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in about 50% of flat x - ray spectrum , broad optical emission line , type 1 agn , ionized matter along the line of sight absorbs the nuclear x - ray continuum ( reynolds et al . , 1997 , george et al . , 1998 ) .
main signatures in the transmitted soft x - ray spectra of these agn are deep and broad k absorption edges , mainly from highly ionized oxygen and neon , but a number of other weaker and/or narrower absorption features are predicted by photoionization and collisional ionization models , including iron l edges , and more than 200 strong k and l resonance absorption lines from c , o , ne , mg , si , s and fe ( nicastro et al . , 1998a ) .
emission features are also predicted , with intensities and equivalent widths highly depending on the geometry .
moreover , these would be the only spectral features revealing the presence of this component if the line of sight was not obscured by the ionized gas ( see also matt , 1998 , this conference ) .
however photoionization models predict quite low emission and absorption line intensities and equivalent widths , with the latter ranging from @xmath2 to @xmath3 ev depending on the geometry and the gas dynamics .
( netzer , 1993 , 1996 , nicastro , fiore & matt , 1998a , matt , 1998 , this conference ) .
for this reason moderate resolution soft x - ray spectrometers ( @xmath4 kev @ 1kev ) , have so far permitted only the detection and marginal separation of the ovii and oviii absorption k edges at 0.74 and 0.87 kev respectively ( the strongest predicted absorption features ) .
this has allowed estimates of the mean ionization degree and column density of the `` warm '' gas ( reynolds et al . , 1997 ) .
many more questions concerning the physical state , the geometry and the source of ionization of the gas are still open , and unambiguous answers can be found only by the next generation of x - ray spectrometers , with greatly improved energy resolution . in this work we present our `` photoelectric+resonant absorption '' model ( nicastro , fiore & matt , 1998a ) , and
show its diagnostics capability in conjuction with high resolution , high collecting area x - ray spectra of agn , as will be available with the next generation of x - ray gratings and calorimeters onboard axaf , xmm and constellation - x .
details of our model are described in nicastro et al .
( 1998a , 1998b ) .
we present here two spectra transmitted from single outflowing ( v=1000 km s@xmath5 ) clouds of photoionized and collisionally ionized gas respectively .
turbulence " velocity is @xmath6 km s@xmath5 ( note that we have not included emission line spectra , since these are highly geometry dependent )
. the column density in both cases is fixed at @xmath7 @xmath8 ( a commonly observed value , reynolds , 1997 ) and we adopt an ionization parameter ( logu=0.1 ) and an electron temperature ( logt@xmath9=6.5 ) such that , in both cases , the relative ionic abundances of ovii and oviii are similar .
we adopt a small covering factor ( as seen from the central source ) of @xmath10 .
this allows us to neglect the contribution of the gas emission .
the 0.1 - 2.5 kev flux is 2.6@xmath11 erg s@xmath5 @xmath8 , corresponding to a rosat - pspc count rates of 2.5 ct / s , similar to that observed from the brightest seyfert 1 nuclei ( e.g. ngc  3516 , mathur et al . , 1997 ) .
1 shows these two transmitted spectra .
the main resonance absorption lines are labeled in both panels .
the different line ratios in the two cases are mainly due to the broader ionizing photon distribution ( a multi - power law ) in the photoionization case , compared with the narrow electron temperature distribution ( a maxwellian ) in the collisional case ( nicastro et al . , 1998b ) .
= 3.6 in = 3.6 in
the new generation of x - ray spectrometers will offer the opportunity to detect and separate narrow ( @xmath12 ev ) absorption features .
we built a simple `` simulator '' to fold our models with the responses of ( a ) the axaf - medium energy grating ( axaf - meg : @xmath13 ev , collecting area = 100 @xmath14 @ 1 kev , `` axaf proposer s guide '' , vs 1.0 , 1997 ) ; ( b ) the xmm - reflecting grating spectrometer ( xmm - rgs 1st order : @xmath15 ( 1.5 in 2nd order ) ev , collecting area @xmath16 ( 200 in 2nd order ) @xmath17 @ 1 kev ) ; and ( c ) the baseline constellation - x calorimeter ( @xmath18 ev , collecting area @xmath19 @xmath17 @ 1 kev , `` the high throughput x - ray spectroscopy ( htxs ) mission '' , 1997 ) .
we added statistical ( and instrumental , when available ) noise .
