document: symbiotic systems ( ss ) are generally composed by a white dwarf ( wd ) , a red giant ( rg ) star , and by circumstellar and circumbinary nebulae . 
   gas and dust radiation from the nebulae appear throughout a large frequency range , from radio to x - rays . 
   z andromedae ( z and ) binary system consists of a cool giant of spectral type m4.5 and a hot compact component with a temperature of @xmath0 1.5 10@xmath1 k. many periods of activity have been reported during more than 100 years , in 1915 , 1939 , 1960 , 1984 , and 2000 as nonuniform eruptions of classical outbursts . 
   sokolowski et al . ( 
   2006 ) claim that the _ outbursts sometimes come in pairs ( as in 1984 and 1986 ) _ , or in a series of eruptions with decreasing maximum brightness and different shapes , separated in time by periods slightly shorter than the orbital one ( kenyon & webbink 1984 ) . 
   the main characteristics of the 1984 - 1986 event are the two maxima which appear in the profile of the visual magnitude ( fernndez - castro et al . 
   1995 , hereafter fc95 , fig . 
   1 ) leading to the ambiguous interpretation of the outburst as a double or a disturbed single one . 
   fc95 suggested , on the basis of the line ratio analysis , that a drop in the wd radiation flux could provoke the observed trend of the light curve in 1986 . 
   they claim that a shell of material was ejected during each of the _ two outbursts _ at 1984 and 1986 . 
   the brightness of the 1984 - 1986 event was exceptionally low ( leibowitz & formiggini 2008 , fig . 
   nevertheless , it was monitored at very close dates by the spectral observations of fc95 . 
   the spectra are rich enough in number of lines to allow a detailed modeling of z and physical conditions .    in this paper 
   we revisit z and system focusing on the physical conditions in the nebulae and their fluctuations in the 1984 - 1986 years . 
   our aim is to reveal unpredicted episodes by the detailed modeling of the spectra .    in z and , 
   the stellar wind of the cool giant has a velocity of @xmath0 25 ( sequist et al . 
   bisikalo et al ( 2006 ) claim that varying the wind velocity from 25 to 30 changes the accretion regime from disk to wind accretion . 
   this is accompanied by a jump in the accretion rate , increasing the hydrogen burning rate . 
   an optical thick wind forms from the wd . this wind is revealed e.g. by the observations of uv and optical spectra during the 2000 - 2002 outburst ( e.g. sokoloski et al 2006 , tomov et al 2008 ) . 
   the shocks which derive from wd and rg wind collision yield an increase in the luminosity of the system . 
   this hypothesis is strengthened by fc95 who did not find evidence for an accretion disk at the 1984 - 1986 epoch but suggested collision episodes . 
   collision of the winds ( girard & willson 1987 , bisikalo et al 2006 , angeloni et al 2010 , contini & angeloni 2011 ) leads to two shock fronts between the stars : the strong one , dominating the spectrum , propagates in reverse towards the wd , while the weakest one propagates towards the rg . 
   moreover , a shock front expands out of the system throughout the circumbinary medium . 
   we calculate the line spectra in sect . 2 in the frame of the wind collision model , adopting the code sumamarcel / suma / index.htm ] , which accounts consistently for shocks and photoionization . 
   the input parameters are : the shock velocity , the preshock density 0 , the preshock magnetic field 0 , the colour temperature of the hot star , the ionization parameter @xmath2 relative to the black body ( bb ) flux reaching the nebula . 
   the geometrical thickness of the emitting nebula @xmath3 , the dust - to - gas ratio @xmath4 , and the abundances of he , c , n , o , ne , mg , si , s , a , fe relative to h are also accounted for . 0=10@xmath5 gauss is adopted . 
   the observations at different epochs of the continuum spectral energy distribution ( sed ) are reproduced in sect . 3 by models constrained by the fit of the line ratios . 
   the adjustment factors determine the distance of the nebulae downstream of shock fronts from the system center , providing a detailed picture of the z and components . 
   discussion and concluding remarks follow in sect . 
   the trend of the spectral observations between 1978 and 1993 ( fc95 , figs . 
