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the process of accretion is the power source driving the luminosities for a wide range of objects , including protostars , binary systems containing accreting white dwarfs , neutron stars or black holes , gamma ray bursts and supermassive black holes in active galactic nuclei . a comprehensive review of this process is provided by frank , king & raine ( 2002 ) .    in most of these systems , most of the time , the accretion process proceeds via an accretion disc which transports angular momentum outwards and matter inwards . the temperature of this disc increases towards the centre , and is a function of accretion rate and central accretor mass . the disc may also produce ( or even be partially replaced by ) at various times a relatively cool disc wind , a very hot corona and a collimated relativistic outflow or ` jet ' . finally , the ` state ' of the accretion flow at the centre of the disc may vary dramatically indicating rapid variations between phases with different geometries , temperatures and outflows . reviews of observation of accretion onto white dwarfs , neutron stars and black holes in binary systems can be found in e.g. warner ( 2003 ) and lewin & van der klis ( 2006 ) .    in x - ray binary systems containing an accreting neutron star or black hole the region of the accretion disc responsible for the optical continuum and emission lines lies at a large distance from the central accretor ( several light seconds , or @xmath3 gravitational radii e.g. hynes et al . many such systems are transients , in that they display phases of very high luminosities lasting typically weeks to months , followed by long periods in quiescence ( e.g. chen , shrader & livio 1997 ) . such cycles are likely to have their origin in accretion disc instabilities associated with hydrogen ionization ( frank et al . 2002 and references therein ) . it is widely accepted that the emission lines in such systems arise in the rotating accretion disc flow . the strongest evidence for this interpretation is in the form of twin - peaked line profiles ( e.g. horne & marsh 1986 ; charles & coe 2006 and references therein ) . the dominant pumping mechanism for these lines is likely to be irradiation by the central x - ray source at high luminosities , as is observed for the optical continuum ( van paradijs & mcclintock 1994 ) . at lower luminosities viscous heating of the disc may contribute significantly . however , the picture may not be so simple . russell et al . ( 2006 and references therein ) have demonstrated that in black hole x - ray binaries ( bhxbs ) in ` hard ' x - ray states synchrotron emission from the jet dominates the continuum in the near - infrared and can contribute significantly in the optical band . wu et al . ( 2001 , 2002 ) argue for three distinct origins for the emission line depending on the source luminosity and spectral states . in their model , in bright / soft x - ray states the line arises in the atmosphere of an optically thick disc , whereas in bright / hard x - ray states the h@xmath0 line arises in a dense outflow . in the faintest quiescent states the whole accretion flow is optically thin .    in a related work , eikenberry et al . ( 1998 ) found a near - linear correlation between br@xmath4 ( 2.166 @xmath5 m ) integrated line flux and the adjacent continuum flux density in the black hole x - ray binary grs 1915 + 105 , during phases when the continuum was likely to be synchrotron emission from the powerful jet in this source . this suggests radiative pumping of the lines by uv emission from the jet . comparison with phases of soft x - ray spectra with no jet emission in the same study were taken to indicate that the jet is a far better source of photoionising photons than the inner accretion disc , presumably due to the fact that the ultraviolet - emitting region of the jet is raised above the disc and illuminates it very efficiently ( this argument does not work if this region of the jet is moving relativistically away from the disc and therefore strongly doppler de - boosted ) . however , a strong change in h@xmath0 ew associated with x - ray state , and therefore jet production , is _ not _ observed in gx 339 - 4 ( wu et al . 2001 ; see also fig 1 ) , possibly arguing against strong pumping of h@xmath0 by the jet .    in this paper we have compiled measurements of the strength of h@xmath0 , the most prominent emission line in the optical spectra of x - ray binaries . our goal was to see how the properties of the emission line varied , if at all , with accretion ` state ' and luminosity of x - ray binary systems , and whether or not there was clear evidence for phases where the lines were formed in an outflow rather than a disc . what we find is evidence for an anti - correlation between the equivalent width ( ratio of line to local continuum flux ) and overall luminosity of such systems , in particular when in the ` hard ' x - ray state ( at luminosities below about 1% of the eddington limit ) .    [ cols="^,^,^,^",options="header " , ]      the approach taken in section 2.1 , in plotting h@xmath0 ew as a function of time for bhxbs is a strong but indirect indication of an anti - correlation of the ew with luminosity . a more direct comparison of luminosity and ew would be desirable , to rule out some unexpected mechanism rather than luminosity changes dominating the observed effect . unfortunately , the majority of the optical spectroscopic observations do not appear to be well flux - calibrated , so we can not directly compare ew with optical continuum luminosity . a second possibility is to compare with an x - ray measurement which has been made contemporaneously , which is what we attempt next . in order to be considered , the x - ray and optical spectroscopic measurements had to be made within one day of each other , except at quiescent levels at which we assumed a more or less stable level had been reached ( quiescent variability can reach factors of several , but this not very significant for these logarithmic plots ) . in fig 4 we plot h@xmath0 ew as a function of @xmath6 for the sample of objects discussed previously ( except for v404 cyg for which we do not have contemporaneous x - ray measurements ) , plus the more unusual object gx 339 - 4 . none of the individual sources show a statistically significant rank ( anti-)correlation ( see table 2 ) . in fig 5 we plot the ensemble of data points , plus a small amount of additional data from the sources v404 cyg , gro j1655 - 40 , 4u 1957 + 11 , gs 2000 + 25 , xte j1650 - 500 and xte j1859 + 226 ( see table 1 for references ) .    a spearman rank correlation test for the complete sample plotted in fig 5 results in a spearman rank correlation coefficient of @xmath7 ( a @xmath8 result ) . this supports the conclusion of section 2.1 . a single power - law fit to the data presented in fig 5 gives    @xmath9    this is close to , but steeper than , the ew @xmath10 estimated in section 2.1 , and indicates a mean x - ray decay e - folding time less than the 30 days given in chen et al . the apparent anti - correlation seems to be dominated by the difference between measurements at @xmath11 erg s@xmath12 , where all sources are in the hard x - ray spectral state , and those at higher luminosities , where transitions between different spectral states can occur ( see e.g. nowak 1995 ; homan & belloni 2005 ; remillard & mcclintock 2006 for a discussion of these states ) . it is somewhat puzzling that the anti - correlation stands out much more clearly in the first analysis ( figs 2 and 3 ) than in the second ( figs 4 and 5 ) , which implies that there may not be a simple one - to - one relation between x - ray luminosity and h@xmath0 ew . in this context it is interesting to note that the low - frequency qpos in some hard state black hole candidates show similar monotonic behaviour with time which is not so simple when compared to x - ray flux ( e.g. xte j1118 + 480 in wood et al . 2001 ) . in any case , the anti - correlation , whether simple or not , is clearly there . far fewer results were available in the literature for neutron star systems ; the details and references are provided in table 1 . in fig 6 we plot the neutron star data alongside the black hole data presented in fig 5 . what we see is approximately similar behaviour , in that there is a suggestion of a similar trend , with a similar ( or slightly lower ) normalisation . in fact a spearman rank correlation test indicates that the sample of seven neutron star measurements are anticorrelated at the @xmath13 level ( @xmath14 ) . so the neutron star data are consistent with , but do not independently establish , the anti - correlation found for the bhxbs .    adding the bhxb and neutron star samples results in a spearman rank correlation coefficient of @xmath15 , corresponding to an anticorrelation at the @xmath16 level .    in summary , although the individual simultaneous @xmath6 and ew measurements do not confirm the anti - correlation further , the effect is significant when considering the ensemble of all data points , moreso when combining neutron star data with the black hole sample . the upper x - axes of figs 5 and 6 also indicate a rough estimate of the optical  infrared luminosities , @xmath17 based on the relations presented in russell et al . ( 2006 ) , and repeated in the figure captions ( without statistical uncertainties ) . note that the relations are slightly different for black holes and neutron stars ( @xmath18 is larger for black holes at any given x - ray luminosity ) . this allows us to get a better idea of how the ew varies as a function of the optical continuum . subject to caveats about the possibly complex nature of the relation to the lines , we can go further . by definition ,    @xmath19    where @xmath20 and @xmath21 are the line and continuum