Title: Spectral and Timing Nature of the Symbiotic X-ray Binary 4U 1954+319: The Slowest Rotating Neutron Star in an X-ray Binary System Show Chinese title

Title: 共生X射线双星4U 1954+319的光谱和时间特性：X射线双星系统中自转最慢的中子星

Abstract: 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 ~5.4h NS spin period is the longest among all known accretion-powered pulsars and exhibited large (~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 Kalpha line during the outburst. Spectral similarities to slowly rotating pulsars in high-mass X-ray binaries, its high pulsed fraction (~60-80%), and the location in the Corbet diagram favor high B-field (>~1e+12 G) over a weak field as in low-mass X-ray binaries. The observed low X-ray luminosity (1e+33-1e+35 erg/s), 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 ~1e+13 G NS, this scheme can explain the ~5.4 h equilibrium rotation without employing the magnetar-like field (~1e+16 G) required in the disk accretion case. The time-scales of multiple irregular flares (~50 s) can also be attributed to the free-fall time from the Alfven shell for a ~1e+13 G field. A physical interpretation of SyXBs beyond the canonical binary classifications is discussed. Show Chinese abstract

Abstract: 共生X射线双星4U 1954+319是一个罕见的系统，包含一个特殊的中子星（NS）和一个M型光学伴星。 其约5.4小时的NS自转周期是所有已知吸积驱动脉冲星中最长的，并在8年内表现出约7%的大幅波动。 在2012年X射线亮度增强期间，通过Swift/BAT检测到了自转趋势的转变。 根据2011年和2012年的60 ks Suzaku观测，在这次爆发前后，该源处于平静和明亮状态。 在爆发期间，观测到的连续谱可以用一个康普顿化模型来很好地描述，并添加了一个狭窄的6.4 keV Fe Kalpha线。 与高质量X射线双星中的慢旋转脉冲星的光谱相似性、其高脉冲分数（约60-80%）以及在Corbet图中的位置支持高磁场（>~1e+12 G）而非低质量X射线双星中的弱场。 观测到的低X射线光度（1e+33-1e+35 erg/s）、可能的宽轨道以及该SyXB的缓慢恒星风使得亚音速沉降区内的准球形吸积成为一个合理的模型。 假设NS约为1e+13 G，这种方案可以在不采用盘吸积情况下所需的磁星样场（~1e+16 G）的情况下解释约5.4小时的平衡自转。 多个不规则耀斑的时间尺度（约50秒）也可以归因于约1e+13 G磁场下从阿尔芬壳层的自由下落时间。 讨论了超越典型双星分类的SyXB的物理解释。