The symbiotic X-ray binary 4U 1954+319 is a rare system hosting a peculiar neutron star ( NS ) and an M-type optical companion .
Its \sim 5.4 h NS spin period is the longest among all known accretion-powered pulsars and exhibited large ( \sim 7 % ) fluctuations over 8 years .
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 ( \sim 60–80 % ) , and the location in the Corbet diagram favor high B -field ( \gtrsim 10 ^ { 12 } G ) over a weak field as in low-mass X-ray binaries .
The observed low X-ray luminosity ( 10 ^ { 33 } – 10 ^ { 35 } \mathrm { erg } \mathrm { 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 \sim 10 ^ { 13 } G NS , this scheme can explain the \sim 5.4 h equilibrium rotation without employing the magnetar-like field ( \sim 10 ^ { 16 } G ) required in the disk accretion case .
The time-scales of multiple irregular flares ( \sim 50 s ) can also be attributed to the free-fall time from the Alfvén shell for a \sim 10 ^ { 13 } G field .
A physical interpretation of SyXBs beyond the canonical binary classifications is discussed .