Discovery of free precession in the magnetar SGR 1806−20 with the ASCA Gas Imaging Spectrometer

Abstract

Abstract Four X-ray datasets of the soft gamma repeater SGR 1806−20, taken with the Gas Imaging Spectrometer (GIS) onboad ASCA, were analyzed. Three of them were acquired over 1993 October 9–20, the last one in 1995 October. Epoch-folding analysis of the 2.8–12 keV signals confirmed the ∼7.6 s pulses in these data, which Kouveliotou et al. (1998, Nature, 393, 235) reported as one of the earliest pulse detections from this object. In the 1995 observation, 3–12 keV pulses were phase modulated with a period of T = 16.4 ± 0.4 ks, and an amplitude of ∼1 s. This makes a fourth example of the behavior observed from magnetars. As in the previous three sources, the pulse-phase modulation of SGR 1806−20 disappeared at ≲2.5 keV, where the soft X-ray component dominates. In the 1993 datasets, this periodic modulation was reconfirmed, and successfully phase-connected coherently across the 11 d interval. As a result, the modulation period was refined to T = 16.435 ± 0.024 ks. The implied high stability of the phenomenon strengthens its interpretation in terms of free precession of the neutron star, which is deformed to an asphericity of ∼10−4, presumably by the stress of toroidal magnetic fields reaching ∼1016 G. Toroidal fields of this level can be common among magnetars.