luminosities respectively . from russell et al . ( 2006 ) ,    @xmath22    where the value of @xmath23 is slightly different for bh and ns systems . we also fit a crude relation of the form    @xmath24    above . if we can assume @xmath25 ( i.e. @xmath26 which is justified for a maximum ew of @xmath27 angstroms and a filter width of @xmath28 angstroms , _ and _ a large fraction of the data in russell et al . ( 2006 ) being in the r - band ) we get    @xmath29    substituting , we get    @xmath30    for the black holes , @xmath31 and @xmath32 , resulting in    @xmath33    given the crudeness of the fit to the ew-@xmath34 relation , there seems little point in propagating uncertainties on these relations . while this analysis is crude , it does demonstrate that the h@xmath0 line luminosity must vary quite strongly with x - ray luminosity , but not quite as fast as the continuum . a key diagnostic of the emission site of a spectral line is the line profile itself . a twin - peaked line profile is a strong indication that the line originates in a rotating flow , such as the atmospheres of geometrically thin accretion discs ( e.g. horne & marsh 1986 ; note however that murray & chiang 1996,1997,1998 have shown that accretion discs with winds can produce single - peaked lines ) .    in fig 7 we indicate which lines , from the sample presented in fig 5 , were reported as twin - peaked . we see that twin - peaked line profiles have been reported at nearly all luminosities , which strongly suggests that the line - emitting region is ubiquitously associated with the rotating accretion flow . as already noted in the introduction , this is not universally accepted , and a more complex picture is put forward by wu et al . ( 2001 , 2002 ) who argue for three distinct origins for the emission line depending on the source luminosity and spectral states . in all cases , the optical continuum and lines are likely to be excited by irradiation from a central hot continuum source at least at high luminosities ( see arguments in van paradijs & mcclintock 1994 , and the more recent global study by russell et al . 2006 ) . as noted in the introduction , however , it has been suggested that the ionising source may in fact be ( at times ) associated with an outflowing jet or corona and not necessarily a static central x - ray source ( see e.g. eikenberry et al . 1998 ; beloborodov et al . 1999 ; markoff et al . 2005 ) . although we do not think it has an important impact on this analysis , we note that double - peaks may be filled in if the inclination is low or additional components produce low velocity emission . similarly , single peaked profiles can also be mainly from a disc and vice - versa an intrinsic single peaked profiles with central absorption could give the appearance of a double - peaked line without needing a disc - like flow .    the observed anti - correlation between h@xmath0 ew and x - ray luminosity ( both direct and inferred ) implies that as the x - ray luminosity decreases the optical continuum flux drops faster than the line flux . the analysis in section 2.3 above demonstrates that , over large luminosity ranges at least , all three quantities ( x - ray luminosity , optical luminosity , h@xmath0 line flux ) are dropping , but at different rates ( x - rays drop fastest , line flux slowest ) . the optical continuum in bhxbs appears to have contributions from both thermal emission from the outer , irradiated , accretion disc plus , in hard x - ray states , a component associated with optically thin synchrotron emission from a jet ( russell et al . the jet contribution is strongest at longer wavelengths , dominating in the infrared band , but probably contributes no more than 30% of the luminosity in the r - band ( corbel & fender 2002 ; homan et al . 2005 ; russell et al . 2006 ) , which contains the h@xmath0 line .    note that at low luminosities the companion star may begin to have a discernible contribution to the continuum luminosity of the systems . russell et al . ( 2006 ) compile the estimated optical luminosities of several x - ray binaries , and find that in most systems the companion only contributes significantly in quiescence ( i.e. @xmath35 erg s@xmath12 ) . marsh et al . ( 1994 ) show how in the quiescent system a0620 - 00 the h@xmath0 ew is modulated at the orbital period , presumably due to varying continuum contributions from the tidally distorted companion . we do not attempt in this paper to subtract such a contribution , noting that ( i ) over the range of luminosities covered in this compilation it is probably not a major effect , ( ii ) any contribution to the continuum by the companion would serve to _ increase _ the true disc ew at low luminosities and strengthen the anti - correlation were it taken into account . however , it is worth noting that in a regime in which an approximately constant continuum level is set by the companion , the true line flux