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the neupert effect is the name given to the observational finding that the rising part of the soft ( sxr ) light curve often resembles the time integral of the hard x - ray ( hxr ) or microwave emission ( , 1968 ; dennis & zarro , 1993 ) . the physical relevance of the neupert effect basically arises from the fact that it is interpreted as a causal connection between the thermal and nonthermal flare emissions , which can be naturally explained within the nonthermal thick - target model ( brown , 1971 ) . in this model , the flare energy is released primarily in the form of nonthermal electrons , and hard x - rays are produced via electron - ion bremsstrahlung when the electron beams impinge on the lower corona , transition region and chromosphere . the model assumes that only a small fraction of the electron beam energy is lost through radiation ; most of the loss is due to coulomb collisions that serve to heat the ambient plasma . as a consequence of the rapid energy deposition a strong pressure imbalance develops between the dense , heated chromosphere and the tenuous corona . the high pressure gradients cause the heated plasma to convect into the corona in a process known as chromospheric evaporation ( antonucci et al . 1984 ; fisher et al .  1985 ) , where it gives rise to enhanced sxr emission via thermal bremsstrahlung . in this case , the hard x - ray flux is linked to the instantaneous rate of energy supplied by electron beams , whereas the soft x - ray flux is related to the accumulated energy deposited by the same electrons up to that time , and we can expect to see the neupert effect . any deviation from the neupert effect , in principle , suggests that the hot sxr emitting plasma is not heated exclusively by thermalization of the accelerated electrons that are responsible for the hxr emission . therefore , investigations of the neupert effect provide insight into the role of nonthermal electrons for the flare energetics . the neupert effect can be expressed as @xmath0 with @xmath1 the sxr peak flux and @xmath2 the hxr fluence , i.e.  the hxr flux integrated over the event duration . the coefficient  @xmath3 depends on several factors , as , e.g. , the magnetic field geometry and the viewing angle , and thus may vary from flare to flare ( lee et al . , however , if @xmath3 does not depend systematically on the flare intensity , then the sxr peak flux and the hxr fluence are linearly related . we utilize the sxr data from the _ geostationary operational environmental satellites _ ( goes ) and the hxr data from the _ burst and transient source experiment _ ( batse ) aboard the _ compton gamma ray observatory_. the x - ray sensor aboard goes consists of two ion chamber detectors , which provide whole - sun x - ray fluxes in the and 18   wavelength bands . batse is a whole - sky hxr flux monitor that , in part , consists of eight large - area detectors . from each detector there are hard x - ray measurements in four energy channels , 2550 , , 100300 and @xmath4300  kev ( schwartz et al . , 1992 ) . for the analysis , the 1-min averaged goes sxr data measured in the 18   channel and the hxr data collected in the batse solar flare catalog , archived in the solar data analysis center at nasa / goddard space flight center for the period 01/199706/2000 are used . the peak and total count rates are background subtracted for the flux below 100  kev . for the sxr events , we used the flux just before the flare start for background . to be identified as corresponding events we demand that the start time difference between a sxr and a hxr event does not exceed 10  min . overlapping events are excluded . applying these criteria , we obtained 1114 events that were observed in both hard and soft x - rays ( for details see veronig et al . , 2002a ) . for each event we determined the difference , @xmath5 , of the peak time of the sxr emission , @xmath6 , and the end time of the hxr emission , @xmath7 . furthermore , the time differences were normalized to the duration  @xmath8 of the respective hxr event , i.e.@xmath9 figure  [ fig_timing ] shows the histogram of the absolute and normalized time differences . both representations of the sxr hxr time difference have its mode at zero . 