Long-term evolution of Sco X-1: implications for the current spin frequency and ellipticity of the neutron star

Abstract

Sco X-1 is the brightest observed extra-solar X-ray source, which is a neutron star (NS) low-mass X-ray binary (LMXB), and is thought to have a strong potential for continuous gravitational waves (CW) detection due to its high accretion rate and relative proximity. Here, we compute the long-term evolution of its parameters, particularly the NS spin frequency ($\hat{I}½$) and the surface magnetic field ($B$), to probe its nature and its potential for CW detection. We find that Sco X-1 is an unusually young ($\sim 7\times 10^6$ yr) LMXB and constrain the current NS mass to $\sim 1.4-1.6{\mathrm{M}}_{\odot }$. Our computations reveal a rapid $B$ decay, with the maximum current value of $\sim 1.8\times 10^8$ G, which can be useful to constrain the decay models. Note that the maximum current $\hat{I}½$ value is $\sim 550$ Hz, implying that, unlike what is generally believed, a CW emission is not required to explain the current source properties. However, $\hat{I}½$ will exceed an observed cut-off frequency of $\sim 730$ Hz, and perhaps even the NS break-up frequency, in the future, without a CW emission. The minimum NS mass quadrupole moment ($Q$) to avoid this is $\sim (2-3)\times 10^{37}$ g cm${}^2$, corresponding to a CW strain of $\sim 10^{-26}$. Our estimation of current $\hat{I}½$ values can improve the CW search sensitivity.

11 pages, 5 figures, 1 table. Accepted for publication in The Astrophysical Journal