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 .