XMM observation of 1RXS J180431.1-273932: a new M-type X-ray binary with a 494 s-pulse period neutron star?

Context: 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.
Aims: Here, we present long-duration XMM observations of the source 1RXS J180431.1-273932.
Methods: 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.
Results: The source is characterized by regular pulses so that it is most likely a neutron star. A modulation of 494.1±0.2 s (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 ~ -7.7 × 10^-4 counts s^-1 h^-1. 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 6.6 keV. The X-ray flux in the 0.2-10 keV energy band is 5.4 × 10^-12 erg s^-1 cm^-2. 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 ~ 10 kpc to be estimated. Other possibilities are also discussed.
Conclusions: Once the distance was estimated, we got an X-ray luminosity of L_X⪉ 6 × 10³⁴ erg s^-1, which is consistent with the typical X-ray luminosity of a symbiotic LMXB system.