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44 Chapter 2. Multiwavelength approach to LS 5039
and 51176, when the flux density of LS 5039 seemed to have varied by more than a
factor of ∼ 2 on less than one day. However, inspection of the GBI operational notes
reveals that technical problems such as cryogenics warming, or weather problems
such as snow on the dishes, were responsible of this behavior. The apparent bump
between MJD 51180 and MJD 51200 is also a result of cryogenics warming. Hence,
it seems that most of the features in the lightcurve are due to technical problems,
because of the relatively low flux density of the source.
Nevertheless, since Pooley et al. (1999) could detect the 5.6 d orbital period
of Cygnus X-1 in similar radio lightcurves, we have performed a timing analysis of
the LS 5039 lightcurves at both frequencies. Given the span of the observations,
the search was restricted between 2 and 100 d. The methods employed were the
Phase Dispersion Minimization (PDM) (Stellingwerf 1978) and the CLEAN algo-
rithm (Roberts et al. 1987). Unfortunately, no convincing period was detected in
this process. In particular, we do not detect the 4.117 d orbital period recently
found by McSwain et al. (2001) from radial velocity measurements. Folding of the
data with such a period does not reveal any significant difference on radio emission
along the orbit. Since the orbit is quite eccentric (e=0.41), and this is a wind fed
system (where the primary does not fill its Roche lobe), we expect the accretion rate
onto the compact object to change significantly between periastron and apastron.
Hence, the non detection of the orbital period in the GBI data can be attributed to
its relatively high noise due to several facts (intrinsic measurement noise, technical
problems, weather conditions, etc.).
Although the 8.3 GHz data is dominated by noise, we can extract some infor-
mation about the spectral index. The weighted mean of individual spectral indices
is found to be α = −0.4 ± 0.3. On the other hand, if we use the mean flux densities
at both frequencies we obtain a spectral index of α = −0.6 ± 0.4. These value
are in good agreement with the results presented in Sect. 2.4.2 (Martí et al. 1998)
obtained a few months before, thus suggesting that the non-thermal radio spectrum
is a persistent property of the source.
Set2 data
This data set is not useful for timing analysis or other purposes due to its limited
time span and total number of points. However, we have explicitly included the