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hercules x-1/hz herculis is a low mass x - ray binary system consisting of a @xmath3 neutron star and a @xmath4 a dwarf star locked in orbit about each other with a period of 1.7  d ( deeter et al . 1991 , and references therein ) . the x - ray source is eclipsed by the a star for 0.24  d of each period , indicating an orbital inclination near 90@xmath5 . the spectral type of the a star varies with orbital phase from late b / early a to late a / early f such that the later spectral types correspond to eclipse and the earlier spectral types correspond to 1/2 an orbit later ( when x - ray heating of the a star should be most apparent  see milgrom & salpeter 1975 ) . mass transfer , which may be periodic , feeds an accretion disk around the neutron star . accretion onto the neutron star itself , which has a magnetic field of @xmath6 gauss , occurs on the magnetic poles ( gruber et al . 1980 ; voges et al . 1982 ) . the x - rays from the system vary on a period near 34.85 d ( gelman 1987 , baykal et al . 1993 ) , with an x - ray high state typically lasting @xmath711  d , and an x - ray low state ( with a secondary maximum of 30%the high state ) lasting @xmath724  d. the time at which the x - ray high state begins varies unpredictably from the mean ephemeris by more than @xmath8  d , an effect that has led to the generation of a number of different models ( e.g. petterson 1975 ; gerend & boynton 1976 ; wolff & kondo 1978 ; kondo , van flandren , & wolff 1983 ; meyer & meyer - hofmeister 1984 ; schandl & meyer 1994 ) . as the 1.7  day modulation of the optical light continues during the x - ray low states ( bahcall &  bahcall 1972 ) , the 35-day cycle probably results from a geometric effect , such as obscuration by a precessing accretion disk , and not an interruption of the accretion , which would cause the x - ray heating of hz  her to cease . in addition to the 35  d cycle , pulsed x - ray emission with a period of 1.24s is also seen , which is generally interpreted as the rotation period of the neutron star ( tananbaum et al . 1972 ; giacconi et al . these pulsations have also been detected optically ( davidsen et al . 1972 ; middleditch 1983 ) , in the uv ( boroson et al . 1996 ) and at optical and infrared wavelengths ( middleditch , puetter , & pennypacker 1985 ) . the ultraviolet spectrum above 1200    has been extensively observed with iue ( dupree et al . 1978 ; gursky et al . 1980 ; howarth & wilson 1983a , b ; vrtilek &  cheng 1996 ) . both the line and continuum components of the spectrum vary with orbital phase . the ultraviolet light from her x-1 comes from at least two main regions within the system : the heated face of the normal star and the accretion disk . according to the analysis of howarth & wilson ( 1983a ) , roughly 1/2 of the hard x - rays striking the a  star are reprocessed into uv and optical continuum emission . reprocessed x - rays are also responsible for most of the uv light from the disk . using the goddard high resolution spectrograph ( _ ghrs _ ) aboard the hubble space telescope , boroson et al . ( 1996 ) showed that the uv lines have broad and narrow components which they interpreted in terms of emission from the disk and the star .    in this paper , we present a spectrum of her x-1 that extends the far - ultraviolet wavelength coverage to the lyman limit at 912   . we have detected , for the first time , the ovi@xmath0  emission lines , and the continuum in this wavelength region . in  2  we discuss the observations and data reduction , and in  3  we discuss our data in the context of other observations of her  x-1 , and we compare the data with models of the line and continuum emission . the observations were carried out with the hopkins ultraviolet telescope ( hut ) as part of the astro-1 space shuttle mission in 1990 december . hut consists of a 0.9  m mirror that feeds a prime focus spectrograph with a microchannel plate intensifier and reticon detector . first order sensitivity covers the region from 850    to 1850    at 0.51    pix@xmath9 , with @xmath73  resolution . the detector can be operated in any of several modes , including a high time resolution mode where individual photons can be time - tagged to a relative accuracy of 1  ms and an absolute accuracy of 3  ms . this was the mode of operation during the observation described here . further details of the spectrograph and telescope and their on - orbit performance and calibration can be found in davidsen et al . ( 1992 ) and kruk et al . ( 1997 ) . the her x-1 observation discussed here took place on 1990 december  6 at 4:22  ut . according to the 1.7 day orbital ephemeris of deeter et al . ( 1991 ) , this corresponds to @xmath10 . we do not know the x - ray state of her  x-1 at the time of our observation . using a period @xmath11  d and the observed x - ray turn - on nearest in time to our observation ( jd  2448478.3 , as given in baykal et al . 