2 shows the 80 ks xmm - rgs simulation ( the intermediate case , as far as the collecting area is concerned ) .
spectra of such and even better signal to noise will be possible for all the known seyfert 1 galaxies with `` warm absorber '' .
spectra of comparable quality can be obtained for source with 2 - 10 flux down to @xmath20 erg s@xmath5 @xmath8 ( @xmath21 targets ) .
= 3.6 in = 3.6 in    fig .
3 shows a particular of axaf - meg and xmm - rgs ( 1st and 2nd order ) simulations of the same model of fig . 1 ( upper panel ) , for two values of the `` turbulence '' velocity @xmath22 and 1000 km s@xmath5 .
the bottom panel of this figure shows the same particular for a 20 ks constellation - x simulation and for the case @xmath23 km s@xmath5 .
= 3.6 in = 3.6 in    the factor of @xmath24 larger collecting area of xmm - rgs , compared to axaf - meg , will allow detailed spectral variability studies of warm absorbers in bright agn significantly changing their intensity state on timescales of @xmath25 ks ( e.g. ngc  5548 ) .
show this capability for two spectra emerging from gas in photoionization equilibrium with ionizing continua differing by a factor 2 in intensity .
= 3.6 in = 3.6 in
the optical depth @xmath26 of the resonance absorption line due to the @xmath27 transition of the ion @xmath28 of the element @xmath29 , at the core frequency @xmath30 is given by the product between the total optical depth and the voigt profile at the core frequency : @xmath31 .
the total optical depth depends linearly on ( a ) the chemical composition a@xmath32 , ( b ) the relative ionic abundance @xmath33 , ( c ) the equivalent hydrogen column density n@xmath34 , and ( d ) the oscillator strength @xmath35 : @xmath36    a measure of both the optical depth and the relative intensity at the center of two observed lines produced by two ions of a same element may then allow for an estimate of the relative ionic abundances .
this in turn allows ( a ) for unambiguously distinguishing between photoionization and collisional ionization ( see table 1 , col .
ii , `` phot .
vs coll '' ) , and ( b ) for a stringent test of time - evolving non equlibrium photoionization model ( see fig .
4 , lower panel ) .
furthermore , the measure of the ratio between two lines due to two transition of the same ion of the same element , will permit to estimate the chemical composition of the gas .
finally , the position of the line and its width can give us information on the dynamical state of the gas .
accurate measure of blueshift or redshift of the absorption lines will allow to estabilish if the gas is outflowing or inflowing respectively .
furthermore line widths greater than predicted by the only thermal motion of the ions will permit to estimate the degree of turbulence of the gas along the line of sight ( e.g. neix , nex , see fig .
we use the data simulated with the axaf - meg , the xmm - rgs ( 1st and 2nd order ) and the constellation - x calorimeter to measure some of the above quantities , and present the results in table 1 .
the model parameters are those described in  2 , except for the turbulence velocity which is here @xmath22 km s@xmath5 .
a number of emission and absorption features are expected to be visible in high energy resolution x - ray spectra of type 1 agn with ionized gas along the line of sight ( so called  warm absorbers " ) . 
 emission strongly depends on the geometrical configuration of the gas , while absorption along the line of sight does not . 
 absorption features include photoelectric absorption k and l edges along with many strong k@xmath0 , k@xmath1 and l resonance absorption lines from the most abundant elements . 
 we present detailed simulations of our `` photoelectric + resonant absorption '' model with the high energy resolution gratings and calorimeters of axaf , xmm and constellation - x , and discuss the relevant physics which can be addressed with the new generation of x - ray spectrometers .