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the average continuum observed in active galactic nuclei ( agn ) is shown in fig . 1 from the optical to x - ray range , as obtained from a composite optical - uv spectrum of francis et al . ( 1991 ) and zheng et al . ( 1997 ) , and from an average ( slightly modified ) soft x - ray spectrum of laor et al . it is widely admitted since several years that the optical - uv part of this continuum is produced by a warm ( temperature of the order of @xmath2 k ) , optically thick or effectively thick medium , most likely an accretion disc . moreover , one deduces from the spectral distribution in the hard x - ray range , in particular the 30 kev hump observed in many seyfert 1 galaxies , and from the presence of other features like the iron k line around 7 kev , that this medium is irradiated by a hard x - ray source , which is partly reprocessed and reemitted ( improperly named  reflected " ) . so one sees a combination of the primary and of the reflected spectra ( ross & fabian 1993 , and many subsequent works ) . in another interpretation the uv - x continuum is produced by clouds from a disrupted disk ( collin - souffrin et al . 1996 , czerny & dumont 1998 ) , and in this case one would also see the spectrum emitted by the non illuminated surface of the clouds . in both interpretations the uv - soft x spectrum is emitted by a dense and thick shell ( density higher than 10@xmath3 @xmath4 , thomson thickness higher than unity ) , irradiated by an x - ray continuum . similar conditions are met in x - ray binary stars . it is necessary to compute the structure of such an irradiated medium to predict the observed spectrum . therefore one has to build `` intermediate''codes , between those designed for planetary nebulae or narrow or broad line regions in agn , and those designed for stellar atmospheres . these codes have to be valid for thick or semi - infinite dense media , eventually hot , irradiated by a non thermal continuum extending in the hard x - rays , to cover the whole range of situations encompassed in agn and in binary stars . owing to the high optical thickness of the medium in several frequency ranges , such codes require that the transfer of both the continuum and the lines be solved in an `` exact '' way , that is avoiding approximations such as local escape approximation ( for the lines ) or one stream approximation ( for the continuum ) . since the medium is generally dense and sometimes close to local thermodynamical equilibrium ( lte ) , they require that all processes and inverse processes be carefully handled . being irradiated by an x - ray continuum , the medium contains a large number of ionic species , from low to high ionization , which should all be introduced in the computation . finally , the medium being hot and thick , not only thomson , but also compton scattering , should be taken into account . we have undertaken to build a code which satisfies these requirements , to compute the emission spectrum produced by irradiated thick and hot media like those commonly assumed in agn close to the black hole , in a wide range of photon energy . precisely we have built several interconnected codes , which allow more flexibility . the ensemble is far from being perfect and still contains several approximations which restrict its use , but we improve it gradually .    titan is designed to study the structure of a warm or hot thick photoionized gas , and to compute its emission - reflection - transmission spectrum from the infrared up to about 20 kev . it solves the energy balance , the ionization and the statistical equilibria , the transfer equations , in a plane - parallel geometry , for the lines and the continuum . then , given the thermal and ionization stratification , the computation of the emitted spectrum from 1 kev to a few hundreds kev is performed with noar which uses a monte - carlo method taking into account direct and inverse compton scattering ( it allows also to study various geometries ) . in a previous paper ( dumont , abrassart & collin 2000 ) titan and its interconnection with noar were described . we recall here the main characteristics of the code , and we describe recent improvements as well as new results , focussing only on the aspects which are not treated in a standard way . we briefly summarize below the physical processes ( section 2 ) , the transfer method and the iteration procedure ( section 3 ) . the influence of the physical parameters , of the atomic data , and of the density distribution , are discussed in section 4 . the coupling of titan and noar is briefly described in section 5 . in titan , the physical state of the gas ( temperature , ionic abundances and level populations of all ionic species ) , is computed at each depth , assuming stationary state , i.e. : local balance between ionizations and recombinations , local balance between excitations and deexcitations , local energy balance , and finally total energy balance ( equality between inward and outward fluxes ) . the ionization equilibrium equations include radiative ionizations by continuum and line photons , collisional ionizations and recombinations , radiative and dielectronic recombinations , charge transfer by h and he atoms , the auger effects , and ionizations by high energy electrons arising from ionizations by x - ray photons . the emission - absorption mechanisms of the continuum include free - free and free - bound processes , two - photon process , and thomson electron scattering . inverse processes ( except autoionization for dielectronic recombinations , and recombinations onto excited levels for some ions , see