1993 ) , our observation took place at @xmath12 , while x - ray turn - on occurs at @xmath13 . this suggests that our observation took place in the x - ray off state ; however , the times of the x - ray turn - ons are known to vary by @xmath146 days ( gelman 1987 ; baykal et al . 1993 ) , so that without simultaneous x - ray observations , we can not be certain . the hut observation was made with an 18@xmath15 aperture placed on the object , and guiding was accomplished using preplanned guide star positions on the hut tv guider . the total integration was 1700  s. orbital viewing constraints required that we observe her  x-1 entirely during the orbital day of hut , which resulted in substantial airglow contamination , leading not only to significant line emission ( including several euv lines seen in second order ) but also a pseudo - continuum from grating - scattered light ( chiefly from the brightest airglow lines at ly@xmath16 and oi@xmath17 ) . the flux calibration is based on observations of the white dwarf star g191-b2b compared with model stellar atmospheres and laboratory calibrations as discussed by davidsen et al . ( 1992 ) and kruk et al . ( 1997 ) . the relative fluxes are accurate to within 5% . the count rate in the continuum from her x-1 ( in the 14001600  range , relatively free from airglow emission ) was not uniform , but peaked at 5.5 counts  s@xmath9 during a five - minute interval . this count rate should be compared with an average rate of 4.3 counts  s@xmath9 ( 14001600 ) in the 1500  s interval that we used for our analysis . data from the hut tv camera indicate sections at the beginning and end of the observation where pointing stability was disrupted . post - flight analysis of video frames stored during the observation show that even during the portion of the observation where the pointing stability was good , the object was not well centered in the slit , especially in the coordinate perpendicular to the dispersion direction . hence , even minor pointing errors could have affected the count rate ( and derived flux ) because of vignetting by the edge of the slit . ( the psf of the telescope produced star images with fwhm@xmath18 . ) we thus suspect that the fluctuating count rate was due to pointing errors , and that the maximum observed count rate is probably representative of the true flux of the object . this analysis implies that the average count rate was low by @xmath727% ; the derived fluxes ( and errors ) were thus multiplied by 1.27 to correct for this effect . since the hut detector is photon - counting , it is straightforward to generate and propagate errors along with the data as various operations are performed . to the extent that flux was being lost even during stable pointing ( due to vignetting ) this corrected flux could still be too low , but not by more than @xmath19% . the corrected , flux - calibrated spectrum is shown as the upper curve in figure  1 .    in order to subtract airglow contamination to first order , we have fit and subtracted a blank day sky observation obtained with the same aperture during astro-1 . we used smooth fits to the known airglow lines in this spectrum instead of the observed airglow background to reduce the contribution of counting noise . an actual smoothing of the background spectrum , instead of a fit to individual lines , would have increased the widths of the strong airglow lines where they overlap emission lines from her  x-1 . we have continued to use the error array associated with the counts in the original data . the result of our fit to the background spectrum is shown as the lower curve in figure  1 . the airglow - subtracted spectrum representing the summed intrinsic data from the source is shown in figure  2 . because the relative intensities of the fainter airglow lines change with orbital latitude and look - angle with respect to the earth limb , the subtraction of line emission is not perfect . also , a variable complex of airglow emissions affects the spectrum in the region near the lyman limit ( cf . feldman et al . . however , our technique should be sufficient to provide a reasonable representation of the intrinsic continuum longward of 930   . this is the spectrum that will be used for continuum fitting below . we have identified emission from the nv@xmath1  doublet and the civ@xmath2  doublet , which have been seen with iue ( boyle et al . 