may be proportional to the measured ew . based on a wealth of observational data , we have an approximate picture of how the emitted spectra and , to a lesser extent , central accretion / outflow geometry , might vary in bhxbs , principally as a function of luminosity . for example , it is known that an approximately steady jet is produced at relatively low accretion rates ( @xmath36 ) which are associated with hard x - ray spectra , whereas at higher accretion rates transitions to softer x - ray states with weaker jets can occur ( fender , belloni & gallo 2004 ; homan & belloni 2005 ) . at lower accretion rates a simple relation between radio and x - ray luminosities appears to hold to at least @xmath37 ( gallo et al . there is however some evidence that the x - ray spectrum does soften at very low luminosities ( corbel , tomsick & kaaret 2006 ; corbel , krding & kaaret 2008 ) . in the case of accreting white dwarfs in cataclysmic variables , williams ( 1980 ) argued that the outer region of the accretion discs may at some times be optically thick in the line but thin in the continuum . if a similar situation exists during the decay phase of x - ray transients outbursts , it may well contribute significantly to the observed ew anti - correlation . the observed anti - correlation is likely to be due to some combination of these changes in the accretion / outflow geometry , the irradiating spectrum , and the optical depth in the outer accretion disc . a key question is whether or not there are changes in the line properties across the accretion state transitions . as noted earlier this was one of the key motivations for this research ; however , our findings are inconclusive . on the one hand the line profile and ew of h@xmath0 in gx 339 - 4 is clearly very similar in bright hard or soft states ( fig 1 ) , but the analysis in section 2.1 suggests that the anti - correlation is better in the hard state below @xmath38 . results for the black hole transient gro j1655 - 40 presented in shrader et al . ( 1996 ) also suggest a large increase in h@xmath0 ew across a soft @xmath39 hard state transition , although the soft x - ray luminosity of the source would also have been fading during this period . the reader is reminded that wu et al . ( 2001 ) claim that the line profile changes from hard to soft x - ray states . a possible explanation for the observed anti - correlation is that a large part of the outer disc becomes cold enough to be optically thin in the continuum , not the line , during the decline ( williams 1980 ) . in that case , the optical continuum drops much more quickly than the ionizing continuum from the inner region ( where the inflow is still optically thick and producing strong continuum emission ) . as a result the ew of the optical lines from the outer ( optically thin ) region should increase . such optically thin regions of the outer disc may well exist in x - ray binaries ( e.g. canizzo & wheeler 1984 ) . shahbaz et al . ( 2004 ) discuss the possibility of regions of different optical depth contributing to the h@xmath0 and h@xmath40 emission from the bhxb a0620 - 00 in quiescence . it is also possible that there is a saturation effect , in that during outbursts most of the accretion disc becomes too hot for the production of h@xmath0 line emission . the effect should therefore be different for higher excitation lines such as heii . although the models mentioned above may well be responsible for the observed anti - correlation , studies over the past decade have indicated that the accretion flow in x - ray binaries , in particular black hole systems , undergo dramatic changes on short timescales which could well have an effect upon observed emission line profiles . for this reason we summarize this empirical understanding below , and comment on whether or not it might affect the observed anti - correlation . if , as we assume , the h@xmath0 is a result of irradiation of the outer disc , then the main source of this irradiation will be euv photons with energies between about 13  25 ev . these photons are not directly observable in nearly all black hole x - ray binaries , which lie at several kpc in the galactic plane and as a result suffer from strong interstellar extinction . lower energy photons can not ionise hydrogen , and photons with significantly higher energies ( i.e. x - rays ) are more likely to contribute to the reprocessed ( reflection ) continuum ( e.g. ross & fabian 1993 ) . we do not consider here the possibility of collisional excitation in e.g. an accretion disc hotspot . how plausible is it that changes in the ionising spectrum are responsible for the observed anti - correlation ? in bright soft states ( @xmath41 ) the x - ray spectra of bhxbs are dominated by thermal ( accretion disc ) components with kt @xmath42 kev . in less bright , but still very luminous ( @xmath43 ) , hard x - ray states the x - ray spectrum is dominated by a component