X-Ray Observations of Magnetar SGR 0501+4516 from Outburst to Quiescence

TL;DR

This study analyzes the spectral and timing properties of the magnetar SGR 0501+4516 during its transition from outburst to quiescence, revealing insights into magnetic field effects and emission mechanisms.

Contribution

It introduces a two blackbody spectral model for magnetar quiescence and demonstrates its applicability to the entire magnetar population, linking temperature trends to magnetic field decay.

Findings

The quiescent spectrum fits a power-law plus two blackbody components.

The hotspot radius decreased from 1.4 km to 0.49 km after outburst.

A correlation exists between hotspot area and X-ray luminosity.

Abstract

Magnetars are neutron stars having extreme magnetic field strengths. Study of their emission properties in quiescent state can help understand effects of a strong magnetic field on neutron stars. SGR 0501+4516 is a magnetar that was discovered in 2008 during an outburst, which has recently returned to quiescence. We report its spectral and timing properties measured with new and archival observations from the Chandra X-Ray Observatory, XMM-Newton, and Suzaku. We found that the quiescent spectrum is best fit by a power-law plus two blackbody model, with temperatures of kT_low~0.26 keV and kT_high~0.62 keV. We interpret these two blackbody components as emission from a hotspot and the entire surface. The hotspot radius shrunk from 1.4 km to 0.49 km since the outburst, and there was a significant correlation between its area and the X-ray luminosity, which agrees well with the prediction by the twisted magnetosphere model. We 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.