NuSTAR Detection of Quiescent Hard X-Ray Emission from SGR 0526–66 in the Large Magellanic Cloud

Abstract

The soft γ -ray repeater (SGR) 0526–66 is the first-identified magnetar, and is projected within the supernova remnant N49 in the Large Magellanic Cloud. Based on our ∼50 ks NuSTAR observation, we detect the quiescent-state 0526–66 for the first time in the 10–40 keV band. Based on the joint analysis of our NuSTAR and the archival Chandra ACIS data, we firmly establish the presence of the nonthermal component in the X-ray spectrum of 0526–66 in addition to the thermal emission. In the best-fit blackbody (BB) plus power-law (PL) model, the slope of the PL component (photon index Γ = 2.1) is steeper than those (Γ ≲ 1.5) for other magnetars. The soft part of the X-ray spectrum can be described with a BB component with the temperature of kT  = 0.43 keV. The best-fit radius ( R  = 6.5 km) of the X-ray-emitting area is smaller than the canonical size of a neutron star. If we assume an underlying cool BB component with the canonical radius of R  = 10 km for the neutron star in addition to the hot BB component (2BB + PL model), a lower BB temperature of kT  = 0.24 keV is obtained for the passively cooling neutron star’s surface, while the hot spot emission with kT  = 0.46 keV dominates the thermal spectrum (∼85% of the thermal luminosity in the 0.5–5 keV band). The nonthermal component (Γ ∼ 1.8) is still required.