NuSTAR DISCOVERY OF A 3.76 s TRANSIENT MAGNETAR NEAR SAGITTARIUS A*

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 Ṗ = (6. 5 ± 1. 4) × 10^ (–12). This implies a magnetic field B = 1. 6 × 10^ (14) G, spin-down power Ė =5 × 10^ (33) erg s^ (–1), and characteristic age P/2Ṗ =9 × 10³ yr for the rotating dipole model. However, the current Ṗ 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 (NH ~ 1. 4 × 10^ (23) cm^ (–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 Lₓ ≾ 10^ (32) erg s^ (–1). The bursting, timing, and spectral properties indicate a transient magnetar undergoing an outburst with 2-79 keV luminosity up to 3. 5 × 10^ (35) 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.