Letter to the Editor

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

XMM-Newton Science Operations Centre, ESAC, ESA, PO Box 50727, 28080 Madrid, Spain 


Received: 4 June 2007
Accepted: 9 August 2007

Abstract

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  6  10³⁴ erg s^-1, which is consistent with the typical X-ray luminosity of a symbiotic LMXB system.