3xmm j181923.7$-$170616: an x-ray binary with a 408s pulsar

TL;DR

This study refines the pulsation period of the X-ray binary 3XMM J181923.7-170616, characterizes its spectral and timing properties, and suggests it is likely a symbiotic X-ray binary with a K-type giant companion.

Contribution

The paper provides the first detailed timing and spectral analysis of 3XMM J181923.7-170616, proposing its classification as a symbiotic X-ray binary based on multi-wavelength data.

Findings

Refined pulsation period to 407.904(7) s.

Detected persistent hard X-ray emission with spectral hardening during flux increase.

Identified a low-mass K-type giant as the likely companion star.

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 determined a period derivative limit of $\dot{P}{\leq} 5.9\pm 5.4\times 10^{-9} s s^{-1}$ ($1\sigma$). The source radiates hard, persistent X-ray emission during the observation epochs, which is best described by an absorbed \powerlaw\ model ($\Gamma \sim 0.2$--0.8) plus faint Fe lines at 6.4 keV 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 N$_H$ $\sim 1.3\times 10^{22} cm^{-2}$ 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 is the favored nature of 3XMM J181923.7$-$170616 and can essentially explain the low luminosity of ${2.78\times 10^{34} d_{10}^{2} erg s^{-1}}$, slow pulsation, hard X-ray spectrum, and possible K3~III companion. An alternative explanation of the source is a persistent Be/X-ray binary with a companion star no earlier than B3-type.