next section ) are computed through the equations of detailed balance . induced processes are taken into account . energy balance equations include the same processes and compton heating / cooling . hydrogen and hydrogen - like ions are treated as 5 or 6-level atoms . in a first step , in order to save computation time , non hydrogen - like ions were treated with a rough approximation : interlocking between excited levels was neglected and the populations of excited levels were computed separately using a two - level approximation . this approximation does not predict correctly the details of the line spectrum , including the resonance lines themselves . moreover ionizations from excited levels were not taken into account , though recombinations onto excited states were taken into account in the ionization equilibrium . this approach is is not correct close to lte , and even far from lte , if ones wants to predict accurately detailed spectral features ( cf . section 4.2 ) . however it gives a correct overall spectral distribution in this last case . such are the upper layers of an irradiated atmosphere where at the same time the fractional abundances of heavy elements in high ionization states are important , and the ground level of these ions is strongly overpopulated with respect to lte . but it is not the case in the deep layers of a disk atmosphere , emitting the uv part of the spectrum , where the density is high , and the influence of the external irradiation is small . this is why helium - like ions treated as complete 8-level atoms and lithium - like ions treated as complete 5-level atoms have been recently implemented in the code . all processes are thus taken into account for each level . this treatment is valid as far as the upper levels are in lte with the continuum . to handle intermediate cases , we replace the missing upper levels which are very important especially for helium - like ions by adding an additional recombination rate , not compensated by ionization .    in the future we plane to add several other levels , and to sum the contributions of the higher levels to handle at the same time cases close to and cases far from lte . photoionization cross sections are fitted from topbase ( cf . cunto et al 1993 ) . cross sections of neutral and once ionized heavy elements are not well represented , but they do not have any influence on these hot media . the gas composition include 10 elements ( h , he , c , n , o , ne , mg , si , s , fe ) , and all their ionic species .    the slab is divided in about 300 layers with variable thicknesses . the code can work with a prescription for the density or for the pressure , such as a constant pressure or a pressure corresponding to hydrostatic equilibrium . the external illumination can be characterized by an  ionization parameter " , which we choose here as @xmath5 , @xmath6 being the frequency integrated flux incident on one side of the slab and @xmath7 the hydrogen number density at this surface . note that there are several other definitions of the ionization parameter , for instance @xmath8 , or @xmath9 , but they correspond basically to the same concept , namely a parameter which determines the ionization state . it is possible to take also into account a flux incident on the back side of the slab . presently we mimic a semi - isotropic illumination , but with the ali transfer method which is implemented ( cf . below ) it will be possible to take into account the angular distribution of the incident radiation . we are concerned by media with a continuum optical thickness larger than unity in a range of frequencies rich in emission or in absorption lines , and having a very inhomogeneous structure . moreover in the hot photoionized layers , the absorption coefficient is weak at all wavelengths and thomson diffusion is dominant . on the contrary absorption dominates and must be carefully treated to compute the spectrum emitted by the non illuminated cold part of the medium . radiation transfer requires therefore some attention . the transfer equation is    @xmath10    where @xmath11 is the distance to the illuminated surface , @xmath12 is the cosine of the angle between the normal and the light ray , @xmath13 is the absorption coefficient ( for the continuum it is due to photoionizations and free - free transitions ) , @xmath14 is the diffusion coefficient ( here thomson scattering ) , @xmath15 is the emissivity ( due to radiative recombinations and free - free emission ) , and @xmath16 is the mean intensity , equal to @xmath17 ( all these quantities are local , @xmath18 depends on the direction and on the frequency @xmath19 , and j , @xmath13 and @xmath15 depend on the frequency ) .    introducing the source function @xmath20 and the optical depth @xmath21 , eq . 1 can be written :    @xmath22    with    @xmath23    which can be separated between j - dependent and j - independent components :    @xmath24    the formal solution is :    @xmath25      in the previous version of the code ( dumont et al . 2000 ) we used the same method for line and continuum transfer , described in section 3.2 . presently the transfer of the continuum is performed with the * accelerated lambda iteration * method ( ali ) , which was set up with the help of subroutines kindly provided to us by f. paletou ( cf . auer & paletou 1994 , and kunasz & auer 1988 ) .    in this method @xmath26 is written with the help of an operator @xmath27 :    @xmath28.\ ] ]    ali uses an operator splitting , described first by cannon ( 1973 ) , then by olson et al . ( 1986 ) :    @xmath29    where @xmath30 is an approximate @xmath31 operator chosen to facilitate the inversion , and @xmath32 has been calculated in the previous iteration . one obtains :    @xmath33^{-1 } [ s - s^*].