   4,5 ) gives a first hint about the 1984 - 1986 outburst in the frame of a longer activity period . 
   we notice that :    \1 ) the 1984 - 1986 outburst lies upon an event which developed about at 1979 and ended at 1988 . 
   if this long event depends on the wd activity , then the temperature of the wd will not show dramatic changes in the 1984 - 1986 period . 
   \2 ) the oi 1305 resonance line is most probably blended with an upper ionization level line because its behaviour is similar to that of high level lines the oi line is most probably emitted from a different nebula with conditions close to those of the ism . 
   \3 ) the systematic decrease and increase of the cii 1336 line , whose trend is opposite to that of the high ionization lines , indicates that the cii minimum is strongly correlated with the trend of the physical parameters , e.g. the ionization parameter , and the shock velocity . 
   the role of the wd temperature is less prominent . 
   fc95 in their table 3 report an optical spectrum in july 1986 , observed at the same epoch as the uv spectrum presented in their table 2b , row 34 . 
   we start by modeling the uv - optical combined spectrum observed at 11 july 1986 which is constrained by a relatively large number of lines from various ionization levels . 
   this reveals the physical conditions in the emitting nebulae at that time . 
   such conditions will be adopted as a first guess in the modeling of the uv spectra in the next epochs .    [ 
   cols="<,<,<,<,<,<,<,<,<",options="header " , ]     @xmath6 2440000 +     3 ) : red ; ( in units of 10@xmath1 k ) : blue ; @xmath2 : green . 
   dotted black lines define the outburst.,scaledwidth=98.0% ]     3 ) : red ; ( in units of 10@xmath1 k ) : blue ; @xmath2 : green . 
   dotted black lines define the outburst.,scaledwidth=88.0% ]      the visual light curve ( fig . 
   1 ) covers a frequency band accounting for lines and continuum in the optical range . 
   the strongest lines are generally heii 4686 , , [ oiii]5007 , 4959 , etc . , while the optical spectrum ( fc95 ) shows relatively low forbidden lines . 
   so we consider that the main contributors to the visual band are heii 4686 and , which depend strongly on the wd temperature and on the ionization parameter . and 
   @xmath2 affect the spectra in the same way . 
   namely , when they decrease , the high level lines decrease relatively to the low level ones . 
   the increase of at the epochs of the peaks and the decrease at minima would indicate that the active phase is characterised by two different bursts . on the other hand , if does not change significantly and only @xmath2 changes , a disturbing dynamical and/or morphological event between the radiation source and the nebula is most likely the source of the visual curve minimum . 
   the uv band accounts for lines from different levels which are variously modified by and @xmath2 fluctuation . 
   moreover , the shock velocity and the compression downstream affect the stratification of the ions and , consequently , the line ratios . 
   fc95 show that lines from different ionization levels have different trends throughout the 1984 - 1986 time period . 
   we predict that the uv light curve throughout the outburst , would have a profile different from that reported in fig 
   . 1 .    in fig . 
   2 we present the physical parameters obtained by modeling the spectra throughout the 1984 - 1986 outburst . 
   the profiles of the physical conditions are rather unexpected if compared with the visual light curve ( fig . 
   the sudden increase of the density on april 1984 , accompanied by the sudden increase of the velocity , indicates that collision of the wd wind with a shell ejected from the red giant is occurring . 
   the nebula downstream of the reverse shock is heated and ionized by both the shock and the photoionization flux from the hot star . in the next days 
   the density and the shock velocity have opposite trends in agreement with conservation of mass at the shock front ( n1v1=n0v0 , from the rankine - hugoniot equations ) . in the meanwhile the temperature of the hot star is slightly increasing from @xmath0 120,000 k to a maximum of 170,000 k , corresponding to the outburst maximum , in agreement with fc95 . 
   the drop of @xmath2 on 1984 toward july 1986 shows that the spectra during that epoch are dominated by a relatively strong shock , i.e. the reverse shock , which vanishes at a certain time , depending on the outburst characteristics ( chevalier 1982 ) . 