49% of the events lie within the range @xmath10  min , and 65% within @xmath11  min . for the normalized time differences , we obtain that 44% lie within @xmath12  hxr units , and 59% within @xmath13  hxr unit . this outcome suggests that certainly a considerable part of the events coincides well with the expectations from the effect regarding the relative timing . figure  [ fig_fluencepeak ] shows the scatter plot of the sxr peak flux versus the hxr fluence for the complete sample , clearly revealing an increase of @xmath1 with increasing @xmath14 . it can also be inferred from the figure that the slope is not constant over the whole range but that it is larger for large hxr fluences than for small ones . for very large fluences , the slope approaches the value of  1 , indicative of a linear relation between the sxr peak flux and hxr fluence . we stress that the slope at small fluences might be affected by missing events with small sxr peak fluxes ( due to selection effects ) , and thus appear flatter than it is in fact . figure  [ fig_fluencepeak ] reveals an interdependence between the importance of an event and the sign of the time difference . basically all large flares belong to the group of events with @xmath15 , i.e.  the sxr peak occurs before the hxr end . moreover , the flares with @xmath15 reveal a strong tendency to be of long duration . we obtain a high cross - correlation coefficient for the sxr peak flux and hxr fluence relationship , @xmath16 . this coefficient is higher than those for the sxr peak flux and hxr peak flux , @xmath17 . this indicates that the correlation is primarily due to the hxr fluence sxr peak flux relationship , as predicted from the neupert effect , and not , e.g. , due to the fact that flares with large hxr peak fluxes also tend to have intense sxr counterparts . furthermore , it is important to note that the hxr fluence   sxr peak flux correlation is higher for the events with negative time differences , @xmath18 , than for the events with positive time differences , @xmath19 . on the basis of the relative timing of the sxr peak and the hxr end , we extracted two subsets of events . the events of set  1 are roughly consistent with the timing expectations of the neupert effect , and the events of set  2 are inconsistent with it . the two sets are defined by the following conditions : @xmath20 the applied conditions represent a combination of absolute and normalized time differences in order to avoid as much as possible any a priori interdependence with the flare duration and/or flare intensity . out of the 1114 corresponding hxr and sxr flares , 485 ( 44% ) events fulfilled the timing criterion of set  1 ; 270  events ( 24% ) belong to set  2 ; 359 events ( 32% ) are neither attributed to set  1 nor to set  2 .    in figure [ fig_fluencepeak2 ] , we plot the sxr peak flux versus the hxr fluence separately for set  1 and set  2 . the figure reveals that the two sets have very different characteristics besides the different timing behavior . set  1 contains many more large events and shows a steeper increase of @xmath1 with increasing @xmath2 than set  2 . moreover , set  1 contains more events with negative than positive time difference , whereas almost all events of set  2 are characterized by @xmath21 , i.e.  increasing sxr emission beyond the end of the hard x - rays . on average , for small fluences the events belonging to set  2 have a larger sxr peak flux at a given hxr fluence than those of set  1 , indicating an  excess " of sxr emission with respect to set  1 . the cross - correlation coefficients derived separately for the subsets reveal that the correlation among the sxr peak flux and hxr fluence is much more pronounced for the events of set  1 , @xmath22 , than those of set  2 , @xmath23 . 24% of the events have @xmath15 , i.e.  the sxr maximum occurs before the end of the hxr emission . these events are preferentially of long duration . li et al . ( 1993 ) have calculated time profiles of soft and hard x - ray emission from a thick - target electron - heated model , finding that , in general , the time derivative of the sxr emission corresponds to the time profile of the hxr emission , as stated by the neupert effect . however , for gradual events they obtained that this relationship breaks down during the decay phase of the hxr event , in that the maximum of the sxr emission occurs before the end of the hxr emission . this phenomenon can be explained by the fact that the sxr emission starts to decrease if the evaporation - driven energy supply can not overcome the instantaneous cooling of the hot plasma , which is likely to happen in gradual flares . considering our observational findings together with the results from simulations by li et al . ( 1993 ) , presumably most of the events with @xmath15 are consistent with the electron - beam - driven evaporation model . in particular , the high correlation between @xmath1 and @xmath2 , @xmath24 , supports such interpretation . 