\ ] ]    we use here a parabolic interpolation in the evaluation of the integral giving the intensity from the source function , and an acceleration type ng ( in this method , at every fourth iteration a least squares extrapolation is made in order to reduce the residuals , cf . ng 1974 ) .    to perform this computation , one needs to know the opacities and emissivities at each frequency as functions of @xmath11 , hence the temperature and the populations at each depth , so an iteration procedure is required . * for each layer , starting from the illuminated side , the ionization and thermal balance equations are solved by iteration ; the temperature , the opacity and the emissivity are computed ; * when the back side of the cloud is reached , the transfer is solved with the ali method , and @xmath34 is computed ; * the whole calculation is repeated until convergence with the new values of @xmath34 . it is stopped when the energy balance is achieved for the whole slab ( i.e. when the flux entering on both sides of the slab is equal to the flux coming out from both sides ) . this method is almost as slow as the previous one , albeit more secure . 2 shows that the difference in the spectrum computed with the previous transfer method and with ali is very small . it is certainly due to the fact that the continuum optical thickness is never very large ( at most 10@xmath1 ) . this is not the case at the center of intense lines , which are typically 100 times thicker . we are presently implementing ali for the line transfer and we expect to find more important differences for a few lines like oviii l@xmath35 which are not completely converged even after 1000 iterations ( cf . dumont et al . 2000 ) . since ali is presently not implemented for the line transfer , it is treated like in the previous version with a simple method based on the eddington two - stream approximation . in this approximation the transfer equations are written :    @xmath36    the mean intensity @xmath37 is equal to @xmath38 , and the flux @xmath39 defined as usual by @xmath40 is equal to @xmath41 . the  reflected " flux is equal to @xmath42 , and the outward flux to @xmath43 . the optical depth and the total optical depth are defined as :    @xmath44    where @xmath45 is the total thickness of the slab . for @xmath46 the formal solution of the transfer equations between @xmath47 and @xmath11 is :    @xmath48    we approximate the source function @xmath49 by a constant in the interval @xmath50 . it leads to :    @xmath51   \label{eq - trans6}\ ] ]    and :    @xmath52 . \label{eq - trans7}\ ] ]    we calculate @xmath53 and the mean intensity @xmath54 from eq . 3 and from :    @xmath55 , \label{eq - trans10 } \end{aligned}\ ] ]    assuming a constant @xmath56 ( actually it is not constant so the formulae are more complicated but do not deserve to be given here ) . the procedure is then the following :    * for each layer , after computation of the temperature and the source function , @xmath57 is transferred through the layer according to eq . 12 , while @xmath53 and @xmath58 are buffered for each layer and each frequency ; * when the back side of the cloud is reached , new values of @xmath59 are calculated , starting from the back side where @xmath60 is given by eq . 12 , and using eq . 13 with the previous values of @xmath53 and @xmath58 ( actually we use the values given by several previous iterations to accelerate the procedure with a ng method ) . this method requires a large number of iterations to get a complete convergence of the model , and one can not be sure to have reached the correct value . it is why we are now changing for the ali method . we assume presently complete redistribution in the lines , and partial redistribution in intense lines is mimicked by assuming complete redistribution in a pure doppler profile . it will be possible to treat partial redistribution with the ali method . a line profile is represented by a voigt function at several frequencies distributed symmetrically around the line center . the frequencies in the profile are chosen to take into account the fact that the doppler width may decrease by one order of magnitude inside the slab , owing to large temperature variations . moreover , to solve the statistical equilibrium equations one needs to know the total line intensity weighted by the profile @xmath61 , while to compute the thermal and ionization equilibria one simply needs the line intensity integrated over frequencies @xmath62 . for intense lines , the first integral is dominated by the doppler core , and the second by the wings . the set of frequencies chosen to describe the line must therefore cover at least two orders of magnitude . the integrals over the line profiles are achieved with a 15-point gauss - legendre quadrature . we also assume that the lines do not overlap , which is valid as far as multiplets are treated globally , and because the number of lines taken into account in titan is relatively small ( presently 580 , including subordinate lines in the optical - uv range ) .    as shown by fig . 3 , the profiles of @xmath63 and @xmath64 for oviii ly@xmath65 vary considerably from the surface to the deepest layers . we shall call  reference model " a model with the following characteristics :    * it is a plan - parallel slab with a constant density , equal to 10@xmath3 @xmath4 , * its column density equals 10@xmath66 @xmath67 ( corresponding to a thomson thickness @xmath68 of 80 ) , * it is illuminated on one side by an incident power law continuum proportional to @xmath69 from 0.1 ev to 100 kev , * the ionization parameter is @xmath70 erg cm s@xmath71 , * there is no illumination on the other side , * the abundances are ( in number ) : h : 1 , he : 0.085 , c : 3.3 10@xmath72 , n : 9.1 10@xmath73 , o : 6.6 10@xmath72 , ne : 8.3 10@xmath73 , mg : 2.6 10@xmath73 , si : 3.3 10@xmath73 , s : 1.6 10@xmath73 , fe : 3.2 10@xmath73 .    