   the second density peak at december 1985-january 1986 reveals that collision of the wd wind with another rg shell has occurred . 
   the ionization parameter drop by more than two orders of magnitude after june 1985 , accompanied by the simultaneous variation of the dynamical parameters and 0 , suggests that the shock front has suddenly shifted farther from the wd . 
   the matter downstream of the reverse shock front is fragmented by richtmyer - meshkov and kelvin - helmholtz instabilities . 
   high density fragments between the stars eventually prevent the black body flux from reaching the nebula . 
   however , the decline of the bb flux by fragment obstruction could indeed reduce the ionization parameter , but less sharply than predicted by the calculations . 
   so , the whole picture is most likely explained by an unexpected disturbing episode such as the ejection of a new shell from the rg atmosphere and its collision with the wd wind at a relatively large distance from the wd ( sect . 
   the gas composition at the epochs of collision with the shells reveals that the calculated siiii]/niv ] line ratios would better reproduce the data adopting si / h relative abundances lower than solar by a factor of 4 for models m@xmath7 , m@xmath8 and of @xmath0 10 for models m@xmath9 , m@xmath10 , m@xmath11 , m@xmath12 . 
   this indicates that silicate grains are present after collision of the wd wind with the first rg shell . on the other hand si 
   is in the gaseous phase with a solar si / h relative abundance at the epochs corresponding to models m@xmath13 , m@xmath14 , m@xmath15 , m@xmath16 . 
   the grains had not enough time to form at collision with the next shell . 
   grain formation time scales ( gail et al 1984 , scalo & slavsky 1980 ) are as long as @xmath0 1 year . however , in circumstellar shells the chemical equilibrium and consequently grain formation is disturbed by the uv radiation from the wd and by the velocity field ( gail et al . 
   3 we compare the observed continuum sed with model calculations . 
   we refer to the sed of the continuum flux presented by fc95 in their fig . 
   we have used the data in the uv and in the radio range presented by fc95 tables 2a and 4 , respectively . 
   the dates corresponding to observations in the radio and those in the other frequency ranges are not exactly coincident , e.g. we adapted the radio fluxes observed in october 1986 to data observed in july 1986 . 
   each diagram of fig . 
   3 presents the free - free + free - bound fluxes emitted from the nebula at a certain epoch . 
   they are calculated by the same models ( table 2 ) which lead to the best fit of the line spectrum ( table 3 ) . moreover , 
   the bb flux from the rg corresponding to a temperature of 3200 k ( fc95 ) , and the bb flux corresponding to the wd which was derived by modeling the line spectrum are also shown . 
   reprocessed radiation of dust within the nebula calculated consistently with gas emission is added in fig . 
   3 diagrams . 
   this flux is mostly hidden by the bb flux from the rg if dust - to - gas ratios of 10@xmath17 by number ( 4 10@xmath18 by mass for silicates ) are adopted . 
   actually , the dust - to - gas ratios are constrained by the data in the mid - ir . 
   3 shows that the emission from the nebula appears in a small frequency range between the optical and the near - uv ( @xmath0 1 - 3 10@xmath19 hz ) , depending on the epoch . 
   there are no data in the soft x - ray range and beyond it . 
   we can predict that bremsstrahlung from the nebula downstream of shock fronts with @xmath20 160 would be the x - ray source . for lower velocities 
   the sed in the soft x - ray range is the summed flux of bremsstrahlung from the nebulae and bb flux from the wd . 
   the contribution to soft - x - rays up to the x - ray domain of bremsstrahlung from shocked nebulae was suggested for the symbiotic system ag dra by contini & angeloni ( 2011 ) . 
   the fit of calculated to observed data is good enough to confirm a strong self - absorption of free - free radiation from the nebulae for frequencies @xmath2110@xmath22 hz . 
   finally , fig . 
