Maximum gravitational-wave energy emissible in magnetar flares

Abstract

Recent searches of gravitational-wave (GW) data raise the question of what maximum GW energies could be emitted during gamma-ray flares of highly magnetized neutron stars (magnetars). The highest energies (∼ 10 49 erg) predicted so far come from a model [K. Ioka, Mon. Not. Roy. Astron. Soc. 327, 639 (2001)] in which the internal magnetic field of a magnetar experiences a global reconfiguration, changing the hydromagnetic equilibrium structure of the star and tapping the gravitational potential energy without changing the magnetic potential energy. The largest energies in this model assume very special conditions, including a large change in moment of inertia (which was observed in at most one flare), a very high internal magnetic field, and a very soft equation of state. Here we show that energies of 10 48 -10 49 erg are possible under more generic conditions by tapping the magnetic energy, and we note that similar energies may also be available through cracking of exotic solid cores. Current observational limits on gravitational waves from magnetar fundamental modes are just reaching these energies and will beat them in the era of advanced interferometers.

Are gravitational waves from giant magnetar flares observable?

“…Ioka [11] has investigated the maximum gravitational wave energy released by the change in moment of inertia induced by such a mechanism and placed an upper limit of about 10 49 erg under ideal conditions, including optimistic values of the internal magnetic field. More recently, Corsi and Owen [12] found similar values to be possible under more generic conditions, still tapping into the full energy reservoir associated with an instantaneous change in the magnetic potential energy of the star. In contrast, Levin and van Hoven [7] do not find f -mode detection to be very likely in the near future.…”

Methods and results of a search for gravitational waves associated with gamma-ray bursts using the GEO 600, LIGO, and Virgo detectors

“…Sources of this kind are expected to produce some GW energy (≲10 −8 M ⊙ c 2 ) in the 1 kHz-2 kHz range [44][45][46][47].…”

Gravitational Waves from Neutron Stars: A Review

“…and Ciolfi & Rezzolla (2012) performed complementary, general relativistic magnetohydrodynamic simulations using their respective codes (Lasky et al 2011;Ciolfi et al 2011) for catastrophic reconfigurations of the internal magnetic field, also finding that the f -mode is not sufficiently excited to generate a detectable gravitational wave signature. Although these works had different predictions for the gravitational wave scaling as a function of magnetic field strength, they both predicted approximately 10 orders of magnitude less energy being emitted in gravitational waves than Corsi & Owen (2011). Lasky et al 2012, empty black histogram).…”

“…Lasky et al 2012, empty black histogram). Plotted in blue are the optimistic predictions of Ioka (2001) and Corsi & Owen (2011), and the dashed black line is the upper limit on the f -mode gravitational wave energy from the giant flare from SGR 1806-20 (Abadie et al 2011a). wave predictions from Ioka (2001) and Corsi & Owen (2011), while the dashed black line gives the observed upper limit on the f -mode gravitational wave energy from the 2004 giant flare in SGR 1806-20 (Abadie et al 2011a, see below).…”

“…Ioka (2001) assumed an instantaneous change in the star's moment of inertia from a rearrangement of the global, internal magnetic field can excite the neutron star's fundamental f mode (typically at frequencies, f ∼ 1-2 kHz), a study that was backed up by Corsi & Owen (2011). These studies estimated that the energy in gravitational waves could be E gw ∼ 10 48 -10 49 erg, comparable to the energy emitted in electromagnetic waves, and certainly detectable in the Advanced Detector Era.…”