in several examples given below , we also use a smaller value of the ionization parameter , @xmath74 ( corresponding to a value of the usual ionization parameter @xmath8 equal to 0.88 ) , allowing an easier comparison with computations made with cloudy or with xstar .    figs . 4 and 5 show the results corresponding to different values of the ionization parameter , the other parameters being the same as in the reference model . these results were obtained with the previous version of the code . indeed each model takes a long time to run , and all models have still not been recalculated with the ali method . however , we have checked in several cases that the results obtained with the new version differ very slightly from the previous ones . 4 displays the temperature versus the depth . close to the surface the temperature is high and reaches the compton temperature for the highest value of @xmath75 . the extension of this region ( called the  hot skin " ) increases with the value of @xmath75 . its optical thickness reaches @xmath76 for @xmath77 . as we shall see later , this is quite different from the case of constant pressure , where the thickness of this zone is always limited to @xmath78 . this hot skin is almost absent for low values of @xmath75 . below the hot skin the temperature decreases smoothly , again at contradistinction with the case of a constant pressure . this decrease is followed by a region of a quasi constant temperature , which is in a state of  quasi - lte " , i.e. each process important for the energy equilibrium ( lines , free - bound from and onto ground levels , free - free ) is exactly balanced , due to the high optical thickness and relatively high density . however the region is not in lte , in the sense that the fractional ionic abundances do not satisfy saha equations . we shall discuss this issue in more detail in the next section .    on fig . 5 are displayed the `` reflected '' spectrum emitted by the illuminated side of the slab , and the `` outward '' spectrum emitted by the back side . note that the outward spectrum is distinct from the transmitted one , as it includes also _ emission _ ( actually our transfer method does not allow to separate emission and transmission , contrary to approximate methods using for instance  on the spot " approximation ) . the outward spectrum lies only in the optical near uv range , owing to the low temperature of the back side ( cf . this is due to the large column density of the model which does not allow any transmission in the x - ray range . note the strong edges in emission ( including a balmer edge ) , and the cut off of the spectrum at a few tens of ev , due to absorption by heii and by low ionization heavy element species . the reflected spectrum shows a characteristic shape with a smooth  powerlaw " soft x - ray excess , and displays many lines and ionization edges whose intensity decreases with increasing values of @xmath75 , owing to the increasing influence of pure reflection when the ionization state increases . note that the lyman discontinuity is in emission for low @xmath75 and in absorption for high @xmath75 . 6 illustrates the influence of the column density on the reflected and on the outward spectra for @xmath79 , the other parameters being the same as in the reference model . first we see that the reflected spectrum  saturates " above 10@xmath80 @xmath67 , because all the available photons have been absorbed and no longer succeed in escaping from the illuminated side . second , the optical - uv part of the outward spectrum saturates above 10@xmath80 @xmath67 , but not the x - ray part , which is due to transmission . as the column density decreases , the slab becomes more transparent and less emissive , so the outward spectrum becomes basically only the transmitted spectrum , while the reflected spectrum decreases in intensity . note that the  saturation " of the reflected spectrum would occur at a larger value of the column density for a larger @xmath75 , as the hot reflecting skin becomes thicker . it is thus _ necessary to take into account deeper layers , exceeding a thomson thickness @xmath68 of few units , to compute correctly the reflected spectrum in the uv range for @xmath81 , when dealing with constant density slabs_. this has always been overlooked in the past .      in the first version of the code , non hydrogen - like ions were treated with a rough approximation : the populations of the excited levels were computed separately using a two - level approximation , i.e. interlocking between excited levels was neglected , as well as ionizations from these levels . moreover only a very small number of levels ( at most four ) were taken into account . however recombinations onto excited states were taken into account in the ionization equilibrium , and the transfer of these photons was treated in an approximative way , according to an escape probability formula proposed by canfield & ricchiazzi ( 1980 ) . each recombination was also assumed to produce a resonant photon after cascades . hydrogen and hydrogen - like ions were - and are - treated as 6-level atoms . levels 2s and 2p are treated separately , while full l - mixing is assumed for higher levels . all processes including collisional and radiative ionizations and