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the be / x - ray systems represent the largest sub - class of massive x - ray binaries .
a survey of the literature reveals that of the 115 identified massive x - ray binary pulsar systems ( identified here means exhibiting a coherent x - ray pulse period ) , most of the systems fall within this be counterpart class of binary .
the orbit of the be star and the compact object , presumably a neutron star , is generally wide and eccentric .
x - ray outbursts are normally associated with the passage of the neutron star close to the circumstellar disk ( okazaki & negueruela 2001 ) . a review of such systems may be found in coe et al .
( 2000 ) .
the source that is the subject of this paper , the x - ray transient gro j100857 , was discovered on july 14 1993 by the batse experiment on the compton gamma ray observatory ( stollberg et al .
1993 and wilson et al .
the source was observed in the 20200 kev band , and was found to be pulsating at a period of 93.5s .
its spectrum was observed to be consistent with optically thin thermal bremsstrahlung with kt=25 kev . from these x - ray data
it was concluded that the system was a massive binary system - a neutron star with either a be or supergiant primary .
a full report of this discovery outburst was presented in shrader et al .
( 1999 ) .
subsequently the optical counterpart was identified by coe et al , ( 1994 ) who showed it to be a v=15.3 ob star with a strong ir excess and strong balmer line emission . in this paper
we present more than 10 years of optical monitoring of this counterpart , including a blue - end spectrum taken from the southern african large telescope ( salt ) , which enables refinement of the spectral class to much higher accuracy than previous published work .
we also demonstrate how the optical characteristics correlate with the long - term x - ray ( rxte / asm data ) behaviour .
this is supplemented by @xmath2-ray observations from integral , in particular , reporting the details of the june 2004 outburst from gro j100857 .
h@xmath0 data have been collected over the last 13 years from a series of telescopes . the dates and properties of the h@xmath0 line
are presented in table  [ tab : obs ] . in this table the following telescopes and configurations have been used :    * aat - 3.9 m telescope , aat ( australia ) , royal greenwhich observatory spectrograph ( rgos ) , 25 cm camera , 1200v grating and tek ( 1024x1024 ) ccd .
the dispersion was 0.8 / pixel and the signal to noise ratio from the 600s exposure was @xmath340 .
* saao - 1.9 m telescope , sutherland observatory ( south africa ) , spectrograph , site detector , 1200 l / mm grating .
the dispersion was 1.0 / pixel and the signal to noise ratio @xmath310 .
.table of h@xmath0 measurements .
see text for details of observatory / instrument used .
[ cols="^,^ , < , < " , ]     [ tab : spectra ]    two spectral models were fit to the data , a power - law with a high energy cut - off and a thermal bremsstrahlung model .
both models fit equally well and have a reduced chi - square of @xmath31 ; the details of the fit parameters are shown in table  [ tab : spectra ] .
an attempt was made to introduce an absorption component to both , however the fit could not resonably constrain the parameter .
consequently , an absorption component fixed to the anticipated galactic absorbing column density of @xmath31.5 @xmath4 10@xmath5 @xmath6 was added .
the unfolded spectra with the high - energy cut - off power law model fitted is shown in figure  [ fig : intspec ] .
the fluxes given by the spectral models were 5.7 @xmath4 10@xmath7 erg @xmath6 s@xmath8 in the 320 kev band and 4.1 @xmath4 10@xmath7 erg @xmath6 s@xmath8 in the 20100 kev band .
taking these numbers combined with the source distance of 5kpc ( coe et al , 1994 ) produces a flux estimate of ( 12 ) @xmath4 10@xmath9 erg s@xmath8 .
this value is in good agreement with the level of x - ray luminosity estimated from the small accretion - torque driven pulse period changes reported in section 4 .
using the clear outburst profile presented in figure  [ fig : fold ] for the period 2001 - 2006 it is possible to determine an accurate ephemeris .
we assume the binary period is 248.9d ( levine & corbet , 2006 ) and chose the highest bin in the figure to be defined as phase 0.0 .
from this the following outburst ephemeris is determined :    @xmath10 = 50186@xmath14 + 248.9n    where the dates are in modified julian day format and n is the outburst number .
if the time of outburst is compared to the predicted time of periastron determined from the pulse period variations ( see table 2 ) , it agrees within 1.5d for the mjd 49189 outburst , indicating no significant phase shift between these two events .
if we use this rxte / asm ephemeris then we can determine that the phase of the two integral outbursts discussed here are 0.99 and 0.00 .
furthermore , the phase of the peak of the 1993 batse outburst reported in shrader et al .
( 1999 ) is 0.01 .
thus all three reported events fit very comfortably with the above ephemeris and hence are almost certainly type i outbursts .
this is supported by comparing the phase of the peaks of outbursts as seen in the folded lightcurves from batse , rxte / asm and integral all of which seem to match very well .
there is no evidence for the phase changes in outburst seen in exo 2030 + 375 and explained by wilson et al .