1986 , howarth & wilson 1983a , 1983b ) and hst ( anderson et al . 1994 ; boroson et al . 1996 ) , and also the ovi@xmath201031.9,1037.6 resonance doublet and some fainter emission lines at a more marginal detection level . this constitutes the first detection of o  vi emission from the her  x-1 system , and provides a significant new diagnostic of photoionized gas in this system . in order to provide quantitative estimates of the line strengths , we have fit gaussian profiles to the data . since some of the lines , including ovi@xmath0 , are affected by nearby airglow features , we first used our template airglow spectrum to carry out a local subtraction of the airglow in order to produce a flat continuum adjacent to each line . typically , this required us to scale the template spectrum by of order  20%  from the value used above to achieve a reasonable local fit near the lines . each doublet was fit assuming an optically thick 1:1 ratio for the lines and a fixed separation of the doublet components ( although the central wavelength of each feature was allowed to vary ) . observations with the hst show two line components ; for a narrow component , the doublet ratios are typically @xmath21 , while for the broad component , the doublet ratio is more nearly @xmath22 . we found a superior fit ( reduced @xmath23 instead of @xmath24 with 42 degrees of freedom ) when we allowed a @xmath25 doublet ratio for ovi@xmath0 . allowing the doublet ratio to be a free parameter does not improve the fit ( @xmath26 for a ratio of 1.65:1 ) . alternately , the ovi@xmath0  lines could have a 1:1 ratio , but an interstellar cii@xmath27 line could absorb ovi@xmath28 . allowing the wavelength of the interstellar line to vary , but fixing its width as the width of the ovi lines ( the instrumental width ) , we found @xmath29 with 42 degrees of freedom . the @xmath30 value for all the ovi fits may be artificially high , as the errors do not reflect the uncertainty in the subtraction of the nearby strong airglow lines of h  ly@xmath31 , oi@xmath32 , and oi@xmath33 . figure  3 shows the region near ovi@xmath0  with the observed and modelled airglow lines , and figure  4 shows the best fits for all the lines . in addition to ovi@xmath0 , nv@xmath1 , and civ@xmath2 , there was marginal ( 3@xmath34 ) evidence for an ov line at 1371   , at a strength relative to nv and civ similar to that seen with the hst . heii@xmath35 may also have been detected at a marginal level . there is no evidence for siiv@xmath36 , and while some features appear at @xmath37 , they are not significant enough to identify as ciii@xmath38 . any possible ciii@xmath39 is lost in the airglow emission that affects this region of the spectrum . all of the lines intrinsic to her x-1 are blueshifted . after detailed inspection of the video frames , we were unable to determine unequivocably whether or not this shift is real . based on analysis of two guide stars , we infer an offset within the aperture in the dispersion direction that is consistent with a blueshift of 0.0  1.9    ( 0.0  5.6@xmath15 miscentering ) . the discrepant results from the different guide stars are due to a combination of distortions within the tv camera and measurement error . if the blueshift caused by the miscentering is near 1.9   , then the apparent blueshifts of the lines are not real , and it becomes more likely that interstellar cii@xmath27 can absorb ovi@xmath28 . we searched for pulsations in the uv lines from her x-1 , using the data as received in high time rate mode during the time period 1990 dec  6 04:29:00 to 04:48:20 ( ut ) . we added counts in 0.2  s intervals , folded the data over several likely pulsation periods near the neutron star rotation period , and applied the analysis of variance method ( anova , davies 1990 ) to determine the likelihood of pulsations . no significant pulsations were found in the lines or continuum . however , due to the low count rate and the extensive airglow contamination , the limits on pulsation amplitude , 15  35% of the total flux , are not stringent . the detection of ovi in her x-1 is important , but with a single observation at a particular @xmath40 and @xmath41 , we are limited in the conclusions we can draw about the system . in this section we discuss the hut observation in the context of the more extensive iue and hst spectroscopy of her x-1 . howarth & wilson ( 1983a , b ; hereafter hwa and hwb ) discuss a consistently reduced set of iue data on her x-1 that cover a range in both orbital period and the 35 day x - ray cycle . the line and continuum strengths show a clear dependence on orbital phase ( figure  3 of hwa displays the 1500    continuum and figure  2 of hwb displays the nv and civ line strengths ) . her  x-1 has shown similar line strengths at corresponding orbital phases in subsequent hst observations ( anderson et al . 