.8 & 154.0 & 12.4 +   + h151934@xmath37653558 & 0.044 & fwzi & ... & 36.0 & ... & 44.1 & 92.2 & 44.1 + & & fwhm & ... & 13.6 & ... & 9.7 & 9.0 & 14.5 + & & ew & ... & 43.6 & -1.3 & 22.6 & 26.4 & 6.1 +   + h163419@xmath37594504 & 0.341 & fwzi & 144.2 & 48.1 & 72.1 & 56.1 & ... & ... + & & fwhm & 19.9 & 12.9 & 12.9 & 13.7 & ... & ... + & & ew & 33.1 & 19.7 & 30.8 & 267.2 & ... & ... +   + h165043@xmath37043618 & 0.031 & fwzi & ... & 48.1 & 36.1 & 52.1 & 124.2 & 56.1 + & & fwhm & ... & 40.1 & 27.2 & 21.0 & 49.1 & 32.6 + & & ew & ... & 45.9 & 2.8 & 26.6 & 43.0 & 12.3 +   + h165238@xmath37022206 & 0.395 & fwzi & 184.3 & 56.1 & 164.3 & 64.1 & ... & ... + & & fwhm & 70.0 & 24.1 & 129.5 & 21.5 & ... & ... + & & ew & 66.6 & 22.5 & 37.5 & 68.5 & ... & ... +   + h204253@xmath35103826 & 0.363 & fwzi & ... & 72.1 & 184.3 & 76.1 & ... & ... + & & fwhm & ... & 22.5 & 48.5 & 22.3 & ... & ... + & & ew & ... & 15.4 & 73.0 & 201.8 & ... & ... +   + h204435@xmath35102808 & 2.755 & fwzi & @xmath127248.4 & ... & ... & ... & ... & ... + & & fwhm & @xmath127101.9 & ... & ... & ... & ... & ... + & & ew & @xmath12756.6 & ... & ... & ... & ... & ... +   + h222632@xmath37211138 & 0.260 & fwzi & ... & ... & 125.5 & 26.7 & ... & ... + & & fwhm & ... & ... & 41.6 & 7.9 & ... & ... + & & ew & ... & ... & 36.4 & 9.3 & ... & ... +   + h231932@xmath35424228 & 0.101 & fwzi & ... & 68.1 & 40.1 & 56.1 & 172.3 & 64.1 + & & fwhm & ... & 23.1 & 26.3 & 21.8 & 46.9 & 38.4 + & & ew & ... & 20.4 & 5.0 & 24.2 & 77.6 & 7.5 +   + h232729@xmath37084926 & 0.154 & fwzi & ... & 28.0 & 92.2 & 60.1 & 116.2 & 56.1 + & & fwhm & ... & 15.9 & 80.3 & 18.7 & 34.2 & 37.5 + & & ew & ... & 13.0 & 16.8 & 19.1 & 60.9 & 16.2 +   + h232906@xmath37083416 & 0.953 & fwzi & 368.6 & ... & ... & ... & ... & ... + & & fwhm & 76.1 & ... & ... & ... & ... & ... + & & ew & 133.6 & ... & ... & ... & ... & ... +   + h233154@xmath37193836 & 0.475 & fwzi & 344.5 & ... & ... & ... & ... & ... + & & fwhm & 75.0 & ... & ... & ... & ... & ... + & & ew & 93.5 & ... & ... & ... & ... & ... + [ tab5 ]
we present optical spectroscopic identifications of hard x - ray ( 5 - 10 kev ) selected sources belonging to the hellas sample obtained with _ bepposax _ down to a 5 - 10 kev flux limit of @xmath0@xmath1@xmath2 erg @xmath3 s@xmath4 . 
 the sample consists of 118 sources . 
 25 sources have been identified trough correlations with catalogues of known sources . 
 49 have been searched for spectroscopic identification at the telescope . 
 13 fields resulted empty down to r=21 . 
 37 sources have been identified with type 1 agn and 9 with type 2 agn . 
 the remaining are : 5 narrow emission line galaxies , 6 clusters , 2 bl lac , 1 radio galaxy and 1 star . combining these objects with other hard x - ray selected agns from _ asca _ and _ heao1 _ 
 , we find that the local luminosity function of type 1 agn in the 2 - 10 kev band is fairly well represented by a double - power - law - function . 
 there is evidence for significant cosmological evolution according to a pure luminosity evolution ( ple ) model @xmath5@xmath6@xmath7 , with @xmath8=2.12 and @xmath8=2.22 ( @xmath9@xmath10@xmath11 ) in a ( @xmath12,@xmath13)=(1.0,0.0 ) and in a ( @xmath12,@xmath13)=(0.3,0.7 ) cosmology , respectively . 
 the data show an excess of faint high redshift type 1 agn which is well modeled by a luminosity dependent density evolution ( ldde ) , similarly to what observed in the soft x - rays . 
 however , in both cosmologies , the statistic is not significant enough to distinguish between the ple and ldde models . 
 the fitted models imply a contribution of agn1 to the 2 - 10 kev x - ray background from 35% up to 60% .