which peaks at @xmath27 kev and probably has its origin in a comptonising ` corona ' ( e.g. sunyaev & titarchuk 1980 ) , with some possible contributions from a jet ( e.g. markoff , nowak & wilms 2005 ) . it should be noted that there can be strong hysteresis between spectral state and luminosity for @xmath44 ( homan & belloni 2005 ) . as a source drops further in luminosity the accretion disc temperature should drop monotonically until in ` quiescence ' ( @xmath45 ) it is only about 1 ev ( mcclintock , horne & remillard 1995 ; mcclintock et al . as noted above , at most x - ray luminosities this thermal disc component is essentially unmeasurable as it peaks at ( e)uv wavelengths , in which band most distant galactic plane bhxbs can not be observed . in the case of xte j1118 + 480 ( mcclintock et al . 2003 ) this disc component was found to be significantly more luminous than the x - ray component when in quiescence . therefore , at the lowest x - ray luminosities the bulk of the photons released by the accretion process are not able to ionise hydrogen , which  naively  should result in a reduced h@xmath0 ew at the lowest luminosities , _ contrary _ to what is observed . however , the optical continuum is also dominated by reprocessing and so the overall effect on ew will depend upon which component is pumped most effectively by the ionizing continuum , which in some sense returns us to the models dealing with the optical depth of the outer disc ( e.g. williams 1980 ) .    what about changes in the geometry of the accretion flow and outflow with luminosity ? some recent sketches of the accretion flow as a function of luminosity for bhxbs ( esin , mcclintock & narayan 1997 ; done , gierlinski & kubota 2007 ) suggest that as luminosity increases the ability of the central hard x - ray source to illuminate the disc is reduced , as the coronal component shrinks . geometries which include jets ( e.g. fender , belloni & gallo 2004 ; ferreira et al . 2006 ) may be more complete , although it is unclear how significantly any x - ray emission from the jet could contribute to irradiation of the accretion disc or its atmosphere ( markoff , nowak & wilms 2005 ) . such x - ray emitting regions near the base of the jet may be essentially indistinguishable from an outflowing corona ( e.g. beloborodov 1999 ) . nevertheless , it has been suggested that the velocity of the outflow from x - ray binaries may increase with luminosity , and in a more complex way with spectral state . if the jet or outflowing corona is responsible for some irradiation of the disc , then an increase in velocity with luminosity could result in a reduced h@xmath0 ew as an increasing fraction of the x - rays are beamed along the direction of motion , away from the disc . a prime motivation for this research was to investigate whether the profiles of optical emission lines change significantly across the accretion state transitions in x - ray binary systems . such a result may have indicated a strong response to a changing irradiating spectrum and/or geometry of the line - emitting region . however , the results in this respect remain inconclusive .    what we did find , and report in detail , is an anti - correlation between the broadband luminosity ( optical  x - ray ) and the equivalent width of the h@xmath0 emission line in x - ray binaries . possibly the most closely related phenomenon already known is a comparable anti - correlation in the h@xmath40 line for cataclysmic variables ( patterson 1984 ; see also witham et al . 2006 ) , accreting binaries hosting a white dwarf rather than a black hole or neutron star . patterson ( 1984 ) compares this result with the models of tylenda ( 1981 ) , which are similar to those of williams ( 1980 ) in which the outer accretion disc becomes optically thin at low luminosities . this may also turn out to be the appropriate explanation for the x - ray binaries under discussion here , but the complex and hysteretical accretion state changes about which we have learned much in the past decade caution us against drawing such a conclusion at this time . more detailed observations of line profiles across state transitions would clearly be of interest . the reader is further reminded that ( i ) rebrightenings during the decay of transients , ( ii ) the monotonic behaviour of qpos in sources such as xte j1118 + 480 despite varying behaviour in @xmath6 , and ( iii ) the poorer anticorrelation of ew with quasi - simultaneous @xmath6 than with time all suggest that the causal link between luminosity and ew may be complex . it it worth mentioning in passing the similarity with the baldwin effect observed in accretion flows around supermassive black holes ( baldwin 1977 ; see also e.g. mushotzky & ferland 1984 ) . in this case an anti - correlation is observed between the ew some lines originating in the broad line region ( blr ) and the continuum luminosity . a related effect is observed in the x - ray band ( iwasawa & taniguchi 1993 ; page et al . 