Nustar Discovery of a 3.76-second 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 9 days apart yield a spin-down rate of Ṗ= (6.5± 1.4)× 10. This implies a magnetic field B = 1.6× 10 G, spin-down power Ė= 5 × 10 erg s, and characteristic age P/2Ṗ= 9 × 10 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 kTBB = 0.96 ± 0.02 keV and photon index Γ = 1.5 ± 0.4. The neutral hydrogen column density (NH ∼ 1.4 × 10 23 cm) 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. The bursting, timing, and spectral properties indicate a transient magnetar undergoing an outburst with 2 − 79 keV luminosity up to 3.5× 10 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. Subject headings: pulsars: individual (SGR J1745−29) — stars: neutron