| Summary: | 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 over P] = (6.5 ± 1.4) × 10[superscipt –12]. This implies a magnetic field B = 1.6 × 10[superscript 14] G, spin-down power [dot over E] = 5 × 10[superscript 33] erg s[superscript –1], and characteristic age P/2 [dot over P] = 9 × 10[superscript 3] yr for the rotating dipole model. However, the current [dot over 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 [subscript BB] = 0.96 ± 0.02 keV and photon index Γ = 1.5 ± 0.4. The neutral hydrogen column density (N [subscript H] ~ 1.4 × 10[superscript 23] cm[superscript –2]) 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[subscript x] [< over ~] 10[superscript 32] erg s[superscript –1]. The bursting, timing, and spectral properties indicate a transient magnetar undergoing an outburst with 2-79 keV luminosity up to 3.5 × 1035 erg s–1 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.
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