recombinations are taken into account for each level ( cf . mihalas 1978 ) . in the previous version , recombinations onto levels higher than five were not taken into account , which amounted assuming that the higher levels were in lte with the continuum . it was true for hydrogen in the conditions in which this code is presently used , but not for other hydrogen - like ions . we have therefore recently added recombinations onto higher levels , and we intend to implement  fictive " levels to take ionizations from upper levels into account as it is done for instance in cloudy or xstar . level populations are obtained as usual by matrix inversion . the rough treatment of non hydrogen - like ions could have several consequences . * first it should _ not predict correctly the details of the line spectrum _ , since it neglects subordinate lines and even important resonance lines . * also it can lead to a _ bad estimation of the energy losses _ and consequently to a change of the temperature . the recombination continuum corresponds generally to a smaller absorption coefficient than the neglected lines , and therefore can escape more easily from the deep layers . it would correspond to an overestimation of the losses and an underestimation of the temperature . but other parameters , such as the ionization state , play also a crucial role in the energy balance , so the net effect is difficult to predict . fortunately in these models cooling due to lines is not predominant except in some regions . * it can lead to important errors in the _ fractional ion abundances_. this effect is  self - cumulating " , as an increased proportion of a given ion leads to a stronger ionization edge , which in turns inhibits further photoionizations . so again the net effect is not predictable . * finally recombinations onto excited levels are not balanced by ionizations from these levels . it is not important in conditions prevailing in a photoionized slab far from equilibrium ( like for instance in the hot skin of an irradiated accretion disk ) , as the lowest atom levels are strongly overpopulated with respect to lte . but this treatment is not valid in the innermost layers of a disk atmosphere , where the density is very high so complete lte is almost reached . we have recently introduced interlocked multi - level computations for he - like and li - like ions , in order to obtain a better description of the spectrum , and to treat cases closer to lte . however we are for the moment restricted to 8 levels for he - like ions and 5 for li - like ones , which is not sufficient close to lte . we take into account recombinations onto the levels higher than 8 for he - like ions ( respectively 5 for li - like ) , adding them as a contribution to the recombination of the highest level . this treatment is similar to the old one , except that it takes into account many subordinate lines and ionizations from excited levels , and it is valid as far as lte is not reached for the highest levels and ionizations are dominated by the ground level . like for hydrogen - like ions we plane to add several other levels , and to mimic the contributions of the higher levels . we mention also that radiative recombinations are computed differently according to the relative values of @xmath82 to the photon energy bin , in order to get an accurate frequency dependence .    figs . 7 to 10 illustrate the influence of the new treatment with respect to the old one , for the reference model ( except that the thickness is only @xmath83 instead of 80 ; we have already mentioned that above this value nothing varies anymore for @xmath70 , cf . 4 ) . in this model the level populations and the ionization degrees are far from lte , even in the deepest layers . it is quite comparable to a classical photoionized model , where excited levels are populated by radiative recombinations and depopulated by deexcitations onto lower levels . note that these results are obtained with the new transfer treatment . 7 shows the new temperature profile . except at the surface @xmath84 is increased with respect to the previous computation . the change is quite important , reaching a factor two in the transition layer . 8 shows the change of fractional abundances of hydrogen , helium , and iron . it is relatively important in the hot skin for hei , but the abundance of this ion is completely negligible in this region , so it should not induce any change in the emitted spectrum around the ionization edge , i.e. 20 ev . the result of a computation with full he - like and li - like atoms , but without additional recombinations on the highest level , is also shown . except precisely for hei in the hot skin , the results with and without the additional term are quite similar . figs . 9 and 10 display the reflected spectrum for this model , computed with the old and the new atomic treatment , with a low resolution in the whole energy range ( fig . 9 ) , and with a higher resolution in a smallest but interesting energy range ( fig . 10 ) . the spectral distribution does not differ much for the two treatments . this is not the case of the detailed features , as it can be seen on fig . the flux of the main lines of h - like ions in the reflected spectrum are almost not changed ( which is expected since the treatment is the same ) . for the he - like and li - like ions , the total flux for all lines is multiplied by a factor 3 or 4 . to illustrate this effect , fig . 