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46 Chapter 2. Multiwavelength approach to LS 5039
hence supporting the mechanism of synchrotron radiation as the responsible for the
detected radio emission.
Excluding false data due to technical or weather problems, LS 5039 does not
show day-to-day variability larger than ∼ 30%, and no strong radio outbursts have
been detected. No orbital period is detected in the GBI data in the range between
2 and 100 days. In general terms, the observed radio behavior confirms the earlier
suggestions by Martí et al. (1998) concerning the persistent and moderately variable
nature of the radio emission.
2.4.4 VLBA observations, discovery of a microquasar
As we have seen in Sect. 2.4.2, LS 5039 appeared unresolved to the VLA in A
configuration, i.e., with a size ≤ 0.1 ′′ . On the other hand, a lower limit to the source
size of ≥ 0.2 mas was derived to prevent catastrophic inverse Compton losses to
occur. Therefore, we decided to conduct observations using the Very Long Baseline
Interferometry (VLBI) technique, aimed to reveal the mas structure of the source.
Observations and results
We observed 11 LS 5039 at 5 GHz frequency (6 cm wavelength) on 1999 May 8
(MJD 51306) with the 10×25-m antennas of the NRAO 12 Very Long Baseline Array
(VLBA) and the 27×25-m antennas of the VLA in its phased array mode in its D
configuration. In this mode, the VLA has the same sensitivity as a dish of ∼ 115 m
diameter, and provides sensitive baselines with the VLBA antennas. The VLA
phasing process was successful, and it worked without problems as another VLBI
station. On the other hand, in theory the VLA could also record data on its own.
This would allow to perform a very sensitive VLA map and to follow the radio flux
density during the VLBA run. However, there was an air conditioning failure in the
VLA correlator, and it could not record data.
The source 3C 345 was used as a fringe-finder, whereas J1733−1304 was the
phasing source for the VLA. We could not use the VLBI phase-reference technique
11 Published in Paredes, J. M., Martí, J., Ribó, M., & Massi, M. 2000, Science, 288, 2340.
12 The National Radio Astronomy Observatory is a facility of the USA National Science Foun-
dation operated under cooperative agreement by Associated Universities, Inc.