A Compact Star Rotating at 1122 Hz and the r -Mode Instability

Abstract

We show that r-mode instabilities severely constrain the composition of a compact star rotating with a submillisecond period. In particular, the only viable astrophysical scenario for such an object, present inside the lowmass X-ray binary associated with the X-ray transient XTE J1739Ϫ285, is that it has a strangeness content. Since previous analyses indicate that hyperonic stars or stars containing a kaon condensate are not good candidates, the only remaining possibility is that such an object is either a strange quark star or a hybrid quark-hadron star. We also discuss under which conditions submillisecond pulsars are rare. The possibility has been widely discussed in the literature that r-mode instabilities can very efficiently drag angular momentum from a rotating compact star if its temporal evolution on the Q-T plane (angular velocity and temperature) enters the r-mode instability window; see, e.g., Andersson (1998) and Friedman & Morsink (1998). Therefore huge regions of the Q-T plane are excluded. Moreover, the size and position of that window are strictly related to the composition of the star, since it is strongly dependent on the value of the bulk and shear viscosity. It is particularly important to recall that for stars containing strangeness, such as hyperon stars (Lindblom & Owen 2002), hybrid stars (Drago et al. 2005), and strange quark stars (Madsen 2000), there is also a contribution to the bulk viscosity associated with the formation of strangeness. Due to this, the instability window splits into two parts: one which starts at temperatures larger than (7 ‫ע‬ 3) # 10 9 K (high-temperature instability window, HTIW) and a lower temperature window at temperatures smaller than (5 ‫ע‬ 4) # 10 8 K (low-temperature instability window, LTIW). 3 Concerning the left border of the LTIW, its position is regulated by the shear viscosity and by the so-called viscous boundary layers located at the interface between the crust and the fluid composing the inner part of the star (Bildsten & Ushomirsky 2000). In particular, in bare quark stars (composed by either normal or superconducting quark matter), due to the absence of a significant crust, the left border of the LTIW extends to much lower temperatures and the LTIW has a minimum corresponding to a significantly lower temperature than in the case of stars containing a crust.

Generation of strong magnetic fields byr-modes in millisecond accreting neutron stars: Induced deformations and gravitational wave emission

“…hyperons or deconfined quarks, is present in the inner core. In this type of stars the viscosity and thus the r-mode instability window is quite different from that of normal NSs [32]. Moreover, the evolution of the internal magnetic fields and the magnetic properties of the compact star are also expected to be different [8,33].…”

Sequential deconfinement of quark flavors in neutron stars

“…Unmasking neutron star interiors may therefore require observables based on transport properties, which could be strongly modified in the presence of color superconductivity. It has been suggested that such tests of the structure of matter at high density be based on an analysis of the cooling behavior [41][42][43][44] stability of rapidly spinning stars against r-modes [45,46]. It has turned out that these phenomena are sensitive to the details of color superconductivity in quark matter.…”

Quark Matter in Compact Stars: Astrophysical Implications and Possible Signatures

“…93 ), thus XTE J1739-285 contains the most rapidly rotating compact star discovered up to now, and the first with a submillisecond spin period (P = 0.891 ms). This discovery gives a model indipendent observational contraint to EOS dense stellar matter, and it has prompted a number of studies [100][101][102] in this direction. In one of these studies, Lavagetto et al 100 have derived the following upper limit…”