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54 Chapter 2. Multiwavelength approach to LS 5039 of the VLBA observations at the same frequency. In all the images the jets have similar position angles, ∼ 125 ◦ in the VLBA image, ∼ 140 ◦ in the EVN image, and ∼ 150 ◦ in the MERLIN image. These results suggest a bending of the jets with increasing distance from the core and/or precession. The brightness and length asymmetry of the jet components may involve special relativity effects (Rees 1966, Blandford & Königl 1979, see also Sect. 1.2.1). Hence, assuming that this is the reason for the detected length asymmetry of the jets, we can estimate some parameters by using the following equation: β cos θ = µa − µr µa + µr = da − dr da + dr . (2.7) Although we do not know the epoch of ejection of the terminal plasma, we can identify da and dr with the lengths of the approaching and receding jet, respectively. Using Eq. 2.7 and the EVN values in Table 2.3 we obtain β cos θ = 0.17 ± 0.05, and hence β > 0.17 ± 0.05 and θ < 80 ◦ ± 3 ◦ . For the MERLIN values we obtain β > 0.15 ± 0.06 and θ < 81 ◦ ± 3 ◦ . These values are similar to those previously derived from the VLBA image, of β > 0.15 ± 0.04 and θ < 81 ◦ ± 2 ◦ . We have not considered an eventual larger size for the NW jet with a brightness below the image noise level. McSwain & Gies (2002) have recently proposed an inclination of i 30 ◦ for LS 5039. If we assume that the jet is perpendicular to the accretion disk, and that the disk lies in the orbital plane of the binary system, then θ = i = 30 ◦ , and using the values from the EVN image, we obtain β = 0.20 ± 0.06, which indicates a mildly relativistic jet. In this case, the apparent velocity and proper motion of the approaching components would be 0.12c and 7.2 mas day −1 . For the receding components we would have 0.085c and 5.1 mas day −1 . The total length of the EVN and MERLIN jets is ∼ 60 and ∼ 300 mas, respectively. Considering that the source is located at 2.9 kpc (Ribó et al. 2002), these angular lengths translate into linear lengths in the plane of the sky of ∼ 175 and ∼ 870 AU, respectively. Assuming that θ = 30 ◦ we obtain intrinsic total lengths of ∼ 350 and ∼1740 AU, respectively, and lengths of the SE jet arm of ∼ 200 and ∼1000 AU, respectively. Moreover, the jet width is smaller than one synthesized beam even in the EVN image. This implies a jet half opening angle ≤ 6 ◦ . Further discussion on the LS 5039 jets is reported in Sect. 2.8.
2.5. The optical/IR star: LS 5039 55 Conclusions The EVN and MERLIN observations have confirmed the existence of relativistic radio jets in LS 5039, as previously found in VLBA observations (Paredes et al. 2000, see also Sect. 2.4.4). Moreover, the recurrent detection of the jets strongly suggests that in LS 5039 relativistic radio jets are always present. 2.5 The optical/IR star: LS 5039 The star LS 5039 was originally classified as the star number 5039 in the catalog Luminous stars in the southern Milky Way, by Stephenson & Sanduleak (1971). It is an early type star, O6.5V((f)), with a visual magnitude ∼ 11.2. In this section we report photometric and spectroscopic results that have allowed to obtain the spectral type, the distance and the orbital parameters of the binary system. 2.5.1 Photometry Optical photometry of LS 5039 has been carried out by several groups. Here we start by reporting photometry obtained by us, and then provide a compilation of optical and near infrared photometry obtained by other groups. Optical observations with the 1.5 m OAN telescope Soon after the VLA runs presented in Sect. 2.4.2, we carried out optical CCD observations 16 at Calar Alto observatory (Spain). Our main goal was to search for photometric variability that could evidence the active nature of LS 5039 as an X- ray binary. We used the 1.52 m telescope of the Spanish Observatorio Astronómico Nacional (OAN) from 1998 June 1 to 8. The Ritchey-Chrétien focus was available together with a Tektronics TK1024AB chip. This provided a scale factor of 0.4 ′′ per pixel and a 6.9 ′ × 6.9 ′ field of view. Images were acquired through the V RI Johnson filters and they were reduced using standard procedures based on the iraf image processing system. In Fig. 2.10 we show a 30 s exposure of the LS 5039 field in 16 Published in Martí, J., Paredes, J. M. & Ribó, M. 1998, A&A, 338, L71.
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2 Chapter 1. Introduction and backg
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122 Chapter 4. The cross-identifica
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