11 displays the fluxes of the ovi , ovii , and oviii lines . even the resonant lines differ in both treatments , as the energy is distributed among a much larger number of lines : for instance the oviii line at 665 ev decreases by one order of magnitude with the new treatment . the net increase of line flux is simply due to the redistribution of continuum recombination photons into line photons through cascades , an effect which was only partly taken into account in the previous treatment .    besides , we have added the result of a computation performed with the complete treatment of he - li - like ions , when suppressing the recombination rates added on the highest level ( cf . figs . 9 and 10 ) . this suppression does not affect at all the spectral distribution , and only slightly the detailed features of the spectrum . the next further step is to introduce forbidden lines , which could be important for the cooling of the medium in some places . note that all the results given in the following sections are obtained without li - like and he - like complete atom models . :    up to now , we have presented results only for slabs of constant density . it is a peculiar case , as the pressure is thus varying according to the density and the temperature . when the pressure is imposed and not the density , one is faced to the well - known problem of thermal instability described first in the framework of agn by krolik , mckee & tarter ( 1981 ) . if the pressure is imposed , three ( or even more ) solutions to the gain = loss equation can exist , and one of these solutions is unstable . the gas has therefore to shift either to the compton dominated hot solution at a temperature close to the compton value , or to the cold solution dominated by atomic processes , at a few @xmath85 k.    krolik et al . were considering optically thin media with a uniform temperature , whose value is given by the external irradiating spectrum . here we are considering a stationary case , where the temperature changes across the slab because the mean intensity @xmath37 is modified as a function of depth . since @xmath37 is absorbed in the soft x - ray band , its spectral distribution becomes more dominated by uv as the depth increases , and such a spectrum does not lead to the s - shape curve characteristic of thermal instability ( cf . krolik et al . however , even if there is no instability , the temperature decreases very rapidly with increasing depth , as the constancy of the pressure leads to an increase of the density , therefore to an increase of the cooling , and to a further decrease of temperature ( note that the temperature is not strictly inversely proportional to the density , as radiation pressure generally exceeds gas pressure ) . this is illustrated on fig . 12 which shows the gain - loss function versus the temperature for different layers . the model is a slab with a constant pressure , a density at the surface equal to 10@xmath86 @xmath4 , an ionization parameter equal to 1000 . the illuminating spectrum is a power law @xmath87 , chosen because such a flat spectrum is well known to correspond to an  s - shape " curve with a strong instability . however we see that as the temperature decreases with the depth , there are never multiple solutions of the gain = loss equation . note that it is by no means a general result . in particular , a medium with a more rapid variation of the pressure with the depth , like given by hydrostatic equilibrium , could likely lead to a thermal instability , as the transition to the cold region occurs in a region where the spectrum is less absorbed than in a constant density medium ( cf . nayakshin , kazanas & kallman 2000 ) . also if the uv band is absorbed before the soft x - ray one , which is the case when the ionization parameter is small , it should lead to a thermal instability . 13 displays the temperature versus depth for slabs of constant density and slabs of constant pressure , with different ionization parameters . note that it would be better to speak of  ionizing flux " instead of ionizing parameter in the constant pressure case . actually the comparison is performed with the same incident fluxes . as expected , and in contrast with the constant density case where there is a smooth decrease of the temperature with increasing depth , the constant pressure case gives a sharp transition between the hot and dilute , and the cold and dense , parts of the slab . moreover the thickness of the  hot skin " is smaller for the same flux in the constant pressure case , in particular for high illumination . for instance , for @xmath70 the thomson thickness of the hot skin is about unity in the constant density case , and only 0.4 in the constant pressure case . all these effects have strong consequences on the emission spectrum .    for constant pressure the spectrum displays a narrow uv bump , while the constant density the uv bump is more extended in frequency , as it corresponds to intermediate temperatures which do not exist in the constant pressure case ( cf . the difference between the two cases is particularly apparent for high illumination , as expected . it is interesting to note that the lyman edge is quite weak in the constant pressure case , since the uv spectrum is closer to a blackbody . this is a point that constant density models have difficulties to account for in agn . the distribution of fractional ionic abundances is also completely modified . as an illustration , 15 displays the fractional abundances of iron versus depth . the most remarkable difference is the presence at @xmath88 of low ionization species , such as feiv or fev , which are completely absent in the constant density case . one notes also that the extension of highly ionized species is smaller than in the constant density case , as expected . for instance fexiv is present up to @xmath89 for constant pressure , instead of @xmath90 for constant density . as a consequence , the iron k lines produced are different from the constant density , as we shall see later .      :    in order to describe an irradiated accretion disk , we have introduced in collaboration with a. rosanska and b. czerny a variation of the density given by the hydrostatic equilibrium law . titan treats then only the layer at the surface of the disc where the diffusion approximation does not hold . in this case there is a flux incident on the back side of the slab , equal to the flux generated by viscous dissipation inside the disk . iterations are needed with the hydrostatic equilibrium equations ( for more details see raska et al . 2001 ) . 16 shows the results for the following case : @xmath91 m@xmath92 , @xmath93 , @xmath94 , and an x - ray irradiating powerlaw spectrum , with a spectral index 0.9 in flux , and an x - ray flux @xmath95 , i.e. equal to half the viscous flux . note that the temperature decreases abruptly like in the constant pressure case , and that the hot skin is very thin . the temperature reaches that given by viscous dissipation at about @xmath96 . the reflected spectrum displays a prominent blue bump with no soft x - ray excess , and a strong absorption in the uv range due to low ionization species in the deep layers of the atmosphere . it is important to note that such a spectrum differs strongly from that of an irradiated disc in which the density is assumed to be constant ( ross & fabian 1993 , and subsequent works , cf . raska et al . 2001 for a detailed discussion ) . our results are similar to those of nayakshin , kazanas & kallman ( 2000 ) . in order to compute the effect of compton scattering on the high energy part of the reprocessed radiation and to determine precisely the compton heating - cooling rate in the energy balance equation , one needs another numerical approach . in this aim abrassart ( 1998 ) developed a monte carlo code , noar ( cf . dumont et al . the asset of such an approach is also that it enables to investigate an arbitrary geometry and to determine the angular dependence of the observed spectra . moreover , it allows to easily extract time variability information . noar takes into account direct and inverse compton scattering , according to the method proposed by pozdniakov , sobol & sunyaev ( 1983 ) and by gorecki & wilczewski ( 1984 ) . given all the fractional abundances and the temperature in each layer provided by titan , it computes the absorption cross sections in each layer . free - free absorption is taken into account , as well as recombination continua of hydrogen and helium like ions ( the spectral distribution of the recombination continuum is determined by the local temperature ) . the proper yields to account for the competition with the auger effect are used for the fluorescence lines . fluorescence of iron xvii to xxiii is suppressed by resonant trapping ( but not in titan ) . for the pseudo fluorescence of iron xxv and xxvi , a simplifying prescription is adopted which includes the two most probable outcomes of a k shell photoionization , that is : direct recombination on ground level or l - k transition . other line emission is not included . one use of noar is to provide titan with the local compton gains and losses in each layer . this is necessary , because compton heating - cooling is dominated by energy losses of photons @xmath97 20 kev for high values of @xmath75 . in dumont et al . ( 2000 ) it was shown that given an incident spectrum the dependence of the compton heating - cooling over @xmath75 ratio on the thomson depth depends only slightly on @xmath75 except for high values of @xmath75 . the compton heating - cooling rate obtained with noar is fitted analytically as a function of @xmath11 , which is transferred to titan . the spectra computed with titan are then completed . ( cf . fig . 17 , and also figs . 14 and 16 ) . titan and noar lead to similar energy distributions of the reflected spectra in the 1 - 20 kev range . the reflected spectrum above 10 kev exhibits a `` compton hump '' depending on the high energy cut - off of the incident spectrum , which is here 100 kev , and on the ionization parameter . the spectrum in the higher energy range is used for the energy balance . comptonisation leads also to line broadening , and when it is important , to smearing of the ionization edges . direct compton downscatterings produce a red wing , while inverse compton upscatterings produce a blue wing . the effect can be comparable to relativistic broadening near the black hole , and can account partly for the broad profiles of the iron line in agn ( abrassart & dumont 1998 ) .    fig . 18 displays ( with a relatively high resolution , in order to show detailed features ) the spectrum around 7 kev in the constant density case , for different values of @xmath75 , the other parameters being as in the reference model . the feature near 7 kev is made of a mixture of iron edges and of several iron lines :    - for @xmath75 equal to a few hundreds erg cm s@xmath71 , the iron line is dominated by low ionization stages , smaller than iron xvii ; the absorption edge is proeminent above 7 kev ;    - for @xmath75 equal to @xmath98 erg cm s@xmath71 , fexxv and fexxvi lines are apparent , modified by compton scattering . the lines are significantly broadened , the broadening is asymmetric , the profile is skewed towards the red . the absorption edge is still important , but extends from 8 to 10 kev , due both to comptonization and to the influence of several edges ;    - for @xmath75 equal to @xmath85 erg cm s@xmath71 , the line has a large red compton wing and a weak blue wing , and it is weak . the absorption edge is completely erased . 