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

2.4. The radio counterpart: NVSS J182614−145054 39
Flux density [mJy]
50
40
30
20
15
10
May 12
Apr 09
Mar 10
Feb 11
1 2 5 8 10 15 20
Frequency [GHz]
Figure 2.3: The radio spectrum of LS 5039 on different epochs during the 1998 obser-
vations with the VLA. Error bars not shown are smaller than the symbol size.
Astrometric identification
The J2000.0 ICRS coordinates of the LS 5039 radio position derived from the VLA
A configuration maps in Fig. 2.2 were found to be: α = 18 h 26 m 15.056 s ± 0.001 s ,
δ = −14 ◦ 50 ′ 54.24 ′′ ± 0.01 ′′ . These coordinates agree within 0.1 ′′ with those of
LS 5039 in the optical USNO-A1.0 catalog, whose typical astrometric error is about
0.25 ′′ . Therefore, we concluded that the identification of the LS 5039 radio counter-
part on the basis of astrometrical coincidence at the sub-arcsec level was virtually
certain. From source count analysis (e.g., Ledden et al. 1980), the a priori probabil-
ity of having a background extragalactic source with S20cm 30 mJy, or brighter,
within 0.2 ′′ of the LS 5039 optical position is as low as 10 −7 . To our knowledge,
this discovery increased to four the number of confirmed massive X-ray binaries with
associated radio emission at that epoch, the other three being SS 433, Cygnus X-1
and LS I +61 303 (Cygnus X-3 was a rather strange system with a Wolf rayet com-
panion, γ Cas a very peculiar system in many aspects, and the transients V4641 Sgr
and CI Cam were not yet discovered).