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

Abstract

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 =(6.5 ± 1.4) × 10−12. This implies a magnetic field B = 1.6 × 1014 G, spin-down power =5 × 1033 erg s−1, and characteristic age P/2 =9 × 103 yr for the rotating dipole model. However, the current 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 × 1023 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 ≲ 1032 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 × 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.