2007A&A...474L...1N

Low-mass X-ray binaries are peculiar binary systems composed of a compact object and a low-mass star. Recently, a new class of these systems, known as symbiotic X-ray binaries (with a neutron star with a M-type giant companion), has been discovered. Here, we present long-duration XMM observations of the source 1RXS J180431.1-273932. Temporal and spectral analysis of the source was performed along with a search for an optical counterpart. We used a Lomb-Scargle periodogram analysis for the period search and evaluated the confidence level using Monte-Carlo simulations. The source is characterized by regular pulses so that it is most likely a neutron star. A modulation of 494.1±0.2s (3σ error) was found with a confidence level of >99%. Evidence of variability is also present, since the data show a rate of change in the signal of ~-7.7x10^–4counts/s/h. A longer observation will be necessary in order to determine if the source shows any periodic behavior. The spectrum can be described by a power law with photon index Γ∼1 and a Gaussian line at 6.6keV. The X-ray flux in the 0.2-10keV energy band is 5.4x10^–12erg/s/cm2. The identification of an optical counterpart (possibly an M6III red-giant star with an apparent visual magnitude of ≃17.6) allows a conservative distance of ∼10kpc to be estimated. Other possibilities are also discussed. Once the distance was estimated, we got an X-ray luminosity of L_X≲6x10³⁴erg/s, which is consistent with the typical X-ray luminosity of a symbiotic LMXB system.