Discovery of a 36 ks Phase Modulation in the Hard X-ray Pulses from the Magnetar 1E 1547.0$-$5408

TL;DR

This study reanalyzed Suzaku data of magnetar 1E 1547.0$-$5408, discovering a 36 ks phase modulation in its hard X-ray pulses, likely caused by free precession due to strong internal magnetic fields.

Contribution

It presents the first detection of a long-term phase modulation in this magnetar's X-ray pulses, suggesting free precession as a new insight into magnetar behavior.

Findings

Detected a 36 ks phase modulation in hard X-ray pulses.

Modulation amplitude decreases at lower energies, disappearing below 4 keV.

First such detection after 4U 0142+61, indicating a new class of magnetar behavior.

Abstract

The Suzaku data of the highly variable magnetar 1E 1547.0$-$5408, obtained during the 2009 January activity, were reanalyzed. The 2.07 s pulsation of the 15--40 keV emission detected with the HXD was found to be phase modulated, with a period of $36.0^{+4.5}_{-2.5}$ ks and an amplitude of $0.52 \pm 0.14$ s. The modulation waveform is suggested to be more square-wave like rather than sinusoidal. While the effect was confirmed with the 10--14 keV XIS data, the modulation amplitude decreased towards lower energies, becoming consistent with 0 below 4 keV. After the case of 4U 0142+61, this makes the 2nd example of this kind of behavior detected from magnetars. The effect can be interpreted as a manifestation of free precession of this magnetar, which is suggested to be oblately deformed under the presence of strong toroidal field of $\sim 10^{16}$ G.