2013ApJ...770L..23M

We report the discovery of 3.76 s pulsations from a new burst source near Sgr A* observed by the NuSTAR observatory. The strong signal from SGR J1745-29 presents a complex pulse profile modulated with pulsed fraction 27%±3% in the 3-10 keV band. Two observations spaced nine days apart yield a spin-down rate of {dot}P =(6.5±1.4)x10^–12. This implies a magnetic field B = 1.6x10¹⁴ G, spin-down power {dot}E =5x10³³ erg/s, and characteristic age P/2{dot}P =9x10³ yr for the rotating dipole model. However, the current {dot}P may be erratic, especially during outburst. The flux and modulation remained steady during the observations and the 3-79 keV spectrum is well fitted by a combined blackbody plus power-law model with temperature kT_BB= 0.96 ±0.02 keV and photon index Γ = 1.5±0.4. The neutral hydrogen column density (N_H∼ 1.4x10²³/cm2) measured by NuSTAR and Swift suggests that SGR J1745-29 is located at or near the Galactic center. The lack of an X-ray counterpart in the published Chandra survey catalog sets a quiescent 2-8 keV luminosity limit of L_x ≲ 10³² erg/s. The bursting, timing, and spectral properties indicate a transient magnetar undergoing an outburst with 2-79 keV luminosity up to 3.5x10³⁵ erg/s for a distance of 8 kpc. SGR J1745-29 joins a growing subclass of transient magnetars, indicating that many magnetars in quiescence remain undetected in the X-ray band or have been detected as high-B radio pulsars. The peculiar location of SGR J1745-29 has important implications for the formation and dynamics of neutron stars in the Galactic center region.