19 displays the iron line complex for the constant pressure case , to be compared to the constant density case . both a  cold line " at 6.4 kev and fexxv and xxvi lines are present in the constant pressure case , while only highly ionized species are visible in the constant density case . for constant pressure the cold layers are indeed closer from the surface than in the constant density case , so the photons emitted by these layers are able to leave the medium . the coupling of titan and noar allows to compute the structure and the emission of hot compton thick irradiated media , by solving consistently the energy balance of the medium . moreover it allows also to take into account compton broadening of the lines and of the edges in the x - ray range .    in the previous description of the code ( dumont et al . 2000 ) , we had already recalled the importance of the returning flux , which is often neglected , even for relatively low column densities , and we had compared the line transfer treatment for the lines with the escape probability approximation generally used in these problems . here we have in addition stressed the influence of the pressure law , by showing the strong differences existing both in the structure and in the emitted spectrum , between a constant density medium , a constant pressure medium , or a medium in hydrostatic equilibrium . we have also shown that the atom models are of fundamental importance as they determine the ionization state , the energy balance , the temperature , and in fine the detailed spectral features which will be soon accessible through x - ray spectroscopy . the accelerated lambda iteration method is used now for the continuum . we will soon implement it for the lines , which are the most difficult to converge . it will in particular allow to treat partial redistribution in the lines .    in collaboration with s. coupe and m.c . artru ( coupe et al . 2001 ) we have begun to take into account subordinate lines for all ions by solving a complete multi - level atom for li - like and he - like ions , after h - like ions which were already introduced in the previous version of the code . in the future we plan to implement other iso - electronic sequences , and to add to the present ones several other levels . in particular we will try to get a better representation of the highest levels for computations close to lte . we will take into account forbidden lines , which could be important for the cooling of the medium in some places . we also intend to add to the line transfer an option allowing the uv and soft x - ray photons to escape from the medium through compton scattering . abrassart a. & dumont a .- m . 1998 , proceedings of the first xmm workshop on science with xmm " , m. dahlem ( ed . ) abrassart a. 1999 , proceedings of the 32nd cospar scientific assembly , nagoya , japan , advances in space research , vol 25 issue/3 - 4 , p 465 auer h.l . , paletou f. 1994 , a&a 284 , 675 canfield r.c . , ricchiazzi p.j . 1980 , apj 239 , 1036 cannon c.j . 1973 , j. quant . radiat.transfer 13 , 627 cunto w. , mendoza c. , ochsenbein f. , zeippen c.j . 1993 , a&a 275 , l5 coupe s. , dumont a - m . , artru , m - c . , collin s. 2001 , in preparation collin - souffrin s. , czerny b. , dumont , a.  m. & zycki p.t . 1996 , a&a 314 , 393 czerny b. & dumont a .- m . 1998 , a&a , 338 , 386 dumont a .- , abrassart a. , collin s. 2000 , a&a 357 , 823 francis p.j . , 1991 , apj 373 , 465 ferland g.j . , korista t. , verner d.a . , ferguson j.w . , kingdom j.b verner e.m . , 1998 , pasp 110 , 761 gorecki a. , wilczewski w. 1984 , acta astronomica 34 , 141 krolik , j.h . , mckee , c.f . , tarter , c. b. , 1981 , apj 249 , 422 kunasz p. & auer h. l. 1988 , j. quant . transfer 39 , 67 laor , a. , fiore , f. , elvis , m. , wilkes , b.j . , mcdowell , j.c . 1997 , apj 477 , 93 ng k. c. 1974 , j. chem . 61 , 2680 olson g.l . , auer l.h . , buchler j.r . 1986 , j. quant . transfer 35 , 431 pozdniakov l.a . , sobol i.m . , sunyaev r.a . 1983 , astrophysics and space physics reviews 2 , 189 ross r.r . , fabian a.c . , 1993 , mnras 261 , 74 raska a. , czerny b. , dumont a - m . collin , s. 2001 , a&nnna , submitted zheng w. , kriss g.a . , telfer r.c . , grimes j.p . , davidsen a.f . 1997 , apj 475 , 469
we describe a code designed for hot media ( t @xmath0 a few 10@xmath1 k ) , optically thick to compton scattering . 
 it computes the structure of a plane - parallel slab of gas in thermal and ionization equilibrium , illuminated on one side or on both sides by a given spectrum . 
 this code has been presented in a previous paper ( dumont , abrassart & collin 2000 ) , where several aspects were already discussed . 
 so we focus here mainly on the recent developments . 
 presently the code solves the transfer of the continuum with the accelerated lambda iteration method ( ali ) and that of the lines in a two stream eddington approximation , without using the local escape probability formalism to approximate the line transfer . 
 this transfer code is coupled with a monte carlo code which allows to take into account direct and inverse compton diffusions , and to compute the spectrum emitted up to mev energies , in any geometry . 
 the influence of a few physical parameters is shown , and the importance of the density and pressure distribution ( constant density , pressure equilibrium , or hydrostatic equilibrium ) is stressed . 
 recent improvements in the treatment of the atomic data are described , and foreseen developments are mentioned . 
 # 1_#1 _ # 1_#1 _ =    # 1 1.25 in .125 in .25 in