Explaining observations of rapidly rotating neutron stars in low-mass x-ray binaries

Abstract

In a previous paper [M. E. Gusakov, A. I. Chugunov, and E. M. Kantor, Phys. Rev. Lett. 112, 151101 (2014)], we introduced a new scenario that explains the existence of rapidly rotating warm neutron stars (NSs) observed in low-mass X-ray binaries (LMXBs). Here it is described in more detail. The scenario takes into account the interaction between superfluid inertial modes and the normal (quadrupole) m = 2 r-mode, which can be driven unstable by Chandrasekhar-Friedman-Schutz (CFS) mechanism. This interaction can only occur at some fixed "resonance" stellar temperatures; it leads to formation of the "stability peaks" which stabilize a star in the vicinity of these temperatures. We demonstrate that a NS in LMXB spends a substantial fraction of time on the stability peak, that is, in the region of stellar temperatures and spin frequencies, that has been previously thought to be CFS unstable with respect to excitation of r-modes. We also find that the spin frequencies of NSs are limited by the CFS instability of normal (octupole) m = 3 r-mode rather than by m = 2 r-mode. This result agrees with the predicted value of the cutoff spin frequency ∼ 730 Hz in the spin distribution of accreting millisecond X-ray pulsars. In addition, we analyze evolution of a NS after the end of the accretion phase and demonstrate that millisecond pulsars can be born in LMXBs within our scenario. Besides millisecond pulsars, our scenario also predicts a new class of LMXB descendants-hot and rapidly rotating nonaccreting NSs ("hot widows"/HOFNARs). Further comparison of the proposed theory with observations of rotating NSs can impose new important constraints on the properties of superdense matter.

Viscosity of neutron star matter andr-modes in rotating pulsars

“…We have checked that with the assumptions used in Ref. [53] we recover their results. For comparison in Fig.…”

“…Note that values of ν c,η obtained by us are less than those found in Ref. [53] by a factor varying from 1.5 at T = 10 7 K to 2 at T = 10 9 K. Simplifying consideration, Ref. [53] used the density profile taken ad hoc, which corresponds effectively to a substantially higher central density of the star.…”

“…We note, however, that such a strong suppression of the r-modes was questioned in Ref. [53]. Within the model [60] the data on the young pulsar PSR J0537-6910 might be explained with a standard cooling, for a sat being slightly below 10 −1 .…”

“…[53] by a factor varying from 1.5 at T = 10 7 K to 2 at T = 10 9 K. Simplifying consideration, Ref. [53] used the density profile taken ad hoc, which corresponds effectively to a substantially higher central density of the star. Moreover, they ignored density dependence of the proton pairing gap, which results in an increase of the lepton shear viscosity term, because then the proton superfluidity holds up to central densities in all stars.…”

“…The mixture of r-modes with other more stable resonance superfluid inertial modes in the star has been suggested in Refs. [52,53] as a reason for the stability of old recycled pulsars in x-ray binaries. The authors argue that the mixture can only occur at some fixed "resonance" stellar temperatures.…”