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the first extra - solar x - ray source discovered was the low - mass x - ray binary sco  x-1 @xcite .
its optical counterpart , v818  sco , was discovered by @xcite , paving the way for many subsequent multiwavelength studies . the binary period is widely accepted to be 18.9hr based on the discovery of a photometric modulation by @xcite and spectroscopic confirmation by @xcite .
we now know that sco  x-1 contains a low - mass late - type donor transferring mass onto a neutron star at a rather high rate .
the modulation arises from x - ray heating of the donor star , which also manifests as narrow emission lines of niii and ciii moving in phase with the donor star @xcite .
@xcite obtained the period of @xmath0days quite remarkably by examining archival photographic plates from 1889 to 1974 .
a sinusoidal modulation of full amplitude around 0.20.3mag was found in several independent datasets , with considerable scatter around the mean curve @xcite .
while the long baseline of photographic observations defined the period to incredible precision , the sparse sampling left a plethora of aliases , and @xcite identified strong signals at one - day , one - month , and one - year aliases of their favored period .
of these , the one - year alias has been by far the hardest to reject .
several subsequent photometric studies reproduced the modulation , but none improved the ephemeris , or resolved the one - year alias issue @xcite    spectroscopic confirmation of this period was suggested by @xcite and @xcite , and demonstrated conclusively by @xcite , who found a period of @xmath1days , and again by @xcite .
both of these works performed a period search on the data , but in both cases the frequency resolution was limited by only observing over a baseline of a week .
other spectroscopic analyses of these and other data have also found variations at this period , @xcite , but no other groups have performed a rigorous independent period search .
several groups also searched for the orbital period in x - ray data , with initially no success @xcite .
the only positive detection of an orbital period in x - rays came from @xcite based on a multi - year _
rxte_/asm dataset .
they did not find exactly the @xcite period , but instead the one - year alias ( 0.78893days ) with a modulation around 1% .
given the intensive multi - year coverage of _ rxte _ this is surprising , since this dataset should not be susceptible to the one - year alias problem .
@xcite therefore claimed that their period was the true orbital period and that @xcite had misidentified the alias .
while this result was tantalizing , @xcite could not reproduce this period using a larger _ rxte _ dataset .
they did , however , not use as sophisticated an analysis as @xcite , leaving open the possibility that the x - ray period could be real .
surprisingly , then , fifty years after discovery of the prototypical lmxb sco  x-1 , there remain doubts about its most fundamental parameter , the orbital period .
while the original optical ephemeris of @xcite has remained the standard reference for the 37years since its publication , it remains to be resolved whether this , or the x - ray period of @xcite , is the true orbital period . to attempt to resolve these questions , and update the ephemeris of sco  x-1 with modern data , we examine here archival photometry from the all sky automated survey ( asas )
this nine year dataset has both the long baseline to determine a precise period , and coverage of a large enough fraction of a year to finally break the one - year alias problem using optical data .
the all sky automated survey ( asas ) monitored sco  x-1 from 2001 to 2009 @xcite . we note that while sco  x-1 was not included in the asas catalog of variable stars ( acvs ) its photometry is in the asas-3 photometric @xmath2 band catalog in two datasets , 1619551538.4 and 1619551538.5 .
the sco x-1 datasets include 640 observations from 2001 january 22 to 2009 october 5 . with multiyear coverage spanning typically about 270days of the year , it is ideally suited for obtaining an updated ephemeris and breaking the one - year alias .
we performed our analysis for a range of choices of data grades and apertures to optimize our filter criteria . for final analysis , we retained the 567 grade a or b observations , and used the smallest asas aperture .
inclusion of grade c or worse data , or use of larger aperture data , significantly reduced the quality of the fits .
to determine the orbital period we performed a sinuoidal fit to the data points . since the scatter around the model is dominated by intrinsic flickering rather than photometric uncertainties , we assigned a mean uncertainty of 0.30mag to each point to represent the flickering .
this was chosen to produce a minimum @xmath3 equal to the number of degrees of freedom .
we then evaluated sinusoidal fits over a range of trial periods . for each period the best - fitting mean magnitude , amplitude , and phasing were determined using the downhill simplex algorithm @xcite .
we show the results in the vicinity of the disputed periods in fig .
