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

72 Chapter 2. Multiwavelength approach to LS 5039
In HMXBs the presence of an emission line around 6.4 keV is commonly seen,
with a variety of strengths. It is ascribed to fluorescent reprocessing by neutral or
mildly ionized iron in relatively cold circumstellar material (Fe Kα line). A higher
energy implies the presence of highly ionized iron (He-like ∼ FeXXV) or a different
origin than Kα emission. One possibility would be radiative recombinations of H-
like iron followed by cascade processes in a relatively hot corona. Unfortunately, the
PCA energy resolution prevents from distinguishing between these possible models.
On the other hand, the high EWline(Fe) value indicates that a large amount of
circumstellar matter is present in the system. Since LS 5039 lies at low galactic
latitude, we cannot rule out that part of the emission feature comes from the Galactic
ridge (Yamauchi & Koyama 1993). Nevertheless, this contribution cannot account
for the total flux of the line. In addition, Wang et al. (2002) have recently shown
that the ∼ 6.7 keV emission near the Galactic Center region arises from discrete
sources rather than from the diffuse continuum, giving support to the idea that the
∼ 6.7 keV line comes from LS 5039.
A hydrogen column density of ∼ 2 +1
−2 × 10 21 cm −2 is found from the fit. This
value is, however, not very well constrained. In fact, it is consistent with zero. The
difficulty in constraining the hydrogen column density from our X-ray data can be
attributed to the fact that the interstellar gas mainly absorbs X-ray photons with
energies lower than 2–3 keV, i.e., outside of the energy range considered here. We
note that from the λ 4430 and λ 6284 interstellar bands Motch et al. (1997) found
E(B − V ) = 0.8 ± 0.2. Using the relation NH = 5.3 × 10 21 cm −2 E(B − V ) (Predehl
& Schmitt 1995), we obtain that NH ∼ (4 ± 1) × 10 21 cm −2 , which is consistent,
within the errors, with the X-ray observation value. However, if we consider the
obtained photometric values for E(B − V ) quoted in Table 2.6 ( 1.2 ± 0.1), we
obtain NH (6.4 ± 0.5) × 10 21 cm −2 , which seems clearly inconsistent with the
results after the PCA/RXTE observations. In any case, the lack of sensitivity of
the detector below 3 keV may explain why we have detected such low values of NH.
Possible nature of the compact object based on the X-ray data
The non detection of X-ray pulses does not necessarily mean that RX J1826.2−1450
/ LS 5039 contains a black hole, since it could contain a low magnetic field neutron
star. Both types of objects have been found in REXBs.