1994 , boroson et al . 1996 ) and iue observations ( vrtilek &  cheng 1996 ) . plotting the hut data on figure  3 of hwa , figure  2 of hwb , and figure  5 of vrtilek &  cheng demonstrates that the hut fluxes are about a factor of  2 lower than those observed with iue at phase 0.34/0.66 ( i.e. 0.34 from mid - eclipse ) . inspection of optical magnitudes provided by the american association of variable star observers ( aavso ) shows that the 1.7  day optical modulation continued during the period surrounding our observation ( v magnitudes @xmath42 at @xmath43 and @xmath44 , and @xmath45 at @xmath46 ) . as x - ray heating is thought to cause both the optical modulation and the uv emission , this suggests that the decrease in uv flux that we observed was a short - lived phenomenon or unrelated to x - ray heating . from our examination of the data we are confident that the decrease is not due to any known shortcoming of the data acquisition and reduction . in addition to the line fits described in  2 , we have produced fits to the continuum after airglow subtraction . the continuum spectrum below @xmath47 , reported here for the first time , is important because the accretion disk should contribute more to the continuum flux at shorter wavelengths .    in fitting the continuum , we used regions free of strong airglow emission lines . we chose to fit the continuum with 1 ) a power law plus extinction component , and 2 ) a model of an x - ray illuminated accretion disk and roche lobe - filling star . these fits will be described briefly below .    for the power law fit , we specified initial guesses at a normalization , @xmath48 ( the flux at 1000   ) , and the exponent of the power law , @xmath16 , so that @xmath49 . we also applied a seaton ( 1979 ) galactic extinction law , extropolated to the lyman limit in a manner that is consistent with voyager measurements ( longo et al . for the her x-1 hut spectrum , we find @xmath50 erg s@xmath9 @xmath51 @xmath9 , @xmath52 , and @xmath53 . the reduced @xmath30 for this fit was @xmath54 ( with 1125 points and three free parameters ) . the @xmath55 value is inconsistent with uv observations of the 2200    feature ( gursky et al . setting @xmath56 , we find @xmath57 and @xmath58 . this fit is nearly as good ( the reduced @xmath30 is still @xmath54 ) as the fit with @xmath55 as a free parameter . we plot this latter model against the data in figure  5 . we also tried fitting the hut spectrum with a detailed physical model of an x - ray illuminated accretion disk and star ( assumed to fill its roche lobe ) . the model is identical to that presented by cheng , vrtilek , & raymond ( 1995 ) , except that we have used kurucz model atmospheres ( kurucz 1992 ) instead of our library of stellar spectra , which does not extend to the lyman limit . the model solves for the temperature structure of the disk using the method described by vrtilek et al . ( 1990 ) , and includes x - ray heating of the normal star , the eclipse of the star by the disk , and the x - ray shadow of the disk on the surface of the normal star using the methods described in hwa . we used values for the fixed parameters describing her  x-1 given in table  2 of cheng et al . ( 1995 ) , including @xmath59 .    in figure  5 we compare the continuum spectrum produced by the detailed disk model with the power - law fit and with the spectrum observed with hut . the disk contributes @xmath60%  of the optical flux , @xmath61%  of the flux near 1200 , @xmath62%  of the flux near 1100 , and @xmath63%  of the flux near 950 . the only free parameter in the model is @xmath64 , the mass transfer rate onto the neutron star . the best - fit value for @xmath64 is @xmath65 ( in units of solar masses per year ) . this value of @xmath64 is lower than the values reported by vrtilek & cheng ( 1996 ) during an `` anomalous low '' state of her  x-1 ( in their final fit , the lowest value is @xmath66 ) . with @xmath64 fixed at the best - fit value , the model predicts @xmath67 also at @xmath68 , when aavso observers report @xmath42 . the fit of the detailed model is worse than the simple power - law ; we find a reduced @xmath30 of 1.64 . the fit is especially poor below 1200 . we investigated several possible systematic errors that could have caused this poor fit : the value we have used for the interstellar reddening , the flux calibration of hut , the timing of the 35-day cycle , and the subtraction of the background airglow spectrum .    