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74 Chapter 2. Multiwavelength approach to LS 5039
ephemeris. Therefore, the lack of accuracy in the orbital period is not important in
this case in order to compute the orbital phase, which ranges from 0.89 to 0.11. As
discussed in Sect. 2.5.4, the time of an X-ray eclipse if one occurs is near phase 0.06
according to these ephemeris, and its maximum duration, for a star of 40 M⊙ and
an inclination angle of 90 ◦ , is expected to be of ∼ 0.2 d, or ∼ 0.05 phase interval
(McSwain, private communication). Hence, we observed the source just around the
time of a possible X-ray eclipse, but there is no signature of such an event in the
BeppoSAX data.
The non-detection of an X-ray eclipse is consistent with the proposed inclination
of the system of ∼ 30 ◦ (McSwain & Gies 2002). We also note that McSwain &
Gies (2002), based on the broad residual emission in the Hα profile, obtained a
higher mass loss rate of the optical star in 1998 than in 2000, which may explain
the difference in the detected flux between the PCA/RXTE observations (carried
out in year 1998) and the BeppoSAX observations (carried out in year 2000).
Therefore, we conclude that the source was intrinsically faint during these obser-
vations, and that there seem to be no X-ray eclipses in RX J1826.2−1450 / LS 5039,
in good agreement with the proposed inclination of this binary system.
2.7 The γ-ray counterpart: 3EG J1824−1514
The highly energetic processes that take place near compact objects in X-ray binaries
are observable from radio to hard X-rays (Mirabel & Rodríguez 1999) and possibly
beyond. Therefore, soon after the discovery of the microquasar nature of LS 5039, we
inspected the most recent high-energy catalogs in order to find possible counterparts
to LS 5039. While there was no counterpart in the COMPTEL (0.75–30 MeV)
source catalog (Schönfelder et al. 2000), there was a very interesting unidentified
high-energy γ-ray source in the Third EGRET Catalog (E > 100 MeV) (Hartman
et al. 1999), namely 3EG J1824−1514.
In Fig. 2.18 we show the location map of 3EG J1824−1514, together with the
X-ray sources from both the ROSAT All Sky Bright Source Catalog (RBSC) (Voges
et al. 1999) and Faint Source Catalog (RFSC) (Voges et al. 2000), as well as the
radio sources brighter than 20 mJy (for clarity) from the NRAO VLA Sky Survey
(NVSS) (Condon et al. 1998). As can be seen, there is only a RBSC source, namely