Astron. Astrophys. 347, 518-523 (1999)

1. Introduction

The star LS 5039 is the most likely optical counterpart to the X-ray source RX J1826.2-1450. Such an association was originally proposed by Motch et al. (1997), hereafter M97, as a result of a systematic cross-correlation between the ROSAT All Sky Survey (Voges et al. 1996) and several OB star catalogues in the SIMBAD database. The unabsorbed X-ray luminosity, at an estimated distance of 3.1 kpc, amounts to (0.1-2.4 keV)  8.110³³ erg s^-1, and the hardness of the source is well consistent with a neutron star or a black hole, accreting directly from the companion's wind (M97). In the optical, LS 5039 appears as a bright  11.2 star with an O7 V((f)) spectral type. Based on this evidence, M97 proposed the system to be a high mass X-ray binary (HMXRB).

In addition, this system has been found to be active at radio wavelengths. Its radio counterpart (NVSS J182614-145054) is a bright, compact and moderately variable radio source in excellent sub-arcsecond agreement with the optical star (Martí et al. 1998). All these facts point to the peculiar nature of RX J1826.2-1450/LS 5039, and suggest a classification among the selected group of radio emitting HMXRB.

In order to explore how this source behaves compared to other members of its class (e.g. Cygnus X-1, LS I+61^o303 and SS 433), we have analyzed the corresponding X-ray data from the All Sky Monitor (ASM) and the Proportional Counter Array (PCA) on board the satellite Rossi X-ray Timing Explorer (RXTE). In Sect. 2 we present an X-ray timing analysis based on both the ASM and the PCA instruments. The ASM data are suitable to study the long-term (days to months) temporal behavior of the source, whereas X-ray variability on shorter time scales (seconds to hours) is better investigated with the PCA. In Sect. 3 a PCA spectroscopic analysis is presented, with the different spectral models that fit the data being examined and discussed.

In the radio domain, RX J1826.2-1450/LS 5039 was included at our request in the list of radio sources routinely monitored at the Green Bank Interferometer (GBI) ¹. At the time of writing, the radio light curves cover  4 months of observations. In Sect. 4 we present the GBI radio data so far acquired with some discussion on the source variability and spectral index properties. Finally, we conclude in Sect. 5 with a brief comparative discussion of RX J1826.2-1450/LS 5039 versus other radio loud HMXRB.

Hereafter, we will refer to the source as RX J1826.2-1450 when discussing the X-rays. In the optical/radio context the LS 5039 designation will be preferred.

© European Southern Observatory (ESO) 1999

Online publication: June 30, 1999
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