Magnetar Twists: Fermi/gbm Detection Of Sgr J1550-5418

In 2008 October and in 2009 January and March, SGR J1550-5418 (previously known as AXP 1E 1547.0-5408) went into very active bursting episodes. The most intense episode was observed on 2009 January 22, during which Fermi/GBM recorded 450 soft gamma-ray bursts from the source. Using the GBM data during this intense bursting period, we identified an enhanced persistent emission that exhibited intriguing timing and spectral properties: (i) clear pulsations up to 110 keV at the spin period of the neutron star, (ii) an additional (to a power-law) blackbody component required for the enhanced emission spectrum with a temperature of 17 keV, and (iii) pulsed fraction that is strongly energy dependent and peaks in 50-74 keV. A total isotropic-equivalent energy emitted during this enhanced emission is estimated to be 10⁴⁰ erg. We also estimate the area of the blackbody emitting region to be 0.4 km² (roughly a few × 10^-5 of the neutron star surface area): this small "hot spot" may correspond to the size of magnetically-confined hot plasma in the twisted magnetic field near the star's surface.