X-ray observations of magnetar SGR 0501+4516 from outburst to quiescence

Abstract

Magnetars are neutron stars having extreme magnetic field strengths. Study of their emission properties in the quiescent state can help understand effects of a strong magnetic field on neutron stars. SGR 0501+4516 is a magnetar discovered in 2008 during an outburst and has recently returned to quiescence. I report its spectral and timing properties measured with new and archival observations from the Chandra X-Ray Observatory, XMM-Newton, and Suzaku. I found that the quiescent spectrum is best fit by a power-law plus two blackbody model, with temperatures of kT_low ∼ 0.25 keV and kT_high ∼ 0.61 keV and a photon index of Γ ∼ 3.2. I interpret these two blackbody components as emission from a hotspot and the entire surface. The hotspot radius shrinked from 1.5 km to 0.5 km since outburst and there is a significant correlation between its area and the X-ray luminosity, which well agrees with prediction by the twisted magnetosphere model. I applied the two temperature spectral model to all magnetars in quiescence and found that it could be a common feature among the population. Moreover, the temperature of the cooler blackbody shows a general trend with the magnetar field strength, which supports the simple scenario of heating by magnetic field decay