56% of the events have @xmath21 ; these events are preferentially of short and weak hxr emission . in principle , the fact that the sxr emission is still increasing although the hxr emission , i.e.  the electron input , has already stopped indicates that an additional agent besides the hxr emitting electrons is contributing to the energy input and prolonging the heating and/or evaporation . however , mctiernan et al . ( 1999 ) have shown that the sxr time profile depends on the temperature response of the used detector : an increase of the sxr emission of low - temperature flare plasma after the hxr end may arise due to cooling of high - temperature plasma . thus , we can not attribute all events with @xmath21 as inconsistent with the electron - beam - driven chromospheric evaporation model . instead , we consider as inconsistent only flares which show strong deviations from @xmath25 , i.e.  the events belonging to set  2 . this means that for at least one fourth of the analyzed events an additional heating agent besides nonthermal electrons is suggested . a probable scenario is that energy is transported from the primary energy release site via thermal conduction fronts , which initiate chromospheric evaporation but do not produce hard . the finding that for a considerable fraction of flares , preferentially weak ones , an additional heating agent other than electron beams is suggested , is not only relevant for the flare energetics but also for parker s idea of coronal heating by nanoflares ( parker , 1988 ) . hudson ( 1991 ) pointed out that if the corona is heated by flare - like events of different sizes , then the flare energy distribution must have a power - law slope @xmath26 . if the sxr flux does not vary systematically with temperature and density , then the sxr peak flux is linearly related to the maximum thermal energy of the flare plasma ( see lee et al . , 1995 ; veronig et al . , 2002a ) . on the other hand , hxr fluence distributions can be considered as representative for the energy contained in nonthermal electrons . in figure [ fig_distr ] , we show the sxr peak flux and the hxr fluence distributions derived from 1114 corresponding sxr / hxr flares , finding @xmath27@xmath280.13 and @xmath29@xmath280.13 , respectively . the discrepancy between the slopes of the hxr fluence and the sxr peak flux distributions was already pointed out and discussed in lee et al . ( 1993 , 1995 ) and veronig et al . ( 2002b ) . the present analysis provides an explanation for this difference in power - law slopes : the relationship between the sxr peak flux and the hxr fluence is not linear , whereby the deviations from a linear correlation are strongest for weak flares ( cf .  figure  [ fig_fluencepeak ] ) .    the soft x - ray flare emission increases due to energy supply by electron beams as well as any other heating agent , whereas the hard x - ray emission contains only information on the energy provided by electrons . together with our finding that particularly in weak events an additional heating agent besides electron beams is suggested , this strongly suggests that soft x - ray peak flux distributions are a more meaningful indicator of flare energy distributions than hard x - ray fluence distributions . furthermore , we have shown that weak flares have different characteristics than large flares , in the sense that electrons are less important for their energetics . therefore , it is possible that the power - law slope of nanoflare frequency distributions differs from that derived for observed flares , which is close to the critical value of  @xmath30 . in this respect it should be worthwhile to investigate flare frequency distributions of sxr flares without hxr counterparts , i.e.  without detectable particle acceleration , since these flares possibly provide the link to the smallest flare - like energy release events . a.  v. , m.  t.  and a.  h.  gratefully the austrian _ fonds zur frderung der wissenschaftlichen forschung _ ( fwf grant p15344-phy ) for supporting this project . antonucci e. , gabriel a. h. , dennis b. r. , 1984 , apj 287 , 917 brown j. c. , 1971 , solar phys . 18 , 489 dennis b.r . , zarro d.m . , 1993 , solar phys.146 , 177 fisher g. h. , canfield r. c. , mcclymont a. n. , 1985 , apj 289 , 425 hudson h. s. , 1991 , solar phys . 133 , 357 lee t. t. , petrosian v. , mctiernan j. m. , 1993 , apj 412 , 401 lee t. t. , petrosian v. , mctiernan j. m. , 1995 , apj 448 , 915 li p. , emslie a.g . , mariska j.t . , 1993 , apj417 , 313 mctiernan j. m. , fisher g. h. , li p. , 1999 , apj 514 , 472 neupert w. m. , 1968 , apj 153 , l59 parker e. n. , 1988 , apj 330 , 474 schwartz r. a. , dennis b. r. , fishman g. j. , et al . , 1992 , nasa cp-3137 , 457 veronig a. , vrnak b. , dennis b.  r. , temmer m. , hanslmeier a. , magdaleni j. , 2002a , a&a revised veronig a. , temmer m. , hanslmeier a. , otruba w. , messerotti m. , 2002b , a&a 382 , 1070
based on simultaneous observations of solar flares in hard and soft x - rays we studied several aspects of the neupert effect . 
 about half of 1114 analyzed events show a timing behavior consistent with the effect . for these events , a high correlation between the soft peak flux and the hard fluence 
 is obtained , being indicative of electron - beam - driven evaporation . however , 
 for about one fourth of the events there is strong evidence for an additional heating agent other than electron beams . 
 we discuss the relevance of these findings with respect to parker s idea of coronal heating by nanoflares .