( 2002 ) as related to global one arm oscillations in the circumstellar disk .
though there are changes in the disk , evidenced by the overall shape changes of the h@xmath0 profile seen in figure  [ fig : ha ] , they are obviously not great enough to result in such outburst phase shifting .
figure  [ fig : ew2 ] shows the h@xmath0 equivalent width measurements over the last 13 years . although the h@xmath0 equivalent width is not a direct measurement of the size of the circumstellar disk the data in this figure can be used as an indicator of growth and decline in the disk . to this effect
we interpret the significant decline in the h@xmath0 flux , followed by a strong recovery as a period of disc shrinking and regrowth .
this idea is consistent with the lack of observed x - ray outbursts of gro  j1008@xmath1157 during 19962001 - see figure  [ fig : fold ] .
another feature to take note of in figure  [ fig : ew2 ] is the maximum measured h@xmath0 equivalent width .
reig , fabregat & coe ( 1997 ) show that this maximum h@xmath0 equivalent width is correlated with the orbital period in a be / x - ray binary and the observed correlation can be explained by a viscous circumstellar disk truncated by the resonant torque of the orbiting neutron star ( negueruela et al . , 2001 ) .
the system under discussion here spends a large fraction of the observed timescale at an h@xmath0 equivalent width of close to @xmath12 , while undergoing type  i x - ray outbursts .
this may suggest that the @xmath12  ew measurement describes the circumstellar disk at a size that puts it close to the @xmath13 point at periastron .    to estimate the size of the h@xmath0 emitting region of the circumstellar disk we make used of the peak separation measurements in table  1 and the relation from huang ( 1972 ) .    where @xmath14 is the disk radius , @xmath15 is the inclination and @xmath16 is the peak separation .
the average of the three measurements taken while the system was in normal outburst was determined to be 236@xmath115 km / s and used in the following determination of the disk size .    from the blue spectrum
we determine the spectral type of the optical counterpart of gro  j1008@xmath1157 to be b1 - b2 ve . from allen ( 1973 )
we estimate the mass and radius of such a star to be 15m@xmath17 and 7r@xmath17 respectively .
then , using the measured values of @xmath18 and assuming that the be star disk is in the plane of the orbit we find an inclination angle of @xmath19 for the circumstellar disk of gro  j1008@xmath1157 .
these stellar parameters give an estimated h@xmath0 disk size of 72r@xmath17 and a ratio of disk size to l@xmath20 point ( measured from the centre of the be star ) of r@xmath21/r@xmath22 @xmath3 0.64    in comparison , if the data for 1997 are used with a @xmath23v of 350 km / s , then this results in a much smaller disk of the order of 33r@xmath17 in size .
even the 72r@xmath17 is much smaller than the truncation radius predicted by the viscous disk theory for this be / x - ray binary by okazaki & negueruela ( 2001 ) . using slightly different stellar and orbital parameters
those authors predict that the circumstellar disk of gro j1008 - 57 is most likely truncated at the 7:1 or 8:1 resonance radius . at such a radius
the disk verges on the l@xmath20 point at periastron , thus explaining the frequently observed type i outbursts .    exploring the upper and lower limits of the disk size estimates presented in this work requires the identification of the major uncertainties affecting the results.the uncertainty in the spectral types introduces a range of possible masses for the be star of 10  18 m@xmath17 . using extreme upper limits for both the stellar mass and @xmath18 , with the lower limit on @xmath23v
, we find that r@xmath21/r@xmath22 @xmath3 0.86 . at the other end of the range we find r@xmath21/r@xmath22 @xmath3 0.51
however , it is worth bearing in mind that , contrary to the assumption in most disk models , the be star disk is probably not isothermal .
thus the physical size of the disk is not constrained to the h@xmath0 emitted size of the disk .
for example , in the case of the isolated be star @xmath24 per the h@xmath0 disk size has been measured by direct interferometry to be around ten stellar radii ( quirrenbach et al . , 1997 ) , whereas similar radio observations ( dougherty & taylor 1992 ) suggest that material may be present as far away as hundreds and even thousands of stellar radii .
so , in the case of gro j100857 the totality of the disk may extend beyond the radius measured here from the h@xmath0 data .
this is indirectly confirmed by the very fact that we see regular x - ray outbursts , suggesting the disk material must , indeed , reach the l@xmath20 point .
the study of pulse period changes in gro j100857 from the batse data have allowed a precise orbital determination .
the orbital period found agrees , within errors , with that determined by levine & corbet ( 2006 ) from x - ray outburst cycles .
these outbursts seem to be very reliable in nature , showing no variation with x - ray energy , nor are they correlated with the changes in the h@xmath0 profile , as was seen in the case of exo 2030 + 375 ( wilson et al , 2002 )
. however , the substantial changes in the h@xmath0 ew correlate nicely with the existence and changing levels of the x - ray outburst sizes .
in addition , using the peak separation measured from the h@xmath0 profiles an indication was provided of the disk size which is in general agreement with the detailed models of okazaki & negueruela ( 2001 ) .
the results are a direct confirmation of the value of multiwavelength observations of these complex systems .
this paper uses observations made from the south african astronomical observatory ( saao ) , and x - ray data provided by the asm / rxte team .
vam acknowledges support from the south african nrf and the british council in the form of a salt / stobie studentship .
multiwavelength observations are reported here of the be / x - ray binary pulsar system gro j100857 . over ten years worth of data 
 are gathered together to show that the periodic x - ray outbursts are dependant on both the binary motion and the size of the circumstellar disk . in the first instance 
 an accurate orbital solution is determined from pulse periods , and in the second case the strength and shape of the h@xmath0 emission line is shown to be a valuable indicator of disk size and its behaviour . 
 furthermore , the shape of the emission line permits a direct determination of the disk size which is in good agreement with theoretical estimates . 
 a detailed study of the pulse period variations during outbursts determined the binary period to be 247.8@xmath10.4d , in good agreement with the period determined from the recurrence of the outbursts .    
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