[ periodfig ] .     as a function of trial period for sinusoidal fits to asas data .
we show the @xcite period of 0.787313days and the @xcite period of 0.78893days for comparison .
we also show calculated one - year aliases of the preferred period at 0.78562 and 0.78901days .
the @xcite period is strongly favored by the asas data . while some signal is seen at the alternative periods , all are rejected at greater than 5-@xmath4 confidence.,width=336 ]    we see that the @xcite period is reproduced exactly to within the limits of our frequency resolution . our formal best period is @xmath5days .
the uncertainty quoted is a formal 1-@xmath4 error determined from the @xmath6 confidence range in period .
we verified the uncertainty using the bootstrap method with 30 resamplings of the data .
this gave a consistent 1-@xmath4 uncertainty ( @xmath7 ) .
we also show the period of @xcite , and the one - year aliases with which they associated it .
we find that none of these alternatives are consistent with the asas data , and all can be rejected at better than 5-@xmath4 confidence .
we therefore can not directly improve on the period of @xcite using the asas data , which is not surprising as that used data drawn from nearly a hundred year baseline .
we can , however , overcome the limitation of that dataset in its vulnerability to one - year aliases , as the asas data has much wider coverage within a year .
using the same @xmath3 approach , we determine a mean time of minimum of @xmath8 .
this corresponds to an offset of very close to 17057 cycles from the time of minimum of @xcite .
if we project their time of minimum forwards we predict @xmath9 , with equal contributions to the uncertainty from their time of minimum ( quoted as 0.022 cycles ) and their period ( @xmath10days ) .
our time of minimum is completely consistent with theirs ( a remarkable testament to the accuracy of their historical ephemeris ) , but at this point our modern measurement of the time is somewhat better constrained for use with modern data .    finally , we show in figure  [ lightcurvefig ] the asas lightcurve folded on our derived ephemeris , together with the best fitting sine wave .
the mean @xmath2 band brightness is 12.63 , and the full - amplitude is 0.26mag , comparable to that found by @xcite and @xcite .
we have established that in optical photometry the 0.787313day period produces a stable modulation over 120years of observation .
the ephemeris of @xcite reliably and precisely predicts the time of minimum in the asas data , over 17,000 intervening cycles .
it is hard to imagine any clock other than the orbital period providing this stability .
this has to be the true orbital period .
the question then arises as to what , if anything , @xcite detected .
we of course should allow that it was a spurious detection , until it can be reproduced with data from the remainder of the _ rxte _ mission .
@xcite failed to reproduce it , but also did not use all the techniques that @xcite used . associating it with an alias of the true orbital period seems unlikely , as _
rxte_/asm data on sco  x-1 are rather well sampled through the year ( just as asas data are )
.    one possible explanation might be if the x - ray signal came at the beat frequency between the orbital period and a superorbital period of around a year .
many x - ray binaries have indeed shown super - orbital periods of tens to hundreds of days ( see e.g. * ? ? ?
* ) , although typically all are shorter than a year .
the only claim of such a long period in sco  x-1 , came from early _
rxte_/asm data , from which @xcite suggested a 37day period .
this detection has not been sustained in subsequent data , and no super - orbital period was found by @xcite in _ swift_/bat data . on longer timescales , @xcite and @xcite both independently suggested a @xmath11year x - ray modulation is present in _ rxte_/asm data , although this is too long to account for the @xcite period .
this explanation therefore seems unlikely , and it remains to be seen if the x - ray period can be reproduced from the full _ rxte _ mission - long dataset .
we have analyzed asas data of sco  x-1 spanning nine years .
we can confirm the period of @xcite , while rejecting its one - year aliases , and also the putative x - ray period of @xcite .
our updated ephemeris is @xmath12 .
the orbital period of sco  x-1 was first identified by @xcite . 
 while this has been confirmed on multiple occasions , this work , based on nearly a century of photographic data , has remained the reference in defining the system ephemeris ever since . 
 it was , however , called into question when @xcite claimed to find the one - year alias of the historical period in _ rxte_/asm data and suggested that this was the true period rather than that of @xcite . 
 we examine data from the all sky automated survey ( asas ) spanning 20012009 . 
 we confirm that the period of @xcite is in fact the correct one , at least in the optical , with the one - year alias strongly rejected by these data . 
 we also provide a modern time of minimum light based on the asas data .