3XMM J181923.7–170616: An X-Ray Binary with a 408 s Pulsar

Abstract We carry out a dedicated study of 3XMM?J181923.7–170616 with an approximate pulsation period of 400?s using the XMM-Newton and Swift observations spanning across nine years. We have refined the period of the source to 407.904(7) s (at epoch MJD 57142) and constrained the 1σ upper limit on the period derivative . The source radiates hard, persistent X-ray emission during the observation epochs, which is best described by an absorbed power-law model (Γ?～?0.2–0.8) plus faint Fe lines at 6.4 and 6.7 keV. The X-ray flux revealed a variation within a factor of 2, along with a spectral hardening as the flux increased. The pulse shape is sinusoid-like and the spectral properties of different phases do not present significant variation. The absorption ( ) is similar to the total Galactic hydrogen column density along the direction, indicating that it is a distant source. A search for the counterpart in optical and near-infrared surveys reveals a low-mass K-type giant, while the existence of a Galactic OB supergiant is excluded. A symbiotic X-ray binary (SyXB) is the favored nature of 3XMM?J181923.7–170616 and can essentially explain the low luminosity of , slow pulsation, hard X-ray spectrum, and possible K3?III companion. An alternative explanation of the source is a persistent Be X-ray binary (BeXB) with a companion star no earlier than B3-type