2004 ) , and maybe also in the strong stellar winds of wolf - rayet stars ( morris et al . 1993 ) . however , there is little evidence for a blr - like region in x - ray binary systems , probably due to the high ionisation state of the gas in the inner disc region ( e.g. proga , kallman & stone 2000 ) . therefore the physical origins of the two effects are almost certainly rather different . further observations would obviously be of interest to understand better this effect . these should include observations of other lines and in other bands . observations in the near - infrared , where the continuum should have a much stronger jet contribution ( e.g. corbel & fender 2002 ; russell et al . 2006 ) should show a much stronger reaction to accretion state changes ( e.g. we would expect a large jump in ew of a line like br @xmath4 as the jet switches off in a hard @xmath39 soft state transition ) . it is interesting to note that the spectra in fig 1 hint at a similar anti - correlation for hei , and heii would be even more interesting , given the higher ionisation potentials . the observed anti - correlation holds the promise of being able to estimate the luminosity of a source from the h@xmath0 ew , something which could prove invaluable in distance determinations for faint x - ray binaries . however , the relation obviously needs to be significantly improved before that is possible , and may turn out to have enough intrinsic scatter to reduce its usefulness in this aspect ( as is in fact the case for the baldwin effect in agn ) . in any case it is promising that low - luminosity accreting sources should be clearly identifiable in e.g. h@xmath0 surveys by their large equivalent widths .    finally , it is interesting to note that observations of the variation of the h@xmath0 line strength with luminosity may in fact be our best way to track the behaviour of the accretion disc component at intermediate luminosities . below @xmath46 the accretion disc component peaks in the ( e)uv spectral regime and is unobservable for nearly all sources . for two sources only has it been observed in quiescence , at which point it appears that it no longer extends to the isco but is truncated ( mcclintock et al . 1995 , 2003 ) . although well - defined models exist ( e.g. dubus , hameury & lasota 2001 ) , exactly how and where this truncation begins is observationally very uncertain ( e.g. miller et al . 2006 ) . however , since the h@xmath0 line responds preferentially to photons of energies 1325 ev , it may be the best way to study the evolution of this disc component at luminosities @xmath47 . we would like to thank cristina zurita for providing tables of data for xte j1118 + 480 , and tariq shahbaz and gavin dalton for providing some of the data for fig 1 . we would like to thank guillaume dubus , daniel proga and kinwah wu for careful reading of , and useful comments on , an earlier version of this work , and the anonymous referee for advice which helped to improve this work . wu k. , soria r. , johnston h. , hunstead r. , 2002 , in : the ninth marcel grossmann meeting . proceedings of the mgixmm meeting held at the university of rome `` la sapienza '' , 2 - 8 july 2000 , eds . : vahe g. gurzadyan , robert t. jantzen , remo ruffini . singapore : world scientific publishing , p 2274 ( astro - ph/0101390 )
we report an anticorrelation between continuum luminosity and the equivalent width ( ew ) of the h@xmath0 emission line in x - ray binary systems . the effect is evident both in a universal monotonic increase in h@xmath0 ew with time following outbursts , as systems fade , and in a comparison between measured ews and contemporaneous x - ray measurements . 
 the effect is most clear for black hole binaries in the low / hard x - ray state , which is prevalent at x - ray luminosities below @xmath1% eddington . 
 we do not find strong evidence for significant changes in line profiles across accretion state changes , but this is hampered by a lack of good data at such times . 
 the observed anti - correlation , highly significant for black hole binaries , is only marginally so for neutron star systems , for which there are far less data . 
 comparison with previously established correlations between optical and x - ray luminosity suggest that the line luminosity is falling as the x - ray and optical luminosities drop , but not as fast ( approximately as @xmath2 ) . 
 we briefly discuss possible origins for such an effect , including the optical depth , form of the irradiating spectrum and geometry of the accetion flow . 
 further refinement of the relation in the future may allow measurements of h@xmath0 ew to be used to estimate the luminosity of , and hence the distance to , x - ray binary systems . beyond this , further progress will require a better sample of spectro - photometric data . 
 accretion , accretion discs  x - rays : binaries