Gravitational Waves from Newborn Accreting Millisecond Magnetars

TL;DR

This paper explores the formation and gravitational wave emission of accretion columns on newborn millisecond magnetars, revealing their properties, potential detectability, and impact on magnetar spin evolution within seconds after birth.

Contribution

It provides the first detailed analysis of accretion column characteristics and their GW signals on newborn magnetars, including their evolution and observational prospects.

Findings

Accretion columns reach about 1 km height and are neutrino-cooled.

GW strain from columns could be detectable within 1 Mpc by next-gen detectors.

Magnetar survival time with accretion columns is a few tens of seconds.

Abstract

Two accretion columns have been argued to form over the surface of a newborn millisecond magnetar for an extremely high accretion rate $\gtrsim1.8\times10^{-2}M_\odot\ {\rm s^{-1}}$ that may occur in the core-collapse of a massive star. In this paper, we investigate the characteristics of these accretion columns and their gravitational wave (GW) radiation. For a typical millisecond magnetar (surface magnetic field strength $B\sim10^{15}$ G and initial spin period $P\sim1$ ms), we find (1) its accretion columns are cooled via neutrinos and can reach a height $\sim1$ km over the stellar surface; (2) its column-induced characteristic GW strain is comparable to the sensitivities of the next generation ground-based GW detectors within a horizon $\sim1$ Mpc; (3) the magnetar can survive only a few tens of seconds; (4) during the survival timescale, the height of the accretion columns increases rapidly to the peak and subsequently decreases slowly; (5) the column mass, characteristic GW strain, and maximum GW luminosity have simultaneous peaks in a similar rise-fall evolution. In addition, we find that the magnetar's spin evolution is dominated by the column accretion torque. A possible association with failed supernova is also discussed.