Observable QPOs produced by steep pulse profiles in Magnetar Flares

TL;DR

This paper investigates how steep pulse profiles in magnetar flares can amplify quasi-periodic oscillations, but concludes that the observed QPOs are likely due to emission modulations rather than direct starquake signals.

Contribution

It introduces a mechanism showing how pulse profile steepness can enhance QPO amplitudes, but finds this insufficient to explain observed signals, implying emission modulations are responsible.

Findings

Steep pulse profiles can amplify underlying oscillations.

Amplification depends on starquake amplitude and emission geometry.

The mechanism cannot fully account for observed QPOs, indicating emission modulations are the cause.

Abstract

Strong quasi-periodic oscillations in the tails of the giant gamma-ray flares seen in SGR 1806-20 and SGR 1900+14 are thought to be produced by starquakes in the flaring magnetar. However, the large fractional amplitudes (up to ~20%) observed are difficult to reconcile with predicted amplitudes of starquakes. Here we demonstrate that the steeply pulsed emission profile in the tail of the giant flare can enhance the observed amplitude of the underlying oscillation, analogously to a beam of light oscillating in and out of the line of sight. This mechanism will also broaden the feature in the power spectrum and introduce power at harmonics of the oscillation. The observed strength of the oscillation depends on the amplitude of the underlying starquake, the orientation and location of the emission on the surface of the star, and the gradient of the light curve profile. While the amplification of the signal can be significant, we demonstrate that even with uncertainties in the emission geometry, this effect is not sufficient to produce the observed QPOs. This result excludes the direct observation of a starquake, and suggests that the observed variations come from modulations in the intensity of the emission.