(News & views on the paper by Yu & Xu 2010, vol.10, 815-820)
Challenges from a
low-mass millisecond pulsar
Xiang-Dong Li (Nanjing University)
Isolated pulsars are remarkable clock-like rotators, the spin periods of which range from 1.4 milliseconds to about 10 seconds. Pulsars spinning at periods less than about 10 milliseconds are called millisecond pulsars, which are suggested to be the result of spin-up due to accretion in binaries (Alpar, Cheng, Ruderman & Shaham, 1982; Radhakrishnan & Srinivasan, 1982). Of course, the millisecond pulsars should be more massive than normal pulsars in this conventional recycling scenario, probably with mass higher than 1.4 solar masses.
Certainly, it is very difficult to measure the mass of millisecond pulsars. Even in a binary system, we might only know the mass functions of each of the components due to their unknown inclination angle. Nevertheless, recently, the first amazing, example of an eclipsing accretion-powered 1.93-millisecond X-ray pulsar system, SWIFT J1749.4-2807 (Schady et al. 2006), was found which may provide us with a perfect opportunity for measuring the pulsar mass and the inclination angle. Making use of the timing parameters of SWIFT J1749.4-2807 (Markwardt & Strohmayer 2010; Altamirano et al. 2010; Ferrigno et al. 2010), Yu & Xu (2010) re-calculated the pulsar mass (to be about one solar mass) and the inclination angle, based on the assumption that the companion star is on the main-sequence and is filling its Roche-lobe. Such a relatively low pulsar mass seems to be in contradiction with the conventional recycling scenario for the origin and evolution of millisecond pulsars.
This low mass example would have profound implications on the nature of pulsars, namely neutron or quark stars. Different equations of state may result in different mass-radius relations (Lattimer, 2007) of compact stars. Furthermore, according to general relativity, the rotation-mode (i.e., r-mode) instability could be excited in fast relativistic stars and they may effectively spin down through radiating gravitational waves (Andersson et al. 2009). Yu & Xu (2010) then concluded that SWIFT J1749.4-2807 could be a quark star if the ratio of the critical frequency of r-mode instability to the Keplerian one is higher than 0.3, since quark stars may sustain faster spins than neutron stars. It will surely be helpful to understand the non-perturbative behaviors of the elementary strong interaction if SWIFT J1749.4-2807 is really a quark star.
In summary, the derived relatively low mass (around one solar mass) of SWIFT J1749.4-2807 has challenged the theories of both the origin of millisecond pulsars and the inner structure of pulsar-like compact stars.
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