if @xmath69 , then a greater value of @xmath64 is required to fit the data . the higher resulting temperature of the heated disk and star can compensate for the spectral change introduced by the reddening . for @xmath70 , the model predicts an observed flux from 10001100that is @xmath71%  greater than in the case with no reddening ; however , the fit is still not good , and @xmath72 . increasing the flux in the hut spectrum by a factor of 2 to reflect some unknown flux calibration problem results in a better fit ( @xmath73 ) and @xmath74 , which is more typical of her  x-1 . at @xmath68 , @xmath75 . the spectrum predicted by this model is shown in figure  5 . the 35-day cycle of her  x-1 does not repeat exactly ( baykal et al . thus there is some uncertainty in our choice of @xmath41 , which determines the orientation of the accretion disk . if we allow @xmath41 to vary as a free parameter , we find @xmath76 for the best - fit values @xmath77 , @xmath78 . although the fit is much improved , @xmath79 is further out of the range of previous observations . using this 35-day phase , more of the hot , central disk is visible , and so the flux at shorter wavelengths is increased . the uncertainty in extrapolating the 35-day ephemeris through @xmath80 cycles can be approximated as 0.9 @xmath81  d ( baykal et al . 1993 ) . the difference between the best - fit value of @xmath41 and that expected from the nearest reported x - ray turn - on is @xmath82d , or a deviation of @xmath83 . as described in  2 , we subtracted from the her  x-1 spectrum a smooth model of the airglow spectrum based on a blank - sky observation . there is marginal evidence in the blank - sky spectrum for weak airglow lines of ly@xmath84 near 930,938,950 , where our fit to the data is poor . we tested our method of background subtraction by smoothing the blank - sky spectrum with a savitzsky - golay filter . while this method introduces errors near the wings of the strong airglow lines , it can approximate the airglow background even when strong lines are not present . using this method of background subtraction does not change our derived value of @xmath64 , and the model still predicts too low a flux below 1200 , but now @xmath85 . furthermore , this shows that there may be significant errors introduced by the background - subtraction .    in summary , we find that the her  x-1 far - uv spectrum was unusually faint at the time of our observation , resulting in model fits with values of @xmath64 lower than previously reported . the flux @xmath86  is stronger than expected from detailed models . uncertainties in the 35-day phase , the centering of her  x-1 in the aperture , or the background - subtraction may account for these discrepancies . we have compared the observed emission line fluxes with those predicted by theoretical models of x - ray illuminated accretion disks ( raymond 1993 ) . the results are shown in table  1 . the `` cos '' model for the emission lines uses cosmic abundances , while the `` cno '' model uses a fixed enhancement of he and n and a fixed depletion of c to represent abundances due to cno processing . in computing the emission line strengths expected from the models , we have assumed that her  x-1 is at a distance of 5.5  kpc , and have made no correction for interstellar reddening . the spectrum confirms the basic predictions of the x - ray illuminated accretion models . in particular , the interpretation of the nv / civ ratio as an element abundance effect , rather than a problem with the predicted ionization or density structure , is borne out by the line strengths . while the nv@xmath1  doublet is stronger than civ@xmath2 , the ovi@xmath0  lines may be slightly weaker . this supports the conclusion that the unexpected strength of the nv lines relative to civ does not result from a dependence of line flux on ionization potential . the presence of ovi emission lines in her x-1 is interesting because ovi is