NuSTAR discovery of a 3.76-second 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 9 days apart yield a spin-down rate of Pdot = (6.5+/-1.4)x10^{-12}. This implies a magnetic field B = 1.6x10^14 G, spin-down power Edot = 5x10^33 erg/s, and characteristic age P/2Pdot = 9x10^3 yr, for the rotating dipole model. However, the current Pdot 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 = 1.5+/-0.4, respectively. The neutral hydrogen column density (nH ~ 1.4x10^23 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. The bursting, timing, and spectral properties indicate a transient magnetar undergoing an outburst with 2-79 keV luminosity up to 3.5x10^35 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.

Discovery of a Transient Magnetar Near Sagittarius A*

The paper "NuSTAR Discovery of a 3.76-Second Transient Magnetar Near Sagittarius A*" presents a detailed account of the identification and characterization of a transient magnetar, SGR J1745−29, situated in close proximity to the supermassive black hole at the center of our galaxy, Sagittarius A*. The discovery was facilitated by observations conducted using the NuSTAR Observatory, complemented by follow-up data from Swift and Chandra.

Key Findings

The NuSTAR observations uncovered periodic X-ray pulsations with a period of 3.76 seconds from this newly identified source, signaling its nature as a magnetar. The reported pulsed fraction is 27 ± 3% within the 3-12 keV band. The magnetar exhibits a spin-down rate of Ṗ = (6.5 ± 1.4) × 10^-12, which suggests a magnetic field strength of B = 1.6 × 10^14 G and a characteristic age of approximately 9,000 years assuming a dipole braking model. These findings are indicative of the intense magnetic fields that define magnetars and fuel their energetic emissions.

The spectral analysis conducted across the 3-79 keV band reveals that the source's emission can be modeled as a combination of blackbody radiation and a power-law component. The temperature of the blackbody component is found to be kT_BB = 0.96 ± 0.02 keV, while the photon index of the power-law component is Γ = 1.5 ± 0.4. The total unabsorbed luminosity in the 2-79 keV range is calculated to be 3.5 × 10^35 erg s^-1 assuming a distance of 8 kpc, consistent with its location at the Galactic Center.

Implications and Future Directions

The discovery of SGR J1745−29 adds to the growing roster of transient magnetars, a subclass characterized by rapid outbursts and relatively low quiescent luminosity. Such transient magnetars may remain undetected during quiescent phases unless observed at radio wavelengths, as demonstrated by the additional radio observations capturing pulsations at the same period. This emphasizes the need for comprehensive multiwavelength monitoring to unveil the complete lifecycle of these enigmatic objects.

The peculiar location near Sagittarius A* prompts questions about the astrophysical processes governing neutron star formation and dynamics within the chaotic environment of the Galactic Center. The formation of a neutron star in such a region may involve interactions with the dense stellar population and supermassive black hole, influencing its evolutionary trajectory and behavior.

Long-term monitoring of SGR J1745−29 will be crucial in understanding the temporal evolution of magnetars and their interaction with the surrounding medium. Continued observations using X-ray and radio telescopes are anticipated to provide valuable insights into the mechanisms driving magnetar outbursts and power generation, thereby enriching the theoretical frameworks regarding neutron stars and their magnetic phenomena.

Conclusion

This paper contributes significant observational data that enhance our understanding of magnetar physics, particularly within the unique environment of the Galactic Center. It underscores the importance of coordinated multi-instrument observations in identifying and studying transient astrophysical phenomena. Future research should aim to expand the sample of known magnetars and refine models of neutron star formation and magnetic field evolution in such dense regions.