Quarks and Compact Stars (QCS)
Oct. 20-22, 2014, KIAA at Peking University, Beijing - P. R. China

The mass and radius of neutron star in 4U 1746-37

Zhaosheng Li

Department of Astronomy, School of Physics, Peking University, Beijing 100871, P. R. China

Abstract: Glitch (sudden spin-up) is a common phenomenon in pulsar observations. However, the physical mechanism of glitch is still a matter of debate because it depends on the puzzle of pulsar's inner structure, i.e. the equation of state of dense matter. Some pulsars (e.g. Vela like) show large glitches but release negligible energy, whereas the large glitches of AXPs/SGRs are usually (but not always) accompanied with detectable energy releases manifesting as X-ray bursts or outbursts.We try to understand this aspect of glitches in a starquake model of solid quark stars. There are two kinds of glitches in this scenario: bulk-invariable (type I) and bulk-variable (type II) ones. The total stellar volume changes (and then energy releases) significantly for the latter but not for the former. Therefore, glitches accompanied with X-ray bursts (e.g. that of AXP/SGRs) could originate from type II starquakes induced probably by accretion, while the others without evident energy release (e.g. that of Vela pulsar) would be tospheric radius expansion (PRE) bursts have already been used to constrain the masses and radii of neutron stars. \textit{RXTE} observed three PRE bursts in \object{4U 1746-37}, all with low touchdown fluxes. We discuss here the possibility of low mass neutron star in \object{4U 1746-37} because the Eddington luminosity depends on stellar mass. With typical values of hydrogen mass fraction and color correction factor, a Monte-Carlo simulation was applied to constrain the mass and radius of neutron star in \object{4U 1746-37}. \object{4U 1746-37} has a high inclination angle. Two geometric effects, the reflection of the far side accretion disc and the obscuration of the near side accretion disc have also been included in the mass and radius constraints of \object{4U 1746-37}. If the reflection of the far side accretion disc is accounted, a low mass compact object (mass of $0.41\pm0.14~M_{\odot}$ and radius of $8.73\pm1.54~\rm km$ at 68\% confidence) exists in \object{4U 1746-37}. If another effect operated, \object{4U 1746-37} may contain an ultra low mass and small radius object ($M=0.21\pm0.06~M_{\odot},~R=6.26\pm0.99~\rm km$ at 68\% confidence). Combined all possibilities, the mass of \object{4U 1746-37} is $0.41^{+0.70}_{-0.30}~M_\odot$ at 99.7\% confidence. For such low mass NS, it could be reproduced by a self-bound compact star, i.e., quark star or quark cluster star.