SPECTRAL AND TIMING NATURE OF THE SYMBIOTIC X-RAY BINARY 4U 1954+319: THE SLOWEST ROTATING NEUTRON STAR IN AN X-RAY BINARY SYSTEM

The symbiotic X-ray binary (SyXB) 4U 1954+ 319 is a rare system hosting a peculiar neutron star (NS) and an M-type optical companion. Its similar to 5.4 hr NS spin period is the longest among all known accretion-powered pulsars and exhibited large (similar to 7%) fluctuations over 8 yr. A spin trend transition was detected with Swift/BAT around an X-ray brightening in 2012. The source was in quiescent and bright states before and after this outburst based on 60 ks Suzaku observations in 2011 and 2012. The observed continuum is well described by a Comptonized model with the addition of a narrow 6.4 keV Fe-K alpha line during the outburst. Spectral similarities to slowly rotating pulsars in high-mass X-ray binaries, its high pulsed fraction (similar to 60%-80%), and the location in the Corbet diagram favor high B-field (greater than or similar to 10(12) G) over a weak field as in low-mass X-ray binaries. The observed low X-ray luminosity (10(33)-10(35) erg s(-1)), probable wide orbit, and a slow stellar wind of this SyXB make quasi-spherical accretion in the subsonic settling regime a plausible model. Assuming a similar to 10(13) G NS, this scheme can explain the similar to 5.4 hr equilibrium rotation without employing the magnetar-like field (similar to 10(16) G) required in the disk accretion case. The timescales of multiple irregular flares (similar to 50 s) can also be attributed to the free-fall time from the Alfven shell for a similar to 10(13) G field. A physical interpretation of SyXBs beyond the canonical binary classifications is discussed.