formed in a hotter region than any of the other uv lines . the ratio of ovi@xmath0  to ov@xmath87 provides a temperature diagnostic . the ov line is produced from recombination of ovi and thus has the same dependence on the ovi ionization fraction and on the electron density as the ovi@xmath0  lines . the strength of the ovi@xmath0  lines depends on the strongly temperature - dependent excitation rate of the upper level of the transition . using the ovi recombination rates of nussbaumer &  storey ( 1984 ) and the standard excitation rate expressions and cross - section found in osterbrock ( 1989 ) , we find temperatures of @xmath88  k and @xmath89  k for the ovi@xmath0  fluxes inferred assuming , respectively , a 2:1 doublet ratio and a 1:1 ratio with interstellar absorption . correcting for reddening ( using @xmath70 ) increases ovi / ov by a factor of 1.44 . this only increases the temperature estimate to 15,000  k.    hwb model the line emitting region , and for the nv region find @xmath90 , and @xmath91 , assuming a cosmic abundance of n and that all n is in the nv state . our temperature estimate has assumed that the ovi@xmath0  lines are optically thin ; if this assumption is not right , then the temperature may be much higher . further , we have assumed that the ovi lines and the ov line are emitted in the same regions or in regions with similar temperatures and densities , which may not be the case . both the cos model and the cno model predict observable ciii@xmath38 emission ( 7.5@xmath92 and 4.2@xmath92 erg  s@xmath9  @xmath51 respectively ) . the weak features that we saw had total flux @xmath93 erg  s@xmath9  @xmath51 . interstellar ciii@xmath38 absorption would probably decrease the strength of this line by @xmath94% . we note that anderson et al . ( 1994 ) observed the ciii@xmath95 line with hst ( when the nv and civ lines were twice as strong as observed with hut ) , and found a flux of @xmath96  erg  s@xmath9  @xmath51 , whereas cos and cno predict fluxes of @xmath97 and @xmath98  erg  s@xmath9  @xmath51 , respectively . a further decrease in the c / n ratio beyond @xmath99 as assumed in model cno to c / n@xmath100 would improve the agreement between observations and models , but the evidence is not overwhelming . this observation with the hopkins ultraviolet telescope was the first to study the flux from this interesting source below the lyman  @xmath16 line , where a large fraction the continuum emission probably arises in the disk , and where there are strong emission lines especially sensitive to x - ray illumination . future observations with fuse will be able to detect these features with greater sensitivity and resolution . it is a pleasure to thank the many members of the hut team at jhu and at the jhu applied physics laboratory whose years of work paid off in two successful astro missions . we also thank the spacelab operations support group at marshall space flight center and the crew of the astro-1 mission for their support before and during the flight . we would like to thank the referee , ian howarth , for a careful reading and for suggestions on improving the manuscript . the hopkins ultraviolet telescope project is supported by nasa contract 5 - 27000 to the johns hopkins university . bb and sv were supported in part by nasa ( nag5 - 2532 , nagw-2685 ) , and nsf ( dge-9350074 ) .
we have obtained a far - ultraviolet spectrum of the x - ray binary hercules  x-1/hz  herculis using the hopkins ultraviolet telescope aboard the astro-1 space shuttle mission in 1990 december . 
 this is the first spectrum of her x-1 that extends down to the lyman limit at 912   . 
 we observed emission lines of ovi@xmath0 , nv@xmath1 , and civ@xmath2 , and the far uv continuum extending to the lyman limit . 
 we examine the conditions of the emitting gas through line strengths , line ratios , and doublet ratios . 
 the uv flux is lower by about a factor of  2 than expected at the orbital phase of the observation . 
 we model the uv continuum with a simple power - law and with a detailed model of an x - ray - illuminated accretion disk and companion star . 
 the power - law provides a superior fit , as the detailed model predicts too little flux below 1200 . 
 we note , however , that there are uncertainties in the interstellar reddening , in the background airglow spectrum , and in the long - term phase of the accretion disk . 
 we have searched the data for uv  line and continuum pulsations near the neutron star spin period but found none at a detectable level .