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2.6. The X-ray counterpart: RX J1826.2−1450 73
On the other hand, the multicolor disk model, although formally fitting the data,
does not help either. First, given the low luminosity (LX < 10 35 erg s −1 in the energy
range 2–10 keV) the system would be in the so-called low/hard state. We would
not expect then to detect a strong soft component. Second, the fit provides an
unrealistic value of the disk internal radius of Rin cos 1/2 (θ) ∼ 0.3 km.
Unfortunately, the source is too faint at energies above 30 keV to be detected
with the HEXTE/RXTE instrument. Thus, we cannot confirm from the present
data whether the hard tail that usually characterizes the energy spectrum of black
holes at high energies is indeed present.
2.6.3 BeppoSAX observations
With the aim to confirm and improve the obtained results after the PCA/RXTE
observations, we observed 22 RX J1826.2−1450 / LS 5039 with the BeppoSAX X-
ray satellite during ∼ 80 ksec on 2000 October 8 (MJD 51825.29–51826.21). Un-
fortunately, while the flux detected during the PCA/RXTE observations was of
∼ 5 × 10 −11 erg s −1 cm −2 , it was only of ∼ 5 × 10 −12 erg s −1 cm −2 during the
BeppoSAX observations, i.e., one order of magnitude lower. Hence, we could not
confirm the detection of the ∼ 6.7 keV iron line due to lack of signal-to-noise ra-
tio. However, we could improve the value of the hydrogen column density, NH =
1.0 ± 0.3 × 10 22 cm −2 , and obtain a new value of the photon index, Γ = 1.8 ± 0.2 (in
the energy range 3–10 keV).
Using the relation by Predehl & Schmitt (1995) the NH value implies E(B−V ) =
1.9±0.6, nearly compatible with the E(B−V ) 1.2±0.1 values derived from optical
photometry (see Table 2.6) and much higher than the E(B − V ) 0.4 +0.2
−0.4 value
derived from the PCA/RXTE observations, which was poorly constrained. On the
other hand, the value of Γ is compatible with the one found in the PCA/RXTE
observations, and typical of black holes in the low/hard state, although we note
that McSwain & Gies (2002) have recently suggested a neutron star nature for the
compact object in LS 5039.
The BeppoSAX observations took place in JD 2451825.79–2451826.71, hence
very close to the T =JD 2451822.12 value used in the McSwain et al. (2001)
22 Reig, P., Ribó, M., Paredes, J. M., & Martí, J. 2002, in preparation.