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The Astrophysical Journal Letters June 2013. Volume 770, Issue 2, article id. L23, 5 pp. (2013).

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Mori, Kaya; Gotthelf, Eric V.; Zhang, Shuo; An, Hongjun; Baganoff, Frederick K.; Barrière, Nicolas M.; Beloborodov, Andrei M.; Boggs, Steven E.; Christensen, Finn E.; Craig, William W.; Dufour, Francois; Grefenstette, Brian W.; Hailey, Charles J.; Harrison, Fiona A.; Hong, Jaesub; Kaspi, Victoria M.; Kennea, Jamie A.; Madsen, Kristin K.; Markwardt, Craig B.; Nynka, Melania; Stern, Daniel; Tomsick, John A.; Zhang, William W.

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2013

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) × 10-12. This implies a magnetic field B = 1.6 × 1014 G, spin-down power dot{E} =5 × 1033 erg s-1, and characteristic age P/2dot{P} =9 × 103 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 ~ 1.4 × 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.