   3 diagrams show that the bb flux from the rg and from the nebula dominate the sed in the visual , while the flux from the wd appear in the uv and soft x - ray range . 
   this indicates that strong variations in the wd temperature that generally accompany the outbursts can not be directly revealed by the visual light curve presented by fc95 , but only indirectly by the spectra emitted from the nebula .      in previous works on ss , 
   e.g. ch cyg ( contini et al 2009b ) , ag dra ( contini & angeloni 2011 ) , etc . 
   we calculated the distance r of the nebulae from the center of the ss adjusting the continuum fluxes calculated by the models at the nebula to those observed at earth . 
   we defined the adjusting factor @xmath23 by r@xmath24=10@xmath25 d@xmath24 , where is the filling factor and d the distance to earth .    adopting a distance of z and to earth of 1.12 kpc ( viotti 1982 ) , 
   the best fit to the data of the bremsstrahlung calculated from the nebulae downstream of the shock front , gives r= 1.9 10@xmath26 cm at april 1981 , r=1.4 10@xmath26 cm at april 1984 , r=9.4 10@xmath19 cm at december 1985 , and r= 2.7 10@xmath26 cm at july 1986 . 
   the distances were calculated by a filling factor = 1 . 
   considering that : @xmath27 ( r@xmath28/r)@xmath24 = @xmath2 n c ( where @xmath27 is the flux from the wd , r@xmath28 is the radius of the wd , n is the density of the gas and c the speed of light ) , the results suggest that the drop of the ionization parameter @xmath2 between november and december 1985 most probably derives from the sudden increase of the distance from the hot source . 
   this is explained by the ejection of the next shell in the rg atmosphere . 
   the collision between the wind from the wd and the two shells from the rg occurred within less that 2 years , from march 1984 to november 1985 . 
   the shock front has therefore expanded from @xmath0 2 10@xmath26 to @xmath09 10@xmath19 cm , by an average velocity of @xmath0 130 , in rough agreement with the observations . 
   the 1984 - 1986 outburst of z and is revisited by a detailed modeling of the spectra . comparing the visual light curve of z and during the 1984 - 1986 outburst with those in other epochs from 1895 up to 2007 september , the 1984 - 1986 event is remarkable for its double peaked structure and for its low brightness . 
   we explain the burst double structure in the light of red giant pulsation , by the collision of the wd wind with two ejected shells . 
   the minimum of the light curve between the two peaks is accompanied by the dip of the ionization parameter which is constrained by the detailed modeling the uv line spectra at november - december 1985 . 
   the sharp drop of @xmath2 is due to the sudden increase of the distance between the wd and the downstream nebula reached by the wd black body flux , namely , the collision of the wd wind with a new shell in the rg atmosphere . 
   since the shell was recently formed at the time of observations , the grains did not have enough time to develop . 
   this is revealed by the solar relative abundance of si / h . 
   we suggest that the outburst did not attain its maximum luminosity because the collision of the wind network was distorted by the oncoming of the next shell . 
   the small peaks in the light curve before and after the 1984 - 1986 burst are most probably due to rg shell ejections . 
   the periodicity of the maxima is in fact of @xmath0 300 - 400 days .    during the 2000 - 2002 outburst of z and , broad wings ( up to 2000 ) developed in the line profile ( tomov et al 2008 ) . 
   moreover , collimated bipolar jets appeared and disappeared throughout the 2006 outburst ( skopal et al 2009 ) . 
   broad lines were observed in other ss , e.g. ch cyg , bi cru , ag dra , etc . 
   they originate from the high velocity matter accompanying the outbursts . 
   exceptionally broad and line profiles were explained by wd explosion ( contini et al . 
   2009 a , c ) , while collimated jets at some epochs reveal the presence of the accretion disk . both broad lines and jets 
   were not observed during the 1984 - 1986 outburst of z and . 
   we are grateful to sharon sadeh for helpful advise . 
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summary: the visual magnitude profile of the symbiotic system z and during the 1984 - 1986 activity period appears double peaked and the flux intensity is low compared to outbursts in other epochs . 
   the detailed modeling of the observed spectra , accounting for the shells ejected by the red giant star , shows that the outburst is intrinsically single but distorted by the collision at different phases of the white dwarf wind with two close shells . 
   binaries : symbiotic - stars : individual : z and