Universitat de - Departament d'Astronomia i Meteorologia ...

48 Chapter 2. Multiwavelength approach to LS 5039
Flux density at 2.25 GHz [mJy]
80
60
40
20
VLBA run 1999 May 8
0
51290 51295 51300 51305 51310 51315 51320
Modified Julian Date [JD−2400000.5]
Figure 2.7: GBI radio monitoring of LS 5039, at 2.25 GHz, during the weeks before and
after the date of our VLBA observations, indicated by the vertical bar.
final synthesis map (see Fig. 2.6) shows a two-sided jet emerging from a central core.
A deconvolved angular size of about 2 mas is estimated for the core. The jets extend
over 6 mas on the plane of the sky, oriented along a position angle of ∼ 125 ◦ with
respect to the North, and they account for 20% of the total 16 mJy flux density. At
a distance of ∼ 3 kpc the total projected length of the jets is ∼ 20 AU.
Discussion
To obtain some order of magnitude estimates, we will assume that the overall size
of the radio source is approximately 6 mas×2 mas. This implies a high brightness
temperature of ∼ 9.4 × 10 7 K, indicative of synchrotron radiation. As has been seen
in Sects. 2.4.2 and 2.4.3, the LS 5039 radio spectrum as a function of frequency
often displays a negative spectral index which is in agreement with a non-thermal
optically thin emission mechanism (Martí et al. 1998, Ribó et al. 1999).
The detection of jets occurred at a time when the source was at its typical
persistent level of radio emission, and only moderately variable, as inferred from
concurrent radio monitoring by the Green Bank Interferometer (GBI) (see Fig. 2.7).
The absence of any precursor outburst for the radio jets strongly suggests that they
are always present and continuously emanating from the core.