Evidence for a 36 ks phase modulation in the hard X-ray pulses from the magnetar 1E 1547.0-5408

Abstract

The Suzaku data for the highly variable magnetar 1E 1547.0-5408, obtained during the 2009 January activity, were reanalyzed. The 2.07 s pulsation, detected in the 15-40 keV HXD data, was found to exhibit phase modulation, which can be modeled by a sinusoid with a period of 36.0^{+4.5}_{-2.5}ks and an amplitude of 0.52 ± 0.14 s. While the effect is also seen in 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 second example of this kind of behavior detected from magnetars. The effect can be interpreted as a manifestation of torque-free precession of this magnetar, which is suggested to be prolately deformed under the presence of a strong toroidal field of ∼10¹⁶ G.